Planet formation and destruction via gravitational instability

This graphic shows a computer simulation of a circumstellar disk that orbits a young star, with each snapshot showing the state of the disk 1,500 years later. This disk is gravitationally unstable, which means that the disk's gravitational attraction for itself causes material to contract into spirals that can also clump up further to form Jupiter-mass protoplanets. The black circles in the above (click picture to zoom in) follow one such clump that first forms at a distance of 300 AU (ie 300 times the Sun-Earth distance) from the central star, which then spirals inwards due to its interactions with the disk. This simulation illustrates one of the difficulties in forming giant planets via gravitational instability, since the clumps that do form by this process also tend to get driven inwards by the disk, where they might accrete onto the central star. For additional details, see the preprint by Zhu and colleagues.

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.