From: LARRY KLAES (ljk4_at_msn.com)
Date: Fri Oct 26 2007 - 08:01:50 PDT
http://www.skyandtelescope.com/news/10740281.html
Lonely Planets of the Cosmos
October 23, 2007by Laura Kinoshita
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Scientists think free-floating planets — occasionally with moons — should
exist in the cold depths of interstellar space, but without any sunlight
they will be very difficult to find.
A brief letter in Nature was John Debes's inspiration. The 1999 piece, by
David J. Stevenson (Caltech), proposed that planets with liquid water oceans
— and even life — could exist in the cold, dark depths of interstellar space
far from any star. Based on the knowledge that some fraction of planets must
get gravitationally ejected from their systems during the systems'
formation, the paper theorized that some of these ejected planets could,
with enough internal heat, keep their atmospheres and stay warm enough to
support liquid water below a thick frozen crust.
What might happen if such an outcast had a big moon? To find out, Debes (at
the Carnegie Institution of Washington) ran 2,700 computer simulations based
on an Earth-mass planet and a lunar-mass companion.
"The ejection process can be very intense," says Debes. "It wasn't clear to
us if any bound systems would actually survive." But in 123 of the cases, or
between 4 and 5 percent of the time, the "Earth-Moon" system did survive
ejection from its solar system intact.
"Anytime something happens in astronomy a few percent of the time, it is
interesting to us because on the grand scale of things, it means it's
happening a lot and people should probably know about it," says Debes.
And these pairs have a better chance to harbor life, since the dissipation
of tidal energy between the moon and the spinning planet causes the interior
of the planet to warm. Debes' models predict that this heating would match
what happened in Earth's case more than 4 billion years ago, when the young
Moon was much closer and Earth was rotating faster.
In the October 20th Astrophysical Journal Letters, Debes and Steinn
Sigurdsson write that the heating would likely be localized in hot spots of
volcanism or other geothermal processes. Biologists are finding many
examples on Earth of life surviving on these energy sources, such as at
mid-ocean ridges. Could such "extremophiles" be the most common form of life
in the universe?
The team's best-case scenario found that an ejected Earth-Moon system can
sustain its heat for up to 250 million years — long enough for life to
arise. But is it long enough for that life to adapt to the eventual
decreasing temperatures?
And if dark, free-floating Earths exist, when will astronomers be able to
detect one? It's not as impossible as you might think. Debes estimates the
next generation of space-based microlensing surveys will have around a 2%
chance.
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