Planets may form from gas shed by dying stars, new observations suggest. The finding is surprising because planets were thought to form only around very young stars.
Standard theories posit that planets take shape in a disc of dust and gas around newborn stars, where the disc is made of material left over from the star's own birth.
Now, astronomers have discovered a disc around a Sun-like star that was actually captured from a dying companion, called a red giant. Michael Ireland of Caltech in Pasadena, US, and colleagues used the Keck Observatory in Hawaii, US, and the Gemini South telescope in Chile to make the discovery.
They observed a star system called Mira. It consists of an ordinary star about half as massive as the Sun, called Mira B, which orbits a red giant, called Mira A, at about three times the distance of Pluto from the Sun.
Like other stars in the advanced red giant phase of life, Mira A is sloughing off large amounts of gas and dust - about one Earth mass every seven years. The new observations have revealed that some of this material has been captured into a disc that orbits its companion at about the distance of Saturn from the Sun.
The disc is the sort of structure where planets are thought to form. And if planets do exist there, they appear to be endowed with some of the raw materials necessary for life - including carbon, in the form of carbon monoxide.
"Of all the elements other than hydrogen and helium, carbon monoxide and water vapour are the dominant materials," Ireland says.
The disc currently contains less than a Jupiter's worth of material. But before the red giant completely loses its outer atmosphere to become a white dwarf a million years from now, it will have dumped three to five Jupiters' worth of matter into the disc, he says.
This is "in the ballpark of what you need to form a planetary system like our own", he says.
In addition to the light from the ordinary star, any planets that form will be bathed in the pale glow of the white dwarf, which would appear about as bright as a crescent Moon, Ireland says.
Although this is the first disc discovered to have formed this way, it may not be so unusual. Two thirds of bright star systems are binaries, and about a quarter of these should evolve into systems like Mira, with a red giant donating material to an ordinary star, Ireland says. "This should be a very common phenomenon," he says.
The results were presented on Tuesday at a meeting of the American Astronomical Society in Seattle, Washington, US.
