First stars had no planets

By Dr David Whitehouse
BBC News Online science editor

The first stars to form in the Universe had no planets. Only later generations of stars, that contained more metal, were able to have planetary companions, according to new research.

Planets formed as the Universe got older

To reach this conclusion, astronomers looked at 754 nearby stars like our Sun. Some of the surveyed stars had planets but most did not.

The results showed that the more iron and other metals there are in a star, the greater the chance it has a companion planet.

The implication is that planets did not form around the metal-poor stars that formed when the Universe was very young.

"Stars forming today are much more likely to have planets than early generations of stars," says astronomer Jeff Valenti. "It's a planetary baby boom."

'Under-appreciated stars'

Following the discovery of the first extrasolar planet in 1992 - about 100 stars are now known to have planets - astronomers noticed that stars rich in metals were more likely to have planets.

Iron and other metals are created by nuclear reactions inside stars and subsequently scattered into space by stellar explosions.

This means that stars with metals were extremely rare in the early history of the cosmos. Over time, however, each successive generation of stars became richer in metals, increasing the chances of forming a planet.

The new survey of metal abundances in stars by Debra Fischer of the University of California, Berkeley, and Jeff Valenti of the Space Telescope Science Institute, is the first to cover a statistically large sample of 61 stars with planets and 693 stars without planets.

Exploding stars scatter metals into space

"People have looked already in fair detail at most of the stars with known planets, but they have basically ignored the hundreds of stars that don't seem to have planets. These under-appreciated stars provide the context for understanding why planets form," says Jeff Valenti.

"We now know that stars which are abundant in heavy metals are five times more likely to harbour orbiting planets than are stars deficient in metals. If you look at the metal-rich stars, 20 percent have planets. That's stunning," says Debra Fischer.

Heavy metal stars

"Whether a star has planetary companions or not is a condition of its birth, those with a larger initial allotment of metals have an advantage over those without, a trend we're now able to see clearly with this new data," Fischer adds.

It has been suggested that metals allow dust, rocks and eventually proto-planets to form around new stars.

Since the young star and the surrounding disc of dust and gas would have the same composition, the metallicity observed from the star reflects the abundance of raw materials, including heavy metals, available in the disc to build planets.

"These results tell us why some of the stars in our Milky Way galaxy have planets while others do not," says Geoffrey Marcy of the University of California, Berkeley.

"The heavy metals must clump together to form rocks which themselves clump into the solid cores of planets."

The research is being presented to the International Astronomical Union (IAU) meeting in Sydney, Australia.