Comets herded by invisible planet

A CLOUD of carbon monoxide may have just revealed a strange, comet-shepherding exoplanet. Radio observations show a huge clump of carbon monoxide gas close to the star Beta Pictoris. 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 (Science, doi.org/rsz). Or two icy planets laden with gas may have previously collided, and pieces are still hitting each other today. Studies that can make out the cloud’s shape and orbit may help distinguish between these.