30 March 2000
The Hunt For New Planets
by Kate Melville
Planet-hunting astronomers have crossed an important threshold in planet detection with the discovery of two planets that may be smaller in mass than Saturn.
Of the 30 extrasolar planets around Sun-like stars detected previously, all have been the size of Jupiter or larger. The existence of these Saturn-sized candidates suggests that many stars harbor smaller planets in addition to the Jupiter-sized ones.
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 Geoff Marcy of the University of California at Berkeley. "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 the nagging possibility open 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 these "Jupiters" are only the tip of the iceberg, and there are many more planets to be found that are 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," said Marcy.
The discovery was made by planet-sleuths Marcy, Paul Butler of the Carnegie Institution of Washington, and Steve Vogt of the University of California, Santa Cruz, using the mighty Keck telescope in Mauna Kea, Hawaii. They discovered a planet at least 80 percent the mass of Saturn orbiting 3.8 million miles from the star HD46375, 109 light-years away in the constellation Monoceros, and a planet 70 percent the mass of Saturn orbiting 32.5 million miles 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 relatively rapid discovery.
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 Keck, 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 -- rhythmically moving toward and then away from Earth -- is only 36 feet per second, a little faster than a human sprints.
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. The planet orbiting 79 Ceti has an average temperature of 1530 degrees Fahrenheit (830 degrees Celsius). The planet orbiting HD46375 has an average temperature of 2070 degrees Fahrenheit (1130 degrees Celsius).
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, because Marcy and Butler's detection technique favors finding massive planets in short-period orbits. This seems to be the case for approximately six percent of the stars surveyed so far.