 
Planet
hunters on trail of worlds smaller than Saturn
29
Mar 2000
By
Ray Villard, NASA and Robert Sanders, Public Affairs
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.
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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