A 1.3 Jupiter-mass planet orbits HD 217107 at only 0.07 AU. -- Artwork copyrighted by Lynette Cook
|Ten years ago there were none, now there are 33. Extrasolar planets are telling astronomers how planetary systems form and evolve.|
|By Geoff Marcy and Paul Butler|
The most remarkable tool astronomers use might not be a telescope nor a computer, but the human imagination. With it, we unconsciously yearn to stitch together a quilt of cosmic knowledge based on patchy research in astronomy, physics, chemistry, and biology. We want to learn how our brief lives are intertwined with the planets, stars, nebulae, and galaxies.
During the past year, our team and others have discovered a dozen or so planets orbiting other stars. Our team alone recently added five new planets to the list. These discoveries highlight a key element in our connection to the universe. The 33 extrasolar planets now known clearly tell us that the Milky Way Galaxy's 200 billion stars must harbor billions or possibly trillions of planets.
Surely there must be many lukewarm worlds that can host complex organic chemistry, leading to replicating molecules that compete with one another for energy and resources. The seeds of life are sewn in such environments, and there is no reason to doubt that increased biological complexity follows naturally.
But we're interested in more than just life. We'd like to know how planetary systems form and evolve. Despite our successes, our planet search is still in its infancy. Still, with 33 planets already in hand, we see a few general trends. Planet formation appears to be a chaotic process that often tosses Earth-size planets out of their systems entirely, leaving Jupiter-mass brutes behind in highly elongated (eccentric) orbits. Failed stars called brown dwarfs appear to be infrequent companions to solar-type stars. We have found the first full-fledged system of planets around a sunlike star, and other stars also show signs of multiple Jupiters. Looming on the horizon are techniques to find Earthlike planets, a quest bound to raise more puzzles for astronomy and biology.
The parade of extrasolar planets around normal stars began in 1995 when Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland discovered a planet at a distance of only 4.6 million miles, or 0.05 astronomical unit (AU), from the star 51 Pegasi (one AU is the average Earth-sun distance). This was followed shortly thereafter by our discovery of planets around 47 Ursae Majoris and 70 Virginis. The star-hugging orbit of the 51 Pegasi planet and the eccentric orbit of the 70 Virginis planet shook our myopic, heliocentric expectation that planetary systems would be nearly identical to ours. Instead, planets fill orbital niches of unimagined diversity.
We don't have the technology yet to image these planets; they're swamped by the glare of their host stars. Instead, we find planets by looking at how they yank their stars as the they go around in their orbits. The jerky motion of a star is revealed in the star's spectrum. As the star approaches Earth in response to the planet's gravity, its light is shifted toward the blue end of the spectrum. As the star recedes, its light is shifted toward the red. This subtle Doppler signature woven into the light of the parent star allows us to reconstruct the orbit and minimum mass of an otherwise hidden planet.
The first true planetary system around a solar-type star was found around Upsilon Andromedae. The star has at least three planets. -- Artwork copyrighted by Lynette Cook
The nearly circular orbits of the planets in our solar system led astronomers to expect that planets around other stars would reside in circular orbits too. After all, planets probably form in circular protoplanetary disks, such as the disks seen in the Orion Nebula. The gas and dust in these disks follow circular orbits, and friction within these disks would circularize the orbits of newly forming planets in much the same way that friction circularizes the flow of water going down a bathtub drain.
But most of the extrasolar planets found so far reside in highly eccentric orbits, not circular. Indeed, the 18 extrasolar planets with the largest orbits all reside in eccentric orbits. Most of those orbits are more than twice as elongated as the orbits of Earth, Jupiter, or Saturn.
Why are other planetary systems dominated by elliptical orbits rather than circular? The best clue comes from comets in our own solar system. Comets reside in orbits so elongated that they visit the inner solar system only rarely. But comets did not form in elliptical orbits. Rather, they formed in circular orbits in the protosolar disk. Comets were gravitationally flung into their present-day elliptical orbits when they ventured too close to planets, in much the same way spacecraft receive gravitational assists from planets.
