Sterrekunde

Hoe kan ons die helling van die wentelvlak van die eksoplanete leer met behulp van radiale spoedmetode?

Hoe kan ons die helling van die wentelvlak van die eksoplanete leer met behulp van radiale spoedmetode?


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Ek het 'n paar jaar gelede gehoor van die radiale spoedmetode om eksoplanete te ontdek, maar hierdie vrae het my bly verwar. As ons 'n planeetster van gemors ken $ m $ wentel 'n ster deur die radiale spoed van die sentrale massa te interpreteer, hoe kan ons sien of dit 'n selfs massiewe voorwerp is waarvan die wentelbaan 'n groter helling het? Verder kan ons die rigting van die baan bepaal, selfs al ken ons die hoek van ons sig en die as van die baan?

Dankie by voorbaat.


Meestal kan u nie die helling van 'n planeet bepaal wat deur die radiale snelheidsmetode opgespoor word nie, maar in sekere gunstige gevalle sal die dinamiese wisselwerking tussen twee planete in dieselfde stelsel u in staat stel om die helling te beperk. Dit is gebruik om die helling van die resonante planete in die Gliese 876-stelsel te bepaal, bv. Rivera et al. (2005).

Andersins, moet u radiale snelhede kombineer met 'n ander opsporingsmetode, byvoorbeeld deurgange of astrometrie.


Ek neem aan jy praat oor die radiaal snelheidsmetode.

Analise van 'n radiale snelheidskurwe lewer die minimum massa van 'n planeet $ M sin i $, waar $ M $ is die ware planetêre massa en $ i $ is sy baanhelling (90 grade sou beteken dat die baanvlak in ons siglyn is).

Oor die algemeen is daar geen ander inligting wat u kan sê nie. In prinsiep $ M $ enige waarde kan hê $ geq M sin i $.

Natuurlik toenemend klein waardes van $ sin i $ is toenemend onwaarskynlik en die gemiddelde $ sin i $ van 'n ewekansige georiënteerde bevolking is $ pi / 4 $.

As u 'n vervoer sien, weet u dit $ i $ is naby aan 90 grade en kan in werklikheid skat $ i $ vanaf die transito-duur.

As u die meet sterre rotasieperiode en kan die geprojekteerde ekwatoriale snelheid skat vanaf die spektraallynverbreding, dan kan jy die neiging tot die siglyn skat. 'N Mens sou dan kon aanneem dat die planeet in die ekwatoriale vlak van die ster wentel en dieselfde hellings vir sy baan gebruik.


Moet dit in lyn wees met die aarde (moet die planeet tussen die aarde en sy ster beweeg) om eksoplanete met transito-fotometrie te ontdek? As dit so is, beteken dit dan dat planete wat in 'n vlak wentel waar dit nie tussen die aarde en sy ster beweeg nie, nie waarneembaar is nie?

Ja, die transiterende planeet moet min of meer tussen ons teleskope en die ster beweeg, wat natuurlik 'n beperking is.

Daar is 'n paar alternatiewe waarvoor die planeet nie ons siglyn moet sny nie, soos Astrometrie. Dit behels die meet van die wiebeling van die ster terwyl die planeet om hom wentel. Dit meet die periodieke verandering in posisie van die ster direk. Aangesien die slinger in twee dimensies opgespoor kan word, gee dit sterrekundiges 'n manier om die helling van die planeet se baan te meet.

As die baanvlak nooit ons siglyn sny nie, kan ons steeds sien hoe die ster rondom die massamiddelpunt van die ster-planeetstelsel beweeg.

Is dit moontlik om planete op aarde te bespeur met hierdie metode? Dit wil voorkom asof 'n planeet groot moet wees om 'n merkbare uitwerking op die ouerster te veroorsaak.

Dankie vir die verduideliking!

Ek neem aan dat die wentelvlak van planete heeltemal ewekansig is en dus eweredig versprei word oor alle moontlike vlakke. Is dit reg?

Gestel 100% van die sterre het planete.

Watter persentasie van daardie sterre kon ontdek ons ​​planete rondom die transito-metode?

Watter persentasie van daardie sterre doen ontdek ons ​​planete rondom die transito-metode?

As die baanvlak vir ons normaal is soos in die visualisering, hoe sou u die wiebeling van die ster meet? Het u nie 'n beweging in en uit die vliegtuig nodig om die rooi / blou verskuiwing van die lig wat van die ster af kom, te meet nie?

Juis. Dit is waarom slegs ongeveer een uit die honderd Aarde-planete waarneembaar sou wees met behulp van die transito-tegniek. En waarom die tegniek gewoonlik nie gebruik word om planetêre opsporing te probeer doen nie. Die rede waarom dit saam met die Kepler-missie gebruik is, is dat dit een van die min tegnieke is wat gebruik kan word terwyl duisende sterre gelyktydig waargeneem word. Die hoofdoel van Kepler was om vas te stel hoe algemene planete statisties is, wat u kan bereken as u die waarskynlikheid van opsporing ken.

Ja, ten minste met behulp van die vervoermetode kan dit nie opgespoor word nie. Die ander opmerking is korrek deur te sê dat die radiale snelheidsmetode (die meting van die oorsaak van die Doppler-verskuiwing deur die ster & # x27s & quotwobble & quot) 'n wyer verskeidenheid wentel hellings kan opspoor, waaronder sommige wat nie deur die vervoer waargeneem kan word nie, maar dit beteken nie dat dit planete in 'n arbitrêre hoek op te spoor. Daar kan baie stelsels wees waarvan die fundamentele vlak (byna) loodreg op ons siening is en ons sal nooit weet dat die planete eers bestaan ​​nie.

Pret feit: ons baanvlak is loodreg op die aansiglyn vanaf Alpha Centauri. As iemand ons hier op een van hierdie maniere probeer opspoor, kan hulle dit nie doen nie.

As die baanvlak van die planeet egter & oppervlak & quot is wanneer dit vanaf die aarde waargeneem word, sal die hele wankeling van die ster loodreg op 'n waarnemingslyn wees. Alhoewel die ster aansienlik binne die baanvlak kan beweeg, sal geen deel van die beweging na die aarde toe of daarvandaan wees nie. Geen spektrumverskuiwing sal opgespoor word nie, en die aardgebonde waarnemer sal onkundig bly oor die aanwesigheid van 'n planeet wat om die ster wentel.

Direkte beelding kan egter steeds werk. Dit is ingewikkeld omdat planete BAIE oorskadu word deur hul ouerster, en omdat ons so ver weg is, lyk dit asof hulle baie naby daaraan is. Infrarooi beelding kan wel 'n bietjie beter vaar: planete word nog steeds oorskadu deur die ster, maar die verskil is minder ekstrem.


Hoe kan ons die helling van die wentelvlak van die eksoplanete leer met behulp van radiale spoedmetode? - Sterrekunde

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Hoe benader sterrekundiges hul soeke na lewe in die heelal? Wat het ons geleer uit die oplewing van eksoplanete-ontdekkings? Hoe waarskynlik is dit dat die aarde nie die enigste lewe in die heelal huisves nie? In hierdie kursus ondersoek ons ​​die veld van astrobiologie, 'n opkomende multidissiplinêre veld. Vordering in astrobiologie word aangedryf deur teleskope op die grond en in die ruimte, en deur nuwe insigte oor hoe lewe op aarde ontstaan ​​het en die diversiteit daarvan. Die onderwerpe in hierdie kursus wissel van die wetenskap oor hoe eksoplanete opgespoor word, tot die chemie wat die argument ondersteun dat die bestanddele vir die lewe algemeen in die heelal voorkom. Ons sal die ontledings van kundiges in chemie, sterrekunde, geologie en argeologie volg om 'n sterk grondslag van begrip te bou. Teen die laaste opdrag sal studente toegerus wees met die nodige kennis om te identifiseer wat 'n planeet bewoonbaar maak en hoe waarskynlik die lewe daar is. Studente studeer af van hierdie kursus, ingelig oor een van die opwindendste vakgebiede in die hele wetenskap, en is gereed om die huidige nuusberigte en ontdekkings oor eksoplanet te bespreek.

