NASA has announced the detection of gases that appear to be all that is left after comets or asteroids were vaporized from getting too close to the star HD 172555. The discovery strengthens our confidence in models of early solar system formation, and suggests the young star probably has at least one giant planet in the process of formation.
HD 172555 is located 95 million light-years away and forms part of the 23-million-year-old Beta Pictoris Moving Group of stars, all of which formed together. By most people's standards, that is an enormous age and distance, but to an astronomer it is the equivalent of a toddler living in the next block. Consequently, the Hubble Space Telescope has been spending some of its valuable time examining this star and the system forming around it.
At the winter meeting of the American Astronomical Society in Grapevine, Texas, Dr Carol Grady of the Goddard Spaceflight Center reported that Hubble has detected gaseous silicon and carbon, adding to an earlier discovery of calcium. These are similar to what we see left behind when comets are destroyed in close approaches to the Sun. Although we have no way of confirming the gases' origins directly, Grady is confident that something similar is happening to HD 172555, but with many objects falling onto the star at once, allowing us to detect their legacy at this distance.
Grady noted that similar observations have been found for two other stars, both also less than 40 million years old, providing enough of a sample to suggest that such young stars frequently experience a rain of comets. “Watching these events gives us insight into what probably went on in the early days of our Solar System, when comets were pelting the inner solar system bodies, including Earth,” she said in a statement. “In fact, these star-grazing comets may make life possible, because they carry water and other life-forming elements, such as carbon, to terrestrial planets."
The three elements so far detected passing across HD 172555's face could originate from either comets or asteroids, leaving the identity of the doomed objects unknown. Grady considers comets the more likely source, and intends a follow-up study looking for the presence of hydrogen or oxygen, which would confirm the source as being cometary rather than rocky.
Since comets form far enough out from the star for H2O to be solid, the observations raise the question of what might disrupt their orbits, sending them Icarus-like towards their sun. A likely explanation is the disruptive gravitational force of a planet that's Jupiter-sized or larger, but this has yet to be confirmed.