February 9, 1997

Scientists Step Up Search for Extrasolar Planets

In This Article:
  • The Quest: Seeking Worlds Around the Stars
  • The Methods: Planets' Effects Give Them Away
  • The Competition: The Unexpected And a Jump-Start
  • The Discoveries: Better Instruments For Better Data
  • The New Projects: 'Goldilocks Orbits' Are Just Right
  • The Prospects: Getting Ready To Cross a Line

    MOUNT HAMILTON, Calif. -- Standing outside the dome of Lick Observatory on this lofty summit, two astronomers gazed beyond the foothills to the far horizon where California meets the Pacific Ocean. As the solid world at their feet rotated east, the great red sphere of glowing hydrogen seemed to sink perilously close to a doomsday collision, only to slip harmlessly out of sight in the west.

    At the moment of sunset, birds somewhere in the trees broke into song, life sounding retreat at the loss of light. The astronomers turned back to the dome and the telescope within. Time to go to work, time to search the heavens for other stars not unlike the Sun and see whether some of them also have companion worlds -- other places where night follows day, where there might be air and water, mountains and shore, even life and song.

    The two astronomers, Dr. Geoffrey W. Marcy and Dr. R. Paul Butler of San Francisco State University, began another night of work at Lick Observatory, near San Jose, Calif., with the quiet confidence of professionals at the top of their game. In little more than a year, they and other teams in this country and Switzerland have for the first time detected planet-size objects -- at least eight confirmed, and possibly two more -- orbiting other stars like the Sun.

    Even if a few skeptics still question whether these objects, called exoplanets, qualify as true planets, Dr. Alan P. Boss, a theoretical astrophysicist at the Carnegie Institution of Washington, expressed the prevailing view: "I truly believe we have indeed identified the first extrasolar planets."

    All of a sudden, astronomers have turned a big corner and glimpsed in the dim light of distant lampposts a universe more wondrous than they had previously known. Other worlds are no longer the stuff of dreams and philosophic musings. They are out there, beckoning, with the potential to change forever humanity's perspective on its place in the universe.

    Although no likely habitable planets beyond the solar system have been detected so far, the discoveries, coupled with fresh evidence of the possibility of early life on Mars, have already renewed enthusiasm in the search for extraterrestrial life. The first of a fleet of spacecraft and robotic landers took off late last year to resume Mars exploration. Space telescopes being planned for the next decade should be able to see planets as small as Earth elsewhere and examine their atmospheres for signs of life.

    More powerful telescopes on the ground and in space, especially the Hubble Space Telescope, and more sensitive electronic detection instruments are sharpening the view of the cosmos for other astronomers as well. Every few weeks brings more spectacular pictures from the depths of space, pictures of the moons of Jupiter, the nurseries of newborn stars and the galaxies taking shape when the universe was young. They embolden cosmologists, the historians of the universe, in the audacious belief that many answers to questions of cosmic origin and evolution may be within their grasp.

    Little wonder scientists today feel justified in proclaiming this to be a new golden age of astronomy.

    In that spirit, Marcy and Butler were now setting their sights on more planetary discoveries. Among the billions of galaxies, Earth's own Milky Way galaxy alone is populated with 100 billion stars, and a few hundred of these stars are close enough -- less than 100 light-years away -- to be in range of technology's new gifts of vision, making it possible to detect large planets there.

    In the dark control room below the 120-inch Lick telescope, the two astronomers studied the light of target stars as they appeared, one by one, on a video screen. It is slow, tedious work. They were looking for faint variations, no more than 1 part in 100 million, in the frequencies of starlight, betraying the wobbling motions of a star caused by an unseen gravitational force nearby, something the size of a Jupiter or several Jupiters.

    There could be no thought of an instant cry of "Eureka!" Hours of computer analysis and months of repeat observations precede any announcement of the discovery of one of these objects, which astronomers are calling extrasolar planets, or exoplanets. But in the wee hours, the two men reflected on the exhilaration of being young on a mountaintop on the only planet known to harbor life and having a part in discoveries with transforming implications.

    Butler, 35, remembered the morning just over a year ago when he was "completely blown away" by the realization that a computer analysis showed that an object more than six times the mass of Jupiter was orbiting close to the star 70 Virginis, 80 light-years away. He could imagine the greats of astronomy nodding in awe. "I really felt the presence of Kepler standing at my shoulder," he said.