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Uitstekende kursus Chris Impey is 'n wonderlike innemende tutor. Het albei sy kursusse aangebied wat aangebied is deur die Universiteit van Arizona en lees tans sy eerste boek. Eersteklas.

Een van die beste kursusse in die Astronomie-tema wat ek gesien het. Met baie opdragte en vasvrae, is hierdie module beslis die moeite werd om in te skryf. Die notas, video's is regtig wonderlik!

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Chris Impey

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In hierdie lesing praat ons oor die transito-metode om eksoplanete op te spoor. In die transito-metode monitor sterrekundiges die sterlig van 'n ster vir periodieke verduistering, wat aandui dat 'n eksoplanet 'n fraksie van die sterlig blokkeer terwyl dit wentel. Dus, ons gebruik die terme transito-metode of Eclipse-metode ewe veel. Tot die lansering van Kepler is die meeste eksoplanete volgens die radiale snelheidsmetode gevind. Hierdie grafiek toon die fenomenale hoeveelheid data wat pas in die eerste jaar van die Kepler-missie vrygestel is. Terwyl die eksoplanet om die ster beweeg, beweeg dit periodiek tussen ons, die waarnemers en die ster, en blokkeer die sterlig. Let nou op dat die verduistering van die verduistering met die transito-metode slegs van toepassing is in 'n deelversameling van die stelsels waar die baanvlak na ons toe gekantel is, sodat ons in die ekwatoriale vlak kyk. As ons van bo af reguit na die stelsel sou kyk, sou daar geen deurgang of verduistering wees nie. Hierdie metode is dus slegs moontlik vir 'n klein subset van die eksoplanete, en sterrekundiges moet die ontbrekende data regstel. Hier is die opsporing van 'n enkele eksoplanet wat deur 'n sonagtige ster gaan. Nou kyk ons ​​in die ekwatoriaalvlak waar die transito as 'n kort duik gesien kan word terwyl dit voor die ster verbyloop. Dit gebeur vir 'n klein fraksie van die baan van die eksoplanet. Daar moet dus baie sterre gelyktydig waargeneem word om 'n statistiese kans te hê om hierdie effek te sien. Maar daar is 'n probleem met hierdie metode vir tipiese stelsels, tensy u die sonnestelsel as voorbeeld gebruik. Laat ons Jupiter en die son gebruik. Laat ons ons voorstel dat ons die sonnestelsel van ver af waarneem en hoop om Jupiter op te spoor. Jupiter beweeg teen 13 kilometer per sekonde. As u die Doppler- of radiale snelheidsmetode gebruik, is dit die Doppler-verskuiwing wat u sal moet opspoor om Jupiter op te spoor, maar as Jupiter beskou word as die son deurvoer, gesien vanaf die kant wat hy elke 12 jaar sou doen, neem dit minder as 'n dag om die son se oppervlak oor te steek, gesien van ver af. Dus, in 'n baan van 12 jaar, dit is ongeveer 1 / 500ste van die tyd, moet u fyn dophou. U kan dus sien hoe moeilik hierdie tegniek is omdat die transito vir 'n massiewe planeet plaasvind wat ver van sy ster af wentel vir so 'n klein fraksie van die wentyd. U spandeer die meeste van u tyd om na die ster te kyk, daar sal niks gebeur nie, en dan skielik, vir 'n klein rukkie, knip en u mis dit, daar is 'n deurreis en 'n verduistering. Die oplossing wat sterrekundiges vir hierdie probleem gebruik, is nie om na enkele sterre te kyk nie, maar om na soveel sterre as moontlik te kyk, sodat een of meer van hulle gemiddeld in enige tydperk wat u dit waarneem, deurgang gee. Die seldsame verduisterings word dus in wese aangetrek deur goeie statistieke en 'n groot aantal gelyktydige waarnemings. Met die Kepler-ruimtetuig is dit gedoen deur na een hemelruim te staar waar meer as 150 000 sterre was, wat almal tegelyk bestudeer is. Hier word twee voorbeelde getoon wat om sonagtige sterre wentel. In een geval veroorsaak 'n klein planeet 'n relatiewe klein duik of deurvoer. In die ander geval veroorsaak 'n groter planeet 'n groter duik. Die diepte van die verduistering of die duik stem ooreen met die verhouding van die oppervlaktes van die planeet in verhouding tot die ouerster. Dus, die grootte van eksoplanete hou natuurlik verband met die verduistering. Ons stel ons voor dat die planeet ondeursigtig is en dat dit dus 'n paar breuke van die ster se lig blokkeer as dit die gesig van die ster kruis. Die verhouding is baie eenvoudig. Dit is die dwarsdeursnee van die planeet gedeel deur die dwarsdeursnee van die ster. Dit is die fraksie van die ster se lig wat [onhoorbaar is] en dus die persentasie van die ster se lig wat verdof word wanneer die transito plaasvind. Kleiner planete het kleiner radiusse en blokkeer dus nie soveel sterlig tydens hul deurgang nie. In hierdie animasie voeg ons 'n detail by die transito-metode. Waar, behalwe dat die eksoplanet die gesig van sy ster kruis en 'n aansienlike duik veroorsaak, is daar 'n tweede duik omdat die eksoplanete in weerkaatsde lig skyn en die weerkaatsde lig verdwyn as dit agter die ster verbygaan en 'n kleiner sekondêre duik veroorsaak. Hierdie effekte is nou gesien in tientalle eksoplanete deur die Kepler-ruimtetuig. Wat gebeur as 'n ster verskeie planete het? Hier het ons 'n bo-onder-aansig, maar as ons in die ekwatoriale vlak is, is daar die moontlikheid dat een of meer van die planete kan deurvoer. Hulle sal oorgaan met frekwensies wat deur hul wenteltydperke gegee word, en die ligkromme aan die linkerkant wys die relatiewe dalings wat ooreenstem met die grootte van daardie eksoplanete. In werklike data van die Kepler-satelliet is daar baie gevalle van veelvuldige planete op hierdie manier gevind. Hier is die data van die transito en dit is die soort data wat u van die Kepler-ruimtetuig kry wat werklike data toon, en hier sien u 'n transito-waarneming waar die dip iets meer as een persent is, tussen een en twee persent. Jupiter is tien keer kleiner as die son. As Jupiter dus die gesig van die son oorsteek, sal dit die son met 1% verdof. U kan dus dadelik sien dat as dit 'n sonagtige ster was, die planete wat gesien word in 'n transito-rigting effens groter is as Jupiter omdat die verduistering effens meer as een persent is. U kan ander voorbeelde hier sien. U kan sien dat die individuele data van die CCD-detektore nogal luidrugtig is. Maar omdat u gemiddeld is met die tyd wanneer daar geen verduistering is nie, is die klein streek waar daar 'n verduistering is, nog steeds baie maklik om op te spoor, en u kan die data ook met 'n kurwe modelleer. Hier is 'n paar regte data-groottes gemeet deur die Kepler-ruimtetuig. Aanvanklik het Kepler baie meer Jupiter-grootte eksoplanete gevind as aardagtige planete. U kan die verspreidings hier sien. Die wenteltydperke, dit is natuurlik natuurlik planete van die aarde op baie kort wentelbane. Hulle is dus warm planete, niks soos bewoonbaar nie. Dit is planete van Jupiter-grootte, ook op kort vinnige wentelbane. Hulle is dus warm Jupiters. Soos ons gesien het, was warm Jupiters die eerste eksoplanete wat ontdek is. Net soos warm Jupiters makliker met die Doppler-metode opgespoor kan word omdat hulle die grootste massas en die kortste wentelbane het, is dit ook die maklikste om met die transito-metode op te spoor. Want as u dink aan die geometrie van 'n planeet wat om 'n ster wentel, en wanneer 'n planeet van 'n gegewe grootte nader aan die ster is, is die kans groter om 'n verduistering waar te neem, hoër, en dit blyk dat ongeveer een uit elke 10 planete hul sterre in 'n warm Jupiter-scenario, in teenstelling met 'n klein fraksie van 'n persent van die aarde-agtige planete wat hul ster kruis as u op soek is na baie kleiner voorwerpe. Net soos met die radiale snelheidstegniek, verkies die transito-metode sterk om warm Jupiters te vind, en dit is inderdaad wat Kepler vroeg al vind. Net soos met die radiale snelheidstegniek, verkies die transito-metode sterk om warm Jupiters te vind, en dit is inderdaad wat Kepler vroeg al vind. Die vervoermetode het ook beperkings. Planete met 'n groot radius lewer duidelike dalings wat maklik opspoorbaar is. U kan baie sterk waarnemings hier sien, waar die geraas op die data amper onsigbaar is en die vervoer regtig duidelik is. Maar as u afkom na Super Aarde en Aarde-grootte planete, is die data baie meer geraas en is dit baie moeiliker om daardie duik op te spoor, wat natuurlik in die geval van 'n aardagtige planeet en 'n sonagtige ster is 100ste persent, een deel uit tien tot die vier. Dit & # x27s redelik subtiel. Die voordeel is nou dat u verskeie waarnemings kan neem omdat die baan herhaal, en as u die wentelperiode ken, weet u wanneer u die data moet byvoeg vanaf die volgende keer dat die transito plaasvind, en u kan die verduisterings bymekaar tel. In die geval van Kepler was die span nie bereid om 'n eksoplanet op te spoor nie, tensy 'n verduistering twee keer bevestig is. Met ander woorde, een enkele meting hierdeur was nie voldoende om die opsporing van 'n eksoplaneet te verklaar nie, en selfs die herhaling daarvan nog nie voldoende was nie. Dit sal u natuurlik toelaat om die periode te bepaal, maar sal u die periode eers bevestig voordat u dit 'n derde keer gesien het. Dus, Kepler het die derde verduisteringsmeting van 'n gegewe voorwerp gebruik om dit as eg te verklaar en natuurlik die data bymekaar te tel. As u 'n derde gehad het, kon u 'n vierde, 'n vyfde ensovoorts kry as u genoeg data gehad het. As u praat oor klein wenteltydperke, in die missie van Kepler, het u moontlik tientalle of soms selfs honderde gange. Maar in die geval van 'n aardse situasie met 'n baan van een jaar, was die Kepler-missie nie lank genoeg om meer as 'n paar gebeurtenisse te vind nie. Planete op toenemende wentelafstand blokkeer minder lig van hul ster. Dit word dus al hoe moeiliker om deurgange op te spoor namate die planete verder van hul ster af is. Die transito-meetkunde kom ook voor vir 'n baie klein stel wentelings. Dit is dus baie skaarser om op te spoor, en u moet baie meer waarnemings insamel om daardie seldsame gevalle te vind. Laat ons 'n idee kry van die ongelooflike diversiteit van die eksoplanete wat Kepler ontdek het. Elke klein diagram toon 'n baan van 'n eksoplanet om 'n sonagtige ster of op baie plekke 'n dwergster. Die grootte van die kolletjies stel die grootte van die eksoplanete voor, die spoed van die wentelbane en die spoed van hul wentelbane is natuurlik nie tot die regte tyd nie. U kan multi-planeetstelsels in 'n aantal gevalle sien. Dit is slegs 'n deelversameling van die duisende planete wat deur die Kepler-ruimtetuig gevind is. Sommige van die vinnigste bane relatief tot die stadigste bane is fantastiese vinnige bane van minder as 'n dag. Die langste wentelbane, maande of jare. Ons sal inzoom op sommige van die intensste, vinnigste van hierdie eksoplanete. Die musiek is gepas van Star Wars. Om hierdie lesing op te som, meet die transito-metode klein duik in die ligkrommes van sterre om eksoplanete op te spoor. Die eksoplanet kruis die gesig van die son en blokkeer 'n klein fraksie van sy lig wat lei tot 'n duik in die helderheid en die grootte van die exoplanet word met hierdie metode vervaardig. Let op: hierdie metode gee net die grootte, maar nie die massa nie, terwyl die radiale snelheidsmetode die massa gee, maar nie die grootte nie. Hulle is dus aanvullend in daardie sin. Eksoplanete se grootte beperk die waarneming. Vir 'n aardagtige planeet en 'n sonagtige ster is die verduistering 100 persent, en die Kepler-satelliet is ontwerp om dit op te spoor, maar nie veel minder nie. Dit word die limiet van die tegniek. Dit is ook moeilik om eksoplanete op groot orbitale afstande op te spoor omdat 'n baie klein fraksie van die orbitale meetkunde deurgange of verduisterings gee.


