Rocky planets like Earth may form in a hurry, suggests observations of one young star.
In the journal Nature, a team led by Carl Melis of the University of California, San Diego, looks at a "young, Sun-like star" called TYC 824126521, located some 456 light years from Earth (where one light year is about 5.9 trillion miles). About 10 million years old, TYC 824126521 was surrounded by a dense dust disk in observations made before 2009. Most early stars possess such dusty disks, from which planets are thought to later form, just as they did in our own solar system some 4.5 billion years ago.
But based on observations from ground telescopes and two satellites, the team reports that the infrared heat signature from the dust disk surrounding the young star has dropped by about a factor of 30, in less than two years time. "The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust," says the study.
What happened? A startlingly-rapid period of planet formation, the astronomers suggest, where most theories about planet formation (which has never been observed) usually estimate that thousands to millions of years are required to form worlds."The disappearance of the excess infrared radiation from TYC 824126521 in less than two years is incredibly fast by our current understanding," says Margaret Moerchen of the Leiden Observatory, in a commentary accompanying the report, to which she adds, "perhaps the most exciting possibility is that the brightness drop represents a stage of terrestrial-planet formation that occurs so quickly that we have not been lucky enough to glimpse it until now."
The study suggests two runaway collision models for planet formation may explain the sudden disappearance of the dust disk surrounding the star but notes both have trouble explaining the rapidity of the process seen at TYC 824126521.
Says Moerchen:
"More plausible is the collisional cascade model, in which gravitationally bound dust grains experience successive cratering or wholly destructive collisions that eventually yield grains small enough to be blown out of the system. This model predicts that a brightness drop of the level observed should occur on a timescale several times longer than the 2.5 years documented. However, refining the size distribution of the pre-event dust population could help to match this theory to the data. One benefit of this model is that it predicts cyclical behaviour, such that dust replenishment could be expected to yield an observable signature within two decades of the time of depletion."
So, by that measure, all we have to do is keep watching the star to see if a new dust clouds forms from the aftermath of collisions between young planets surrounding the star within the next twenty years or so, an eye-blink in astronomical time, to confirm the rapid planet-formation theory.
"Although the exact circumstances are not yet clear, this system has clearly undergone a drastic event that promises to provide unique insight into the process by which rocky planets form," concludes the study.