    Professionally, Marcy, 42, mused, "for us this is the best that will be, at our young ages."

    And the discoveries may be only beginning. One recent study suggested that planets might be lurking around half the Milky Way's stars. Astronomers have already seen enough to suspect that their definition of planets may have to be broadened considerably to encompass the new reality. As soon as they can detect several planets around a single star, they are almost resigned to finding that the Sun's family, previously their only example, is anything but typical among planetary systems.

    The Quest: Seeking Worlds Around the Stars

    The Epicurean philosophers of classical Greece would probably not have been surprised by such a turn of events. They believed that the chance conglomerations of infinite atoms in an infinite universe must form "innumerable worlds." Metrodorus of Chios, a disciple of Epicurus, wrote, "It would be strange if a single ear of corn grew in a large plain or were there only one world in the infinite."

    When the Polish scholar Copernicus determined in the early 16th century that Earth orbited the Sun, a revolutionary idea began to take root in modern Western thinking: Earth might not be the center of the universe or even unique as an abode of life. Acceptance of that idea was grudging. Giordano Bruno was burned at the stake in 1600 for, among other things, the heresy of speculating about other inhabited worlds.

    Many scientists, atheists and believers alike, came to agree on the likelihood of a plurality of worlds. One of them, the Frenchman Bernard de Fontenelle, is remembered for two things. He lived a vigorous life to the age of 100 and even in his 90s, upon meeting an attractive young woman, is supposed to have sighed, "Ah! If I were only 80 now." Also, in 1686, he wrote a popular book making a case for the existence of Venusians and Jupiterians and, more to the point, of planets around distant stars.

    More recently, scientists have been encouraged in the search for other worlds by the recognition that the laws of physics appear to be universal and that life is a phenomenon based on natural chemical processes that need not be confined to one planet around one rather ordinary star. Since the 1960s, a few radio astronomers have been patiently listening to the heavens, seeking signal patterns that just might come from intelligent extraterrestrials.

    Prospects for finding worlds around other stars improved substantially in the 1980s. Dr. Bradford A. Smith, of the University of Arizona in Tucson, and Dr. Richard J. Terrile, of the Jet Propulsion Laboratory in Pasadena, Calif., made infrared observations of a thick rotating disk of dust around the young star Beta Pictoris. This appeared to be a planetary system in the making and thus confirmation of the standard theory for the origin of such systems.

    According to the theory, as a cloud of interstellar matter compresses and collapses, forming a new star, rotational forces cause the residual dust and gas to form a flat disk and then fragment and coalesce as planets. Similar disks were soon observed around other young stars. Some stellar clusters show signs of such disks around at least 60 percent of their stars. But detecting these broad disks has been easier than finding planets, relatively small objects obscured by the bright light of their parent stars.

    The Methods: Planets' Effects Give Them Away

    Planetary companions cannot be seen directly in visible light or other wavelengths. Their presence must be inferred from their gravitational effects on the motions of their stars. The spectroscopic method, used by most astronomers, including Marcy and Butler, involves measurements of changes in the frequency of a star's light, the Doppler shift that reveals slight motions of the star toward or away from the observer. The simplest explanation for such radial motion is that a companion tugs at the star. The size of the frequency change in the starlight gives the amplitude of the star's motion, which in turn provides an estimate of the mass of the orbiting object.

    Other scientists use the astrometric method, which requires years of tracking a star's course through space. Astronomers try to discern any speeding up or slowing down of a star, a sign that an orbiting object, like a planet, is alternatively holding back the star, then pulling it faster along its trajectory. Over the years, this technique has raised many false hopes, as when two companions were supposedly detected earlier in the century around Barnard's Star. They later proved to be artifacts of the telescope.

    As it turned out, astronomers using radiotelescopes, not spectroscopy or astrometry, were the first to strike pay dirt in the search for planets, in 1994.

    Dr. Alexander Wolszczan, a radio astronomer at Pennsylvania State University, reported detecting two and perhaps three planet-size objects orbiting a star in the Virgo constellation. He called this "a final proof that the first extrasolar planetary system has been unambiguously identified."

