NEW YORK: A new survey suggests that low-mass 'corpses' of stars are much more common in the universe than previously thought.

The finding is detailed in an upcoming issue of the Astrophysical Journal, and it could give planet hunters a new lead in their search for alien worlds.

These stellar runts are the product of a process that reduces metal-rich suns into skeletons of their former selves, but until now was thought only to kick-in in the distant future.

Smouldering white dwarf

Near the end of their lives, stars with around the mass of our Sun burn up most of their hydrogen fuel and swell to become bloated red giants. After a while, most red giants expel their outer gas shell, and what's left behind is a smouldering white dwarf.

For reasons that are not well understood, the more metal a star contains, the more mass it will lose during the red giant phase. This means that stars that are rich in metals can form 'runt' sized white dwarfs, which are over 30 per cent smaller than normal.

Until recently, these stellar runts existed only in theory, because no existing star was known to have a high enough metal content to form them.

"Metals in the universe are created as a result of supernova explosions," said study co-author Mukremin Kilic, an astronomer at Ohio State University in Columbus, USA. "These explosions seed the newly forming stars with metals, therefore each generation of stars become more metal rich."

But last year, another team of astronomers announced that they discovered 'single low-mass white dwarfs' – aka stellar runts – in NGC 6791, a metal-rich star cluster in the constellation Lyra.

Now, a more comprehensive sky survey by Kilic's team suggests these stars are relatively common elsewhere too. "We found that about 5 per cent of [all] stars have similar metallicities as NGC 6791," Kilic said.

Source of planets

The abundance of these stellar runts could give planet-hunters a new type of star to hone in on when searching for new worlds, because they are a more common source of planets.

"We know that metal rich stars tend to have more planets compared to metal-poor stars," Kilic said. "If these single, low-mass white dwarfs actually form from metal-rich progenitor stars, then they are more likely to have planets assuming that the planets survive the late stages of stellar evolution."

But whether they survive these late stages remains to be seen, as it's predicted that many planets (such as Earth) would be devoured by the red giant stage of their star's evolution.

Jason Kalirai, an astronomer at Lick Observatory in California who was not involved in the study, said the sky survey results are also important because they could help explain the confusing excess of ultraviolet light emitted by some elliptical galaxies.

The same channel that leads to the formation of single, low-mass white dwarfs also produces a rich population of bright blue, helium-burning stars, Kalirai said. "One of the primary mechanisms for forming these helium-burning stars was through binary [star system] interaction. These studies show in fact that it's possible to form these stars through single stellar evolution."