Den igangværende søgen efter beboelige exoplaneter

En lun Florida aften, og min familie og jeg stod på Cocoa Beach, ser nordpå mod Cape Canaveral Air Force Station. Vi var en del af en folkemængde ved havet, der var samlet for at overvære opsendelsen af ​​NASAs Kepler-rumteleskop. Da ildkuglen dukkede op og langsomt begyndte at stige i det fjerne, vi jublede sammen med vores medobservatører. Cirka 30 sekunder senere, vi mærkede jorden buldre og hørte det dybe brøl, at se Delta II-raketten klatre op på nattehimlen og accelerere, mens den er på vej ud over havet.

Kepler fortsatte med at tilbringe ni år i det dybe rum på at søge efter galaktiske naboer som os:planeter på størrelse med Jorden, der kredser om sollignende stjerner. Kepler så en del af Mælkevejsgalaksen, der omfattede millioner af stjerner. Det tilbagesendte data om næsten 200, 000 af dem og fundet mere end 2, 300 exoplaneter - planeter uden for vores solsystem.

"Med data fra Kepler, vi har mere præcise og detaljerede oplysninger, end vi nogensinde havde haft før, " siger astrofysiker Eric Ford, som var en del af Keplers videnskabsteam. Ford og hans kolleger ved Penn State's Center for Exoplanets and Habitable Worlds bygger på arven fra Evan Pugh-professor Alex Wolszczan, som opdagede de første kendte exoplaneter i 1992 ved hjælp af undersøgelser fra jordbaserede instrumenter. "Kepler fandt tusindvis af planeter, " siger Ford. "Astronomer ville elske at lære mere om dem alle, men der er ikke nok teleskoptid. Da folk er særligt interesserede i at lære mere om dem, der kan ligne Jorden, vi planlægger at koncentrere os om at karakterisere planeter i deres planetsystemers beboelige zoner."

Den beboelige zone er et område i et solsystem - en afstand ikke for tæt på og ikke for langt fra en sol - hvor en planet ville have de nødvendige betingelser for at have flydende vand på sin overflade, et vigtigt krav for eksistensen af ​​kulstofbaseret liv, som vi kender det. James Kasting, Evan Pugh professor i geovidenskab, var en af ​​de tidlige udviklere af konceptet. Planetens overfladetemperatur skal være over vandets frysepunkt og under kogepunktet. Andre forhold spiller også ind, inklusive planetens masse, rotation, og atmosfære. Blandt Kepler exoplaneterne, der er blevet analyseret indtil videre, flere dusin anses for at være i deres stjernes beboelige zone.

Erik Ford, medlem af Keplers videnskabsteam, studerer, hvordan planeter dannes og udvikler sig, både i vores solsystem og i andre. Mange af systemer fundet af Kepler er meget forskellige fra vores, rejser nye spørgsmål om, hvordan planetsystemer udvikler sig, og hvorfor de opstår i så forskellige former. Kepler-instrumentet er opkaldt efter den tyske astronom Johannes Kepler, som i begyndelsen af ​​1600-tallet formulerede tre love for planeternes bevægelse.

Sådan finder du en exoplanet

I sin søgen efter exoplaneter, Kepler-missionen brugte transitmetoden, ved hjælp af digitalkamera-lignende teknologi til at registrere og måle små fald i en stjernes lysstyrke, når en planet krydser foran stjernen. Med observationer af transitplaneter, astronomer kan beregne forholdet mellem en planets radius og dens stjerne - i det væsentlige størrelsen af ​​planetens skygge - og med det forhold kan de beregne planetens størrelse. "Vi kender størrelsen på tusindvis af planeter takket være transitmetoden, " siger Ford.

Selvom dens solcelledrevne elektronik kunne fortsætte med at fungere i lang tid, det sidste efterår, Kepler løb tør for det hydrazinbrændstof, der var nødvendigt for at orientere sig præcist, og NASA pensionerede rumfartøjet. Det er nu 94 millioner miles væk, i en bane, der følger Jorden rundt om Solen. Men missionen producerede nok data til at holde astronomer beskæftiget i de kommende år. Og nu, en ny NASA-mission udvider Keplers optælling af exoplaneter ved at målrette tættere på, lysere stjerner.

