Image: San Francisco State University
The great voyages of discovery shrank our planet from a fearsome void to a familiar orb. More recently, explorations of the planets, moons, comets and asteroids in the solar system have reduced our turf in the cosmos to a relatively cozy corner.
Now it's the Milky Way's turn. The announcement on April 14 that a nearby star known as Upsilon Andromedae, quite like our Sol, sports a trio of planets, carries with it the long-sought conclusion that our tiny neighborhood may not be a cosmic quirk--there are other solar systems. As a result, the galaxy has grown a bit smaller, but a new vista, comparing ourselves to these significant others--call it comparative solar systems--has opened to us.
The discovery of the first multiple planet system ever found around a normal star, other than the nine planets in our solar system, was revealed in a joint press conference in San Francisco by two independent research teams. "We are witnessing the emergence of a new era in human exploration," said Geoffrey W. Marcy , an astronomer from San Francisco State University, whose team has identified most of the 20 extrasolar planets found so far. "We are embarking on a reconnaissance of planets around other stars in our own Milky Way Galaxy."
It was a group headed by Marcy and his colleague R. Paul Butler, now a staff astronomer at the Anglo-Australian Observatory, that first found evidence of an extrasolar planet orbiting Upsilon Andromedae--a star 44 light-years away from Earth that is roughly three billion years old, two thirds the age of the sun--in 1996. Using the telescope at the Lick Observatory near San Jose, Calif., and a technique known as Doppler spectroscopy, which detects planets orbiting distant stars by their gravitational effects on the host star's velocity, they reported that an object with three quarters of the mass of Jupiter was zipping around the star every 4.6 Earth-days in the extremely close orbit of only 0.06 astronomical units (the distance from Earth to the sun, or about 93 million miles).
But when the astronomers looked at the data over a longer period of time, they observed that there were a large number of data points that did not fit the predicted curve. This "noise" indicated that there could be an additional object circling Upsilon Andromedae. The San Francisco State researchers notified other astronomers and set to work to find the answer by analyzing 11 years of spectrographic data collected by Lick Observatory. Meanwhile a group at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the High Altitude Observatory in Boulder, Colo., had been studying Upsilon Andromedae for more than four years with data collected by the Smithsonian's Whipple Observatory near Tucson, Ariz.
Both groups drew the same conclusion--there are three huge, Jupiter-like planets orbiting Upsilon Andromedae. The middle planet, designated c, contains at least twice the mass of Jupiter and takes 242 days to orbit the star once. It resides approximately 0.83 AU from the star, similar to the orbital distance of Venus. The outermost planet, d, has a mass of at least four Jupiters and completes one orbit every 3.5 to 4 years, placing it 2.5 AU from the star. The two outer planets have elliptical orbits, a characteristic of nine other extrasolar planets in distant orbits around their stars. "Now we have a multiple system, and maybe a Rosetta stone to understand a lot of the weird planets that we're finding," Butler says.
Although the researchers were unsure that the signals they were seeing were actually planets when only one was detected around a star, the finding of multiple objects and the two independent confirmations has them convinced that these are really planets. "Here are two teams, using two completely different sets of apparatus, different software, a similar technique, it's true, but basically two completely independent data sets, that show you the same thing," says Timothy Brown of the High Altitude Observatory. "Imagine giving your taxes to two different accountants and having them come up with the same numbers at the end."
The two teams have submitted a joint paper to the Astrophysical Journal, but they raise far more questions than they answer. "Is our own solar system unusual in some way? Is the architecture of our solar system some sort of cosmic quirk of nature? We don't know," Marcy says. "The most moving question is whether or not our own Earth, with its lukewarm temperatures, allowing for water in liquid form, is a common type of planet."
Indeed, the most intriguing feature of the new findings is the way they challenge current ideas about the formation of planets. Based on astronomers' conclusions from looking at our solar system, massive planets composed largely of gas, such as Jupiter and Saturn, would form only in the cold outer reaches of a solar system. But these gas giants seem to exist perilously close to the inferno of the host star.
The planets may have formed close to the host star or, like balls on a billiard table, may have scattered off each other, migrating into their current orbits from a more distant place of origin. The discovery of this multiple planet system suggests a new paradigm for planet formation, in which many small seed planets known as planetesimals might develop in the disk of matter surrounding a star. Those planets that grow fastest would engage in a gravitational tug of war that weeds out some of the smaller worlds and determines which planets ultimately remain in orbit. "The Upsilon Andromedae system suggests that gravitational interactions between Jupiter-mass planets can play a powerful role in sculpting solar systems," Butler says.
Moreover, the extrasolar Jupiters tend to have elliptical orbits, not the neat circular orbits of our system. It is unlikely that an Earth-like planet could exist in the Upsilon Andromedae system--the giant Jupiter-type planet crossing its orbit would tear it apart. "I am mystified at how such a system of Jupiter-like planets might have been created," Marcy says.
So what are the prospects for Earth-like planets? Marcy, for one, is undaunted. "Of all the stars that we've surveyed, only about 5 percent of them harbor these marauding Jupiters," he says. "So it's our nondetections that give us the best hope--95 percent of all the stars in our galaxy, at least plausibly, could harbor Earths in stable orbits."
Our Milky Way Galaxy contains 200 billion stars. If even a small percentage of those planetary systems do harbor Earth-like planets, then our Milky Way Galaxy surely contains billions of Earth-like planets, and some of them, perhaps, are at just the right distance from their host star to be warmed up to lukewarm temperatures where the water could be in liquid form and perhaps life could flourish.
The answer will have to wait. As sensitive as current spectrographic systems may be, the cannot detect the telltale tugs of small, Earth-like planets. And even those Jovian giants could harbor moons, like our Jupiter's Europa, whose water might be the biological petri dish of distant solar systems.