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Two planets smaller than Saturn found orbiting distant stars

Artist's concept of an extrasolar planet and moon  

March 30, 2000
Web posted at: 8:40 AM EST (1340 GMT)

In this story:

Fast orbits, blistering heat

Searching for gentle giants


WASHINGTON -- Astronomers on Tuesday announced the discovery two Saturn-sized planets outside the solar system, a milestone in the search for distant celestial bodies that resemble Earth.

The previous 30 planets found outside the solar system are at least the size of Jupiter, Earth's largest neighbor. But the new finding lends credence to the theory that small planets outnumber giants in the galaxy, according to NASA.

The team of astronomers indirectly detected the new planets, more than 100 light years away, by studying their gravitational pull on host stars.


Using Hawaii's Keck Observatory, they measured changes in the velocity of the stars as small as 36 feet per second, about the speed of a bicyclist, according to one of the planet sleuths, Geoff Marcy, a University of California, Berkeley astronomer.

The finding of Saturn-sized planets supports the idea that large planets accrete from small ones, using gas and dust that encircle stars, according to NASA. The theory predicts that more small planets exist than large ones; researchers are increasingly finding this trend in the data.

Fast orbits, blistering heat

The newly discovered planets have quick and close orbits around their host stars. One of them, with at least 80 percent the mass of Saturn, circles a star known as HD46375 from a distance of 3.8 million miles and completes an orbit roughly every three days. In contrast, Earth is 93 million miles from the sun.

A second planet with 70 percent of Saturn's mass orbits the star 79 Ceti from 32.5 million miles. Its three-day orbit, like that of most of the known extrasolar planets, is highly elliptical. It completes an orbit every 75 days.

"It's very different than solar system planets. Ours are in beautiful, concentric orbits," said Paul Butler of the Carnegie Institute in Washington, one of the planet discoverers.

Because they are so close to their parent stars, the new planets, presumably gas giants made of hydrogen and helium, undoubtably experience blistering temperatures and couldn't harbor life, according to NASA.

The larger planet has an average temperature of about 2,000 degrees Fahrenheit, the smaller one about 1,500 degrees Fahrenheit.

The new planets likely formed farther away from their stars, where they accumulated cool gas. They then migrated to their present locations, possibly disrupting the orbits of smaller planets along the way, according to NASA, which sponsored the research.

The astronomers made the discovery as part of a multi-year project to look at the wobbles of more than 1,000 stars within 300 light-years, using the Keck Observatory in Mauna Kea, Hawaii.

The team spotted 21 extrasolar planets before the latest two. But with the 79 Ceti planet, "For the first time we have one with less wobble than that between the sun and Jupiter," Marcy said.

The researchers said ground-based observations will help them find planets the size of Neptune within a few years. They think many smaller planets exist, but said advanced telescopes, deployed in space, would be required to find them.

Searching for gentle giants

Marcy said the discovery of a planet like Jupiter, with a concentric orbit, could prove crucial in the search for terrestrial-like planets. Unlike marauding jovian cousins in other systems, Jupiter does not affect the orbit of its neighbors and in fact protects planets in the inner solar system from dangerous space debris, he said.

Gas giants elsewhere could likewise absorb killer comets and meteorites that otherwise might slam into a smaller neighbor, perhaps one that resembles Earth, he said.

Observatory reveals storms on Neptune, oceans on Titan
January 18, 2000
NASA announces missions to seek planets, study gamma rays
October 15, 1999
Hubble picture reveals seeds of planet-making
June 2, 1999

NASA's Origins Program
Discovery of Extrasolar Planets

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