29, 2000 -- 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.
Above: 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 in an elliptical orbit about the star, which carries
it closer to the star than Mercury is to our sun. Credit: Greg
information from the Hubble Space Telescope Science Institute]
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.
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
Mass distribution of Known Extrasolar Planets (Table format)
Mass of planet (Jupiter = mass
Number of Planets
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).
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
Right: Diagram shows the orbit of planet of HD 46375
as compared with the orbits of Mercury and Venus. The orbit of
this planet is very close to its star - approximately 1/5th of
the diameter of Mercury's orbit around our Sun.
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.