[meteorite-list] Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Tue, 06 Nov 2007 21:56:27 -0400
Message-ID: <vj62j3ligo86fflq5u5muniduasjflc4dr_at_4ax.com>


Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri

ScienceDaily (Nov. 6, 2007) ? Astronomers have discovered a record-breaking
fifth planet around the nearby star 55 Cancri, making it the only star aside
from the sun known to have five planets.

The discovery comes after 19 years of observations of 55 Cancri and represents a
milestone for the California and Carnegie Planet Search team, which this year
celebrates the 20th anniversary of its first attempts to find extrasolar planets
by analyzing the wobbles they cause in their host star.

The team's long history of measurements - more than 300 for 55 Cancri alone -
made the discovery of a five-planet system possible, said UC Berkeley astronomy
professor Geoffrey Marcy, who with Paul Butler, now at the Carnegie Institution
of Washington, began observations of many nearby stars at the University of
California Lick Observatory in 1987.

The unique 55 Cancri system, located 41 light-years away in the direction of the
constellation Cancer, is notable also because its clutch of four inner planets
and one giant outer planet resembles our own solar system, though without an
Earth or Mars.

"This system is interesting because there's a giant planet at 6 AU and four
smaller planets inward of 0.8 AU, with a huge remaining gap in between, right
where we would expect to find an Earth-sized planet," Marcy said.

An AU, or astronomical unit, is the average distance between the Earth and the
sun, about 93 million miles.

According to lead author Debra Fischer, assistant professor of astronomy at San
Francisco State University, the fifth planet is within the star's habitable zone
in which water could exist as a liquid. Though the planet is a giant ball of
gas, liquid water could exist on the surface of a moon or on other, rocky
planets that may yet be found within the zone. "Right now, we are looking at a
gap between the 260-day orbit of the new planet and the 14-year orbit of another
gas giant, and if you had to bet, you'd bet that there is more orbiting stuff

Fischer noted that what occupies this gap has to be another planet around the
size of Neptune or smaller, because anything larger would have destabilized the
orbits of the other planets. All of the planets around 55 Cancri are in stable,
nearly circular obits, like the eight planets in our solar system. Jupiter is
located at 5.2 AU from the sun, while Mercury and Venus are closer than 0.72 AU.
Earth and Mars are in the gap at 1 AU and 1.5 AU.

"We haven't found a twin of our solar system, because the four planets close to
the star are all the size of Neptune or bigger," Marcy said, but he added that
he's optimistic that continued observations will reveal a rocky planet within
five years.

The new discovery, using data from the Lick Observatory and the W. M. Keck
Observatory in Hawaii, has been accepted for publication in the Astrophysical
Journal. The authors are Fischer, Marcy and their colleagues at the Carnegie
Institution, San Francisco State University, UC Santa Cruz, Tennessee State
University and UC Berkeley.

Fischer and Marcy also discussed their findings today during a media
teleconference hosted by NASA.

"This work marks an exciting next step in the search for worlds like our own,"
said Michael Briley, an astronomer at the National Science Foundation. "To go
from the first detections of planets around sun-like stars to finding a
full-fledged solar system with a planet in a habitable zone in just 12 years is
an amazing accomplishment and a testament to the years of hard work put in by
these investigators."

In 1996, when Marcy and Butler found a Jupiter-sized planet orbiting close to 55
Cancri and circling every 14.6 days, it was only the fourth known star with an
exoplanet. The second planet discovered in 2002 around the star turned out to
circle in a more distant orbit, like our own Jupiter does, although the planet
was four times the weight of Jupiter. The third, also discovered in 2002, was
smaller, about half the size of Saturn, and was orbiting near the star with an
orbit of 44 days, slightly farther than the first planet. The fourth planet,
found in 2004, was so close to the star as to be hellishly hot - a Neptune-sized
planet (14 times Earth's mass) with a 2.8 day period discovered in collaboration
with a team led by Barbara McArthur of the University of Texas.

Although astronomers have found nearly 250 exoplanets, only one other star, mu
Ara in the southern sky, is known to have four planets.

The newly-found fifth planet around 55 Cancri is also large - around half the
size of Saturn, or at least 45 times the mass of Earth - and orbiting at about
0.785 AU in 260.8 days. Because the star 55 Cancri is older and dimmer than our
sun, the habitable zone - the region in which planetary temperatures can be
favorable for liquid water - is closer to the star than is our sun's habitable
zone, and includes the new planet.

Finding multiple planets around a star is difficult because each planet produces
its own stellar wobble. Marcy compares detecting the wobble within wobbles that
are caused by one of several planets to picking out a single musical note from
many played simultaneously. While the ear can do that, it took Marcy more than
10 months to convince himself that a fifth wobble was buried in the data.

The Doppler technique used by the search team sees this wobble as a change in
the speed with which a star moves toward or away from us. The search team can
detect velocities as small as 1 meter per second, which is walking speed.

55 Cancri has produced "a rat's nest of radial velocity data," Fischer said. "We
probably still don't have all the planets. We are pulling out one thread at a
time, disentangling all these orbits, and it has taken a lot more data and time
than we predicted. I think it's amazing what we have been able to do with the

Coauthors with Fischer, Marcy and Butler are Steven S. Vogt and Greg Laughlin of
UC Santa Cruz; Jason T. Wright, John A. Johnson and Kathryn M. G. Peek of UC
Berkeley; Gregory W. Henry of Tennessee State University's Center of Excellence
in Information Systems; and David Abouav, Chris McCarthy and Howard Isaacson of
San Francisco State University.
Received on Tue 06 Nov 2007 08:56:27 PM PST

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