TESS (Transiting Exoplanet Survey Satellite), som blev lanceret i april sidste år, scanner næsten hele himlen, et plaster ad gangen, leder efter transitplaneter omkring de nærmeste stjerner. Mens de typiske stjerner, Kepler observerede, var fra 300 til 3, 000 lysår væk (et lysår er omkring seks billioner miles), TESS ser på stjerner, der kun er snesevis af lysår væk. Og i stedet for at bruge årevis på at se på én plet af himlen, som Kepler gjorde, TESS vil flytte sit syn fra den ene del af himlen til den næste.

Ved at bruge TESS-observationer af lysere stjerner - i gennemsnit 30 til 100 gange lysere end de stjerner, Kepler undersøgte - vil astronomerne være i stand til at inspicere planeter nærmere og nemmere foretage opfølgende observationer. "Med TESS, vi fokuserer på at søge efter planeter omkring stjerner, der er tættere på os, da vi vil være i stand til at karakterisere dem mere effektivt, " siger Ford. Data fra TESS vil give information om en planets størrelse og omløbsperiode, og opfølgende observationer med andre instrumenter vil give forskere mulighed for at måle masserne og beskrive disse planeters atmosfærer.

Men lige så værdifuld som transitmetoden er for planetariske undersøgelser, det har sine begrænsninger. "Transitser lader dig kun se planeter, der tilfældigvis krydser mellem os og den stjerne, vi ser på, " forklarer astrofysiker Fabienne Bastien. "Radiale hastigheder gør det muligt for os at se planetsystemer i andre orienteringer."

Også kaldet Doppler-spektroskopi, den jordbaserede metode til radial hastighed var faktisk den første teknik til at detektere exoplaneter med sollignende stjerner. Det er baseret på det faktum, at en stjerne slingrer let som reaktion på en kredsende planets gravitationstræk. Disse små bevægelser påvirker stjernens lysspektrum, eller farvesignatur. Når stjernen bevæger sig lidt væk fra en observatør, bølgelængden af ​​dets lys forlænges lidt, skiftende mod den røde ende af spektret. Mens den kredsende planet trækker stjernen lidt mod iagttageren, stjernens lys skifter mod det blå. Gennem gentagne observationer af ændringer i stjernens spektrum, forskere kan beregne planetens masse.

Bastien, hvis forskning fokuserer på værtsstjernerne i planetsystemer, kombinerer transitdata med studier af radial hastighed for at lære mere om fjerne sole. "Disse sole har pletter og udbrud og alle former for aktivitet, der enten kan efterligne eller maskere et exoplanetsignal, " siger hun. "Meget af mit arbejde involverer at adskille planetsignalet fra stjernesignalet, så vi kan bekræfte, at det faktisk er en planet, vi ser. Penn State er allerede et kraftcenter for radial hastighed, og jeg er begejstret for to nye spektrografer, der er meget mere følsomme end det, vi har haft til dato, og som vil dramatisk fremme vores studier."

Disse nye verdensklasse, meget følsomme spektrografer, bygget af et Penn State-hold ledet af astrofysiker Suvrath Mahadevan, er ved at ændre det radiale hastighedslandskab. De vil måle radiale hastigheder ekstremt præcist for at karakterisere lavmasseplaneter i eller nær deres stjerners beboelige zoner. En spektrograf er designet til optisk undersøgelse af nærliggende sollignende stjerner, og den anden til at detektere køler, svagere, stjerner med lavere masse ved hjælp af infrarødt lys.

"I can't wait to use these spectrographs to explore some ideas I have for finding habitable exoplanets, " Bastien says. "I want to start a planet search around some stars that haven't received much attention because they're too noisy—there are complicating factors around them that make them difficult to study. The group here is enthusiastic and collaborative and open to new ideas, so there are all sorts of possibilities."

Fabienne Bastien studies the host stars of planetary systems. It's fairly easy to find a star, but knowing whether it has planets orbiting around it is much harder. Two approaches Bastien uses are the transit method and the radial velocity method.

All planetary systems are not alike

As researchers learn more about potential habitable zones of distant solar systems, they also want to learn about how those systems might have formed and evolved. That's the research focus of astrophysicist Rebekah Dawson. "It's an exciting time because so many new planets have been discovered in other solar systems and they're very different from the planets in our solar system, " she says. "Exoplanet discoveries forced us to change our understanding of solar system and planet formation."

For eksempel, Kepler found a lot of planets with sizes between that of Earth and Neptune (about four times Earth's diameter), that are as close to their stars as Mercury is to the Sun, or even closer. "These planets are common in other planetary systems, and we have nothing like them in our solar system, " Dawson says. "So we're going back to the drawing board with some of our theories for how planets form and what happens early in planetary systems, now that we don't have just our solar system to judge these theories against."

