Planet Formation Observed Around Massive Stars

As the old saying goes – big stars live fast and die young. And, according to new research presented at the 215^th meeting of the American Astronomical Society, so do their planets.

A team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the National Optical Astronomy Observatory (NOAO) examined a star forming region called W5, which lies about 6,500 light years away in the constellation Cassiopeia, using NASA’s Spitzer Space Telescope and the ground-based Two Micron All-Sky Survey (2MASS) to look for signs of dusty planetary disks around more than 500 massive stars of A and B spectral types – which are generally between 2 and 15 solar masses.

The team found that about ten per cent of all the stars examined had dusty disks – and of these 15 stars showed signs of a central gap suggestive of a new born Jupiter-scale planet clearing its orbit.

“The gravity of a Jupiter-sized object could easily clear the inner disk out to a radius of 10 to 20 astronomical units, which is what we see,” said Lori Allen of NOAO. (An astronomical unit is the average distance between the Earth and the Sun).

The research team have also suggested that all massive stars may begin their life with a sizeable dusty disk of accreting material. However, the powerful radiation and stellar winds generated by such massive stars tend to destroy these disks rapidly. The stars observed in the W5 region are thought to be only two to five million years old, but most have already lost the dusty disk needed to make planets. On this basis, it seems that, at least for type A and B stars, planets must form quickly or not at all.

Prospects for finding life on such planets are slim. While the massive stars may foster a habitable zone of some kind – which in the case of life forms depending on liquid water as a chemical solvent, would be considerably further out from these stars than the Earth is from the Sun. However, such life forms would have limited future.

Life on Earth needed over three billion years just to evolve to the early differentiated body forms seen in the Cambrian explosion. Life on an exoplanet orbiting massive A or B type stars would have between 10 and 500 million years before its star grows to a red giant or a supernova.

“These stars aren’t good targets in the hunt for extraterrestrials,” said Xavier Koenig of the Harvard-Smithsonian CfA, who presented the research in a press conference at the AAS meeting today, “but they give us a great new way to get a better understanding of planet formation.”