April 3, 2000
Planet hunters on trail of worlds smaller than Saturn
By Don Savage, NASA
Planet-hunting astronomers announced last week that they have crossed an important
threshold in planet detection with the discovery of two planets that may be smaller
in mass than Saturn.
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| This is an artist's concept of a giant planet recently discovered orbiting the
sun-like star 79 Ceti, located 117 light-years away in the constellation Cetus the
sea monster. The planet was not directly photographed but indirectly detected by
its gravitational pull on the star. The planet is one of the smallest found so far;
it is at least 70 percent the mass of Saturn. Image: Greg Bacon/STScI |
"It's a harbinger of things to come because it shows that as we continue to push this technique we're able to see smaller planets, so it's very exciting," said Steven Vogt, UCSC professor of astronomy and astrophysics.
The discovery was made by planet-sleuths Geoffrey Marcy of UC Berkeley, Paul Butler of the Carnegie Institution of Washington, and Vogt, using the Keck I telescope in Mauna Kea, Hawaii.
Finding Saturn-sized planets reinforces the theory that planets form by a snowball effect of growth from small ones to large, in a star-encircling dust disk. The 20-year-old theory predicts there should be more smaller planets than large planets, and this is a trend the researchers are beginning to see in their data.
"It's like looking at a beach from a distance," explained Marcy. "Previously we only saw the large boulders, which were Jupiter-sized planets or larger. Now we are seeing the 'rocks,' Saturn-sized planets or smaller. We still don't have the capability of detecting Earth-like planets, which would be equivalent to seeing pebbles on the beach."
Jupiter alone is three times the mass of Saturn. This has left open the nagging possibility that some of the extrasolar planets might really be stillborn stars, called brown dwarfs, which would form like stars through the collapse of a gas cloud. But now researchers are better assured that these "Jupiters" are only the tip of the iceberg, and that there are many more planets to be found that have the mass of Saturn or smaller.
"Now we are confident we are seeing a distinctly different population of bodies that formed out of dust disks like the disks Hubble Space Telescope has imaged around stars," Marcy said.
The researchers discovered a planet at least 80 percent the mass of Saturn orbiting the star HD46375, 109 light-years away in the constellation Monoceros, and a planet 70 percent the mass of Saturn orbiting around the star 79 Ceti (also known as HD16141) located 117 light-years away in the constellation Cetus.
These planets are very close to their stars and so have short orbits. They whirl around their parent stars with periods of 3.02 days and 75 days respectively. This allowed for their discovery in a relatively short order.
The astronomers detected the small wobble of a star caused by the gravitational tug of the unseen planets. For the past five years Marcy and Butler have used this technique successfully to catalog 21 extrasolar planets. Boosted by the light-gathering power of the Keck telescope and the "HIRES" spectrograph designed by Vogt, they have steadily increased the precision of their measurements so they can look for the gravitational effects of ever smaller bodies. In this latest detection, the change in a star's velocity as it moves rhythmically toward and then away from Earth is only 11 meters per second, a little faster than human sprinting speed.
The Saturn-mass planets are presumably gas giants, made mostly of primordial hydrogen and helium, rather than the rocky material Earth is made of. They are so close to their parent stars they are extremely hot, and are not abodes for life as we know it.
They probably formed at a farther distance from the star, where they could accumulate cool gas, and then migrated into their present orbits. Along the way they would have disrupted the orbits of any smaller terrestrial planets like Earth. These "marauding" gas giants seem more the rule than the exception among the planets surveyed so far. That's because Marcy and Butler's detection technique favors finding massive planets in short-period orbits. And that seems to be the case for approximately six percent of the stars surveyed so far.
Their research is part of a multi-year project to look for wobbles among 1,100 stars within 300 light-years of Earth. The project is supported by NASA and the National Science Foundation.
--Additional reporting by Tim Stephens
