Two families of exocomets in the Beta Pictoris system

The young planetary system surrounding the star β Pictoris harbors active minor bodies1–6. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as in the early ages of the Solar System7. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies8–11, i.e. exocomets. Here we report the analysis of over a thousand spectra gathered between 2003 and 2011, which provides a sample of about 6000 variable absorption signatures due to transiting exocomets. Statistical analysis of the observed properties of these exocomets allow us to identify two populations with highly different physical properties. A first family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted comets trapped in a mean motion resonance with a massive planet; the second family consists of exocomets producing deep absorption lines, which are possibly 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 Pic system are analogous to the comets in our own Solar System. From 2003 to 2011, a total of 1106 spectra of β Pictoris have been obtained using the HARPS spectrograph. Observations of the Ca II doublet (K-3933.66Å; H-3968.47Å) show a large number of variable absorption features (Fig. 1) varying on timescales of one to six hours. These features simultaneously detected in both Ca II K and Ca II H lines are interpreted as exocomets transiting in front of the stellar disk7–11. Since the β Pic Ca II spectrum is typically observed to be stable on 30 minutes timescales, we averaged together spectra in distinct 10 minutes time intervals to limit any possible spectral variability. This results in a total of 357 spectra with signal-to-noise ratio greater than 80. In order to characterize the profile of these transient absorption lines, we divided each of the 357 averaged spectra by a reference spectrum of β Pictoris (Extended Data Fig. 1 and 2) assumed to be free of transiting exocomet’s absorption signatures. Given the HARPS resolution and sensitivity, each β Pictoris spectrum shows an average of about 6 variable absorption features due to exocomets. These features have radial velocities ranging from -150 to +200 km s−1 with respect to