A team of planet hunters has announced the discovery of an Earth-sized planet (three times our mass) orbiting nearby star Gliese 581 at a distance that places it squarely in the middle of the star's 'habitable zone', where liquid water could exist on the planet's surface.The most interesting of the two new planets announced is Gliese 581g, with a mass just over three times that of the Earth and an orbital period of just under 37 days.
Its mass indicates that it is probably a rocky planet with a definite surface and that it has enough gravity to hold on to an atmosphere.
The surface gravity would be about the same or slightly higher than Earth's, so that a person could easily walk upright on the planet
