September 6, 2004
Astronomers discover first Neptune-sized planets
outside our solar system
By Tim Stephens
A team of astronomers announced last week the discovery of some of
the smallest planets yet detected beyond our solar system. The two newly
discovered planets represent a new class of extrasolar planets, and
their discovery is a significant advance in the quest for such objects.
Steven Vogt, a professor of astronomy
and astrophysics, is a codiscoverer of one of the new planets.
"These are important milestones on our way toward being able to
detect planets the size of Earth," said Steven Vogt, a professor
of astronomy and astrophysics at UCSC and codiscoverer of one of the
Each of the newly discovered planets is roughly comparable to the planet
Neptune in our own solar system, said Geoffrey Marcy of UC Berkeley,
a codiscoverer of both planets.
Although the new planets are about 20 times the size of Earth, they
are much smaller than the roughly 120 other extrasolar planets (also
called exoplanets) that have been discovered to date. Most of those
objects are considerably larger than the largest planets in our solar
system, Jupiter and Saturn.
Vogt and Marcy (who was Vogt's first graduate student at UCSC, earning
his Ph.D. in 1982) are members of the California and Carnegie Planet
Search Team, which has discovered the majority of the known extrasolar
planets. These discoveries rely on a strategy pioneered more than a
decade ago by Marcy and team member Paul Butler of the Carnegie Institution
of Washington. The researchers monitor a long list of candidate stars
looking for a telltale "wobble" caused by the gravitational
tug of an orbiting planet. The wobble shows up as subtle "Doppler
shifts" in the starlight and can only be detected with sensitive
spectrographs that separate the light gathered by telescopes into its
component colors or frequencies.
The first of the new Neptune-mass planets was discovered by Butler,
Vogt, Debra Fischer of San Francisco State University, and Marcy at
the Keck I Telescope in Hawaii, using the HIRES spectrograph designed
by Vogt. It is only the second exoplanet found around a type of cool,
reddish, low-mass star known as an M dwarf. M dwarf stars are very faint,
and only the largest telescopes, such as the Keck, have the light-gathering
capacity needed to detect Doppler shifts in their dim light.
The astronomers have been using the Keck Telescope to monitor 950 nearby
stars, of which 150 are M dwarfs. The star harboring the new Neptune-mass
planet is called Gliese 436. It lies about 33 light-years from Earth
in the direction of the constellation Leo. The planet, with a mass at
least 21 times that of Earth, orbits the star once every 2.64 days at
a distance of about 4.5 million kilometers, only about 3 percent of
Earth's distance from the Sun.
"This discovery is particularly exciting because we've been looking
for planets around M dwarf stars for a long time. It's a common type
of star, so there are a lot of them out there," Vogt said.
The second of the new Neptunes was found in orbit around 55 Cancri,
a yellow, sun-like star that lies about 41 light-years from Earth in
the direction of the constellation Cancer.
In 2002, the same team had already announced their discovery of three
planets around 55 Cancri, based largely on observations made by Fischer
at UC's Lick Observatory on Mt. Hamilton. (Vogt also designed the spectrograph
used by the planet hunters at Lick Observatory.) Over time, however,
they found that their subsequent Doppler measurements were beginning
to vary from the predicted values in a way that suggested a fourth planet.
The team then sent all their accumulated observations to University
of Texas astronomer Barbara McArthur, who has been organizing a major
campaign to study the 55 Cancri system. Combining that dataset with
another 100 Doppler measurements taken with the Hobby-Eberly Telescope
in Texas, McArthur and her colleagues then confirmed the star does indeed
have a fourth planet. It has 18 Earth masses, an orbital period of 2.81
days, and an orbital radius of just 3.8 percent that of Earth.
The composition of the two new Neptune-mass planets is unknown. They
could be gas giants, similar to Jupiter and Saturn, or more like Neptune
itself, which has a core of rock and ice. It could even be a barren
ball of rock and iron without an envelope of gas.
"With these smaller planets, we are now down to the level where
they could almost be rocky planets," Vogt said.
A recent major detector upgrade to the HIRES spectrograph will lead
to even more discoveries of such planets, he added.
"We just finished the first tests, which show much greater precision
than we had before. The higher precision of the instrument will allow
us to detect even lower-mass planets than these, carrying us down into
the realm of terrestrial or rocky planets," Vogt said.
The researchers will publish their results in December as two separate
peer-reviewed papers in the Astrophysical Journal. The authors
of the first paper, in addition to Butler, Vogt, Marcy, and Fischer,
are Jason Wright of Berkeley, Gregory Henry of Tennessee State University,
Greg Laughlin of UCSC, and Jack Lissauer of NASA Ames Research Center.
The authors of the second paper, in addition to McArthur, are Michael
Endl, Fritz Benedict and William Cochran, University of Texas, Austin;
Fischer, Marcy, and Butler; Dominique Naef, Michel Mayor, Didier Queloz,
and Stephane Udry, Observatoire de Geneve, Switzerland; and Thomas Harrison,
New Mexico State University.
The new discoveries were publicly announced at a NASA Science Update
on August 31. European astronomers earlier last month announced preliminary
evidence for the detection of one other Neptune-sized planet.
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