Less Massive than Saturn? - astronomers detect light extrasolar planets

Science News, April 1, 2000 by Ron Cowen

Astronomers pass a milestone in the search for new worlds

Planet hunters Geoffrey W Marcy and R. Paul Butler have become frequent visitors to a telescope atop Hawaii's Mauna Kea. They come to search for alien worlds but often feel like they've landed on one. Rising 14,000 feet above the palm trees and lush vegetation, the windswept summit of this extinct volcano is nearly as pockmarked as the moon and as strewn with reddish rocks as Mars.

From this desolate perch, the astronomers use the Keck I Telescope to track the motion of several hundred nearby stars. A telltale stellar wobble betrays the tug of an unseen planet. That wobble shows up as a Doppler shift, pushing the wavelength of starlight alternately toward the bluer and redder end of the spectrum as the star moves toward and away from Earth.

The 35 planets that Marcy and Butler's team and other researchers have so far detected in this way have little in common with those in our solar system. Many of the planets circle their parent stars more tightly than Mercury does the sun, and nearly all are more massive than Jupiter, the solar system's giant. That's not surprising, since the biggest planets and those nearest to a star exert the strongest tug and induce the largest wobble (SN: 8/8/98, p. 88).

This week, however, Marcy, Butler, and a colleague announced that they've passed a milestone. After several years of searching for tiny wobbles with high-precision spectrographs, the team has discovered what may be the two lightest planets ever found outside our solar system. Each could be less massive than Saturn, the ringed planet that weighs one-third as much as Jupiter. The extrasolar orbs detected from Mauna Kea are beginning to seem more familiar.

Marcy, who is at the University of California, Berkeley, and his colleagues Butler of the Carnegie Institution of Washington (D.C.) and Steven S. Vogt of the University of California, Santa Cruz unveiled the findings March 29 at a NASA press briefing.

Butler estimates that with further perseverance his team can detect planets as light as Neptune--just 17 times the mass of Earth--if they lie within 0.1 astronomical units (AU) of their parent stars. One AU is the distance between Earth and the sun.

"This absolutely demonstrates that we ... can detect solar system analogs," declares Butler. "At this point, it's simply a matter of collecting data for another decade. The technique is there."

Other astronomers agree. "The big thing is that by pushing into the regime of very high-precision detection, they're really pushing into an area where they have demonstrated they can find solar system analogs," says theorist Alan P. Boss of the Carnegie Institution. "We sort of knew they could do it, but now they've proved it."

"Finding `Saturns' is a wonderful breakthrough," Marcy adds. "But more profound is that we are finding increasing numbers of planets having smaller and smaller mass. This points toward a plentitude of Earth-sized planets yet to be found."

At the same time, theorists are gaining new insight into some of the rather odd planets--unlike those in the solar system--already detected.

Tightly hugging its parent star, one of the new planets has an orbit similar to that of the first extrasolar planet discovered, an object several times the mass of Jupiter (SN: 10/21/95, p. 260). However, the new planet's mass may be as small as 84 percent that of Saturn. It whips around the sunlike star HD 46375, which resides 109 light-years from Earth in the constellation Monoceros. Once every 3.024 days, the planet circles the star at a distance one-tenth that at which Mercury orbits the sun.

The other newly detected planet, which took several years to detect, has a slightly smaller minimum mass, about 74 percent that of Saturn. It also has a more leisurely orbit, making one complete revolution around star HD 16141 every 75.8 days. Following an elliptical path, it resides at an average distance from it's parent star just slightly less than Mercury's separation from the sun. HD 16141, also known as 79 Ceti, lies 117 light-years away from Earth in the constellation Cetus.

It's the second find that has most elated the researchers. Marcy, Butler, and Vogt detected this planet even though the tug it exerts on its parent star induces only a modest wobble of 11 meters per second, a little faster than a person's sprinting speed. That's a smaller yank than Jupiter's pull on the sun.

In other words, the planet hunters have now demonstrated that their detectors have enough sensitivity to find a Jupiterlike planet orbiting a sunlike star at a Jupiterlike distance.

Butler says his team has several such candidates, but a definitive detection is several years off. That's because a planet with an orbit similar to Jupiter's takes 12 years to complete a single revolution around its star.

Finding lighter-weight planets lends credence to a basic theory about planet formation--that planets were built up by the agglomeration of smaller objects. "Any kind of process that works like that is going to give you a lot more smaller objects than bigger objects," notes Butler. "It's just as though when you come upon a beach, you see the boulders from a long way away, but there's a lot more grains of sand."