Die metodes van opsporing

Gedurende die afgelope twee dekades is verskillende tegnieke gebruik om eksoplanete op te spoor. Soos vroeër genoem, spoel die lig wat deur 'n ouerster uitgestraal word, altyd die bietjie lig wat deur sy planeet (te) weerkaats word. Daarom moes wetenskaplikes met alternatiewe en indirekte metodes vorendag kom om eksoplanete op te spoor, aangesien dit amper onmoontlik is om dit direk waar te neem. Hierdie hoofstuk gee 'n oorsig van die mees gevestigde metodes wat sukses behaal het, asook die logika en wetenskap daaragter, terwyl die voor- en nadele van elke metode bespreek word.


Ander sonnestelsels Ander Aarde

Duisende jare lank het ons nadink oor ons plek in die uitgestrektheid van die heelal. Ons het die blinkende sterre hierbo as ander wêrelde voorgestel, bevolk deur wesens wat soortgelyk is aan onsself. Maar in werklikheid het ons vir die meeste van daardie eeue in die verlede nie eens geweet of daar wêrelde rondom die sterre was nie, wat nog te sê mense daarop. Dit het alles in 1995 verander, ten minste tot by die aanwesigheid van planete. Nuus is regoor die wêreld gestuur wat die ontdekking van die allereerste planeet wat 'n ander ster as die son omring, aankondig. Sedertdien het die aantal van hierdie sogenaamde eksoplanete vermenigvuldig tot op die punt dat hulle nou 25 jaar later in duisende is.

Hierna sal ons die ontdekking van hierdie verre uitheemse wêrelde uiteensit, tesame met 'n oorsig van hoe ons onderneem het om dit op te spoor en te bestudeer. Ons eindig met die toekomsvooruitsigte om die bekende aantal te vergroot en ons begrip daarvan te verdiep.

Begin

Die eerste eksoplanet wat ontdek is wat om 'n normale ster wentel, is in Desember 1995 deur die Switserse sterrekundiges Didier Queloz en Michel Mayer aangekondig. Hulle het die planeet reeds weke tevore bespeur, maar gewag op bevestiging deur ander sterrekundiges voordat hulle die wêreld vertel het. Radiale snelheidsgegewens het aangedui dat die ster 51 Pegasi, 'n sonagtige ster in die sterrebeeld Pegasus, saamgespan is deur 'n ongesiene planeetgenoot.

Die bewyse vir hierdie planeet bestaan ​​uit 'n effense beweging van die ster na en dan weg van die aarde in 'n sikliese patroon. Die beweging was soortgelyk aan dié wat in spektroskopiese binêre sterre gesien is, behalwe dat daar in hierdie geval slegs een stel sterre lyne aanwesig was. Wat meer is, die klein bewegings het voorgestel dat die metgesel baie kleiner as die ster was. Kleiner, in werklikheid, as die planeet Jupiter. Soos tipies geword het vir die benoeming van buite-solare planete, is hierdie ontdekking 51 Peg b genoem.

