A red dwarf star 20 light-years away is again providing hints that it hosts the first definitively habitable planet outside our Solar System. The planet Gliese 581d is at the colder outer edge of the Goldilocks zone in which liquid water can be sustained. Now a study in Astrophysical Journal Letters suggests its atmosphere may keep things warm enough for water. The solar system also hosts another contender for habitability, unconfirmed planet Gliese 581g announced in 2010. However, the existence of that planet has since been called into question. Gliese 581d is less controversial; it was discovered along with the planet Gliese 581c in 2007, occupying the outer and inner edges of the Goldilocks zone, respectively. Gliese 581c was soon determined to be too close to its host star to sustain water, with a surface temperature exceeding 1,000C. Conversely, the outlying planet 581d - with a mass about six times that of the Earth and twice its size - was initially taken to be too cold to have liquid water. Now, French researchers have run computer simulations of the planets atmosphere, arguing that it is likely to contain high concentrations of carbon dioxide. They contend that conditions could be suitable for oceans of liquid water as well as clouds and rainfall. However, Gliese 581ds denser air and dim red light from its host star would make for a murky environment that would be toxic to humans. BBC