Rocky planets like our own are thought to form by accretion. Small dust grains agglomerate to make bigger grains, then pebbles, rocks, boulders--all the way up to objects the size of Mars. At that threshold--about 1/10 of Earth's mass--a rocky protoplanet has consumed all the locally available material. To grow further, Mars-sized protoplanets must collide and merge. There's ample evidence throughout the solar system for impacts of the scale needed to complete Earth's formation. Now, a team led by Carey Lisse of the Johns Hopkins University in Baltimore has found evidence of a giant impact outside the solar system, in a dusty disk around a nearby star. Lisse and his colleagues did not detect the impact itself, shown here in an artist's impression. Rather, they found IR spectral features attributable to the impact's blasted and ground-up debris: amorphous silica dust. They also detected the product of vaporized planetary crust: silicon monoxide gas. From those spectral features, Lisse could deduce that the impact spalled a mass of dust at least as big as Pluto, 1022 kg. If the impactor had the same mass and enough kinetic energy to create the observed SiO, then its impact speed must have been 10 km/s. Such high relative velocities are not uncommon in the solar system. Indeed, the Mars-sized object that struck Earth to form the Moon had a relative velocity of that magnitude. (C. M. Lisse et al., Astrophys. J. 701, 2019, 2009.)--Charles Day
