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Chemistry of other stars’ planets coming to light

Dec. 9, 2008
Courtesy NASA
and World Science staff

Car­bon di­ox­ide has been iden­ti­fied the at­mos­phere of a plan­et out­side our so­lar sys­tem, a key step to­ward find­ing chem­i­cal signs of pos­si­ble ex­tra­ter­res­tri­al life, as­tro­no­mers say.

The Jupiter-sized plan­et, called HD 189733b, is too hot for life. But the ob­serva­t­ions are a “proof of con­cept” that the bas­ic chem­is­try of life is meas­ur­a­ble on plan­ets or­bit­ing oth­er stars, ac­cord­ing to sci­en­tists.

An artist's im­pres­sion of the plan­et HD 189733b be­hind and slight­ly be­neath its par­ent star from our point of view. As­tro­no­mers us­ing the Hub­ble Space Tel­e­scope have meas­ured car­bon di­ox­ide and car­bon mon­ox­ide in the plan­et's at­mos­phere. The plan­et is a "hot Jupiter," which is so close to its star that it com­pletes an or­bit in on­ly 2.2 days. This type of ob­ser­va­tion is best done when the plan­et's or­bit car­ries it be­hind the star (as seen from Earth), which al­lows an op­por­tu­ni­ty to sub­tract the light of the star alone (when the plan­et is blocked) from that of the star and plan­et to­geth­er pri­or to eclipse, as­tro­no­mers say. This al­lows as­tro­no­mers to iso­late the in­fra­red emis­sion of the plan­et and chem­i­cal­ly an­a­lyze the day-side at­mos­phere. (Cred­it: ESA, NA­SA, M. Ko­rn­messer (ESA/Hub­ble), and STScI)


On Earth, car­bon di­ox­ide is re­leased when an­i­mals breathe. It and oth­er car­bon-based com­pounds can be byprod­ucts of var­i­ous life pro­cesses, so their de­tec­tion on an Earth-like plan­et could some­day pro­vide the first ev­i­dence of life be­yond Earth.

The re­search­ers used NASA’s and the European Space Agency’s Hub­ble Space Tel­e­scope to search the light spec­trum from the plan­et for tell­tale sig­na­tures of spe­cif­ic chem­i­cals. 

Pre­vi­ous ob­serva­t­ions of HD 189733b by Hub­ble and the Spitzer Space Tel­e­scope found wa­ter va­por. Ear­li­er this year Hub­ble found meth­ane in the plan­et’s at­mos­phere.

“Hub­ble is al­low­ing us to see mol­e­cules that probe the con­di­tions, chem­is­try, and com­po­si­tion of at­mos­pheres on oth­er plan­ets,” said Mark Swain of The Je­t Pro­pul­sion Lab­o­r­a­to­ry in Pas­a­de­na, Calif. “Thanks to Hub­ble we’re en­ter­ing an era where we are rap­idly go­ing to ex­pand the num­ber of mol­e­cules we know about on oth­er plan­ets.” 

Swain and other as­tro­no­mers used an in­stru­ment on Hub­ble known as the Near In­fra­red Came­ra and Mul­ti­-Object Spec­trom­e­ter to study in­fra­red light emit­ted from the plan­et, which lies 63 light-years away. A light-year is the dis­tance light trav­els in a year.

Iden­ti­fica­t­ion of spe­cif­ic chem­i­cals in ce­les­tial ob­jects is pos­si­ble be­cause par­tic­u­lar com­pounds ab­sorb light of only cer­tain en­er­gies. This leaves a mark on the light spec­trum we see from that ob­ject: it’s de­void of light from those en­er­gies, or wave­lengths. The team iden­ti­fied not only car­bon di­ox­ide, but al­so car­bon mon­ox­ide.

“The car­bon di­ox­ide is kind of the main fo­cus of the ex­cite­ment, be­cause that is a mol­e­cule that un­der the right cir­cum­stances could have a con­nec­tion to bi­o­log­i­cal ac­ti­vity as it does on Earth,” Swain said. “The very fact that we’re able to de­tect it, and es­ti­mate its abun­dance, is sig­nif­i­cant for the long-term ef­fort of char­ac­ter­ize plan­ets both to find out what they’re made of and to find out if they could be a pos­si­ble host for life.”

This type of ob­ser­va­tion is best done when the plan­et's or­bit car­ries it be­hind its host star, as seen from Earth, members of Swain’s team said. This provides an op­por­tu­ni­ty to sub­tract the light of the star alone, when the plan­et is blocked, from that of the star and plan­et to­geth­er pri­or to eclipse. This in turn al­lows as­tro­no­mers to iso­late the in­fra­red light of the plan­et.

The findings are to appear in an up­coming is­sue of The Astro­phys­ical Jour­nal.

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Carbon dioxide has been identified the atmosphere of a planet orbiting another star, a key step toward finding chemical tracers of possible extraterrestrial life, astronomers say. The Jupiter-sized planet, called HD 189733b, is too hot for life. But the observations are a “proof of concept” that the basic chemistry of life is measurable on planets orbiting other stars, according to scientists. On Earth, carbon dioxide is released when animals breathe. It and other carbon-based compounds can be byproducts of various life processes, so their detection on an Earth-like planet could someday provide the first evidence of life beyond Earth. The researchers used the Hubble Space Telescope to search the light spectrum from the planet for telltale signatures of specific chemicals. Previous observations of HD 189733b by Hubble and the Spitzer Space Telescope found water vapor. Earlier this year Hubble found methane in the planet’s atmosphere. “Hubble is allowing us to see molecules that probe the conditions, chemistry, and composition of atmospheres on other planets,” said Mark Swain of The Jet Propulsion Laboratory in Pasadena, Calif., a member of the research team. “Thanks to Hubble we’re entering an era where we are rapidly going to expand the number of molecules we know about on other planets.” The international team of astronomers used an instrument on Hubble known as the Near Infrared Camera and Multi-Object Spectrometer to study infrared light emitted from the planet, which lies 63 light-years away. A light-year is the distance light travels in a year. Identification of specific chemicals in celestial objects is possible because particular compounds absorb light of only certain energies. This leaves a mark on the light spectrum we see from that object: it’s devoid of light from those energies, or wavelengths. The team identified not only carbon dioxide, but also carbon monoxide. “The carbon dioxide is kind of the main focus of the excitement, because that is a molecule that under the right circumstances could have a connection to biological activity as it does on Earth,” Swain said. “The very fact that we’re able to detect it, and estimate its abundance, is significant for the long-term effort of characterizing planets both to find out what they’re made of and to find out if they could be a possible host for life.”