A new map of Beta Pictoris reveals an asymmetric clump of carbon monoxide likely produced in cometary collisions. It provides a rare glimpse at the chaotic birth of a planetary system.
The bright young star Beta Pictoris hosts an archetypal edge-on debris disk: a dusty circling plane where planets form. The star is about 20 million years old, which is relatively young for a star whose life expectancy is billions of years. Located a mere 63 light years from Earth, its proximity allows astronomers to pinpoint detailed features and analyze the birth of a planetary system.
The Beta Pictoris system hosts dust rings, empty gaps within those rings, and even a planet, Beta Pictoris b, whose orbit is 25% larger than Saturn’s. But one of the most puzzling features was the detection of carbon monoxide gas. While debris disks initially form from both dust and gas, current models predict that after 10 million years or so, all of the primordial gas should fall onto the star or get caught up in what will later become an orbiting gas giant.
The carbon monoxide present in Beta Pictoris remained a mystery until the arrival of the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile. The telescope’s high sensitivity enabled an international team of astronomers to detect the emission from carbon monoxide, as opposed to previous studies which were only able to probe absorption along the line of sight. This combined with its high spatial resolution provided the first detailed map of molecular gas in the edge-on disk.
“When I first saw these data, it completely blew my mind,” says coauthor Aki Roberge (NASA Goddard Space Flight Center). The ability to resolve the gas’s emission with a resolution of 12 astronomical units, or 12 times the distance between Earth and the Sun, wasn’t possible before the advent of ALMA.
The carbon monoxide gas orbits Beta Pictoris in a broad belt, with the inner radius beginning at a distance just beyond our Kupier belt and extending out 3 times further. (Its inner and outer radii correspond to 50 and 160 a.u. from the star.) The gas has a total mass equivalent to a third of the Moon's mass, and 30% is found in a single clump 85 a.u. away from the star.
Here’s the rub: carbon monoxide sublimates at temperatures much lower than those measured for the debris disk. And once it turns into gas, ambient starlight will break the molecule down on a timescale of 120 years — substantially less than the 600-year orbital period at 85 a.u. So the gas astronomers detected must have been either recently released or continuously replenished.
The likely culprits are comets, which can trap pockets of carbon monoxide within their water-icy reservoirs. Any collisions would then release the gas. This explanation is supported by mid-infrared images, which show that tiny-micron sized dust particles coexist with the carbon-monoxide clump. These dust particles have a very short lifetime, so they could also be the result of recent cometary collisions.
The authors suggest two likely scenarios: either a huge, unseen planet is shepherding swarms of comets into asymmetric clumps (where they easily collide with each other) or the observed gas clump is the remnant of one recent, massive collision.
In the first scenario the outward migration of a planet forces comets into resonant orbits that clump them on either side of the planet — similar to the Trojan asteroids, which are trapped into orbits that lead or trail behind Jupiter. Comets would frequently collide within these clumps, constantly replenishing the sites with carbon monoxide. Calculations show the destruction of one large comet every five minutes would sustain the clump’s gas reservoir.
The second scenario requires a recent collision between two Mars-sized icy comets that released a massive amount of carbon monoxide in one go. While both scenarios provide plausible explanations, the research team favors the first, as the second would mean we’re observing the system at a very lucky time. It’s not impossible, just unlikely.
Either way, both scenarios indicate a period of chaotic activity within this young debris disk. It was only a decade or so ago when we thought planet formation was slow, calm, and orderly, notes Roberge. There weren’t large motions of planets sweeping from the outer system to the inner system, and there weren’t moon-forming impacts.
We now know the birth of a planetary system is much more dynamic. And Beta Pictoris provides astronomers with the remarkable opportunity to probe the chaotic environment of a young, bright, and nearby disk. The team next plans to use ALMA to look for prebiotic molecules such as methane.
Dent W. R. F., Wyatt M. C., Roberge A., et al. “Molecular Gas Clumps from the Destruction of Icy Bodies in the Beta Pictoris Debris Disk" Science, 2014