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- As a wealth of new
data flooded in about planets beyond
our solar system in 1999,
scientists gained a clearer sense of what they
don,t know --
particularly about whether extrasolar planets might harbor
life.
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- A
dozen extrasolar planets were discovered in 1999, bringing
the total
number to 28 and confirming that planets are not an extreme rarity
in
the universe. The year saw the first detection of a planet crossing
in
front of a star, and the first discovery of a multi-planet system around
a sunlike star.
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- All of the planets that have been discovered are gas
giants,
and many of them are "hot Jupiters," gas giants that
travel
around their stars in close, swift orbits. "What's becoming
clear
is that hot Jupiters are reasonably common," says John Bally,
a
professor of astronomy at the University of Colorado.
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- Hot Jupiters are generally not
considered likely abodes
for life. But their existence, notes Bally,
may be only the "tip of
the iceberg." Present-day instruments
are unable to detect any smaller,
Earthlike planets that may exist, and
even gas giants are hard to detect
if they travel in orbits distant
from their stars.
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- Some gas giants are found in the "habitable zone,"
orbiting at roughly the same distance from their stars as the Earth is
from the sun. But their presence there carries mixed implications for the
possibility of life.
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- "Giant gas planets will scatter small planets out
of their path, very much like cars are going to scatter flies out of their
path," says William Borucki, a research scientist at NASA's Ames
Research
Center. Thus, he says giant gas planets in the habitable zone
"preclude
habitable planets around those stars."
-
- "It's
interesting to ask whether a solar system
like ours is a requirement
for technological life." Chris Chyba, SETI
Institute
-
- On the other hand,
such planets might have moons that
are habitable. "If these
Jupiters are in stable orbits, which they
seem to be, then although the
planet might not be suitable for life, any
moons that might orbit
around them might be," says Bally, who adds
that this is
"pure speculation."
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- Currently, it is not known whether any extrasolar
planets
have moons, or whether such moons would be large enough to
maintain thick
atmospheres that provide a shield against a gas giant's
radiation.
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- How similar to our own solar system?
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- A further question is whether
gas giants are common in
orbits far from their stars.
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- "Under the most
propitious situation, the giant
planets form and stay out a long
distance," says Borucki, noting that
Jupiter helps divert comets
away from Earth and our inner solar system.
However, he adds, it may be
that numerous Earthlike planets can form in
a solar system if there are
no gas giants there at all.
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- Earthlike planets or moons, moreover, may not be the
crucial conditions for life. Even if all life requires liquid water,
"you
probably don,t need solar systems like ours," says Chris
Chyba, holder
of the SETI Institute's Carl Sagan Chair for the Study of
Life in the Universe.
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- There may be extrasolar worlds similar to Jupiter's moon
Europa, which appears to have an ocean beneath layers of ice, Chyba points
out. "It's clear from what we've learned about life on Earth that
you can have ecosystems that flourish underground as long as you have
liquid
water."
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- He adds: "It's interesting to ask whether a solar
system like ours is a requirement for technological life. Can you imagine
a technically intelligent species evolving underneath a kilometer of ice
on a world like Europa? That's much harder to envision, although our
understanding
here is very limited."
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