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April 16, 1999

At Long Last, Another Sun With a Family of Planets
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  • In New Discoveries, a Planetary Mystery

    By JOHN NOBLE WILFORD
    The solar system is not alone as an array of planets orbiting in the gravitational embrace of a shining star.

    Laying to rest any lingering notion of the Sun's family being a singular phenomenon in the universe, astronomers announced on Thursday the detection of three large planets around Upsilon Andromedae, a solar-type star 44 light-years away.



    Retuers
    An image of Upsilon Andromedae, a solar-type star with three large planets orbiting it.
    Solitary planets had been observed around several other stars in the last four years, but this is the first clear evidence showing another star accompanied by multiple planets in a stable system bearing some resemblance to the Sun's.

    Two of the planets are several times more massive than Jupiter, the solar system's giant, which is 318 times heftier than Earth. The third planet, with at least three-quarters the Jovian mass, is so close to the star that it completes a full orbit -- its year -- every 4.6 Earth days. Astronomers said that they would not be surprised if they eventually find other, more distant objects around the star.

    Other astronomers greeted the discovery with unbridled enthusiasm. They called it a major milestone in planetary science. Here, finally, was what they had eagerly been seeking: another planetary system to compare with their own. They expected further study of the Upsilon Andromedae system to challenge some theories of planet formation and evolution, and probably hatch new ones.

    Of even greater philosophical as well as scientific importance, the discovery encouraged astronomers in their growing belief that the universe abounds in stars with planetary systems. This, in turn, increased the probability that some of them include habitable worlds, scientists said, though no such claim is being made for the newly discovered system.

    "The single planets we found around other stars was a glorious discovery, but the architecture of other planetary systems had been missing," Dr. Geoffrey Marcy, a leader of the discovery team, said in an interview. "Here for the first time, we can see a kinship between these planets and our own solar system."

    Dr. Alan P. Boss, a theorist of planetary systems at the Carnegie Institution of Washington, who must come to grips with the implications of the findings, said simply, "This is exciting stuff."

    The discovery was made independently by two teams, one from San Francisco State University and the other from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the National Center for Atmospheric Research in Boulder, Colo. They joined in announcing the results at a news conference in San Francisco.

    NEW TERRITORY
    Astronomers have detected three large planets orbiting the star Upsilon Andromedae, marking the first time that a multi-planet solar system has been observed around a normal star, other than the Sun. As seen in the diagram below, the innermost of these giants orbits much closer to its star than Mercury does to the Sun, completing an orbit every 4.6 Earth days.

    The New York Times
    Source: San Francisco State University

    A full report, which has already been reviewed by more than a dozen independent astronomers, has been submitted for publication in The Astrophysical Journal.

    "Having two completely independent sets of observations gives us confidence in this detection," said Dr. Debra Fischer, of the San Francisco team. And Marcy, the team leader and most prolific discoverer of extra-solar planets, said, "I would bet my house on it."

    Although Upsilon Andromedae is a nearby bright star visible to the unaided eye, the three planets cannot be seen even with the most powerful telescopes. Astronomers infer their existence, orbits and minimum masses from years of careful study of their gravitational effects, characterized as reflex motions, on the host star. In their orbital courses, the planets tug first one way and then the other on the star, causing ever-so-slight changes in the star's velocity.

    This observational technique has been responsible for the detection of 18 Jupiter-class extra-solar planets since 1995, when Swiss astronomers found the first planet around another normal star, 51 Pegasi.

    Dr. Robert Noyes, a Harvard-Smithsonian astronomer, said the new observations should dispel any doubts that these objects are true planets.

    "A nagging question was whether the massive bodies orbiting in apparent isolation around stars really are planets," Noyes said in a statement. "But now that we see three around the same star, it is hard to imagine anything else."

    Dr. Douglas Lin, a theorist at the University of California at Santa Cruz who has sought to explain how such huge planets could exist so close to their stars, much closer than Jupiter is to the Sun, said the new detections should enable scientists to evaluate their various theories. They are struggling to understand if systems with several super-Jupiter planets, traveling eccentric orbits close to their stars, are more typical than the solar system, with its gaseous giant planets all traveling circular orbits at great distance from the Sun.

    "This is a very, very important discovery," Lin said in an interview. "It tells us that planetary systems are quite ubiquitous, and some of them are quite stable. It also tells us that the existence of habitable planets is highly probable."

    Although Earth-size planets could exist in the Upsilon Andromedae system, astronomers said, they would be undetectable with current search methods. In any event, they would be unlikely to exist in what astronomers think of as the habitable zone of a planetary system, close enough to be warmed by the star, like Earth, but not so close as to be baked like Mercury and Venus. Also, the gravitational forces of Jupiter class objects in that vicinity would more than likely have cleared it any small planets.

    The discovery teams calculated that the middle planet in the Upsilon Andromedae system is in an orbit corresponding to the distance of Venus from the Sun. It is at least twice the mass of Jupiter, making the complete circuit every 242 days. The outermost known planet, at least four times more massive than Jupiter, orbits the star about once every four earth years at a distance comparable to the region between Mars and Jupiter.

    Astronomers suspect that these giant planets are, like Jupiter and Saturn, huge spheres of gas without a solid surface. But, also like Jupiter and Saturn, they could have many large moons. Possibly, Lin speculated, on one of these moons there could be liquid water, and atmosphere and other conditions conducive to life.

    "Who knows," Lin remarked, "on one of those satellites, I would probably have a beach-front property."

    The planet closest to Upsilon Andromedae had already been discovered by Marcy and Dr. R. Paul Butler in 1996. At the time, they detected additional motions of the star suggesting other companions, but only with repeated observations and careful analysis could they be sure.

    Astronomers were less surprised by the discovery than relieved. For several months, they had generally assumed the existence of extra-solar planetary systems. After all, the Sun is a common type of star, one of 200 billion in the Milky Way alone, and beyond lie more than 80 billion other galaxies. It hardly seemed likely that the Sun's planets were unique.

    The latest discovery is expected to tax the ingenuity of theorists. It had been thought that such giant gas bodies could only form at great distances from a star, out where temperatures are low enough for ice to condense and begin the process of planetary formation as gaseous spheres. Finding the single Jupiter-class object near stars had forced scientists to invent migration theories, explaining how the planets might have formed at greater distances and then worked their way in closer to the star.

    "I am mystified at how such a system of Jupiter-like planets might have been created," Marcy said of the Upsilon Andromedae system. "This will shake up the theory of planetary formation."

    Boss of the Carnegie Institution said the discovery could upset conventional ideas explaining giant Jupiter-class planets. "Maybe nature has many ways of making giant planets," he said.

    The next milestone, astronomers said, would be finding evidence of another system with a Jupiter-class planet out at a distance from its star corresponding to Jupiter's from the Sun.

    "Until we do," Marcy said, "there will always be the question of whether the solar system is a cosmic freak."

    Other members of the Harvard-Smithsonian team were Dr. Sylvain Korzennik, Dr. Peter Nisenson and Adam Contos, a graduate student. Dr. Timothy Brown of the National Center for Atmospheric Research worked with them. Their observations were made at the Whipple Observatory near Tucson, Ariz. The San Francisco team used the Lick Observatory near San Jose, Calif. Butler, formerly at San Francisco State and now at the Anglo-Australian Observatory near Sydney, is the lead author of the journal report on the detections.

    It may be a decade or more before spacecraft are in place to look for Earth-size planets of other stars.

    After years of searching and speculation, both fanciful and educated, the discovery of multiple objects orbiting Upsilon Andromedae marked the beginning of the science of comparative planetary systems.



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