Extrasolar planets: More like home

A trove of newly discovered planets orbiting other stars suggests that our solar system may not be the oddball it had begun to seem. Among eight newly identified extrasolar planets, three have large, circular orbits that resemble those of the planets in the solar system. One of the planets is at least 80 percent as massive as Jupiter and orbits the star HD 4208. If it were part of our solar system, it would be about as far from the sun as Mars is. Two other planets, each with a minimum mass 2.8 times that of Jupiter, circle the star HD 23079.

The new finds, announced Oct. 15, bring the tally of extrasolar planets to nearly 80. In August, when the count was at 70, only two of the planets had orbits large enough and circular enough to resemble the paths of planets in the solar system (SN: 8/18/01, p. 100: Astronomers spy familiar planetary system). All the rest either circle their parent star so closely that they graze its outer atmosphere or they have large, highly elongated paths that bring them alternately very close and very far from their parent.

Planets that lie at greater distances from their parent require a longer time–years rather than months–to be detected.

Most of the planetary systems that researchers initially found bear little resemblance to the solar system, says study collaborator Steven S. Vogt of the University of California, Santa Cruz. After years of observations, “we’re starting to see something like second cousins. In a few years, we could be finding brothers and sisters,” he says.

Each orbiting body is detected indirectly. As it circles the star, it pulls the parent to and fro. The resulting wobble shows up as a periodic shift in frequency in the star’s light.

The technique only measures the wobble along one direction, the line of sight to Earth. That means the method can only provide the minimum mass of an orbiting planet.

Because of that limitation, some astronomers have maintained that some declared planets could be considerably heavier than the minimum value, qualifying them either as lightweight stars or failed stars known as brown dwarfs (SN: 10/28/00, p. 277: Are most extrasolar planets hefty imposters?). David C. Black of the Lunar and Planetary Institute in Houston, a longtime critic of the extrasolar-planet findings, has led the naysayers making this argument.

Last year he was joined by a few other astronomers who independently studied data from the Hipparcos satellite, which tracked stars across the sky. They reported that two objects earlier identified as extrasolar planets may be heavy enough to be stars. After further analysis, however, the same researchers now conclude that the Hipparcos data lack the sensitivity to say anything definitive about the extrasolar-planet findings. Shay Zucker and Tsevi Mazeh of Tel Aviv University in Israel describe these findings in an upcoming Astrophysical Journal.

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