Some stars have a high level of comet activity around them,
and that could spell doom for any life trying to take root on any local planets.
Ongoing research is trying to determine what fraction of stellar systems may be
uninhabitable due to comet impacts.
Many of our own solar system's comets are found in the Kuiper
Belt, a debris-filled disk that extends from Neptune's orbit (30 AU) out to
almost twice that distance. Other stars have been shown to have similar debris
disks.
"The debris is dust and larger fragments produced by
the break-up of comets or asteroids as they collide amongst themselves,"
says Jane Greaves of the University of St. Andrews in Scotland.
Roughly 20 percent of nearby sun-like stars have debris disks
that are more substantial than our Kuiper Belt, according to data from the
Spitzer space telescope. More debris means more comets, but does this also mean
more killer impacts for any Earth-like planets that might be orbiting these
stars?
The answer depends on whether there are any gas giant planets
around.
Jupiter is known to shield Earth from some comets by
deflecting them out of the solar system. However, scientists showed in 2007
that Jupiter also injects
other comets into Earth-crossing orbits. In fact, if Jupiter were Saturn's
size, the number of impacts on Earth would have been much higher.
Greaves has been modeling how comets are generally affected
by gas giants. Her early results indicate that comets will be a major problem
around a few percent of sun-like stars.
Comet sweep
Early in our solar system's history, there were plenty of
remnants left over from planet formation. All this debris led to a heavy
bombardment of comets and asteroids on the inner planets, as evident in the
crater record of the moon (on Earth, most of these scars have eroded away with
time or have disappeared due to tectonic activity).
The number of impacts eventually tapered off around 3.8
billion years ago, 700 million years after the solar system formed.
The cause of this decrease may have been a shift in the
orbits of the gas giants that cleared away many of the comets. Jupiter and
Saturn appear to have migrated outwards, pushing out on the orbits of Uranus
and Neptune. This in turn perturbed the Kuiper Belt and ejected many of the
comets into interstellar space, Greaves says.
"This might be a very peculiar event, or it might
happen in other star systems — we don't know yet, because we have limited
information about their giant planets," she says.
Catastrophic impact
Still, our planet has not been completely immune to deadly
impacts.
Many scientists believe the dinosaurs were snuffed
out by a 4-20 kilometer-wide comet or asteroid that struck 65 million years
ago at a point on the Yucatan peninsula. The impact led to a global firestorm
and the eventual extinction of more than half of the planet's life forms.
A 100-kilometer impactor would have left no survivors. Such
a "catastrophic impact" would destroy the entire crust of the Earth
and eject the atmosphere into space.
The Earth likely experienced a few of these catastrophic
impacts very early on, before life as we know it had even begun.
"While 'dinosaur-killer' class impacts occur about
every 100 million years [on Earth], we would be unlikely to experience another
100-km class event in the lifetime of the sun," Greaves says.
How much higher would the impact rate on a planet need to be
to prevent life from ever forming?
Greaves thinks that life could not evolve on a planet where 10-100
kilometer-size impactors hit every 20 million years. This kind of bombardment
doesn't allow organisms enough time to recover between blows. The level of
biodiversity remains low, so there's less probability that any species will
survive the next devastating impact.
In previous work, Greaves and her colleagues speculated that
Tau
Ceti—a nearby sun-like star that has been a favorite target of SETI
searches—is uninhabitable
due to the large number of comets that appear to be buzzing around it (although
this assessment may have been overly pessimistic, she now says).
Her team is currently looking at the general threat posed by
comets. They have modeled various representative planetary systems (both with
and without gas giants). From this, they estimate that at least a few percent
of stars are going to be too comet-stricken to be in the running as possible
hosts for life.
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