Ons moet daarop let dat daar vroeër, in 1992, buite-sonplanete bevestig is, maar dit wentel om die pulserende PSR1257 + 12, nie 'n normale ster nie. Die opsporing daarvan was moontlik omdat die baanbeweging van die stelsel effense verskuiwings in die periode van die pulsar veroorsaak het, 'n metode wat ongeveer analoog is aan, maar anders as, die metode wat gebruik word om die planeet op te spoor wat om 51 Peg wentel. Verskeie ander pulse met planete is bekend, maar omdat die aard van hierdie stelsels verskil van dié rondom normale sterre, sal ons dit verder bespreek.

Nadat die ontdekking van 51 Peg b aangekondig is, het addisionele opsporings vinnig gevolg. Dit was moontlik omdat die planete in plat sig in die datastelle wat reeds in die besit van sterrekundiges was, weggekruip het. Die eerste wat aangekondig het, was Geoff Marcy en Paul Butler, twee sterrekundiges van die San Francisco State University in Kalifornië. Deur hul eie radiale snelheidsmetings van talle sterre te ondersoek, het hulle bewyse gevind vir verskeie planete wat vroeër ongesiens verbygegaan het.

Hulle het, net soos ander sterrekundiges, nie verwag om baie massiewe planete te sien wat baie naby hul sterre wentel nie. As gevolg hiervan is vroeëre soektogte nie geoptimaliseer om sulke stelsels op te spoor nie. Maar hul gegewens, soos dié van Queloz en Mayer, het hierdie verwagting gekantel. In die loop van die jaar het Marcy en Butler baie meer buitesolêre planete ontdek, net soos ander sterrekundiges. Die soorte planete wat hulle ontdek het, was meer uiteenlopend as die eerste paar.

Snel vorentoe, twee dekades, en daar is duisende planete rondom baie verskillende soorte sterre. Dit sluit sonagtige sterre in, maar ook sterre sowel as kleiner. En anders as die eerste planete wat gevind is, is sommige van die onlangse ontdekkings vergelykbaar met die aarde. Maar voordat ons dit alles raaksien, moet ons van naderby kyk hoe planete rondom ander sterre in die eerste plek gevind word.

Radiale snelheid

Ons dink oor die algemeen aan planete wat om sterre wentel, en in ons gedagtes dink ons ​​waarskynlik aan die planeet wat beweeg terwyl die ster stil bly. Dit is nie 'n akkurate beeld van wat gebeur nie. In plaas daarvan wentel planeet sowel as ster om hul gemeenskaplike massamiddelpunt. Hierdie massamiddelpunt is 'n geweegde gemiddelde van hul posisies.

As u hierdie idee probeer begryp, is dit waarskynlik, ten minste eers, makliker om te dink dat die twee voorwerpe dieselfde massa het. Daarbenewens vereenvoudig dit die begrip as albei in sirkelbane beweeg. Geen van hierdie vereenvoudigings sal ons gevolgtrekkings minder algemeen maak nie, maar slegs bedien om die begrip makliker te maak.

Stel u dus 'n binêre sterstelsel voor. Die massamiddelpunt (kortweg COM) vir die stelsel is op 'n punt halfpad tussen die twee sterre. Dit is waarskynlik maklik genoeg om voor te stel. Terwyl die sterre om die COM wentel, volg hulle identiese sirkelpaadjies, elkeen direk oorkant die ander. Elkeen neem dieselfde tyd om te wentel (wat sou gebeur as hulle dit nie gedoen het nie), sodat elkeen op dieselfde snelheid om sy baan beweeg. Dit is eenvoudig soos 'n stelsel kan wees.

Stel jou nou voor dat die twee sterre verskillende massas het. Hulle sal steeds om hul gemeenskaplike COM wentel, maar nou sal dit nie halfpad tussen hulle wees nie. As die een ster twee keer massiewer is as die ander ster, sal die COM nader aan die massiewe ster wees. In werklikheid sal die COM een derde wees van die middelpunt van die massiewe ster na die middel van die minder massiewe ster. As die massiewe ster nege keer die massa van sy metgesel is, sal die COM tien persent van die afstand van die massiewe ster tot die laermassa-ster lê, ensovoorts.

Aangesien albei sterre om die COM wentel, sal die ster wat nader daaraan is, 'n kleiner baan hê om te bedek as die sterre. Albei benodig steeds dieselfde tyd, een wentelperiode, om hul baan te voltooi. As gevolg hiervan sal die massiewe ster stadiger beweeg as die minder massiewe ster omdat dit 'n kleiner afstand beweeg.

Die situasie word in die onderstaande diagram geïllustreer. Daar word van die sterre gebruik gemaak dat dit ongelyk is, en vir die eenvoud sal ons aanneem dat dit op sirkelbane beweeg. Hierdie aanname, hoewel dit nie nodig is nie, vermy komplikasies, maar dra steeds die kernpunte van die metode oor.

Die waarnemer word op 'n groot afstand van die skerm af links bevind. Die twee sterre wentel om mekaar in die vlak van die skerm. Hul wentelbane word as streeplyne getoon. Op die oomblik getoon beweeg die massiewe ster na die waarnemer en die minder massiewe ster beweeg weg. Hul spoed word onderskeidelik deur die blou en oranje pyltjies voorgestel, en die lengte van elke pyl dui op spoed.


'N Halwe periode na die oomblik hierbo uitgebeeld, sal die twee sterre hul oriëntasie omgekeer het. Die blou ster sal bo die oranje een in die diagram wees, en dit sal na die waarnemer beweeg in plaas van weg, maar met dieselfde spoed. Hierdie situasie word in die onderstaande figuur getoon. Hierdie siklus sal onbepaald herhaal word.

Die waarnemer kan die Doppler-effek op sterre-absorpsielyne gebruik om die beweging van die sterre na en van haar af te bepaal. As snelhede teenoor haar negatief is, en snelhede van haar af positief is, kan die snelhede teenoor tyd geteken word. Die sterre se snelhede volg 'n sinusgolf. Albei het dieselfde periode, maar hulle sal met presies 'n halwe periode van mekaar af verskuif word. As die een ster 'n positiewe snelheid het (dit beweeg weg), sal die ander ster 'n negatiewe snelheid hê, wat dui op die rigting van die waarnemer. Verder sal die amplitude van die snelheidskurwe van een ster (die massiewe een) kleiner wees as die amplitude van die ander ster se snelheidskurwe. Moontlike voorbeelde van die krommes wat die waarnemer kan teken, word hieronder getoon.

Vir die voorbeeldkurwes is die massaverhoudings 1: 1, 2: 1, 10: 1 en 100: 1. Hierdie verhoudings word regs bo in elke plot getoon. Wanneer die sterre dieselfde massa het, is hul snelhede dieselfde. Maar as hulle ongelyke massas het, dan is hul snelhede ook ongelyk.
Aangesien die massiewer ster stadiger beweeg as die laermassa-ster, is die verhouding van die amplitude van hul radiale snelheidskurwes die omgekeerde van die verhouding van hul massas. Die snelheidsdiagramme toon hierdie omgekeerde verhouding duidelik. Ster 1, die massiewer van die twee, is in rooi geteken. Namate die massa toeneem in verhouding tot die massa van ster 2, neem die maksimum snelheid af in verhouding tot die massaverhouding. Dit is natuurlik alles te wyte aan die feit dat sy baanradius (sy afstand vanaf die COM) kleiner is, en sy baan ook kleiner is.