We now suspect that most planets themselves engage in this slingshot activity, leaving them in disturbed, elliptical orbits. If two or more massive planets form in orbits a few AU apart, this fate is inevitable. One planet will be scattered inward, the other outward. If even one planet suffers this slingshot effect, it will likely travel close enough to neighboring planets to disturb them as well. This theory explains why the large majority of the extrasolar planets found to date reside in noncircular orbits.
Unfortunately, Jupiter-mass planets toss Earth-mass planets around like Tinkertoys in a train wreck. The resulting chaos may result in some planets being gravitationally ejected from the planetary system altogether, leaving only the most massive survivors behind. Our Galaxy must be filled with trillions of Earth-size rogue planets - dark, rocky hulks wandering aimlessly through interstellar space.
The predominance of elliptical orbits implies that planetary systems with circular orbits may be the exception rather than the norm. Apparently, our nine planets were just far enough apart and low enough in mass to avoid this chaos. The nine planets do perturb one another, but not enough to cause close passages. The planetary house of cards that we call our solar system may be one of the rare systems that remains just barely stable (see "Crack in the Clockwork," May 1998).
If our solar system is unusual in its circular orbits, we humans would seem to be extraordinarily lucky to be here. After all, the circular orbit of Earth keeps solar heating nearly constant, minimizing temperature fluctuations. Perhaps biological evolution would not have proceeded to intelligence if Earth's temperature were fluctuating wildly. It may be that Darwinian evolution toward complex organisms is enhanced by relatively quiescent climates enabled by circular orbits. If so, we owe our existence to Earth's stable orbit.
Ultimately we would like to directly "see" these planets and take their spectra. But until technology catches up with our dreams, there is still much information to be gleaned from a statistical analysis of our current harvest of planets.
We use a "mass histogram" to categorize the planets that have been discovered. We create a mass histogram by putting each planet in a "bin" according to its mass. The number of planets in each mass-bin indicates whether higher- or lower-mass planets are more common. The 29 extrasolar planets in the mass histogram on page 45 represent all the planets found around normal stars (the other four planets orbit pulsars). Four groups in the United States and Europe have made these discoveries: our team, the Swiss team led by Michel Mayor, a team led by Robert Noyes of the Smithsonian Astrophysical Observatory and Tim Brown of the National Center for Atmospheric Research, and a team led by Bill Cochran and Artie Hatzes of the University of Texas at Austin and Martin Kürster of the European Southern Observatory.
All the surveys are primarily sensitive to Jupiter-mass planets with orbital periods of three years or less. Massive companions are the easiest to detect because they tug harder on their host stars. Our surveys can find virtually all stellar companions with three or more Jupiter-masses and orbital periods of three years or less. But we could easily overlook planets of one Jupiter-mass or less, and we can't detect Neptune-mass planets. This selection effect strongly favors the discovery of planets with five Jupiter-masses or more relative to those of one Jupiter-mass. But the mass histogram clearly shows that we are finding more of the lower-mass planets. This implies that the planets discovered to date represent just the tip of the planetary iceberg and that as our techniques improve, we will find many more lower-mass planets.
Remarkably, all the companions found by our team's velocity survey are less than 10 Jupiter-masses. When our Lick Observatory planet search around 107 sunlike stars began in 1987, we expected to find both planets and brown dwarfs. Brown dwarfs are failed stars of 10 to 80 Jupiter-masses (see "The Little Stars that Couldn't," August 1999). Because brown dwarfs have higher masses than planets, brown dwarfs are much easier to detect. But during the past 12 years of our Lick survey, we haven't found a single brown dwarf companion around any of our 107 stars. This proves that only about one in 200 solar-type stars have brown dwarf companions within five AU.
The solar system's rich diversity of planets displays an astonishing range of environments, from pancake-shaped volcanic domes on Venus to the frozen rings of Saturn. But to an impartial observer, the solar system essentially consists of the sun, Jupiter, and some leftover debris. If alien astronomers are looking at the solar system with technologies similar to our own, they will only detect the sun, Jupiter, and maybe Saturn. We therefore shouldn't be surprised that the first rush of discoveries involved single planets.
