Thursday, July 1, 1999 Published at 05:02 GMT 06:02 UK
Life lost in space
Planets could be ejected as the solar system formed
Planets that could harbour life may be roaming the voids of interstellar space, according to a report in Nature magazine.
Planetary scientist David Stevenson thinks it is possible that rocky masses the size of the Earth are ejected from solar systems as they form - kicked out by the gravitational effects of much larger planets.
However, even without the energy supplied by sunlight, he believes these nomadic worlds could still maintain the conditions necessary for life.
Mr Stevenson, of the California Institute of Technology, says the ejected planets would possibly carry with them an atmosphere of hydrogen. This could act to trap any internal heat. It may even exert enough pressure to maintain oceans of liquid water on the planet's surface.
With ammonia and methane gases in the atmosphere, and volcanoes bubbling heat and molten rock to the surface, these lonely planets would look remarkably similar to the Earth when life began here some four billion years ago.
All these planets would lack is the energy supplied by a sun.
Writing in Nature, Mr Stevenson says: "If life can develop and be sustained without sunlight (but with other energy sources, plausibly volcanism or lightning in this instance), these bodies may provide a long-lived, stable environment for life (albeit one where the temperatures slowly decline on a billion-year timescale).
"The complexity and biomass may be low because the energy source will be small, but it is conceivable that these are the most common sites of life in the universe."
Oceans on the nomadic planets would have surface pressures and temperatures similar to those found at the bottom of the Earth's oceans, places where we know life can exist, thriving on the heat and nutrients supplied from hot water vents.
Mr Stevenson's ideas are controversial but it will be virtually impossible to test them. Despite emitting some degree of radiated heat, and possibly radio waves, the planets would be almost invisible.
Artist's impression of a developing solar system taken from the BBC Science TV series The Planets.