ALMA finds a surprise gas clump around a young star
Sen—Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile have discovered an unexpected clump of carbon monoxide gas in the dusty disc around the star Beta Pictoris. Such gas is expected to be rapidly destroyed by starlight, so scientists believe frequent collisions between small, icy objects such as comets must be causing the gas to be continuously replenished.
Beta Pictoris is already hailed as the archetypal young planetary system. It is known to harbour a planet that orbits some 1.2 billion kilometres from the star, and it was one of the first stars found to be surrounded by a large disc of dusty debris.
New observations from ALMA show that the disc is permeated by carbon monoxide gas. The presence of carbon monoxide, which is so harmful to humans on Earth, could indicate that the Beta Pictoris planetary system may eventually become a good habitat for life. The cometary bombardment that its planets are currently undergoing is likely providing them with water.
Carbon monoxide is easily and rapidly broken up by starlight — it can only last about 100 years where it is observed in the Beta Pictoris disc. Seeing it in the 20-million year old Beta Pictoris disc is a complete surprise. So where did it come from, and why is it still there?
"Unless we are observing Beta Pictoris at a very unusual time, the carbon monoxide must be continuously replenished," said Bill Dent, an ESO astronomer at the Joint ALMA Office in Santiago, Chile, and lead author on a paper published in the journal Science. "The most abundant source of carbon monoxide in a young solar system is collisions between icy bodies, from comets up to larger planet-sized objects."
But the rate of destruction must be very high: "To get the amount of carbon monoxide we observe, the rate of collisions would be truly startling - one large comet collision every five minutes," noted Aki Roberge, an astronomer at NASA's Goddard Research Center, and co-author of the paper. "To get this number of collisions, this would have to be a very tight, massive comet swarm."
The ALMA image of carbon monoxide around Beta Pictoris (above) can be deprojected (below) to simulate a view looking down on the system, revealing the large concentration of gas in its outer reaches. Image credit: ALMA (ESO/NAOJ/NRAO) and NASA's Goddard Space Flight Center/F. Reddy.
ALMA's unique ability to simultaneously measure both position and velocity also mapped the carbon monoxide's location in the disc. The gas is concentrated in a single compact clump that lies 13 billion kilometres from the star, which is about three times the distance of Neptune from the Sun. Why the gas is in this small clump so far from the star is a mystery.
"This clump is an important clue to what is going on in the outer reaches of this young planetary system," says Mark Wyatt, an astronomer at the University of Cambridge, UK, and a co-author on the paper. "Either the gravitational pull of an as yet unseen planet similar in mass to Saturn is concentrating the cometary collisions into a small area, or what we are seeing are the remnants of a single catastrophic collision between two icy Mars-mass planets."
Both of these possibilities give astronomers reason to be optimistic that there are several more planets waiting to be found around Beta Pictoris.