Less Massive than Saturn?

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."

"If we had found no `Saturns,' then we would have to doubt that the extrasolar planets are planets at all," says Marcy. He and his colleagues would have had to contend with the possibility that many of the objects they have found are in fact much heavier than Jupiter.

Using the Doppler method, astronomers can only compute a lower limit to a planet's mass. That's because they don't typically know in what plane the planet's orbit lies. For an orbit that happens to be edge-on as viewed from Earth, scientists see the full magnitude of the wobble and can infer an accurate mass. When the orbit lies at an angle to the line of sight, however, the observed wobble is less than the actual wobble, and the inferred mass is smaller than the planet's true mass.

If the extrasolar objects that astronomers have detected are significantly weightier than the minimum mass calculated, they might not be planets at all. They might qualify instead as stillborn stars called brown dwarfs orbiting sunlike stars. In contrast to planets, brown dwarfs form as stars do, from the fragmentation of gas clouds.

"If we had found only gigantic planets without the small ones, then something would be terribly out of kilter, either with our solar system's planets or with the theory of planet formation," notes Marcy.

Butler views the new discoveries as a key but incremental step. "We would like now to push down to much lower masses and begin to see if ... the small, rocky planets that we know and love are part of a continuous distribution of objects that includes the [massive] objects that we've found to date," he says.

If our own solar system is any example, finding a "Jupiter" at a Jupiterlike distance from a star boosts the odds that any smaller planets lying nearer the star are able to support life, notes Boss.

Several years ago, George W. Wetherill of the Carnegie Institution showed that giant Jupiter, in its lonely outpost five times as far from the sun as Earth is, protects the inner planets from bombardment by comets. Jupiter's gravity either flings the icy bodies out of the solar system or draws them close, so they crash into it rather than Earth.

Moreover, a Jupiterlike planet on a circular orbit can force lighter, inner planets to adhere to circular paths. Such orbits, which by definition keep a planet at the same distance from the sun, maintain a constant temperature, making it easier for life to thrive.

Tracking the motion of 1,100 stars from three locations--Mauna Kea, the Lick Observatory on Mount Hamilton, Calif., and the Anglo-Australian Telescope near Coonabarabran, Australia--Marcy, Butler, and their colleagues have surveyed just 5 percent of the sunlike stars within 300 light-years of Earth. Other astronomers are watching several hundred more. With such a small fraction of the stars being examined, it's too soon to say whether our solar system is an average Joe or an oddball. By using Magellan I and II, the twin 6.5-meter telescopes now nearing completion at Las Campanas Observatory in Chile, Marcy and Butler hope by 2003 to double the number of stars that their team monitors.

In the meantime, theorists have closely analyzed several systems in which an extrasolar planet has so small an orbit that it actually grazes the outer atmosphere of its parent star.

The work was prompted by the discovery last year of a large extrasolar planet dimming the light emitted by its parent star each time it passed in front of the luminous body (SN: 11/20/99, p. 324). That first-of-a-kind measurement unequivocally provided the mass and radius of the planet, which comes within roasting distance of the star HD 209458. Although the planet has a mass about 70 percent that of Jupiter, its diameter is 1.4 times as large.

Contrary to what some scientists had thought, the planet's bloated appearance does not stem from the heating and expanding of its atmosphere by intense starlight. Rather, the heat from the star prevents the planet, plump at birth, from cooling off and contracting, report Adam Burrows of the University of Arizona in Tucson and his colleagues, including Tristan Guillot of the Observatory of the Cote d'Azur in Nice, France, and Didier Saumon of Vanderbilt University in Nashville.

These researchers subscribe to the notion that planets aren't born in the hot seat but migrate inward after forming in the outer, cooler regions surrounding a star (SN: 12/16/95, p. 412). Had the newborn planet stayed put far from its parent, its hot core would have quickly cooled and the object would have rapidly shrunk. Burrows and his colleagues calculate that to remain puffed up, the planet must have spiraled inward no more than 10 million years after it formed.

Once a planet moves closer to the stellar furnace, the heat beating down on it stops the core from cooling any further, leaving the planet with a sizable girth. The researchers describe their analysis in an upcoming ASTROPHYSICAL JOURNAL LETTERS.

"Because this object has a radius significantly larger than that of Jupiter, it cannot have cooled far from its star, like Jupiter did," notes Guillot.

For a less massive planet, such as a "Saturn," to remain just as bloated as a hot '"Jupiter," it would have to have formed even more rapidly and have spent even less time far from its star, Guillot says.

Ten million years is more than enough time to build a Jovian planet from itinerant gas and dust, notes Boss. Whether or not expansive Saturnlike planets would have enough time to form in the traditional manner--and how common they are--remains to be seen.

"We don't know [if the typical planetary system] could be a solar system just like ours or completely different," says Boss. But over the next decade, as astronomers continue their search from outposts like Mauna Kea, they'll find out just how our system of planets measures up.

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