Dawson's research on planetary systems can in turn inform and provide context for studies of individual planet formation. By understanding what might have been happening early on in a planetary system, she and her colleagues can develop theories about how planets might form in that system. For eksempel, as giant planets gravitationally interact with each other, they could be sending asteroids and comets into regions where terrestrial planets are forming, and that could influence the composition of those planets.

Among Dawson's research interests are hot Jupiters, some of the first exoplanets ever discovered. Similar in mass to our Jupiter, these giant gas planets are much closer to their sun than Jupiter is to our Sun. They complete an orbit in three to four days. "That's not where we expected to find giant gas planets in their solar systems, " Dawson says. "We're trying to understand their origin and how they could be so close to their star. One theory is that after these hot Jupiters formed, they were put into an extremely elliptical orbit that would bring them close to their star, and then tidal friction—tides raised on the Jupiter by the star—caused the orbit to shrink and become more circular.

"I sometimes think of a planetary system as an ecosystem that could support a potentially habitable planet, and we have to understand how the whole thing functions to really understand if that planet is habitable and what its formation history is, " Dawson continues. "When we started to learn about those hot Jupiters and how their orbits might have been altered, that has implications for the rest of the planetary system. If that were happening, it would probably wipe out any planets in between the hot Jupiter and the star, so that region wouldn't be a likely place to find a habitable planet"—even if it's the right distance from the star to be in the habitable zone.

Rebekah Dawson studies how planetary systems formed and evolved. Kepler has revealed that many of the planets in other systems are very different from the planets in our own solar system, and that just because a planet is in a system's habitable zone doesn't mean that it is habitable.

Hvor går vi hen herfra?

Fabienne Bastien recalls the sense of wonder she felt when, as a graduate student, she heard Kepler scientist Natalie Batalha speak of her own realization that the stars we see at night are more than distant suns. "Now we know that they're not just stars, they're planetary systems, " she says—each one potentially home to habitable worlds.

With everything astronomers have learned about that potential, there's still much that remains a mystery. Current methods are just beginning to characterize the atmospheres of exoplanets and determine whether a planet in the habitable zone might have a surface that is conducive or hostile to life. Recent progress gives scientists a better idea of what questions to ask and what kinds of instruments are needed to address them.

"When astronomers have just discovered a planet, we could say it's potentially habitable, but that is more a statement of our limited knowledge than of the properties of the planet, " Ford says. "We want to design a hypothesis that is testable through observations we're able to make. If we can find 100 rocky planets in the habitable zone and characterize their atmospheres to look for water and biomarkers, then we might find some really fascinating planets—but there's also the possibility that we conclude that none of them are suitable for Earth-like life."

One long-term goal for astronomers is direct detection of exoplanets, rather than having to infer their existence through transit or radial velocity studies. Dawson is now serving on a team laying the groundwork for a Large UV Optical Infrared Surveyor (LUVOIR), a multi-wavelength space observatory concept being studied by NASA's Goddard Space Flight Center. LUVOIR is envisioned to be a twelve- to fifteen-meter diameter telescope that would operate about a million miles from Earth. It would allow scientists to recognize planets directly, as small bright bodies against the dark of space. Once a planet is identified, other techniques could then be used to measure its mass and examine other important features.

As researchers look to new technologies such as the new spectrographs, LUVOIR, and other future missions, they're optimistic that one day we'll know whether our solar system is a rare phenomenon or if life does indeed exist on other planets.

"If you think about it, it's amazing that Earth has both continents and oceans, as well as an atmosphere and climate that sustain life, " Ford says. "Is that significant? Is it just the right balance? Is Earth a great coincidence or does planet formation often produce similar planets?"

"Before exoplanets were discovered, I think a lot of us expected every planetary system to look like the solar system, or we thought most stars don't have planets, " Dawson adds. "But instead, what we're seeing is that most stars do have planets, and a lot of these planetary systems are very different from our solar system. Does that make the solar system unusual? We don't know yet. Despite our best instruments and technology, we're still only looking in our own little neighborhood of the galaxy.

"Luckily, I don't think we necessarily need to look at all the stars in the galaxy to know whether our solar system is unusual. And every time there's a new mission or a new instrument that can do something different or dramatically improve the quality of data, there's something surprising that keeps us excited."