Wanneer ons planete bespreek wat om sterre wentel, is die massaverhoudings baie groter as die verhoudings in hierdie voorbeelde. Die son is ongeveer 300 000 keer massiewer as die aarde. Die COM van die Aarde-Son-stelsel is dus diep binne-in die son. Die amplitude van die son se snelheid rondom daardie punt is baie klein, slegs 'n paar sentimeter per sekonde. Ter vergelyking kan die wentelsnelheid van twee sterre in 'n binêre stelsel dikwels tien kilometer per sekonde of meer wees. Ter vergelyking is die wentelsnelheid van die aarde rondom die son 30 km / sek. Dit is duidelik dat dit moeilik is om 'n planeet soos die aarde wat 'n ster soos die son wentel, op te spoor, omdat die spoed van die ster so klein is.

Die radiale snelheidsmetode is baie sensitiewer vir die opsporing van groot planete, soos byvoorbeeld Jupiter. Dit is om hierdie rede dat die eerste eksoplanete wat gevind is groot planete was, groter as Jupiter. Maar in teenstelling met Jupiter, wentel hierdie liggame baie naby aan hul sterre, met wentelstrale baie kleiner as die aarde. Hul groot massas het beteken dat hul gasheersterre omwentelingsnelhede groot genoeg was om gesien te word, en hul klein omwentelingsradius & # 8211 en belangriker, klein omlooptydperke & # 8211 het beteken dat die snelheidskurwes maklik op relatief kort tydperke gesien kon word, baie , baie bane kan gesien word in slegs 'n paar jaar se data.

Die onderstaande figuur (van Marcy and Butler, Astrophysical Journal, 464, L147-L151, 1996) toon die snelheidskurwe vir die planetêre metgesel aan die ster 47 Virginis. Baie wentelperiodes (P

117 dae) pas binne die reeks waarnemings van 8 jaar wat hier uiteengesit word. Die planeet het 'n massa van 6,6 keer die van Jupiter.


Destyds was sterrekundiges 'n groot verrassing om reuse-planete soos Jupiter rondom hul gasheersterre te wentel. Dit het veroorsaak dat hulle hul idees oor planeetvorming en die evolusie van planetêre stelsels heroorweeg het. Terugskouend sou ons dalk verwag het dat dit sou gebeur. Voor die ontdekking van eksoplanete het ons net 'n enkele voorbeeld gehad van hoe 'n planetêre stelsel kan lyk en die een wat ons bevat! Soos dikwels die geval is, is dit moeilik om universele waarhede oor die wêreld uit een enkele voorbeeld af te lei. Sodra ons planete in ander stelsels opspoor, is ons dus dramaties bewus gemaak van hoe parochiaal ons denke was.

Transitte

Radiale snelhede is nie die enigste tegniek wat die aanwesigheid van planete wat om sterre in die omtrek wentel, kan openbaar nie. 'N Ander metode is voorgestel jare voordat planete werklik gevind is. Maar omdat dit onwaarskynlik was om positiewe resultate te lewer, is dit nooit in praktyk gebring nie. Dit het alles verander sodra planete begin het, of dit lyk asof dit amper soos reën uit die hemel val.

Gevoude ligkromme van Algol (Beta Persei), 'n verduisterende binêre ster waarin die sterre gereeld voor mekaar verbygaan soos gesien vanaf die aarde. As gevolg van hierdie verduisterings ondergaan die oënskynlike helderheid van die stelsel periodieke daling. Let daarop dat die tydas (horisontale as hier) baie tydperke op mekaar vou in hierdie diagram, en dus word die baanfase eerder as tyd geteken. Krediet: AAVSO Variable Star Astronomy, Hoofstuk 11.
Dit was sedert laat in die negentiende eeu bekend dat sommige sterre so wentel dat die een gereeld voor die ander verbygaan. Hierdie verduisterende binêre stelsels is redelik skaars, want dit vereis 'n noukeurige belyning van die sterbane rondom die rigting na die aarde. Hulle

'N Skematiese diagram van die Algol-stelsel wat die meetkunde vir verskillende dele van die ligkromme aandui. Planetêre deurgange werk op dieselfde manier, maar die daling in helderheid is baie minder uitgesproke. Krediet: Departement Fisika en Sterrekunde U. Tennessee, Knoxville.

reveal themselves most obviously by periodic dips in brightness of what appears as a single star from Earth. The most well-known of these eclipsing systems is the star Algol in the constellation Perseus. It has been known to vary in brightness for thousands of years, though its true nature has been known for only just over a century.

When a planet passes in front of its star we refer to the passage as a transit, not an eclipse. Transits of Mercury and Venus occur occasionally when they cross in front of the Sun. A series of images of Mercury transiting the Sun in 2019 are shown in the image at right: the apparent curve in its path is caused by the rotation of the Sun in the image plane due to the type of telescope mount used. The path of Mercury is actually a straight line across the Sun.

These transits are quite rare – the next one for Venus, for example, will not happen for almost two hundred years. However, when they do happen, the planetary disc blocks a small amount of light from the Sun, and this decrease in solar brightness can be measured.

William Borucki, an astronomer at the NASA Ames Research Center in Mountain View, California, had proposed to search for planets around other stars. The method he proposed relied upon the transit phenomenon. Borucki reasoned that transits for planets around other stars could be detected by a minute dimming of the starlight. This dimming would give away the presence of a planet that was itself too faint to see directly.

After many failed attempts, William Borucki’s proposal to build a dedicated planetary transit mission was finally accepted by NASA. It became the Kepler mission, launched on March 7, 2009. Kepler spent the next decade staring at a small patch of sky in the constellation Cygnus. Kepler was trying to catch tiny telltale dips in brightness in more than 150,000 stars. These dips would reveal the presence of unseen planets around those stars. During its lifetime Kepler saw more than 2600 of them.

Unlike the radial velocity planet search method, the transit method is not especially sensitive to planets that are particularly close to their host star. However, it does have greater sensitivity to large planets. This is because large planets have larger areas, and thus they cover more of the star during the transit. That means the decrease in brightness will be greater, making the transit easier to detect.


A series of images of Mercury transiting the Sun on November 11, 2019. The curved path is an artifact of using an alt-azimuth mount to make these images. Credit: K. McLin

Some numbers will make this effect more concrete. We can imagine ourselves in a nearby planetary system, looking at the solar system and trying to detect planets around the Sun. Ons

will take Earth and Jupiter as representative examples of a “small” and “large” planet, respectively. The relevant numbers are shown in the table.

From the table, we see that Jupiter would be much easier to detect as it transited the Sun. It would cause a 1% decrease in brightness as seen from some distant viewer. Small, but not compared to Earth. Earth would diminish the light of the Sun by a meager 0.008%, a tiny amount. The ability to detect these changes depends on the quality of the data, of course. This in turn depends on a number of different things, but primarily upon the amount of light from the star that can be collected by the telescope. In any event, it is clear that large planets are easier to find using transits than small planets.

To date, the number of exoplanets found orbiting normal stars exceeds 4000 (as of the date of this article, February 2020), and these are found in more than 3000 different stellar systems. Kepler, which is no longer operating, was responsible for the majority of these systems.

However, other ground and space-based telescopes have also contributed. One interesting example is a project called MEarth. It uses a collection of small robotic telescopes in Arizona to conduct a transit survey of red dwarf (spectral type M) stars, looking for Earth-like companions. Hence the name… M-Earth, or MEarth. So far, MEarth has not found another Earth orbiting a nearby M-dwarf star, but it has found a number of rocky planets as well as some gas giant planets, and it is still searching.


Photo of Kepler Space Telescope credit: NASA

From space, the search for new exoplanets is being carried out by the Transiting Exoplanet Survey Satellite, or TESS. Like Kepler, and as one can infer from its name, TESS employs the transit method to detect planets. However, unlike Kepler, TESS is looking over 85% of the entire sky, and it is directing its attention primarily to stars that are much closer to Earth than the stars Kepler viewed. In this respect it is somewhat similar to the MEarth project, but it has a much larger scope, monitoring more than 200,000 stars for transits.

