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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.
Photo: Tim Stephens

"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 new planets.

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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