Marcy & Butler - 2

Donald Savage
NASA Headquarters, Washington, DC                 March 29, 2000
(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 HUNTERS ON TRAIL OF WORLDS SMALLER THAN SATURN

     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.
                                - end -
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