Launched in July, 2018, TESS is expected to discover many thousands of new planets over its two year primary mission. And because TESS’s program stars are much closer than Kepler’s stars, following up TESS detections with ground-based observations will be much easier. As a result, astronomers will be able to characterize the properties of the new-found planets more precisely than was the case with many Kepler exoplanets.

The figures below show TESS light curves for two planets orbiting the star HD 21749. At left is a confirmed exoplanet with properties that lie between a super Earth and gas dwarf. On the right is a candidate Earth-analogue slightly smaller than Venus. If confirmed, this object would be the first Earth-sized exoplanet discovered by TESS.

TESS data for the star HD 21749. At left is the light curve for HD 21749b, a confirmed super Earth planet. At right is the light curve for the candidate exoplanet HD 21749c. Its radius is about 90% of Earth’s, so a little smaller than Venus. Note the decrease in brightness for each case the drop is only around 0.01%. From Dragomir, et al, Astrophysical Journal Letters, 875, L7, 2019

The census of planets is ongoing. The information obtained to date has given scientists a rudimentary understanding of the various kinds of stellar systems that exist in the space relatively near to the Sun. Based on our current crude understanding, we know that there are basically three categories of bodies detected: the super earths, the gas dwarfs and the gas giants. Super earths are rocky planets that are a few times more massive than Earth, the gas dwarf planets are similar to Neptune and Uranus. They are bigger than the rocky worlds, but smaller than the gas giants, which span upward in mass to objects that start to become more like stars, the brown dwarfs. Further divisions are also possible, but these categories give the general trend basically, there are planets found at size scales that range from near-terrestrial to sub-stellar.

At the low-mass end we know there should be planets like our own terrestrial planets, with masses like Earth and Venus, and lower down to minor planets and asteroids. Our current detection methods do not allow us to find these sorts of bodies, though TESS should be able to find Earth/Venus analogues if they are present in its survey data.

Coronagraph

One of the primary goals astronomers have set for the coming decades is to capture direct images of planets around nearby stars. Because these planets are much fainter than their host stars, imaging missions generally employ a sort of mask that blocks the starlight. These socalled coronagraphs use a combination of disks and optics to occlude the direct starlight, allowing the telescope to see the faint reflected light from nearby orbiting planets that lie outside the masked area. The method is similar to the one used by space-based solar telescopes, which use a disk to block the bright photosphere of the Sun, allowing them to see the much fainter solar corona. The accompanying image illustrates the method, though in this instance the detection turned out to be a false positive.


Above is an image of the solar corona taken using a disk to block the bright solar surface, allowing the faint corona to be observed. A similar technique has been proposed to block the light of stars, allowing the faint planets orbiting them to be seen. Courtesy HAO/SMM C/P project team & NASA. HAO is a division of the National Center for Atmospheric Research, which is supported by the National Science Foundation.

The star Fomalhaut was known to have a disk of material orbiting it, and the sharp inner edge of the disk suggested the presence of a planet. Images taken using the Hubble Space Telescope and ground-based telescopes between 2004 and 2008 seemed to confirm the presence of the planet. The HST image below, taken using the coronagraph method in 2012, shows clear evidence of the planet the inset gives its position for several epochs between 2004 and 2010, and its position at the time of this image is marked by the arrow.

Unfortunately, even apparently solid evidence can sometimes evaporate into empty space. Subsequent imaging of the system over the next two years showed that the planet had disappeared. The figure below, at left, shows HST images from 2014. On the right is a model simulation. What has been taken for a planet was apparently a dust cloud, the result of a violent collision between two proto-planetary bodies. Over time the cloud expanded, becoming brighter for a while, and then simply dissipating. The event offers a cautionary tale common when working at the frontiers of knowledge: sometimes early data can be misleading, and new findings require repeated verification before they can be fully confirmed.


Image Credit: NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona)

Despite the mistaken case of Fomalhaut-b, the direct imaging method has advantages over the transit and radial velocity methods. First, it is not affected by the orientation of the system. Both the wobble and transit methods require a particular alignment that allows their planets and stars to line up with Earth. Only with this alignment will transits occur, and only with nearalignment will the radial velocity shifts be large enough to detect. Direct imaging can detect planets no matter what the inclination of the system is. However, it is not without its own set of limitations.

First, it is difficult to survey many systems at once. The need to block the light from the star makes multi-star monitoring effectively impossible. Second, the method is more sensitive to larger planets that are farther from their star. Planets that are too small are more difficult to see because they are faint. Those that are too close to the star will be blocked by the obscuring coronagraph. But these limitations are complementary to those of the other methods. Furthermore, larger telescopes in space and on the ground will be able to detect fainter planets than current telescopes can. By harnessing the capabilities of larger telescopes, like the James Image Credit: NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona) Webb Space Telescope, direct imaging promises to greatly expand our understanding of exoplanets.

Microlensing

An additional detection technique utilizes the gravitational effect of planets on background stars. Whenever a planet passes in front of a distant star (not the star it orbits) it will cause a At left is an image of the solar corona taken using a disk to block the bright solar surface, allowing the faint corona to be observed. A similar technique has been proposed to block the light of stars, allowing the faint planets orbiting them to be seen. Courtesy HAO/SMM C/P project team & The planet Fomalhaut b is revealed in an image taken using the Hubble Space Telescope. To obtain this image an occulting disk (coronagraph) was used to block the starlight. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute) temporary increase in that star’s brightness. This gravitational lensing effect has already been used to good effect to search for dark stellar-mass objects in our galaxy. It can also be used to search for planet-sized bodies, and it can reveal many of them at a time. All that is required is that many stars be monitored and checked for the tell-tale brightening that would indicate a passing planet. The image below shows an example light curve from a gravitational micro-lens that is part of a survey from the 1990s called MACHOs (for MAssive Compact Halo Objects). These objects were typically much larger than planets, but the method could be used to reveal the presence of planetary bodies, too.

These light curves show gravitational brightening of a background star by passage of an otherwise invisible foreground object, called a MACHO (MAssive Compact Halo Object). The same effect could be used to detect faint planets using their gravitational lensing of background stars. Figure from Alcock et al, Astrophysical Journal, 486, 697, 1997.

Astrometrie

The tug of a planet causes its star to move slightly, and this can be detected through radial velocity measurements. This is a method we have already discussed. However, if we have very precise measurements of the positions of stars, then we can see their position on the sky change slightly as they engage in the gravitational dance with their family of planets. This method of detection is called the astrometry method. In principle it can reveal the presence of planets that are too faint to see. It can also reveal the presence of planets around many stars at once. That is a big advantage. We only have to be able to see the tiny shifts in the position of the stars being surveyed, and that is the crux of the matter.

A star like the Sun would not be detectable with this method because the point it orbits around (the solar system center of mass) is actually inside the star. The method is more sensitive to stars with large orbital motion, and that means stars with large planets that are orbiting at great distances. For systems like that, the center of mass can be outside the star (see discussion above about the radial velocity detection method), and that means the star will undergo larger motions.

However, even given its advantages, the method is quite difficult in practice. The truth of this statement is underlined by the fact that, though this is the exoplanet method that has been in use the longest – since the 1940s – it has yet to find any confirmed exoplanets. Several false alarms have been reported, but none has stood up to additional study and analysis.

This could change with the current and oncoming observational instruments. For example, the European survey satellite Gaia has the required positional sensitivity. It was designed to measure parallaxes and proper motions of stars. Astronomers expect that as it continues to collect data it will reveal the minuscule stellar motions caused by orbiting planets as well.

