The Sciences

The Planet at 51 Peg

By Jeffrey WintersJan 1, 1996 6:00 AM


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Ever since astronomers realized that stars were like the sun, just farther removed, they have wondered whether these other suns might have their own planets--perhaps inhabited ones. So when astronomers Michel Mayor and Didier Queloz of the University of Geneva quietly reported in October that they had detected a Jupiter-size planet circling a fairly mediocre star 57 light-years away, the news did not stay quiet long. It spread through the astronomical community like a tidal wave--one that spilled right onto the likes of Nightline and the Today show. The enthusiasm was more than justified, says astronomer Geoff Marcy of San Francisco State University, who has himself spent much of the past decade trying to locate extrasolar planets--and who quickly confirmed the Swiss find. This is a discovery that any human being who is curious about the universe should rejoice in, Marcy says. Everyone can now look up in the night sky and look at the stars and say, ‘There are planets out there.’

The planet that Mayor and Queloz found orbiting Star 51 in the constellation Pegasus (51 Peg to its friends) is not the first extrasolar planet--just the first one discovered around an ordinary star like the sun. In 1994 astronomers reported convincing evidence of two or three planets orbiting a star in Virgo. But that star was a pulsar--the dense spinning remnant of an exploded star--which meant that it was probably raining lethal radiation on its companions. That put a damper on all dreams of finding extraterrestrial life.

Finding a planet around a homier star, though, is if anything even harder than finding one around a pulsar. In neither case can astronomers actually hope to see the thing. It would be too far away, too faint, and in the case of a sunlike star, too swamped by the visible glare of the star itself. The only way to detect an extrasolar planet is from the way its gravity pulls the star ever so slightly back and forth. In the case of a pulsar, this induces minute changes in the timing of the star’s otherwise regular pulses of radiation. If the star is sunlike, astronomers have to look for the subtle compressing and stretching of its light as it is pulled first toward, then away from Earth.

Marcy and others have spent many years doing just that. Mayor got into the planet-hunting race just a couple of years ago--but with a new and more sensitive type of spectrometer that immediately made a contender of his relatively small telescope at the Haute-Provence Observatory in southern France. By then he had already surveyed a large number of nearby stars, weeding out the ones that either were unlikely to have planets (because they had companion stars instead) or whose planets would be undetectable (because the star’s light output wasn’t steady enough). That left him with a list of stable, single, middle-aged stars like the sun--140 of them. If you are buying some tickets to the lottery, Mayor says, the more you buy, the more you increase your chances. His diligent survey paid off: 51 Peg was on the list. Other planet search parties had passed it over because it had been misidentified in star catalogs as a subgiant--a star whose pulsations would obscure the wobble caused by a planet.

When Mayor and Queloz examined the spectrum of 51 Peg, they saw a large, fast wiggle. At the start we were extremely suspicious and looking for different explanations, says Mayor. The first reaction was not to say, ‘Oh! We have a planet!’ At the start you say, ‘Oh, something is wrong.’ It’s only after weeks or months that you start to be convinced.

By early October, he and Queloz were convinced enough to announce their find at a meeting in Florence. Hearing reports of that talk, Marcy and his colleague Paul Butler went to Lick Observatory near San Jose to look at 51 Peg for the first time. It took them only a weekend to confirm the discovery: the wobble in the spectrum was so large as to be unmistakable. The field has suffered from false claims and hopes being raised and then dashed, Marcy says. In this case there is no question.

So a second solar system seems to have been found at last--but not a second haven for life. The planet orbiting 51 Peg has a mass between half and twice that of Jupiter, yet its orbit is only one-sixth the size of Mercury’s; it circles the star every four days. Its large mass and proximity to 51 Peg explain why its pull on the star is so large and readily detectable. At that distance from its sun, though, the planet must sizzle at more than 1800 degrees Fahrenheit--hot enough to glow like a toaster coil.

No one knows yet whether the planet has always been so close to the star, or even what it is made of. Some theorists think it is essentially a huge, largely molten rock. Others think the planet is a gas giant, much like Jupiter, that formed 100 times farther away and was bounced toward 51 Peg through a near-collision with a yet-to-be-confirmed second planet or companion star. Or it may be a companion brown dwarf star (see box above) that has been whittled down to planet size by intense blasts from its big brother.

With so many questions unanswered, astronomers will be falling over one another to get data on the new planet. They’ll also keep hunting planets around other stars--ideally ones that are in more comfortable orbits. I’m not happy yet, Marcy says. What we really want is to find a planet that is a little more similar to the planets in our own solar system. We’re human. We’re really looking for ourselves in these planets.

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