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I'm Aki Roberge,
an astronomer at NASA's Goddard Space Flight Center. I've been studying
a young nearby planetary system around the bright star Beta Pictoris.
Located 63 light-years away, and only about
20 million years old, the star is surrounded by a vast disk of
gas, dust and comet-like bodies that we view edgewise.
We know of one planet in there too, it's a giant planet tracking along
an orbit nearly as large as Saturn's. I'm part of a team
studying Beta Pic's disk using the ALMA observatory in Chile.
We've found something odd: a belt of carbon monoxide
gas centered about three times farther from the star than Neptune's
distance from the sun. The total amount of gas is about
one-sixth the mass of all the water in Earth's oceans. What's
interesting is that incoming ultraviolet light should break up the carbon
monoxide molecules in little more than a century, on average.
This means that the carbon monoxide must be resupplied by the breakup of icy
comets. To produce the amount of gas we detect, we're looking at the
equivalent of the total destruction of a large comet every 5 minutes.
From our data, we can tell that much of the carbon monoxide
is in one or two massive clumps, which was very surprising.
Because we're viewing the disk edge-on, we can't be sure if its one or
two. Regardless, the comets suppling the gas must also
be concentrated into clumps. How could this happen?
If there is one clump, we think we're seeing the aftermath of collision
between two icy planets about the mass of Mars.
Such a collision would have occurred about half a million years ago, releasing large
quantities of gas and small, comet-like fragments. The second
--and we think more likely--scenario is that the carbon monoxide
exists in two clumps and is continually replenished by by collisions in
huge comet swarms. We believe the comets are shepherded together
by an as-yet-undetected second planet whose gravity confines
the comets into a small region so the frequently collide. A planet
with roughly Saturn's mass could do the job. Other observations
hint that the brightest clump is moving in a way that makes the two clump
scenario more likely. Further observations will track it in better detail
and help us confirm this dramatic picture.
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