Invisible Saturn-sized planet given away by comet army
A cloud of carbon monoxide may have just revealed a strange, comet-shepherding exoplanet.
Radio observations made with the ALMA telescope in Chile show a huge clump of carbon monoxide gas near Beta Pictoris, the first star known to be surrounded by a disc of dusty debris created from smashed-up asteroids and comets. Starlight rapidly breaks down carbon monoxide, so such a large clump would have to be regularly replenished, says Mark Wyatt at the University of Cambridge.
One possibility is that an unseen, Saturn-sized planet is attracting comets, which are then smashing together and releasing trapped gas. “The gravity of the planet gives them a nudge every time they go round their orbits, and that corrals them all into one location,” Wyatt says. The comets would converge in two otherwise empty regions called resonance points, one in front of the planet in its orbital path and one trailing behind it. This would create two gas clouds on opposing sides of the star.
It is also possible that two icy planets laden with gas collided in the region about half a million years ago, and the shattered pieces are still hitting each other. In that case, the icy remains would be orbiting the star together, and the lone cloud they are creating would leave a faint tail of gas and dust as it moves. Studies that can make out the cloud’s shape and orbit may help distinguish between the two scenarios.
The shepherding planet model is more likely, says Michiel Hogerheijde at Leiden Observatory in the Netherlands. But determining the truth may be difficult for ground-based astronomers, because the relatively flat plane of the star system is viewed edge-on from Earth. “They cannot distinguish whether there are two positions on each side of the star, or whether it is one blob with a tail,” he says.