Distant Planets

In the last five years, the idea of finding planets orbiting other stars has gone from a science-fiction fantasy to a reality frequently...

In the last five years,
the idea of finding planets orbiting other stars has gone from a science-fiction fantasy to a reality frequently reported in the daily paper (see "Orbiting Other Suns," September-October 1998, page 16). But until last fall, these "discoveries" of extrasolar planets were all based on indirect detection: astronomers surmised that "wobbles" in some stars' orbits were caused by the gravitational influence of a large, nearby planet. Between 1995 and 1999, says David Charbonneau, a fourth-year Harvard doctoral student in astronomy, the existence of 35 of these planets was announced. But scientists knew that a few of these planets should have orbits whose orientation with respect to Earth would make them directly detectable--and none had been. Some researchers, says Charbonneau, noting that "the light coming to Earth from distant stars can be intrinsically variable, published papers questioning whether these stars really were in a gravitational dance with invisible planets." The doubts were put to rest when Charbonneau made the first direct detection ever of an extrasolar planet--bigger than Jupiter--orbiting the star HD209458.


Last September, Harvard graduate student David Charbonneau became the first person ever to directly detect an extrasolar planet. The illustration is artist Lynette Cook's imagined rendering of the moment when the giant planet transited its parent star, HD209458. Illustration ©1999 Lynette Cook

Originally he had hoped to observe the starlight reflected from a different large, close-in planet just as it passed behind its parent star. This light, he hypothesized, would be added to the light from the star itself, creating a detectable increase in the total light coming from the star. Charbonneau searched for such a reflected signal in data obtained from the Keck Telescope in Hawaii by his research supervisor, professor of astronomy Robert W. Noyes. However, he was able to ascertain only that the attendant planet was either smaller than Jupiter or less reflective.

Undeterred, he tried a different approach. "The planets we've been studying are dissimilar to those in our own solar system," says Charbonneau. "They are big, and very close to the star. Because of this, one in 10 "should be lined up so that it will cause a mini-eclipse as it passes directly in front of its star," he says. An earthbound observer could hope to measure a dip in the star's apparent brightness during the few hours that a Jupiter-size or larger planet passed in front of it.

To seek out such transits, Charbonneau drove to Colorado to work with a special telescope built by astronomer Timothy Brown of the National Center for Atmospheric Research. "Appropriately called STARE," says Noyes, "it is designed to 'stare' at a patch of sky with many thousands of stars for nights on end, under the expectation that a few hundred of them may harbor close-in giant planets, and that a few of these may have orbits oriented just right to transit their stars. Just as Dave set off to Colorado, David Latham of the Harvard-Smithsonian Center for Astrophysics discovered yet another close-in, giant planet orbiting the star HD209458." Latham passed the information about this new discovery to Charbonneau, who used the STARE telescope to look, not at its intended survey star field, but at the piece of sky contain-ing HD209458. The result is history.

"It is hard to overstate the importance of this discovery," Noyes asserts. "Seeing the dip in brightness due to the transit proved that all the recently discovered objects called extrasolar 'planets' really are giant planets, rather like Jupiter. From the details of the transit, it was possible to determine not only the planet's radius, but also its mass, and hence its density and surface gravity. This is a watershed event that will change the face of research into extrasolar planets forever."

Charbonneau, who has been granted director's discretionary time on the Hubble space telescope to study the planet further and to search for moons, hopes to develop techniques to determine the atmospheric composition of distant planets by analyzing the spectrum of light passing through their atmospheres. This could lead to calculations of their dayside surface temperatures.

Asked about the ramifications of his discovery in the search for planets like Earth, Charbonneau sounds a cautionary note. "We don't know at all what the incidence of earth-like planets is," he says. Because such planets would be too small and too far from their stars, today's telescopes aren't powerful enough to detect them--but that could change. A decade from now, when space telescopes such as the proposed Kepler Mission go on-line, scientists could well announce the discovery, using Charbonneau's method, of a place that looks a lot like home.

Read more articles by: Jonathan Shaw

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