Discovered: Two More Planets

SF State-Berkeley Astronomy Duo Find Them in Virgo, Ursa Major

by Robert Sanders

Astronomers have discovered two new Jupiter-sized planets within about 35 light years of Earth, both at temperatures that mean they could harbor water in liquid form.

The discoveries were announced Jan. 17 at a meeting of the American Astronomical Society in San Antonio, Texas, by astronomers Geoffrey Marcy, professor of physics and astronomy at San Francisco State and a visiting scholar here, and Paul Butler, a postdoctoral researcher with a joint appointment at Berkeley and San Francisco State.

Marcy and Butler were the ones who last year confirmed the first discovery of a planet outside our solar system, 51 Pegasi.

"After the discovery of 51 Peg everyone wondered if it was a freak, a one in a million observation," Marcy said. "The answer is no. Planets aren't rare after all."

The two new planets were found around the stars 70 Virginis, in the constellation Virgo, and 47 Ursae Majoris, in the Big Dipper or Ursa Major.

Both stars are visible to the naked eye, but the planets are too small and dim to be seen against the glare of their parent star.

Nevertheless the planets create a telltale wobble in the stars' motion, which the astronomers were able to detect with sensitive equipment mounted on the 120-inch Shane reflector telescope at the UC's Lick Observatory."These new discoveries are important because they spawn a new subfield of astrophysics, the study of planetary systems," Marcy said. "We can now probe the characteristics of these planets, such as their orbits and masses."

Discovery of the first extrasolar planet was announced Oct. 6 at a meeting in Florence, Italy, by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland and confirmed a week later by Marcy and Butler at Lick Observatory.

Circling the star 51 Pegasi about 40 light years distant, the planet was unofficially dubbed Bellerophon by Marcy, in keeping with the name 51 Peg B given by Mayor and Queloz, and the convention of naming planets after Greek and Roman mythological figures. Bellerophon was a Greek hero who rode the winged horse Pegasus to slay the fire-breathing Chimera.

Unlike Bellerophon, which is about half the mass of Jupiter and orbits its star in 4.3 days--so close it is baked to 1000 degrees Celsius (1800 degrees Fahrenheit)--the two new planets are several times the mass of Jupiter and orbit their stars at a distance typical of our own solar system.

The planet around 70 Vir orbits the star in an eccentric, elongated orbit every 116 days and has a mass about nine times that of Jupiter. Using standard formulas that balance the sunlight absorbed and the heat radiated, Marcy and Butler calculated the temperature of the planet at about 85 degrees C (185 degrees F).

"That's cool enough to permit complex molecules to exist, ranging from carbon dioxide to complicated organic molecules," Marcy said. "And because 85deg.C is below the boiling point of water, this planet could conceivably have rain or even oceans."

The star 70 Vir is nearly identical to the Sun, though several hundred degrees cooler and perhaps three billion years older.

The planet around 47 UMa was discovered after analysis of eight years of observations at Lick. Its period is a little over three years (1100 days), its mass about three times that of Jupiter, and its orbital radius about twice the Earth's distance from the Sun.

This planet too probably has a region in its atmosphere where the temperature would allow liquid water.

"There is going to be a zone where a cauldron of organic molecules cooks with water," Butler said. "This system is the closest thing we've seen to anything like our own solar system."

Marcy notes also that during their recent observations they failed to detect a second planet around 51 Peg, in apparent contradiction to data obtained by the Swiss team that discovered it.

The San Francisco State and Berkeley astronomers expect to announce the discovery of more new planets in the coming years, now that they have perfected a unique system for detecting the tiny wobbles in distant stars that indicate the presence of a planet. The two started their efforts in 1987 and have been monitoring 120 stars--all between 10 and 100 light years from Earth--for up to eight years in search of periodic changes in the star's speed.

The wobble in a star's velocity is determined from the Doppler shift in the star's spectrum. The Doppler shift is a shift of wavelength or color caused by the motion of a star toward or away from the observer. Objects moving away have their entire spectrum shifted toward the red, while those moving toward us have their spectrum shifted toward the blue.

Within the past year and a half Marcy and Butler have refined their technique to the point where they can measure this velocity within an accuracy of three meters per second--the speed of a brisk walk.

Jupiter, for example, produces a 12.5 meter per second wobble in the Sun's orbit. With a few more years of observations, Marcy said, the team could detect a Jupiter-sized planet orbiting a distant sun--assuming it is located at Jupiter's distance from our Sun and has a similar period of about 12 years.

They achieve this precision by putting a glass chamber of iodine gas in front of the telescope so that light from the star passes through and is partly absorbed, superimposing the absorption spectrum of iodine onto that of the distant star. Spreading out the composite spectrum and cutting it into chunks, they then measure how much the starlight spectrum has shifted to the blue or red relative to the known spectrum of iodine.

Each chunk provides an estimate of the shift, and averaging the results of 500 such chunks yields a very precise measurement of the Doppler shift, Marcy said.

They were greatly aided in these measurements by a state-of-the-art spectrometer on the Lick telescope that was recently refurbished by astronomer Steve Vogt of UC Santa Cruz.


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