April 2007 Archives
An Earth-like planet in the "habitable zone" of another star was discovered, according to an announcement earlier today by a team of scientists from the European Southern Observatory. The planet orbits the star Gliese 581, a red dwarf located 20.5 light years away, in the constellation Libra.
The habitable zone of a star is the distance where the temperature in the surface of a planet allows the existence of liquid water. In our solar system, the habitable zone includes Venus, Earth and (barely) Mars. In Gliese 581, this zone is much closer to the star, due to its lower temperature: the planet (called for now Gliese 581 C) is 0.07 AU away from the star, or 1/14th of the distance from Earth to the Sun, and it orbits the star once every 13 days.
At that distance, the estimated surface temperature is between 0 and 40°C; the planet is 5 times as massive as the Earth (but only about 1.5 times larger), which means that its gravity is similarly stronger and it should have a dense atmosphere; there's no word on the possible composition of this atmosphere as yet, though (and, since the planet does not move in front of the star as seen from Earth, determining the composition is a hard problem). With this density, the planet certainly is a rocky world (like Earth), not a gaseous one.
The planet is called Gliese 581 C because it's the third one to be discovered around this star; astronomers already knew of a large, Neptune-like planet orbiting very close to the star (a "hot Jupiter" planet) and another one about eight times as large as Earth orbiting further away, outside the habitable zone.
The Parkes dish looked at this star system, back in 1995, looking for signs of intelligent life, and found nothing; so did the Greenbank Radio Telescope, in the US, with the same result. Still, it is probably going to be at the top of the list of observations when the Allen Telescope Array starts operations later this year.
Update: the original announcement is here