The discovery of multiple-planet systems requires a combination of precision, patience, and luck. Reliable detection of an extrasolar planet by our Doppler technique requires that observations span a time covering at least two orbits. Because our survey is only 12 years old, we haven't been searching long enough to detect planets with orbital periods longer than six years. We have only been searching long enough to detect those systems with multiple Jupiter-mass planets orbiting within 4.5 AU of their parent stars.
Still, last year our team and Robert Noyes's team codiscovered the first true extrasolar planetary system, around the solar-type star Upsilon Andromedae just 44 light-years away. Three planets orbit the star at distances of 0.06, 0.82, and 2.4 AU respectively. Our team originally discovered the innermost planet in June 1996 as part of our Lick survey. This planet has a scorching hot 4.6-day orbit and a minimum mass of 0.7 Jupiter. Like the four other 51 Pegasi-like planets with orbital periods of less than five days, the orbit is circular because of tidal interactions with the parent star.
Because our data indicated that there were more planets in the system, our group and Noyes's group independently monitored Upsilon Andromedae as often as possible. By early 1999, the two groups had collected more than 140 observations. This enabled us to untangle the complicated pattern of additional velocities, which revealed not one but two additional planets. The middle planet is a 2-Jupiter-mass planet in a 241-day eccentric orbit, while the outermost planet is a 4-Jupiter-mass planet in a 3.5-year eccentric orbit.
With the possible exception of the companion to 47 Ursae Majoris, all the extrasolar planets found to date are either "51 Peg-like" or eccentric. In the Upsilon Andromedae system we have both types of planets, perhaps making this a Rosetta stone that will allow us to understand the formation and evolution of these alien planetary systems. At present we have virtually no idea how the Upsilon Andromedae system formed with its current architecture. As our planet searches improve their precision, time baseline, and sample size, we will find many more multiple planet systems.
Before 1995, our primary goal was simply to find an extrasolar planet. Now, with 33 planets known, we're pushing the goal posts back. We want to find solar system analogs, marked by Jupiter-mass planets in distant (5 AU) circular orbits. We also want to find Saturn- and Neptune-mass planets, and ultimately, Earth-mass planets. We want to broadly understand the formation and evolution of planetary systems. We also want to know if our solar system is a common type of planetary system or some quirk of star formation (implying that advanced life might be rare in the Galaxy).
To answer these questions we will need to extend our current surveys and develop new technologies. Over the next 10 years our tried-and-true Doppler technique will continue to provide the bulk of extrasolar planet discoveries. Within the last three years both the Swiss group and our group have expanded our surveys to about 1,000 stars, with telescopes covering both the northern and southern hemispheres. By 2010 these surveys will provide the first hints about the fraction of planetary systems similar to the solar system, with giant planets orbiting out to 5 AU and beyond.
Over the next decade, optical and near-infrared interferometry will finally become a reality, with systems operating at the twin 10-meter Keck Telescopes and the European Southern Observatory's Very Large Telescope (VLT). By combining beams of light from several 8- to 10-meter telescopes, these interferometers will be able to measure the position of stars 10 to 100 times more precisely than the current state-of-the-art techniques. This level of precision is required to detect the wobble in a star's position induced by an orbiting Saturn- or Neptune-mass planet.
Making interferometric observations above the blurring effect of Earth's atmosphere should allow even more precise measurements. NASA will launch SIM (Space Interferometry Mission) between 2006 and 2008. SIM will be a five-year mission capable of detecting planets as small as 10 Earth-masses with orbital periods of five years or less.
Advancing technology will eventually put our Doppler method out of business. But our velocity surveys will point the way to missions such as NASA's proposed Terrestrial Planet Finder, which will directly image both Jupiter-size and Earth-size extrasolar planets and look for the spectral signature of life (see "Looking for Life," December 1999). The time is not far off when we can replace the artists' conceptions of extrasolar planets with photographs of the planets themselves and gaze upon landscapes as bizarre and unexpected as those on our nine planets.
Geoff Marcy is an astronomer at the University of California at Berkeley and San Francisco State University. Paul Butler is an astronomer at the Carnegie Institution in Washington, D.C. Marcy and Butler lead a team that has discovered or codiscovered 19 extra-solar planets, more than any other group.
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