Tuesday, August 11, 2009
Planetary-sized impacts around HD 172555?
Casey Lisse (JHU/APL) and colleagues recently used the Spitzer Space Telescope to collect infrared spectra of the dust that orbits the relatively young 12 million year-old star HD 172555. Their observations are described in this preprint. Their spectra shows that this star's circumsteller dust is, as expected, rich in silicate, which is the principle ingredient in circumstellar dust. What is surprising here is that this dust is a glassy silicate, like tektite or obsidian, which tends to form when rocky bodies collide at high speeds of ~10km/sec. These spectra also indicate the presence of ample amounts of SiO gas, which is vaporized rock. From these spectra, Lisse and colleagues infer that this system suffered a recent giant impact via the collision of two large ~1000km bodies (the size of Ceres, the largest asteroid in our Solar System). They estimate that this giant impact occurred within the past ~100 thousand years. There are two possible interpretations of these observations. (1) Collisions among ~1000km-sized protoplanets at HD 172555 indicate that this system is currently undergoing planet formation. This is an important step in the planet-formation process, and is necessary if one wishes to ultimately produce a system of ~10,000km-sized terrestrial planets. (2) Alternatively, giant impacts are instead destroying the protoplanets that orbit HD 172555, and that astronomers are witnessing the collisional destruction of a young planetary system. Which outcome is more likely is presently unclear. See this Spitzer page for more details, as well as the above artist's rendition of a giant impact.