For the first time, scientists have found more than one planet orbiting a star other than our own sun.
The discovery of three massive planets revolving around Upsilon Andromedae, a sun-like star 44 light years from Earth, will have a profound impact on how astronomers view the heavens.
Indeed, the fact that these three planets - each as large or larger than Jupiter - are all relatively close to the star they orbit is forcing scientists to come up with new models for how planets are created.
But perhaps more intriguing, their mere existence implies that many other stars have more than one orbiting planet. And although these three planets are unlikely to harbor life, the discovery supports the theory that another Earth-like planet with conditions conducive to life may exist.
"What makes it a milestone is that this is the first time we have found a planetary system around a star," says Alan Boss, an astrophysicist at the Carnegie Institution in Washington. "That's important because it helps buttress the belief that most of the objects that have been found so far really are planets as opposed to something else."
Moreover, the findings, announced yesterday, are further evidence that planet-finding has become one of the most dynamic fields in astronomy. Considered a fringe science not long ago, researchers have now found 17 planets outside our solar system since 1995 with the help of new telescopes and better technology.
The latest discovery, made by two teams of scientists, came out of a survey of 107 stars in the Milky Way galaxy - and years of minute observations. Using a technique called radial velocity measurements, astronomers watched for a telltale wobble in a star's orbit caused by the gravitational tug of an orbiting body.
It was exactly such a wobble in Upsilon Andromedae's orbit that caught the attention of San Francisco State University astronomers Geoff Marcy, Paul Butler, and Debra Fischer.
They located the closest planet orbiting Upsilon Andromedae in 1996 as part of a handful of major planet discoveries. But their calculations indicated that something else was acting on the bright star, which is sometimes visible from Earth.
Further calculations pointed to the existence of a second and third planet, a finding that was independently confirmed by another group of scientists who had also been studying the star.
To be sure, the discovery is a long way from extraterrestrial life. But it is a steady step along the way to proving that the heavens are dotted with planets. "It suggests that planet formation happens often, happens rapidly, and probably happens around every star like our sun," says Dr. Fischer. "This represents a profound shift in our model of how planets form."
A freak science?
Even a decade ago a finding such as Upsilon Andromedae's planets would have been unlikely, if not impossible. The concept of radial velocity was theoretical and had never been put to practice. Also, few scientists wanted to risk their careers on what was considered a freak science.
"There was a nagging suspicion that anyone who wanted to find out about the planets orbiting other stars was really deep down interested in what the prospects of meeting an alien walking down the street were," says Tim Brown, a senior scientist at the National Center for Atmospheric Research. He was involved in the recent discovery.
A decade later, more powerful computers have made crunching astronomical data easier and enabled a whole array of complex instruments to intricately scan the skies. A new generation of telescopes has made it easier for astronomers to search for planets and planetary systems.
Yet the Upsilon Andromedae discovery has also raised as many questions as it has answered. Astrophysicists, who are still trying to make sense of other recent planet discoveries, are baffled that three massive planets could be orbiting a star so close to each other and to the star.
"The usual picture is that gas giant planets can only form at least four astronomical units [372 million miles] away from a star, where temperatures are low enough for ice to condense and begin the process of planet formation," says Dr. Brown. "But all three ... planets around Upsilon Andromedae now reside inside this theoretical ice boundary."
This discrepancy has further weakened traditional assumptions about planet formation. They held that solid planets formed in clumps as the gas clouds circling a star cooled.
Now scientists are groping toward a new model in which many small planets, known as planetesimals, develop in the gaseous disks that surround a star. According to this theory, faster-growing planets would elbow some of the slower-growing planets out of orbit and subsume others through collisions.
There may, however, be even more ways to form planets. "Nature is very diverse in the type of solar systems that can be formed," says William Cochran, an astronomer at the University of Texas at Austin. "We should not expect things to look like our solar system. We should expect the unexpected."
These types of expectations will likely broaden the scope of the planet search, bringing stars and stellar systems unlike our own under research focus. "The kinds of stars that have been examined right now are pretty much like the sun," says Brown. "It would be very interesting to push the envelope ... and see if stars that are bigger and hotter ... or ... smaller and cooler than the sun have planets."
Still, scientists say they are several years away from being able to detect Earth-like planets. Researchers are working on a project to help the process along, but the actual detection of light from an Earth-like planet is more than a decade away.
For now, scientists must be content hunting bigger planets. "When we find J planets like our Jupiter in a similar orbit, there is some hope that there will be an Earth nearby. That's sort of the Holy Grail," says Dr. Boss.