A 'nomad' planet of the right mass, with the right atmosphere, and some source of heat – perhaps radioactive decay or tectonic activity – could allow for life either on the surface or underground.
The Milky Way may hold a vast number of such sunless planets – perhaps up to 100,000 times more than all the stars in the galaxy, according to a new estimate from researchers in the United States and Britain.
Since 1995, when hunting for planets outside the solar system grew from a fringe field to mainstream astronomy, researchers have uncovered as many as 760 extrasolar planets orbiting other stars – as the Earth orbits the sun – with more than 2,000 additional candidates awaiting confirmation.
The vast majority of these are planets in the classical sense – meaning they orbit host stars, although often arrayed in unusual patterns compared with Earth's home system.
Since 2000, however, astronomers have discovered planets with no obvious stellar home. A group of Spanish astronomers reported that year discovering planets ranging from five to 15 times Jupiter's mass free floating in a cluster of young stars in the constellation Orion. Last year, two groups of astronomers jointly announced the discovery of 10 Jupiter-class planets, the vast majority free of the grip of any host star. The results appeared last May in the journal Nature.
With evidence mounting that these planetary nomads are no observational fluke, “we were interested in asking the question: Is this just the tip of the iceberg?” says Louis Strigari, a researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in Palo Alto, Calif., who led the team making the calculation.
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