HD 172555 is a white-hot A7V star located relatively close by, 95 light years from Earth in the direction of the constellationPavo. Spectrographic evidence indicates a relatively recent collision between two planet-sized bodies that destroyed the smaller of the two, which had been at least the size of Earth's moon, and severely damaged the larger one, which was at least the size of Mercury. Evidence of the collision was detected by NASA's Spitzer Space Telescope.
HD172555 was first recognized in the 1980s as being unusually bright in the mid-infrared by the IRASsky survey. Follow-up, ground-based observations by Schütz et al. and the Spitzer Space Telescope, also in 2004, confirmed the unusually strong nature of the infrared spectral emission from this system, much brighter than what would be emitted normally from the star's surface. As part of the Beta Pictoris moving group, HD172555 is coeval with that more famous system, approximately 20 million years old, and is the same kind of white-hot star as Beta Pic, about twice as massive as our Sun and about 9.5 times as luminous. Comparison with current planetary formation theories, and with the very similar Beta Pic system, suggests that the HD172555 is in the early stages of terrestrial planet formation. But what makes HD 172555 special is the presence of a large amount of unusual silicaceous material – amorphous silica and SiO gas – not the usual rocky materials, silicates like olivine and pyroxene, which make up much of the Earth as well. The material in the disk was analyzed in 2009 by Carey Lisse, of the Johns Hopkins University Applied Physics Laboratory in Laurel, MD using the infrared spectrometer on board the Spitzer Space Telescope, and the results of the Deep Impact and STARDUST comet missions. Analysis of the atomic and mineral composition, dust temperature, and dust mass show a massive amount of warm material similar to re-frozen lava and flash-frozen magma as well as copious amounts of vaporized rock and rubble in a region at 5.8+/-0.6 AU from the HD172555. The material had to have been created in a hypervelocity impact between two large bodies; relative velocities at impacts less than 10 km/s would not transform the ubiquitous olivine and pyroxene into silica and SiO gas. Giant impacts at this speed typically destroy the incident body, and melt the entire surface of the impactee. The implications for the detection of abundant amorphous silica and SiO gas are the following:
Massive hypervelocity impacts happen in young planetary systems. There are a number of examples of such impacts in the Solar System : Mercury's high density; Venus' retrograde spin; Earth's Moon; Mars' North/South hemispherical cratering anisotropy; Vesta's igneous origin ; Uranus' spin axis located near the plane of the ecliptic. Local geological evidence for widespread impact melting includes tektites found on Earth and glass beads found in lunarsoils.
Rocky protoplanets, and possibly planets, exist in the HD172555 system, at about 12 Myr after its formation.
If the collision happened within the last few thousand years, there is likely a protoplanet in the HD172555 system with a liquid magma surface. This is not unexpected; a simple calculation of the gravitational binding energy of the Earth, shows that the energy released in assembling the Earth is about 10x the amount needed to melt it.
