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Wobbly planets could reveal Earth-like moons

Dec. 12, 2008
Courtesy Science and Technology 
Facilities Council
and World Science staff

Moons out­side our So­lar Sys­tem ca­pa­ble of sup­port­ing life may have just be­come eas­i­er to find, thanks to new re­search.

Da­vid Kip­ping, an as­tron­o­mer at Uni­ver­s­ity Col­lege Lon­don, found that such moons can be de­tected through wob­bling mo­tions in the plan­ets they or­bit. The cal­cula­t­ions al­low con­firma­t­ion of wheth­er a plan­et has a sat­el­lite, plus its weight and dis­tance from its host plan­et—factors that de­ter­mine a moon’s likely hab­it­abil­ity, Kip­ping says.

Artist's con­cept of a moon orb­it­ing a gas gi­ant pla­net. (Cour­tesy NA­SA)
Of more than 300 plan­ets out­side our So­lar Sys­tem iden­ti­fied to date, al­most 30 are be­lieved to lie at dis­tances from their host star suit­a­ble for liq­uid wa­ter and thus, pos­si­bly, life. 

The plan­ets them­selves are un­in­hab­it­a­ble, be­ing gas gi­ants like Ju­pi­ter. As­tro­no­mers be­lieve such plan­ets could have hab­it­a­ble moons, though. Some have pro­posed that such moons, rath­er than plan­ets, could of­fer some of the most ex­cit­ing near-term tar­gets in the search for ex­tra­ter­res­tri­al life. 

One po­ten­tial ad­van­tage of moons could be that they’re easy to find, if their par­ent plan­ets are gas gi­ants—which are al­so com­par­a­tively easy to de­tect. A spec­u­la­tive pos­si­bil­ity is that more than one moon of the same plan­et could har­bor life, po­ten­tially al­lowing those crea­tures to set up com­mu­nica­t­ion and trans­porta­t­ion links be­tween moons.

The new cal­cula­t­ions pro­vide the abil­ity to “de­tect an Earth-mass moon around a Neptune-mass gas plan­et,” said Kip­ping, whose re­sults ap­pear in the Dec. 11 is­sue of the re­search jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

“Un­til now as­tron­o­mers have only looked at the changes in the po­si­tion of a plan­et as it or­bits its star. This has made it dif­fi­cult to con­firm the pres­ence of a moon as these changes can be caused by oth­er phe­nom­e­na, such as a smaller plan­et,” Kip­ping said. “By adopt­ing this new meth­od and look­ing at varia­t­ions in a plan­et’s po­si­tion and ve­locity each time it passes in front of its star, we gain far more re­li­a­ble in­forma­t­ion.”

Wob­bles in a plan­et’s po­si­tion and ve­locity would be caused by the gravita­t­ional ef­fect of the moon. While an old meth­od of ana­lyz­ing the fluctua­t­ions al­lowed as­tron­o­mers to search for moons, it did­n’t let them de­ter­mine their mass or their dis­tance from the plan­et, Kip­ping ex­plained.

“It’s very ex­cit­ing that we can now gath­er so much in­forma­t­ion about dis­tant moons as well as dis­tant plan­ets,” said Keith Ma­son, chief ex­ec­u­tive of the Sci­ence and Tech­nol­o­gy Facil­i­ties Coun­cil in Swin­don, U.K., which funded the re­search. “If some of these gas gi­ants found out­side our So­lar Sys­tem have moons... there’s a real pos­si­bil­ity that some of them could be Earth-like.”

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Moons outside our Solar System capable of supporting life may have just become easier to find, thanks to new research. David Kipping, an astronomer at University College London, found that such moons can be detected through wobbling motions in the planets they orbit. The calculations allow confirmation of whether a planet has a satellite, plus its weight and distance from its host planet—factors that determine a moon’s likely habitability, he said. Out of more than 300 planets outside our Solar System identified to date, almost 30 are believed to lie at distances from their host star suitable for liquid water and thus, possibly, life to exist. The planets themselves are uninhabitable, being gas giants like Saturn. Astronomers believe such planets could have habitable moons, though. Some astronomers have proposed that moons, rather than planets, could offer some of the most exciting targets in the search for extraterrestrial life. One potential advantage of moons could be that they’re easy to find, if their parent planets are gas giants—which are also comparatively easy to detect. A speculative possibility is that more than one moon of the same planet could harbor life, potentially allowing those creatures to set up communication and transportation links between moons. The new calculations provide the ability to “detect an Earth-mass moon around a Neptune-mass gas planet,” said Kipping, whose results appear in the Dec. 11 issue of the research journal Monthly Notices of the Royal Astronomical Society. “Until now astronomers have only looked at the changes in the position of a planet as it orbits its star. This has made it difficult to confirm the presence of a moon as these changes can be caused by other phenomena, such as a smaller planet,” Kipping added. “By adopting this new method and looking at variations in a planet’s position and velocity each time it passes in front of its star, we gain far more reliable information.” Wobbles in a planet’s position and velocity would be caused by the gravitational effect of the moon. While the old method of looking at the fluctuations allowed astronomers to search for moons, it didn’t let them determine their mass or their distance from the planet, Kipping explained. “It’s very exciting that we can now gather so much information about distant moons as well as distant planets,” said Keith Mason, Chief Executive of the Science and Technology Facilities Council in Swindon, U.K., which funded the research. “If some of these gas giants found outside our Solar System have moons, like Jupiter and Saturn, there’s a real possibility that some of them could be Earth-like.”