"Long before it's in the papers"
August 03, 2010

RETURN TO THE WORLD SCIENCE HOME PAGE

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)

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.