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Like Earth, if you overlook the lava everywhere?

Feb. 4 , 2009
Courtesy University of Exeter
and World Science staff

A Eu­ro­pe­an sat­el­lite has revealed a plan­et only twice as large as the Earth or­bit­ing a dis­tant star, as­tro­no­mers say.

That would make it the most Earth-like plan­et in size yet found to date out­side our so­lar sys­tem, ac­cord­ing to the sci­en­tists; it may al­so al­so Earth-like in com­po­si­tion, though cer­tainly not in tem­pe­r­a­ture.

One of the meth­ods for de­tect­ing new plan­ets is to look for the drop in bright­ness they cause when they pass in front of their par­ent star. Such a ce­les­tial align­ment is known as a plan­e­tary trans­it. From Earth, both Mer­cu­ry and Ve­nus oc­ca­sion­al­ly pass across the front of the Sun. When they do, they look like ti­ny black dots pas­sing across the bright sur­face. Such trans­its block a ti­ny frac­tion of the light that Co­RoT is able to de­tect, as in this art­ist's view. (Cour­te­sy CNES)


As­tro­no­mers in­fer it must be so hot—over 1,000 de­grees C (1,800 F)—that it should be swad­dled in la­va or su­pe­r­heated wa­ter va­pour. It’s es­ti­mat­ed to lie so close to its par­ent star that it com­pletes an or­bit around it, or a year, in 20 days. The star is slightly smaller than the Sun.

The plan­et’s com­po­si­tion is un­clear, but it’s probably mainly rock and wa­ter, as­tro­no­mers said. 

The find­ing was made us­ing the Co­RoT space tel­e­scope, a proj­ect led by the French Na­tional Cen­ter for Space Stud­ies. The in­stru­ment is de­signed to de­tect the micro-eclipses that oc­cur when plan­ets pass in front of their par­ent stars from our point of view, dim­ming the star­light al­most im­pe­r­cep­ti­bly.

Most of the 330 or so plan­ets out­side our so­lar sys­tem dis­cov­ered so far are gi­ant plan­ets, pri­marily com­posed of gas, like Ju­pi­ter and Nep­tune. The new ob­ject, named Co­RoT-Exo-7b, would be very dif­fer­ent. 

“Find­ing such a small plan­et was­n’t a com­plete sur­prise,” said Dan­iel Rouan of the Par­is Ob­ser­va­to­ry’s Lab­o­r­a­to­ry for Space Stud­ies and As­t­ro­phys­i­cal In­stru­ments, who an­nounced the find­ing Feb. 3 at a con­fer­ence in Par­is. 

“It could be an ex­am­ple of a so-called ocean plan­et, whose ex­ist­ence was pre­dicted some years ago,” said Alain Leg­er of the Mar­seille In­sti­tute of As­t­ro­phys­ics, in France, lead au­thor of a pa­pe­r on the find­ings. Such a plan­et, “made of ice around a rocky core, drifts so close to its star” that the ice melts to form a “fluid en­ve­lope.”

“We were able to see it with Co­RoT be­cause it is in space, with no at­mos­phere to dis­turb the mea­sure­ments or day­light to in­ter­rupt them,” added col­la­bo­ra­tor Roi Alon­so of the As­t­ro­phys­ics Lab­o­r­a­to­ry of Mar­seille.

Such small plan­ets such as this one are ex­tremely hard to de­tect, ac­cord­ing to as­tro­no­mers; it was found be­cause it pas­ses in front of its host star, caus­ing the star to dim just 0.03 per­­cent per or­bit.

The re­search­ers had to make sure they weren’t see­ing one of many oth­er kinds of ob­jects that can mim­ic these micro-eclipses. To rule this out, they used com­ple­men­tary ob­serva­t­ions from the ground. “We ruled out every mim­ic ex­cept for a very improba­ble, al­most per­fect chance align­ment of three stars. All our da­ta so far is con­sist­ent with the tran­sits be­ing caused by a plan­et of a few Earth mass­es, though more da­ta are needed for a pre­cise mass es­ti­mate,” said Su­zanne Aigrain of the Uni­ver­s­ity of Ex­e­ter, U.K., a col­la­bo­ra­tor.

The find­ing was an­nounced Feb. 3 at the Co­RoT Sym­po­si­um 2009 in Par­is and is to be pub­lished in a forth­com­ing spe­cial is­sue of the research jour­nal As­tronomy and As­t­ro­phys­ics ded­i­cat­ed to Co­RoT re­sults.


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A European satellite has identified a planet only twice as large as the Earth orbiting a distant star, astronomers say. That would make it the most Earth-like planet in size yet found to date outside our solar system, according to the scientists; it may also also Earth-like in composition, though certainly not in temperature. Astronomers infer it must be so hot—over 1,000 degrees C (1,800 F)—that it should be swaddled in lava or superheated water vapour. It’s estimated to lie so close to its parent star that it completes an orbit around it, or a year, in 20 days. The star is slightly smaller than the Sun. The planet’s composition is unclear, but it’s probably mainly rock and water, astronomers said. The finding was made using the CoRoT space telescope, a project led by the French National Center for Space Studies. The instrument is designed to detect the micro-eclipses that occur when planets pass in front of their parent stars from our point of view, dimming the starlight almost imperceptibly. Most of the 330 or so planets outside our solar system discovered so far are giant planets, primarily composed of gas, like Jupiter and Neptune. The new object, named CoRoT-Exo-7b, would be very different. “Finding such a small planet wasn’t a complete surprise”, said Daniel Rouan of the Paris Observatory’s Laboratory for Space Studies and Astrophysical Instruments, who announced the finding Feb. 3 at a conference in Paris. “It could be an example of a so-called ocean planet, whose existence was predicted some years ago,” said Alain Leger of the Marseille Institute of Astrophysics, in France, lead author of a paper on the findings. Such a planet, “made of ice around a rocky core, drifts so close to its star” that the ice melts to form a “fluid envelope.” “We were able to see it with CoRoT because it is in space, with no atmosphere to disturb the measurements or daylight to interrupt them,” added collaborator Roi Alonso of the Astrophysics Laboratory of Marseille. Such small planets such as this one are extremely hard to detect, according to astronomers; it was found because it passes in front of its host star, causing the star to dim just 0.03 percent per orbit. The researchers had to make sure they weren’t seeing one of many other kinds of objects that can mimic these micro-eclipses, using complementary observations from the ground. “We ruled out every mimic except for a very improbable, almost perfect chance alignment of three stars. All our data so far is consistent with the transits being caused by a planet of a few Earth masses, though more data are needed for a precise mass estimate,” said Suzanne Aigrain of the University of Exeter, U.K., a collaborator. The finding was announced Feb.3 at the CoRoT Symposium 2009 in Paris and is to be published in a forthcoming special issue of the journal Astronomy and Astrophysics dedicated to CoRoT results.