A dying star has provided astronomers with their first view of water in a planetary system outside our own. The sighting provides hard evidence that our solar system is not unusual in harbouring the ingredients for life.
Infra-red astronomy has already produced pictures of dusty planetary discs orbiting young stars, which theory predicts should coalesce into planets like those in our own solar system. Although some of these systems contain signs of water vapour, astronomers always assumed the water was from the young stars, which are made of hydrogen.
Now NASA鈥檚 Submillimetre Wave Astronomy Satellite has produced the first evidence of water vapour orbiting the brightest star in the night sky, a red giant called CW Leonis. Because CW Leonis contains no hydrogen, astronomers expected it to produce very little water, less than one ten thousandth of the amount SWAS actually saw.
Advertisement
Gary Melnick from the Harvard-Smithsonian Centre for Astrophysics in Cambridge and colleagues on the SWAS team believe the water vapour must have come from planets or comets that orbited the star.
Planets engulfed
This probably happened when CW Leonis began to burn heavy, energetic elements and swelled out to engulf planets and comets in its outer planetary disc. 鈥淲e can expect CW Leonis had a fully developed planetary system,鈥� agrees Alan Boss, who models solar system evolution at the Carnegie Institute in Washington DC.
Most of the water currently around CW Leonis probably comes from icy bodies orbiting beyond the region that its former planets inhabited. These have now been engulfed by the growing star. In our solar system, much of the water is stored in the Kuiper belt, a belt of icy bodies that orbit beyond the planet Pluto.
CW Leonis is now large enough to start heating the entire Kuiper belt, boiling thousands of comets. 鈥淲e are witnessing the apocalyse that will befall our own solar system,鈥� says Melnick.
Boss is especially excited by the SWAS find because it reveals a new capability to sample distant solar systems at different stages of their life cycle. This will enable astronomers to work out just how typical our system is, and what chance there is that conditions suitable for life have evolved elsewhere.