ALIEN lifeforms may be hurtling around the Galaxy on dark, nomadic planets
that have been expelled from their solar systems. 鈥淚t is conceivable that these
are the most common sites of life in the Universe,鈥� says David Stevenson, a
planetary scientist at the California Institute of Technology in Pasadena.
Some astronomers believe that when a new solar system forms, the gravity of
giant planets can catapult Earth-sized embryonic planets out of the system.
These planets could have radioactive elements providing heat at their cores, and
possibly hydrogen-rich atmospheres.
Thinking about these atmospheres, Stevenson realised that they would prevent
a planet鈥檚 internal heat from radiating away quickly. 鈥淭he surfaces of the body
do not cool down,鈥� he says.
Advertisement
Stevenson has now calculated that the temperature at the surface of a nomadic
planet with a hydrogen-rich atmosphere would typically be a comfortable 300
kelvin (Nature, vol 400, p 32). At this temperature, the planet could
host liquid oceans.
鈥淚f life can develop and be sustained without sunlight, maybe this is an
environment where you can imagine life emerging,鈥� Stevenson says. Although there
would be no daylight on a nomadic planet, there could be energy from volcanoes
and lightning, he adds.
Stevenson suggests that astronomers might be able to pick up signs of nomadic
planets if they pass in front of distant stars and dim the starlight. But he
concedes that it would be difficult to detect the planets, much less find signs
of life. He simply hopes his work will encourage scientists to keep an open mind
about possible habitats for alien life. 鈥淚t鈥檚 just an idea that has merit
because it gets people thinking,鈥� he says.
鈥淚t鈥檚 one of those papers that seriously gets the brain working,鈥� agrees
David Hughes, an astronomer at the University of Sheffield. However, he doubts
that a huge amount of material is ejected from solar systems during their
formation, and points out that a nomadic planet would gradually be cooling down:
鈥淚鈥檓 not convinced this is going to be conducive to life.鈥