Opsomming

The past 25 years has seen our knowledge of planets orbiting other stars increase enormously. At first we had no knowledge of extra-solar planets, but once the first planets were discovered, new ones came in a near avalanche. To date we have found myriad systems, and not a one of them looks anything much like our own. In particular, we have not found an Earth twin, though we have discovered some close relatives, second cousins, perhaps. Nor have we yet found any planets on which we think life is present. Finding a close analogue to Earth is one of the primary goals for the future of exoplanet research. But we have only scratched the surface. The 4000 or so exoplanets cataloged so far are almost certainly the merest sliver of all the planets that exist. Our explorations are promising, and the coming years will certainly bring us new discoveries and exciting insights into ourselves and our place in the vastness of the cosmos.


Habitable zones

For a planet to have the potential to support life, it must lie within a region known as the habitable, or ‘Goldilocks’, zone, around its host star. If it’s too close It will be scorchingly hot, but too far away and it won’t receive enough light and heat to support life. In addition, planets beyond the habitable zone of a star tend to be gas giants, as during the formation of a planetary system the colder, gaseous planets can only form farther out, while the rocky terrestrial planets form closer to the star. Our solar system Is a prime example beyond Mars, which is said to be at the edge of our habitable zone, you’ve got Jupiter, Saturn, Uranus and Neptune – all gas giants. However, between Mars and the Sun are Earth, Venus and Mercury. All are rocky terrestrial planets, although only one – Earth – is at the necessary distance for water, and so life, to exist.

The key to finding a liveable exoplanet Is to find one located In the habitable zone of its host star. Due to the primitive methods of finding planets currently at our disposal, the majority of planets found so far have been large, hot gas giants orbiting very close to their host star. It Is only recently that we have begun to find exoplanets of a similar size to Earth, while water-bearing planets have been much scarcer. The farther a planet is from its host star, the harder It is to find. As mentioned earlier, it’s hoped that, eventually, when we are able to directly Image exoplanets, Earth-like ones will appear more regularly.


[edit]Other possible methods

[edit]Astrometry

In this diagram a planet (smaller object) orbits a star, which itself moves in a small orbit. The system’s center of mass is shown with a red plus sign. (In this case, it always lies within the star.)

This method consists of precisely measuring a star’s position in the sky and observing how that position changes over time. Originally this was done visually with hand-written records. By the end of the 19th century this method used photographic plates, greatly improving the accuracy of the measurements as well as creating a data archive. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. Effectively, star and planet each orbit around their mutual center of mass (barycenter), as explained by solutions to the two-body problem. Since the star is much more massive, its orbit will be much smaller. [36] Frequently, the mutual center of mass will lie within the radius of the larger body.

Motion of the center of mass (barycenter) of solar system relative to the Sun.

Astrometry is the oldest search method for extrasolar planets and originally popular because of its success in characterizing astrometric binary star systems. It dates back at least to statements made by William Herschel in the late 18th century. He claimed that an unseen companion was affecting the position of the star he cataloged as 70 Ophiuchi. The first known formal astrometric calculation for an extrasolar planet was made by W. S. Jacob in 1855 for this star. Similar calculations were repeated by others for another half-century [37] until finally refuted in the early 20th century. [38] [39] For two centuries claims circulated of the discovery of unseen companions in orbit around nearby star systems that all were reportedly found using this method, [37] culminating in the prominent 1996 announcement of multiple planets orbiting the nearby star Lalande 21185 by George Gatewood. [40] [41] None of these claims survived scrutiny by other astronomers, and the technique fell into disrepute. [42] Unfortunately, the changes in stellar position are so small and atmospheric and systematic distortions so large that even the best ground-based telescopes cannot produce precise enough measurements. All claims of a planetary companion of less than 0.1 solar mass, as the mass of the planet, made before 1996 using this method are likely spurious. In 2002, the Hubble Space Telescope did succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876. [43]

Future space-based observatories such as ESA’s GAIA may succeed in uncovering new planets via astrometry, but for the time being no planet detected by astrometry has been confirmed.

One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit.

In 2009 the discovery of VB 10b by astrometry was announced. This planetary object was reported to have a mass 7 times that of Jupiter and orbiting the nearby low mass red dwarf star VB 10. If confirmed, this would be the first exoplanet discovered by astrometry of the many that have been claimed through the years. [44] [45] However recent radial velocity independent studies rule out the existence of the claimed planet. [46] [47]

[edit]Eclipsing binary minima timing

When a double star system is aligned such that – from the Earth’s point of view – the stars pass in front of each other in their orbits, the system is called an “eclipsing binary” star system. The time of minimum light, when the star with the brighter surface area is at least partially obscured by the disc of the other star, is called the primary eclipse, and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. These times of minimum light, or central eclipse, constitute a time stamp on the system, much like the pulses from a pulsar (except that rather than a flash, they are a dip in the brightness). If there is a planet in circum-binary orbit around the binary stars, the stars will be offset around a binary-planet center of mass. As the stars in the binary are displaced by the planet back and forth, the times of the eclipse minima will vary they will be too late, on time, too early, on time, too late, etc.. The periodicity of this offset may be the most reliable way to detect extrasolar planets around close binary systems. [48] [49] [50]

[edit]Polarimetry

Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized. [51]

By analyzing the polarization in the combined light of the planet and star (about one part in a million), these measurements can in principle be made with very high sensitivity, as polarimetry is not limited by the stability of the Earth’s atmosphere.

Astronomical devices used for polarimetry, called polarimeters, are capable of detecting the polarized light and rejecting the unpolarized beams (starlight). Groups such as ZIMPOL/CHEOPS [52] and PlanetPol [53] are currently using polarimeters to search for extra-solar planets, though no planets have yet been detected using this method.

[edit]Auroral radio emissions

Auroral radio emissions from giant planets with plasma sources such as Jupiter‘s volcanic moon Io could be detected with future radio telescopes such as LOFAR. [54] [55]


Discovery of Other Planetary Systems

In Star and Planet Formation, we discussed the formation of stars and planets in some detail. Stars like our Sun are formed when dense regions in a molecular cloud (made of gas and dust) feel an extra gravitational force and begin to collapse. This is a runaway process: as the cloud collapses, the gravitational force gets stronger, concentrating material into a protostar. Roughly half of the time, the protostar will fragment or be gravitationally bound to other protostars, forming a binary or multiple star system—stars that are gravitationally bound and orbit each other. The rest of the time, the protostar collapses in isolation, as was the case for our Sun. In all cases, as we saw, conservation of angular momentum results in a spin-up of the collapsing protostar, with surrounding material flattened into a disk. Today, this kind of structure can actually be observed. The Hubble Space Telescope, as well as powerful new ground-based telescopes, enable astronomers to study directly the nearest of these circumstellar disks in regions of space where stars are being born today, such as the Orion Nebula ([link]) or the Taurus star-forming region.

Protoplanetary Disk in the Orion Nebula. The Hubble Space Telescope imaged this protoplanetary disk in the Orion Nebula, a region of active star formation, using two different filters. The disk, about 17 times the size of our solar system, is in an edge-on orientation to us, and the newly formed star is shining at the center of the flattened dust cloud. The dark areas indicate absorption, not an absence of material. In the left image we see the light of the nebula and the dark cloud in the right image, a special filter was used to block the light of the background nebula. You can see gas above and below the disk set to glow by the light of the newborn star hidden by the disk. (credit: modification of work by Mark McCaughrean (Max-Planck-Institute for Astronomy), C. Robert O’Dell (Rice University), and NASA)

Many of the circumstellar disk s we have discovered show internal structure. The disks appear to be donut-shaped, with gaps close to the star. Such gaps indicate that the gas and dust in the disk have already collapsed to form large planets ([link]). The newly born protoplanets are too small and faint to be seen directly, but the depletion of raw materials in the gaps hints at the presence of something invisible in the inner part of the circumstellar disk—and that something is almost certainly one or more planets. Theoretical models of planet formation, like the one seen at right in [link], have long supported the idea that planets would clear gaps as they form in disks.

