2 Neptune-size worlds circle distant stars

Planets: Finds by U.S. astronomers are nearest yet to Earth in mass.

September 01, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

WASHINGTON -- Astronomers racing one another to find the first habitable, Earth-like planet in another solar system announced a significant advance in their quest yesterday -- the discovery of two of the smallest planets ever detected circling sun-like stars.

"Small" is a relative term, of course. The two new planets are whoppers. Scientists think they're about the size of Neptune -- with at least 17 times the mass of Earth and a diameter several times as large.

But NASA was calling them the first in "a new class of planets."

They're much smaller than 135 or so other "exoplanets" that astronomers have found in alien solar systems since 1995. Those planets are usually described in multiples of the mass of Jupiter -- the largest planet in our solar system, with more than 300 times Earth's mass.

And unlike the gas giants found so far, the new planets on the block might be rocky or icy orbs, much more like home.

For astronomers in the race, the latest discoveries constitute a breakthrough -- proof that their search techniques are zooming in on the day when someone will find an Earth-size planet orbiting a sun-like star, at a distance where water is liquid and life as we know it could exist.

"The news that we are announcing today is going to bring us closer to answering the question, `Are we alone in the universe?'" said Anne Kinney, director of the Universe Division of NASA's Science Directorate.

She called it "an astounding amount of progress for nine years of research."

"Ten years ago this field barely existed," Kinney said. "I myself never dreamt we would be making an announcement of Neptune-class planets at this stage."

Both discoveries have been peer-reviewed and accepted for publication in December's Astrophysical Journal.

Yesterday's announcement follows hard on the heels of last week's claim by European scientists that they had found an even smaller planet -- a "super-Earth" with a rocky surface and the mass of Uranus.

Race for first

But Alan Boss, a research astronomer at the Carnegie Institute in Washington, who was not involved in any of the discoveries, said the U.S. discoveries will rank, for now, as the first Neptune-sized exoplanets ever found.

That's because the American findings were submitted for peer review before the European discovery, which has not yet been accepted for publication.

"As soon as that [European] paper does get accepted," Boss said, "they will have full credit for having been the third Neptune-mass planet. But what we have here are the first two."

Swift and hot

Neither of the two new planets unveiled at NASA headquarters yesterday would be very hospitable. Both zip around their stars in less than three Earth days, so close that their surface temperatures would be hostile to life.

The first planet was discovered by a team of planet-hunters led by Paul Butler, of the Carnegie Institute, and Geoffrey Marcy, of the University of California at Berkeley.

They never actually sighted the planet: It is too small and too close to its star to be visible. But using the W.M. Keck Observatory in Hawaii, they surveyed 950 "nearby" stars in our Milky Way galaxy. They eventually zeroed in on one star -- Gliese 436, about 30 light-years from Earth in the constellation Leo.

The star's light exhibited a tell-tale "wobble." Such periodic motions are a signal to planet-hunters that an unseen planet is circling, tugging on the star with each orbit.

From the motion, they calculated that the planet orbits its star once every 2 1/2 days, at a distance of just 2.6 million miles. Earth orbits the sun at a distance of 93 million miles.

The good news about this new planet, Marcy said, is that it is the second found orbiting a so-called M-class "red dwarf" star. The dim M-class stars are one of the most abundant types found in the Milky Way, which contains about 200 billion stars.

Based on the proportion of stars studied so far and found to have planets, Marcy said, "We estimate that something like 20 billion planetary systems exist within our Milky Way galaxy alone, an enormous number of planets."

The second new planet unveiled yesterday was found by a team led by Barbara McArthur, of the University of Texas, Austin, using the Hobby-Eberly Telescope at the McDonald Observatory in West Texas.

Her team zeroed in on the star 55 Cancri, 41 light-years from Earth in the constellation Cancer. Three other planets had already been found circling 55 Cancri. But McArthur and her team were able to detect an extra wobble in the star's light that revealed the presence of a fourth.

McArthur's new planet has a mass about 18 times Earth's mass. It is the closest of the four to its star -- only 3.5 million miles -- orbiting once in just under three days. Its larger and more distant "Jupiter-mass" sister planets have orbits of 15, 44 and 4,520 days.

The system is the first quadruple solar system ever found, McArthur said, making it "a premier laboratory for the study of the formation and evolution of planetary systems."

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