A vast belt of carbon monoxide located at the fringes of the Beta Pictoris system is concentrated in a single clump located about 8 billion miles from the star, or nearly three times the distance between the planet Neptune and our sun.
The total amount of CO observed exceeds 200 million billion tons, equivalent to about one-sixth the mass of Earth's oceans.
The presence of all this gas is interesting because ultraviolet starlight breaks up CO molecules in about 100 years, much faster than the main cloud can complete a single orbit around the star.
The researchers calculated that to offset the destruction of CO molecules around Beta Pictoris, a large comet must be completely destroyed every five minutes. Only an unusually massive and compact swarm of comets could support such an astonishingly high collision rate. Because the disk is viewed nearly edge-on, the ALMA data cannot determine whether the carbon monoxide belt has a single concentration of gas or two on opposite sides of the star. Further studies of the gas cloud's orbital motion will clarify the situation, but current evidence favors a two-clump scenario.
In our own solar system, Jupiter's gravity has trapped thousands of asteroids in two groups, one leading and one following the planet as it travels around the sun. A giant planet located in the outer reaches of the Beta Pictoris system likewise could corral comets into a pair of tight, massive swarms.
"So unless we are observing Beta Pictoris at a very unusual time, then the carbon monoxide we observed must be continuously replenished," said Bill Dent, lead author of the Science Express article and a researcher at the Joint ALMA Office in Santiago, Chile.
"Detailed dynamical studies are now under way, but at the moment we think this shepherding planet would be around Saturn's mass and positioned near the inner edge of the CO belt," said coauthor Mark Wyatt, an astronomer at the University of Cambridge in England.
Astronomers have directly imaged one giant planet, Beta Pictoris b, with a mass several times greater than Jupiter, orbiting much closer to the star. While it would be unusual for a giant planet to form up to 10 times farther away, as required to shepherd the massive comet clouds, the hypothetical planet could have formed near the star and migrated outward as the young disk underwent changes.
"We think the Beta Pictoris comet swarms formed when the hypothetical planet migrated outward, sweeping icy bodies into resonant orbits," explained Wyatt. When the orbital periods of the comets matched the planet's in some simple ratio – say, two orbits for every three of the planet – the comets received a nudge from the planet at the same location every orbit. Like regular pushes on a child's swing, these accelerations amplify over time and work to confine the comets in a small region.
If, however, the gas actually turns out to reside in a single clump, the researchers suggest an alternative scenario. A crash between two Mars-sized icy planets about half a million years ago would account for the comet swarm, with frequent ongoing collisions among the fragments gradually releasing carbon monoxide gas.
Either way, Beta Pictoris clearly has a fascinating story to tell, and ALMA's keen vision will help astronomers delve ever deeper into the tale.