Protoplanetary Disk around HL Tau. (a) This image of a protoplanetary disk around HL Tau was taken with the Atacama Large Millimeter/submillimeter Array (ALMA), which allows astronomers to construct radio images that rival those taken with visible light. (b) Newly formed planets that orbit the central star clear out dust lanes in their paths, just as our theoretical models predict. This computer simulation shows the empty lane and spiral density waves that result as a giant planet is forming within the disk. The planet is not shown to scale. (credit a: modification of work by ALMA (ESO/NAOJ/NRAO) credit b: modification of work by NASA/ESA and A. Feild (STScI))

Our figure shows HL Tau , a one-million-year-old “newborn” star in the Taurus star-forming region. The star is embedded in a shroud of dust and gas that obscures our visible-light view of a circumstellar disk around the star. In 2014 astronomers obtained a dramatic view of the HL Tau circumstellar disk using millimeter waves, which pierce the cocoon of dust around the star, showing dust lanes being carved out by several newly formed protoplanets. As the mass of the protoplanets increases, they travel in their orbits at speeds that are faster than the dust and gas in the circumstellar disk. As the protoplanets plow through the disk, their gravitational reach begins to exceed their cross-sectional area, and they become very efficient at sweeping up material and growing until they clear a gap in the disk. The image of [link] shows us that a number of protoplanets are forming in the disk and that they were able to form faster than our earlier ideas had suggested—all in the first million years of star formation.


1.5. The 51 Peg Revolution

The discovery of 51 Peg b (Mayor & Queloz 1995) clearly marked an explosion of the field 3 . This giant planet (M 0.5 ) in a 4.2 day orbit shocked astronomers, except maybe for the ghost of Otto Struve. It also demonstrated that RV surveys were probing the wrong parameter space (orbital distances of 5 au rather than 0.05 au)—the dangers of planning surveys based on one example, our solar system. The RV amplitude of 50 m s −1 (Figure 1.7) clearly benefited from the increased precision of Doppler measurements. Figure 1.8 shows the discovery rate of exoplanets found using the Doppler method. The sharp increase after the discovery of 51 Peg is for three reasons. First, once astronomers realized that giant planets could occur in short period orbits, they changed their observing strategies so that these short-period planets could be discovered rather quickly. Second, using the Doppler method to detect exoplanets became quite fashionable, with many groups "jumping on the bandwagon." Prior to 1995, there were only a handful of groups using precise stellar RVs to search for exoplanets. Now, the number of such groups is in the dozens. Currently, approximately a hundred exoplanets per year are discovered with the Doppler method. Note that RV measurements also play a key role in the confirmation and mass determination of discoveries made by the transit method.

Figure 1.7. The discovery of 51 Peg b. The RV variations have an amplitude of ≈50 m s −1 and are phased to the orbital period of 4.2 days. The typical measurement error is ≈15 m s −1 . (Adapted by permission from Macmillan Publishers Ltd: Mayor & Queloz 1995.)

Figure 1.8. The rate of exoplanet discovered planets using the Doppler method (transit discoveries are not included data from http://www.exoplanet.eu).

Finally, the increased detection rate of exoplanets using Doppler measurement also benefited from the dramatic increase in RV precision over the past 30 years. The top panel of Figure 1.9 shows precise RV measurements of γ Cep (Walker et al. 1992). These show a scatter of about 15 m s −1 —the measurement error they could achieve in the mid-1980s. The lower panel shows modern RV measurements of Proxima Centauri showing the variations of the Earth-mass companion in an 11.8 day orbit (Anglada-Escudé et al. 2016). These have a scatter of a mere 1 m s −1 . Note that the scale of the y-axis in this lower panel is the size of the error bar in the top panel. An important aspect of this book is to show how this dramatic increase was achieved.

Figure 1.9. (Top) RV measurements of γ Cep phased to the orbital motion using Doppler measurements taken in the mid-1980s. (Bottom) RV measurements of Proxima Centauri phased to the orbital motion as measured in 2016. The box size for this panel is the same as the measurement error for the earlier measurements.

The parameter space in the mass versus semimajor axis of exoplanets discovered with the Doppler method is shown in Figure 1.10. These are only planets discovered through RV measurements and not those from transit discoveries, although Doppler measurements were important for confirming the nature of these discoveries and measuring the companion mass.

Figure 1.10. The detection parameter space for planets found using the Doppler method. This does not include transit discoveries. The method only measures the mass multiplied by the sine of the orbital inclination, i (M sonde i).


Thread: How much does our sun wobble?

Well, it's a standard problem for undergraduate astronomy students to figure out the magnitude of the Sun's motion around its center of mass with Jupiter. The answer turns out to be about 12 meters per second. Since Jupiter is by far the largest influence on the Sun's motion, you can make a rough estimate that at some times, the Sun will appear to be moving toward us at around 12 m/s, and at other times, moving away from us at around 12 m/s.

You could use the ordinary Doppler formula to compute how large a shift in the wavelength of sunlight this motion would cause. And then you could examine the properties of some standard devices for measuring wavelengths (or frequencies) of visible light, and see if those standard devices could detect these shifts.

But is that what DaCaptain is asking? To me, "wobble" means "the axis is tilting", not "the center is moving." To put it another way, "How do the planets affect the precession of the Sun's axis?"

ETA, for that matter, does the Sun's axis precess?

But is that what DaCaptain is asking? To me, "wobble" means "the axis is tilting", not "the center is moving." To put it another way, "How do the planets affect the precession of the Sun's axis?"

ETA, for that matter, does the Sun's axis precess?

StupendousMan had it right, initially what I was wondering was how much the sun wobbles around the center of it's axis. How comparable is that to the wobbles of stars we are finding that have exoplanets?

Does the sun wobble along it's axis as well? Is there any way to determine if it has ever flipped it's axis?

I don't know, but probably, though probably not very much. I think it's a fallacy though to think that is something precesses a lot that it will "flip". That's probably due to the experience of seeing a top flip, but that's because it's anchored to the earth on one end. It wouldn't happen to a top if it was free floating.

And there is no evidence that the sun ever flipped. The sun rotates counterclockwise (seen from the north) as do the orbits of all the planets and the rotations of nearly all the planets.

Most of the radial velocity variations caused by star-planet interactions which we have detected so far are much larger than the 12 m/s caused by Jupiter.
Here, take a look for yourself -- the data in the histogram below was generated just a moment ago on the NASA Exoplanet Data Archive:

Most of the radial velocity variations caused by star-planet interactions which we have detected so far are much larger than the 12 m/s caused by Jupiter.
Here, take a look for yourself -- the data in the histogram below was generated just a moment ago on the NASA Exoplanet Data Archive:

Wow, that really gives you a sense of how small our solar system is compared to those we are finding. Does larger radial velocity also mean the planets are orbiting quicker? Closer to their suns?

It seems it also points out that we can most easily see the larger systems.

Wow, that really gives you a sense of how small our solar system is compared to those we are finding. Does larger radial velocity also mean the planets are orbiting quicker? Closer to their suns?

It seems it also points out that we can most easily see the larger systems.

The radial velocity method preferably detects planets around
1. Small stars
2. with big planets
3. that has close orbits/short orbital periods
4. where the angle between the plane of the orbit and our line of sight is small.

Radial Velocity = Mass of planet * Velocity of planet (depends on mass of star and period of orbit) * sine(inclination of orbit) / mass of star

If any of three first factors is too small or the fourth too large the radial velocity can become very small, especially #4, which is 0 if the orbital plane and our line of sight is perpendicular. As we all know if you multiply any number by zero the answer is zero.