Two families of exocomets in the β Pictoris system

Statistical analysis of over a thousand spectra of the star β Pictoris reveals that it has two kinds of exocomets circling it: old exhausted comets trapped in mean-motion resonance with a massive planet, and fragments of comets. Detection of β Pictoris exocomets The nearby star β Pictoris possesses a young planetary system that appears much like our own would have been few million years after its formation. This analysis of more a thousand archival spectra recorded between 2003 and 2011 reveals variable dust absorption signatures arising from transiting exocomets belonging to two distinct families of comets. First, an old volatile-exhausted population displaying signs of orbital evolution due to interactions with the host planet and second, a volatile-rich population presumably originating from the break-up of a few parent bodies. The young planetary system surrounding the star β Pictoris harbours active minor bodies 1 , 2 , 3 , 4 , 5 , 6 . These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System 7 . Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies 8 , 9 , 10 , 11 , that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.