    Although scientists accepted this assessment, they were nonetheless disappointed. Wolszczan seems to have found planets, but not planets around a normal star like the Sun. They are companions of a pulsar, a dense, rapidly spinning remnant of an exploded star, its thermonuclear furnace dead. The detections were made by observing regular fluctuations in the pulsar's rapid radio signals, indicating the planets' complex gravitational effects on the dead star.

    But a pulsar is no place for supporting life. The environment there would lack warming starlight and be saturated with deadly radiation.

    Meanwhile, other astronomers were busy studying the visible light of nearby stars for telltale variations of their radial velocity, with no success. Marcy and Butler had been at the task since 1987. A group at the University of British Columbia in Vancouver, led by Dr. Bruce Campbell and Dr. Gordon A.H. Walker, who pioneered the spectroscopic search technique, had been looking even longer. Discouraged, the British Columbia group abandoned its fruitless work in April 1995, just before two Swiss astronomers scored the breakthrough.

    The Competition: The Unexpected And a Jump-Start

    On Oct. 6, 1995, Dr. Michel Mayor of the Geneva Observatory in Switzerland announced that he and a colleague, Dr. Didier Queloz, had discovered a planet orbiting a star similar to the Sun, 51 Pegasi, about 40 light-years away.

    The planet has more than half the mass of Jupiter, at least; since the radial-velocity technique can determine only an object's minimum size, the planet's mass could be some 10 times as great as that listed. And, to the surprise and puzzlement of astronomers, the planet is closer to its parent star than tiny Mercury is to the Sun.

    At first, astronomers were wary, but the Swiss astronomers had checked their data with care. Mayor said they had repeated the observations and had also ruled out the possibility that the light variations were caused by the star's pulsations or eruptions like sunspots. When a theorist ran simulations showing that such a large planet could survive intact that near its star, Mayor decided to go public with the results.

    Marcy and Butler raced to Lick Observatory to check it out. Yes, a planet was there. They had been scooped, Marcy said, in part because they had been looking in the wrong place, not expecting that such a large planet could be so close to its star. Their expectations were influenced by the one planetary system they knew well, in which the giant Jupiter, 317.8 times as massive as Earth, is half a billion miles out from its star.

    In astronomy, as in anything else, having a clearer idea of what to look for can improve the odds of success. Marcy and Butler took another look at their old data on 120 stars and conducted new observations. By a year ago, they had two more planets to report: the one around 70 Virginis, in the constellation Virgo, and another around 47 Ursae Majoris, in the Big Dipper.

    More discoveries followed. They detected a planet around Tau Bootis and another around Rho Cancri. In October, Dr. William D. Cochran of the University of Texas at Austin and the Marcy-Butler team announced their independent discoveries of a planet traveling an eccentric orbit around the smaller of twin stars in the constellation Cygnus, designated 16 Cygni B.

    Dr. George Gatewood, of the University of Pittsburgh's Allegheny Observatory, reported finding one and perhaps two planets orbiting Lalande 21185, the fourth-nearest star to the Sun. His report has yet to be confirmed by other observations. But if true, it is the only discovery so far to be made with the astrometric technique, which tracks stars' paths over years.

    The possibility that this could be the first known extrasolar planetary system, not just a solitary planet detection, has generated excitement. "I can't see how it could not be a planetary system," Gatewood said. "But it is all somewhat mysterious."

    The Discoveries: Better Instruments For Better Data

    So, by the latest count, if the two pulsar planets are included, Dr. Adam Burrows of the University of Arizona said, "We now know of more planets outside the solar system than within."

    Astronomers attribute the sudden burst of discovery to significant advances in spectrometers, instruments like prisms that break up starlight into its component colors for detailed analysis; in electronic sensors, known as charge-coupled devices, that record the incoming starlight collected by telescope optics, and in computer software, which has been completely rewritten for discerning more reliably fluctuations in starlight that reveal the telltale motions.

    It also helped to have spent years gathering data on an increasing number of nearby stars. If the British Columbia team had examined at least 40 stars, not just 20, it might have made the first discoveries several years ago. "In retrospect, they were simply unlucky," said Dr. David W. Latham, of the Harvard-Smithsonian Center for Astrophysics.

