Astronomers found the first exoplanet having the same mass as Earth, named KOI-314c. It is the lightest exoplanet ever found, whose the second lightest is Kepler-78b, with 1.7 times Earth's mass.

Located at 200 light-years from the Sun, KOI-314c has a surface temperature of 100°C orbits around its star in only 23 days. Its host star is less massive and cooler than the Sun. The planet has the same mass as Earth's, but is 1.6 times bigger than our planet.

The fact that the orbital period is only 23 days shows that the planet is close to the host star, and so it couldn't contain liquid water, and life. However, according to David Kipping from Harvard-Smithsonian Center for Astrophysics who is the main author of the discover, the exoplanet would have a very thick atmosphere containing hydrogen and helium. The beginning of its life could be the same as Neptune's, then the planet would have lost some of its atmospheric gases over time.

However, Kipping's team of astronomers who participated at HEK Project (Hunt for Exomoons with Kepler) thought that KOI-314c was in the beginning an exomoon orbiting around KOI-314b, an exoplanet having same size as KOI-314c but with 4 times Earth's mass, and orbiting around its host star in only 13 days. The team of astronomers saw then that KOI-314c was finally a planet and not an exomoon.

Sources : Sciences et Avenir (french) and Slate.com

Read also : Exomoon research could start at 1800 light-years from the Sun...

From 1989 to 1995, astronomers discovered the first exoplanets. 1056 exoplanets later, they probably discovered the first exomoon, located at 1800 light-years from the Sun. Pandora could have finally a cousin ...

To see exommons (in the fiction), you just have to search "space wallpaper" or "planets wallpaper" on Google. Or, re-watch the films Avatar (Pandora) and Return of the Jedi (Endor).

But here, we are approaching reality. David Bennett (of the University of Notre Dame, Indiana) and his colleagues report on 13 December that they spotted a microlensing event in 2011, named  MOA-2011-BLG-262, using several telescopes in the world. First, they saw the star's light amplified to 70 times its normal brightness. And an hour later, a second came, smaller increase in brightness. So that would mean that a great object passed in front of the star, followed by a smaller object.

The first scenario that fits in the microlensing data is the following : it's an exoplanet followed by its moon, and both are at a distance of 1800 light-years from the Sun. The exoplanet's mass is about 4 times the mass of Jupiter, while the mass of its moon is half Earth's mass, that is to say 3 x 10^24 kg, so 20 times the mass of Ganymede (which is the heaviest moon of the solar system).
Even weirder : according to this model, the exomoon would be orbiting at about 20 million km from its host planet, while Ganymede orbits at 1,070,000 km from Jupiter. If the scenario is correct, we would have to explain why it is possible that there's a great distance between the planet and its moon.

The second scenario which could work is that the two detected objects are much further away and consists of a very small star or a failed star known as a brown dwarf, orbited by a Neptune-mass planet.

Today we can't confirm any of the 2 scenarios. However, exomoon research interest astronomers because exomoons in more normal planetary systems might even support life. If it is true, we could find then "cousins" of Pandora (whose Jake Sully could explore the oceans in Avatar 2).

Sources : NewScientist and International Science Times...