http://www.spaceviews.com/1999/07/03a.html
Spaceviews
Interstellar Planets Could Support Life
Published: 1999 July 3
9:53
am ET (1353 UT)
Earth-sized planets ejected early in the
history of solar systems could support life
even in the cold depths of interstellar
space, a Caltech scientist has found.
In a paper published in the July 1 issue of the journal
Nature, David Stevenson of Caltech found that such
"interstellar planets" could retain enough heat to support
conditions conducive to the formation of life.
Simulations of the formation of the solar system show that
up to ten planets the size of the Earth could have formed, but
either collided with Jupiter or were ejected from the solar
system during close approaches to the giant planet.
These planets would normally cool as they moved far
away from the Sun or any other star, but a dense hydrogen
atmosphere retained from their formation could act as an
insulating blanket, retaining the heat generated by the
radioactive decay of elements in the interior of the planet.
This could create Earth-like temperatures on the surface of
the planet, even in the absence of an outside heat source.
The planet could have liquid water oceans, but would have a
surface pressure similar to that at the bottom of the Earth's
oceans.
If these planets have geothermal-like heat sources, the
energy could be enough to allow the formation of some small,
simple life forms. How complex the life could be is an open
question, Stevenson believes. "I don't think anyone knows
what is required to drive biological evolution from simple to
very complex systems."
However, other research, including that by Caltech
colleagues Eric Gaidos and Joseph Kirschvink, suggests that
geothermal energy sources may not be sufficient to generate
anything more than the simplest life forms.
Efforts to locate these planets, which may be
commonplace if other solar systems formed like our own,
would require advanced detection techniques, because of
the limited amount of light they emit. Steven suggested
looking for occultations, when the light from a background
star is temporarily dimmed by the passage of one of these
planets.
"I'm not saying that these objects have life," Stevenson
said. "All I'm saying is that, among the places you might want
to consider for sustainable life, you might eventually want to
look at these objects. They could be the most common
location for life in the universe."