June 24, 2002
Astronomers find planet system that reminds them of home
By Robert Sanders, UC Berkeley
After 15 years of observation and lots of patience, the world's premier planet-hunting
team has finally found a planetary system that reminds them of our home solar system.
Led by Geoffrey Marcy at UC Berkeley and Paul Butler at the Carnegie Institution
of Washington, the team includes two UCSC astronomers, Steven Vogt and Gregory Laughlin.
At a press conference June 13 at NASA headquarters, the researchers announced the
discovery of a Jupiter-like planet orbiting a sunlike star at nearly the same distance
as Jupiter orbits our sun.
|This artist's concept depicts a Jupiter-mass planet orbiting the star, 55 Cancri,
about 41 light years from Earth. A possible moon around the planet is shown because
such moons are thought to be common around this type of planet, but no moon has been
detected. Image: NASA/Lynette Cook.
"This is the first near analog to our Jupiter," said Marcy, a UCSC alumnus.
"All other extrasolar planets discovered up to now orbit closer to the parent
star, and most of them have had elongated, eccentric orbits. This new planet orbits
as far from its star as our own Jupiter orbits the sun.''
The planet-hunting team is funded by grants from the National Science Foundation
The star, 55 Cancri in the constellation Cancer, was already known to have one planet,
a gas giant slightly smaller than the mass of Jupiter whipping around the star in
14.6 days at a distance only one tenth that from the Earth to the sun, announced
by Butler and Marcy in 1996. Using the 93 million mile Earth-sun distance as a yardstick,
called an astronomical unit or AU, the newly found planet orbits at 5.5 AU, comparable
to Jupiter's distance from the sun of 5.2 AU. With a mass between 3.5 and 5 times
that of Jupiter, the planet has a slightly elongated orbit that carries it around
the star in about 13 years, comparable to Jupiter's orbital period of 11.86 years.
"We haven't yet found an exact solar system analog, which would have a circular
orbit and a mass closer to that of Jupiter," Butler said. "But this shows
we are getting close, we are at the point of finding planets at distances greater
than 4 AU from the host star. And we found this planet among the 107 stars we first
targeted when we started looking for planets at Lick Observatory in 1987, so I think
we will be finding more of them among the 1,200 stars we are now monitoring."
The team of astronomers passed their data along to Laughlin, who conducted dynamical
calculations that show an Earth-sized planet could survive in a stable orbit between
the two gas giants.
"We tried a hypothetical configuration of a terrestrial planet in the habitable
zone around one AU from the central star and found it very stable," said Laughlin,
an assistant professor of astronomy and astrophysics. "Just as the other planets
in our solar system tug on the Earth and produce a chaotic but bounded orbit, so
the planets around 55 Cancri would push and pull an Earthlike planet in a manner
that would not cause any collisions or wild orbital variations."
For the foreseeable future, any such planet in the habitable zone around 55 Cancri
will remain speculative.
Marcy, Butler, and their team announced a total of 13 new planets at the press conference,
including the smallest ever detected: a planet circling the star HD49674 in the constellation
Auriga at a distance of 0.05 AU and a mass about 15 percent that of Jupiter--nearly
half that of Saturn, or 40 times the mass of the Earth. This brings the total number
of known planets outside the solar system to more than 90.
Discovery of a second planet orbiting 55 Cnc is the culmination of 15 years of observations
using the 3-meter telescope at Lick Observatory, which is owned and operated by the
University of California. Collaborators on the project include Debra Fischer at UC
Berkeley, Greg Henry at Tennessee State University, and Dimitri Pourbaix of the Institut
d'Astronomie et d'Astrophysique, Universite Libre de Bruxelles, in addition to Marcy,
Butler, Vogt, and Laughlin.
Marcy and Butler used a technique that measures the slight Doppler shift in starlight
caused by a wobble in the position of a star, a periodic shift due to a planet yanking
on the star as it orbits. By observing over a period of years, they can infer a planet's
approximate mass and orbital size and period.
The star 55 Cnc is 41 light years from Earth and is about 5 billion years
old. Laughlin speculated that the large inner planet probably formed farther from
the parent star, where ice could form and rocks accrete to form a solid core, and
only migrated inward after it had scooped up a shroud of gas. This inward migration
is a characteristic of giant planets in a disk of gas and dust typical of forming
planetary systems, he said. They create a spiral wake that actually tugs on the planet,
slowing it down and sending it spiraling inward toward the star.
"To me, the question is why it stopped before crashing into the star,"
Laughlin said. Numerous giant extrasolar planets have been found in very short-period
orbits--3 to 3.5 days--when by all rights they should have spiraled to a flaming
There also may be another planet around 55 Cnc, because the two known planets do
not yet explain all the observed Doppler wobbling. The scientists need more data
before they can come to a final conclusion, however.
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