Strange New Worlds

Natural History, Sept, 1999 by Richard Panek

Astronomers are finding more and more Jupiter--class planets orbiting astonishingly close to stars.

Are we alone? Forget for the moment the question of life on other planets. More to the point, are there other planets like the ones in our own solar system?

The idea that we might have something in common with the multitude of stars in the night sky has been around for as long as humans have been able to conceive of stars as individual suns. But now we know we are not alone, at least in terms of belonging to a solar system.

Future historians of astronomy no doubt will recall the 1990s as the beginning of a golden age of planet detection, starting in 1995 with the discovery of a planet orbiting the Sun-like star 51 Pegasi. By the following year, the discovery of extrasolar planets had become so commonplace that Astronomy magazine ran an article with the memorable headline "Ho, Hum, More New Planets."

But this past April, astronomers took planet detection to a new level with the discovery of three planets orbiting Upsilon Andromedae. That's two more than in any other known planetary system, with the exception of our own.

In trying to place this news in a historical context, it might help to remember that the indisputable identification of Earth itself as a planet dates only to Galileo's day. The word "planet" (from the Greek for "wanderer") referred to objects in the night sky that moved against the backdrop of the so-called fixed stars. While we now associate the Copernican revolution primarily with the displacement of Earth from the center of the universe, it's worth remembering where Earth wound up. It, too, became a wanderer.

More than a thousand years earlier, Epicurus had posited the existence of other Earths, with other civilizations, and in the sixteenth century Giordano Bruno had made roughly the same argument, eventually dying at the stake for his "heretical" theories a decade before the invention of the telescope. But only after Galileo had looked through one of the first optic tubes and seen that the planets bore the shape of disks Oust like the Moon) and that the Moon's surface had mountains and valleys (just like the Earth) could the idea that Earth was a planet advance from fanciful speculation to sound foundation. As the young Bernard Le Bovier de Fontenelle wrote in 1686 in A Plurality of Worlds, "Our Sun hath planets that he enlightens, for why therefore should not each fixed star have planets that he enlightens?"

Still, it's one thing to assume, however reasonably, the existence of other planets. It's quite another to prove it. And until you do so, you can't really be certain that we're not somehow an anomaly. When Michael Mayor and Didier Queloz, of the Geneva Observatory, went searching for a first example of a planet outside our solar system, what they found in 1995 (in orbit around 51 Pegasi) matched no known model of a planet. According to their calculations, it was roughly half the mass of Jupiter, yet it was traveling in a pattern that, within our own solar system, would place it in an orbit between Mercury and the Sun. That violated the current model not only of what a planet is but of what a solar system is.

As astronomers have found more and more Jupiter-class planets in astonishingly close proximity to stars, they have begun rethinking such basic definitions. In 1991, four years before the discovery of the planet orbiting 51 Pegasi, researchers had detected three terrestrial companions around the neutron star PSR 1257 12, a former giant sun that at some point had gone supernova and collapsed, leaving behind only a piece of matter that rotated every 6.2 milliseconds--hardly a Sun-like object. Yet were those the first "real" extrasolar planets? Did PSR 1257 12 host the first "real" planetary system around a star other than the Sun?

Even our own ancient definition of "planet" is up for grabs. As premier planet-hunters Geoffrey W. Marcy and R. Paul Butler pointed out last year in an article published in the Annual Review of Astronomy .and Astrophysics, "it seems dangerous to lump Mercury Ia terrestrial mass] and Jupiter [a gas giant] into one class of astrophysical object."

At any rate, we skywatchers can content ourselves with singling out those Sun-like stars that we know are at least somewhat like our own. Upsilon Andromedae, for example, has a magnitude of 4.1, meaning that for an observer in a place sufficiently free of light pollution, it's visible to the naked eye. In September the constellation Andromeda is in the northeast sky. With the help of the photograph (opposite page), you can find Upsilon Andromedae about midway between two Andromeda stars of magnitude 2: Beta (or Mirach) and Gamma (or Almach).

You won't see the orbiting planets themselves, of course; even astronomers working with the most powerful instruments can detect them only through their gravitational influence on the host star. But the sight of Upsilon Andromedae may inspire you to ask the same questions Marcy did during the press conference heralding the detection of the planetary system around this star: "How does our solar system compare to other planetary systems? Is our own solar system unusual in some way? Is it a cosmic quirk of nature? Is our own Earth a common type of planet?"