    Still, astronomers have learned little about the nature of these exoplanets. Some are smaller than Jupiter, but most are larger. Some complete an orbit of their stars in months, while others take years. No one can tell if they are solid bodies, like Earth and Mars, or mainly gaseous spheres, like Jupiter and Saturn. No one can say if they have moons. So far, only one exoplanet, at 70 Virginis, seems to fall into the "habitable zone" -- the region, governed by a planet's distance from its star, where water is liquid rather than solid or gaseous. Even so, the prospects for life there are far from encouraging.

    "Theorists are now going a little bit crazy," Butler said. Observational astronomers seem to take a perverse delight in confounding their theoretically inclined colleagues, and the exoplanet findings have done that.

    Debate continues over whether some or all of these objects are true planets or brown dwarfs. It is more than a question of their sizes and positions in relation to their parent stars; fundamentally, it is a matter of how they were formed, which cannot yet be determined.

    Brown dwarfs form the same way a star does, by the collapse of a compressed interstellar cloud, but they never accumulate enough mass to support nuclear fusion in their interiors. It is conceivable that a brown dwarf may sometimes be as large as 80 Jupiter masses, but no one knows how small one can be. Only a few brown dwarf candidates have been identified, and only one confirmed.

    "If a new object orbiting a star is a gas-giant like Jupiter," Boss, of the Carnegie Institution, wrote in a recent issue of Physics Today, "then in analogy with our own solar system, we would expect that Earthlike planets also formed around that star. However, if a new object is a brown dwarf star, then it is unclear whether or not Earthlike planets also formed -- binary stars are thought to disrupt the planet formation process."

    Dr. David C. Black, director of the Lunar and Planetary Institute in Houston, is especially outspoken in his belief that most of the objects will prove to be brown dwarfs. One discovery, first thought to be an exoplanet, has been revealed to be a brown dwarf companion to the star Gliese 229.

    Black contends that it is "a bit hasty" to conclude that the objects around 51 Pegasi and 70 Virginis, in particular, are planets and not brown dwarfs. If these are Jupiter-class planets, he said, they should be in more circular orbits farther out from their central stars, as is the rule in the solar system.

    A study directed by Dr. Douglas N.C. Lin, of the University of California at Santa Cruz, yielded a possible explanation for finding actual planets in such cozy proximity to their stars: they could have formed farther out and then migrated inward to their observed positions. "In a million-year gravitational tug-of-war among the star, the planet and gas and dust in the rest of the disk," Lin said, "the planet spiraled slowly but relentlessly toward the star. Finally, inward and outward forces on the planet's orbit canceled each other out just before the star would have consumed the planet."

    Dr. Frederic A. Rasio, an astrophysicist at the Massachusetts Institute of Technology, approached the problem by assuming that relatively stable planetary systems like the Sun's, with its one dominant massive planet -- Jupiter -- may be extremely rare. Assume, instead, that many systems start with two Jupiter-size planets in fairly close proximity. In computer simulations, Rasio and a student, Eric B. Ford, showed that the strong gravitational interaction between the two planets could lead to one casting the other out of the entire system, while the survivor headed into a smaller orbit or sometimes crashed into the star.

    One implication of this model, Rasio noted, is that any smaller Earthlike planets "are likely to be lost as a result of the instability." They either escape from the system or collide with the central star. Such a violent history could thus preclude the evolution of advanced life in such systems. By the same token, having only one Jupiter may have been a necessary condition making the solar system sufficiently stable for the evolution of intelligent life.

    Black is skeptical of such explanations. "There is one other possibility, namely that planet hunters have discovered a new class of objects," he said.

    Only when astronomers discover more than one planet candidate around a single star will they learn if what they are seeing are indeed planets and if other planetary systems bear much resemblance to the Sun's family. For all theorists know now, the solar system could be, as Marcy said, "the odd bird in the zoo."

    The New Projects: 'Goldilocks Orbits' Are Just Right

    At a workshop where astronomers discussed new ideas for finding exoplanets, someone asked when would be the earliest anyone might begin detecting objects the size of Earth or Mars orbiting other stars at distances comparable to Earth's from the Sun. Scientists call this habitable zone the "Goldilocks orbit," where conditions should be neither too hot nor too cold but just right for life.

    Without hesitation, Dr. William J. Borucki, of the Ames Research Center in Mountain View, Calif., replied, "2001."

