Donald Savage

NASA Headquarters, Washington, DC

(Phone: 202/358-1547)

Robert Sanders

University of California, Berkeley

(Phone: 510/643-6998)

Andrew Perala

W.H. Keck Observatory, Kamuela, HI

(Phone: 808/885-7887)

RELEASE: 00-47

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

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

“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

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.

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.

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