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Planet hunter from Pitt joins observers in Texas in mapping new object

Sunday, August 13, 2000

By Byron Spice, Science Editor, Post-Gazette

By most counts, the planet that astronomers found orbiting the nearby star Epsilon Eridani would be the 41st planet discovered outside our solar system. But George Gatewood argues it actually may be the first.

Gatewood, a veteran planet hunter at the University of Pittsburgh, maintains that the extrasolar object announced last week is the first one that humans would recognize as a planet if they were to zoom by it.

"This planet would look just like Jupiter," said Gatewood, who has been studying Epsilon Eridani since 1988. He is collaborating with astronomers at the University of Texas, who announced the discovery Monday at a meeting in England.

Moreover, it takes about seven years to orbit its star -- Jupiter takes 12 years to orbit the sun -- suggesting the planetary system is spread out enough to accommodate other planets too.

"It might be the landmark that tells you an Earth might be there," he added.

Just a couple of days ago, Gatewood completed his own calculations, determining the planet has a mass about 1.2 times that of Jupiter.

Astronomers previously have been able only to estimate the minimum mass of extrasolar planets.

But Gatewood was able to determine the actual mass of this planet, because the different measurements made by him and the Texas astronomers complemented each other and allowed him to plot the star's three-dimensional motion.

The motion of stars is the primary tool astronomers use to find planets outside the solar system. Planets are too dim to be discovered by telescopes, but the tug of their gravity causes the stars they orbit to wobble.

By measuring the amount and the rate of the wobble, astronomers can deduce the presence of one or more planets.

Gatewood has been observing Epsilon Eridani with the Allegheny Observatory's 30-inch Thaw Memorial Telescope since 1988.

A star just a bit smaller than our sun, Epsilon Eridani is the ninth-nearest star system to Earth, just 10.5 light-years away, and is a convenient star to study in search of planets.

He could see the star wobbling slightly, but the movement was so slight that he could not be sure that a planet was there.

Artie Hatzes and William Cochran at the McDonald Observatory in Texas also noticed the wobble with their 2.7-meter telescope. By combining their findings with earlier observations by Gordon Walker of the Canada-France-Hawaii Observatory, the Allegheny Observatory and others, they were able to rule out the possibility that the wobble was caused by another star. Something -- presumably a planet no smaller than Jupiter -- seemed to be orbiting the star every seven years.

The McDonald Observatory had measured the star's radial velocity -- its motion toward and away from Earth. The Allegheny Observatory measures angular position -- changes in the star's position on the plane of the sky.

By combining the two measurements, Gatewood was able to plot the three-dimensional motion of the star and calculate the planet's mass.

He continues to study Epsilon Eridani. Though budget cutbacks forced Pitt to lay off the observatory's chief observer and chief technician this spring, support from the university and a small endowment allow observations every clear night. Epsilon Eridani is a bright star, now visible in the southern sky from early morning through dawn.

Most of the other extrasolar planets discovered to date are at least as big as Jupiter and some are much more massive, Gatewood said. These are sometimes called "hot Jupiters," because they have orbital periods measured in days, suggesting they orbit very near to their stars. These objects are red-hot, with temperatures akin to a blast furnace.

Gatewood said he has difficulty calling such an object a planet. "I think the word planet has to denote some sense of being hospitable," he said.

Both the size and the length of the orbit of the newly discovered object at Epsilon Eridani suggest it is probably a real planet, Gatewood said. Further wobbling of the star suggests other planets may be there as well, he noted, and the Texas astronomers have found an irregular dust ring around the star that they think may be due to a second, undiscovered planet.

David Black, director of the Lunar and Planetary Institute in Houston, Texas, said he's not yet convinced a planet is orbiting the star. The orbit is very oblong, he noted, which more closely resembles the orbit of a star-like body than a planet.

Gatewood's calculation of the new object's mass could prove significant, however.

Because Epsilon Eridani is so close by, it should be possible to study the object in several ways, including direct observation, Black said.

These findings could provide important clues about how the object formed -- whether it emerged from a massive cloud of gas like a star, or developed bit by bit like a planet. Some scientists have argued that nothing with a mass as small as a Jupiter could emerge from the process of stellar formation.

What is learned from Epsilon Eridani could either confirm or cast doubt on those theories, Black said, and thus could affect how astronomers interpret future claims about extrasolar planets.

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