March 17, 2014
A large clump of carbon monoxide in a young neighbouring star system indicates that either there is a hidden gas giant unusually far from its host star or two of smaller planets collided, leaving a swarm of comets.
Artist's impression of a yet undetected gas giant (bottom left) and the swarm of colliding comets captured by it. Credit: NASA's Goddard Space Flight Center/F. Reddy
By observing other solar systems, we strive to understand our own system's formation and mechanisms by which water and subsequently life found their way to our planet. In one such system forming around young star Beta Pictoris, 63 light-years away, an international team of astronomers, using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, found a large clump of carbon monoxide 13 billion kilometers from the star.
In the space, unprotected by an atmosphere and bombarded by ultraviolet light, carbon monoxide usually breaks up after about 100 years. Finding it in such vast quantities, 200 billion tons, has many implications.
The most likely origin of carbon monoxide are icy comets that replenish the gas as they collide into one another. We are talking about a big swarm of comets, colliding every 5 minutes. These comets are also rich in water ice, so if there's a rocky planet anywhere in the system, it is possible that it is being bombarded by them, paving a way for life in Beta Pictoris sometime in the future. The system is only 20 million years old and there are probably planets currently being formed.
"Although toxic to us, carbon monoxide is one of many gases found in comets and other icy bodies," said Aki Roberge, team member and an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, USA. "In the rough-and-tumble environment around a young star, these objects frequently collide and generate fragments that release dust, icy grains and stored gases.
"To get the amount of carbon monoxide we observe, the rate of collisions would be truly startling — complete destruction of a large comet once every five minutes. To get this number of collisions, this would have to be a very tight, massive swarm."
The mere presence of carbon monoxide is unexpected, but the fact that it is concentrated in a large clump could mean there is one or more undetected worlds around Beta Pictoris, beside the one gas giant already known – Beta Pictoris b.
From our perspective, we only see what seems to be a single concentration of gas, but there's evidence that there could be two clumps of gas, one behind the other from our perspective. If that is true, this could mean there's a gas giant between them. This is something we can observe in our solar system where Jupiter has trapped asteroids in two swarms, one leading it as it orbits the Sun and one following the planet.
Above is the millimeter-wavelength image of carbon dioxide clump taken by ALMA. Below is what we would see from above if there are indeed two clumps around a gas giant. Credit: ALMA (ESO/NAOJ/NRAO) and NASA Goddard/F. Reddy
If this is the case, it would be very unusual to find a gas giant at this distance from the star, in outer reaches of the system. We need to keep in mind that the planets are still forming there and some of them may not have stable orbit. It is likely this, still undetected, gas giant was formed closer to the star and wandered away, sweeping comets into resonant orbits as it shifted orbits.
"This clump [of carbon monoxide] is an important clue to what is going on in the outer reaches of this young planetary system," says Mark Wyatt, the University of Cambridge, UK. He is the co-author of the study. "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 [carbon monoxide] belt."
If it turns out there's only a single concentration of gas, it could have possibly originated from two icy planets that collided about half a million years ago.
Further such observations with ALMA are planned, which was to be expected. Astronomers hope to better understand the process of gas and water delivery by comets to rocky worlds in inner parts of star systems as they are being formed.