    Borucki's optimism was based on a proposal by him and his colleagues for sending a small satellite into space to focus its telescopic electronic camera on thousands of stars considered to be prime candidates for planetary formation. The instrument should be able to detect a faint drop in the light intensity of a star, suggesting that a planet is passing across its face.

    If the project wins federal approval in the next few months, the spacecraft, called Kepler, could be launched in March 2001; within a few weeks of its launching, it could detect some 2,400 new planets, including perhaps 100 that might have a size and solid surface like Earth's. After a few years, Borucki said, astronomers should have a good estimate of just how common Earthlike planets might be.

    A more ambitious concept is being developed by Dr. J. Roger P. Angel and Dr. Neville J. Woolf of the University of Arizona. They propose putting a large infrared telescope in deep space that would be capable of detecting the radiated heat of exoplanets. The emissions should also reveal the presence of any water, ozone or carbon dioxide on a planet, a combination that could well be evidence of life. But this project may have trouble winning support because of its technological complexity and a cost estimate of $2 billion.

    At the Jet Propulsion Laboratory, engineers are drawing up plans for two space missions they expect will be centerpieces in the National Aeronautics and Space Administration's new Origins Program, a major goal of which is the search for Earthlike exoplanets. The missions are to apply a new technology called optical interferometry for the first time in space science. A variation on the idea that the whole can be greater than the sum of its parts, interferometry involves several small telescopes, separated but operated in unison, to make observations that are as sharp as those of a single telescope that would be so powerful and big, several hundred feet wide, that it would be impossible to build or deploy.

    The first of these, the Space Interferometry Mission, which could be ready for launching in seven years, should give astronomers their most precise measure yet of the positions and motions of stars. Dr. Firouz Naderi, director of the program, put it this way: "If you were looking at the Moon with this system and there was an astronaut with a flashlight on the surface, you would be able to detect his passing the flashlight from one hand to the other."

    The second step, to be taken in about 10 years, would be a mission called the Terrestrial Planet Finder. In a more elaborate application of interferometry, the spacecraft would operate four 60-inch telescopes placed along a 240-foot-long truss. The infrared light they collect would be combined in a way to eliminate light from the star but magnify any radiation from a nearby planet. Astronomers predict that the project should not only be able to discover Earth-size planets but also study their atmospheres for signs of life.

    And that, ultimately, is the goal.

    "What we really want to find -- let's face it -- is an Earthlike planet, habitable and inhabited," Boss said. "Scientists don't talk about it, but they want the same thing. It would be fantastic to happen in our lifetime, but the life we find may be only algae. Life may be close to us in space, but not in time."

    The Prospects: Getting Ready To Cross a Line

    Marcy and Butler know that exoplanets resembling Earth, if they exist, are probably beyond the sensitivity of their technology.

    Even so, they are excited to be in on the beginning of what could be discoveries of Copernican magnitude.

    They and other scientists struggle to find the words to convey their awakening sense of the possible extrascientific meaning of these intimations of new worlds.

    After people accepted that the solar system is not the center of the cosmos, Naderi of the Jet Propulsion Laboratory said, "we drew a new line and said life is unique to us, at least intelligent life." He added: "I think we are beginning to chip away at that notion, too. Which would give us a warmer feeling? That we are something special in the universe? I would feel we are more special if we are part of a grander community."

    In pressing for an accelerated search for exoplanets, Daniel S. Goldin, the NASA Administrator, said that "no human endeavor or thought would be unchanged" by the discovery of some form of life on other planets.

    Boss predicted that the first images of another world like Earth, with the possibility of some form of life on it, "could shape what humans want to do for the next 1,000 years." People would want to explore it, at least with high-velocity robotic probes. They might send back close-up pictures to descendants living a millennium from now. "Hopefully, we won't burn anyone at the stake over it," he said.

    By dawn at the Lick Observatory, at the sound of a bird-song reveille, Marcy and Butler were able to reflect on their night's work.

    Of 20 stars examined, at least 2 looked promising, but it could take another year of observations to be sure.

    Not a bad ratio, though too soon for observing astronomers to relax and leave their discoveries of other worlds to the meditations of philosophers.

    "It's taken a long time getting good at this," Butler said. "Now I just want to find more planets."

    Other Places of Interest on the Web
  • Searching for Extrasolar Planets, the San Francisco State University Planet Research Project
  • The Extrasolar Planets Encyclopedia
  • Lick Observatory

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