Sydney, Australia — A comparison of 754 nearby stars like our sun — some with
planets and some without — shows definitively that the more iron and other
metals there are in a star, the greater the chance it has a companion planet.

"Astronomers have been saying that only 5 percent of stars have planets, but
that’s not a very precise assessment," said Debra Fischer, a research
at the University of California, Berkeley. "We now know that stars which are
abundant in heavy metals are five times more likely to harbor orbiting planets
than are stars deficient in metals. If you look at the metal-rich stars, 20
percent have planets. That’s stunning."

"The metals are the seeds from which planets form," added colleague Jeff
Valenti, an assistant astronomer at the Space Telescope Science Institute
(STScI) in Baltimore, Md.

Fischer will present details of the analysis by her and Valenti at 1:30 p.m.
Australian Eastern Standard Time (AEST) on Monday, July 21, at the
Astronomical Union meeting in Sydney, Australia.

Iron and other elements heavier than helium — what astronomers lump together
"metals" — are created by fusion reactions inside stars and sown into the
interstellar medium by spectacular supernova explosions. Thus, while metals
extremely rare in the early history of the Milky Way galaxy, over time, each
successive generation of stars became richer in these elements, increasing the
chances of forming a planet.

"Stars forming today are much more likely to have planets than early
of stars," Valenti said. "It’s a planetary baby boom."

As the number of extrasolar planets has grown — about 100 stars are now known
to have planets — astronomers have noticed that stars rich in metals are more
likely to harbor planets. A correlation between a star’s "metalicity" — a
measure of iron abundance in a star’s outer layer that is indicative of the
abundance of many other elements, from nickel to silicon — had been suggested
previously by astronomers Guillermo Gonzalez and Nuno Santos based on surveys
a few dozen planet-bearing stars.

The new survey of metal abundances by Fischer and Valenti is the first to cover
a statistically large sample of 61 stars with planets and 693 stars without
planets. Their analysis provides the numbers that prove a correlation between
metal abundance and planet formation.

"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," said Valenti, who is an expert at determining
the chemical composition of stars.

The data show that stars like the sun, whose metal content is considered
of stars in our neighborhood, have a 5 to 10 percent chance of having planets.
Stars with three times more metal than the sun have a 20 percent chance of
harboring planets, while those with 1/3 the metal content of the sun have about
a 3 percent chance of having planets. The 29 most metal-poor stars in the
sample, all with less than 1/3 the sun’s metal abundance, had no planets.

"These data suggest that there is a threshold metalicity, and thus not all
in our galaxy have the same chance of forming planetary systems," Fischer said.
"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."

The two astronomers determined metal composition by analyzing 1,600 spectra
more than 1,000 stars before narrowing the analysis to 754 stars that had been
observed long enough to rule a gas giant planet in or out. Some of these stars
have been observed for 15 years by Fischer, Geoffrey Marcy, professor of
astronomy at UC Berkeley, and colleague Paul Butler, now at the Carnegie
Institution of Washington, in their systematic search for extrasolar planets
around nearby stars. All 754 stars were surveyed for more than two years,
time to determine whether a close-in, Jupiter-size planet is present or not.

Though the surfaces of stars contain many metals, the astronomers focused on
five — iron, nickel, titanium, silicon and sodium. After four years of
analysis, the astronomers were able to group the stars by metal composition and
determine the likelihood that stars of a certain composition have planets. With
iron, for example, the stars were ranked relative to the iron content of the
sun, which is 0.0032%.

"This is the most unbiased survey of its kind," Fischer emphasized. "It is
unique because all of the metal abundances were determined with the same
technique and we analyzed all of the stars on our project with more than two
years of data."

Fischer said the new data suggest why metal-rich stars are likely to develop
planetary systems as they form. The data are consistent with the hypothesis
heavier elements stick together easier, allowing dust, rocks and eventually
planetary cores to form around newly ignited stars. Since the young star and
surrounding disk of dust and gas would have the same composition, the metal
composition observed from the star reflects the abundance of raw materials,
including heavy metals, available in the disk to build planets. The data
indicate a nearly linear relationship between amount of metals and the chance
harboring planets.

"These results tell us why some of the stars in our Milky Way galaxy have
planets while others do not," said Marcy. "The heavy metals must clump together
to form rocks which themselves clump into the solid cores of planets."

The research by Fischer and Valenti is supported by the National Aeronautics
Space Administration, the National Science Foundation, the Particle Physics and
Astronomy Research Council (PPARC) in the United Kingdom, the Anglo-Australian
Observatory, Sun Microsystems, the Keck Observatory and the University of
California’s Lick Observatories.


A bar graph showing the relationship between stellar metal abundance and
likelihood of planets is available at

The percentage of stars that have planets rises with iron abundance. In all,
stars were grouped according to their iron content relative to the sun. The
number above each bar indicates the number of planetary systems in each group.

Credit: Debra Fischer, UC Berkeley/Jeff Valenti, STScI