Other Planets, Other Suns: Astronomers Say Star’s Wobble Tells a Story

Strong new evidence that planets circle stars other than our own sun was presented Wednesday by a team from the Smithsonian Astrophysical Laboratory.

Astronomer David W. Latham told a meeting of the International Astronomical Union in Baltimore that an apparent planet, which is at least 10 times as massive as Jupiter, circles a star named HD 114762 some 90 light years from Earth. The gas giant circles HD 114762 every 84 days, which means that it is about as far from its star as Mercury is from the Sun. It would thus be immensely hot and totally inhospitable to any form of known life.

Many astronomers believe that a large number of stars, if not the majority, are circled by planetary systems like our solar system.

But the existence of such planets has been difficult to prove, because they would typically be too small and too dark to be seen with telescopes on the Earth’s surface. So scientists must infer their existence by looking for possible irregularities in a star’s movement.

Strictly speaking, planets do not orbit stars. The planet and the star both orbit around their center of mass, but since the star is much bigger than the planet, its orbit is quite small.

To an observer on Earth viewing the star through a telescope, it would appear that the star’s velocity through space increases and decreases on a regular basis as it is tugged back and forth by its planets.

This is how Latham and his colleagues at the Smithsonian detected the planet. Over a seven-year period, they observed an almost imperceptible wobble in the star’s motion.

“At first we said, ‘Oh gee, we’ve got a problem,’ ” he said. But in subsequent observations they confirmed a distinct regularity to the wobbles, indicating that they were really seeing evidence of a planet.

“Our data are almost too good to be true,” Latham said in a telephone interview. The observations have been confirmed independently by astronomer Michael Mayor and his colleagues at the Geneva Observatory in Switzerland, Latham said.

Latham’s observations are “very exciting,” said astronomer Richard Terrile of the Jet Propulsion Laboratory, “but I am always a little dubious when people claim that they have found planets.”

Terrile and others cautioned that there is a small possibility that Latham has actually observed a rare object, intermediate in size between a planet and a star, called a brown dwarf. A brown dwarf is, in essence, a star that is not quite big enough for its fusion fire to ignite.

But even that would be a very significant observation, he added. “The biggest unanswered question in astronomy is how common is the situation we have in our solar system?” he said. Whether the new object is a planet or a brown dwarf, “it is one more step toward understanding the planets in our solar system.”

Last year, astronomer Bruce Campbell of the University of Victoria in Canada reported indirect evidence for as many as seven of 16 stars that he had observed. He reported Wednesday at the Baltimore meeting that he had found evidence of two more planets, bringing the total to nine planets for 18 stars examined, all within 100 light years of Earth. Campbell said the planets range from the size of Jupiter to 10 times as large.

Based on his evidence, he suggested that half the stars in the Milky Way galaxy may have “planetary companions.”

But critics have been unconvinced of Campbell’s findings because he has observed the stars for only a short time--and in no case long enough to observe a complete orbit of any planet around its star.

At least two other previous reports of planets circling other stars have been discredited when other astronomers have been unable to reproduce the observations.

Covering 30 Orbits

But Latham’s results are more substantive because they are based on a period of observation that covers about 30 orbits of the proposed planet.

HD 114762 is a yellowish, medium-size star similar to the Sun, but about 10 billion years older. It is too faint to be seen by the naked eye, and was previously thought not to be particularly interesting.

Latham was observing it only because it is one of the so-called “standard stars,” designated by the International Astronomical Union, whose velocities are well known. Astronomers measure the velocity of the standard stars to calibrate their measurements of other stars.

Latham was refining the velocity measurement for HD 114762 and testing out new components for a 50-year-old, 61-inch telescope at the Smithsonian’s Oak Ridge Observatory in western Massachusetts when he observed a discrepancy in its velocity.

But astronomers still have a minor problem, he noted. Because HD 114762’s velocity is constantly changing, it is no longer useful as a “standard star.”

HOW ASTRONOMERS DETECT AN ORBITING PLANET

Even if astronomers cannot see a planet orbiting a star, the planet’s presence can be inferred from stellar motion. The orbiting planet causes the star to move in a tiny counter-orbit around the two bodies’ common center of mass, marked by a cross. The total extent of the stellar motion--or wobble--and length of the orbit can indicate the planet’s mass.