By Ron Cowen
With the discovery last month of three more planets orbiting nearby stars, the number of proposed extrasolar planets now totals more than 50. The three objects were found with a high-precision spectrograph that analyzes starlight beaming into the 4-meter Anglo-Australian Telescope near Coonabarabran, Australia. Researchers have used the telescope since 1998 to search for planets among 200 nearby stars in the southern sky.
The lightest of the three planets is at least 84 percent as massive as Jupiter and lies within roasting distance of its parent star, orbiting at about one-tenth the distance that Mercury circles the sun. That new find whips around its star in a mere 3 days.
The heaviest of the bodies, an object with a minimum mass nearly twice that of Jupiter, has an elliptical orbit and takes just over 2 Earth years to orbit its parent star, mu Ara, in the constellation Altar. The planet’s average distance from its parent is just slightly greater than Mars’ average distance from the sun.
The middle-weight object, which seems the most intriguing, is at least 1.26 times as massive as Jupiter. It has a circular orbit and lies at an Earthlike distance from its parent star, epsilon Reticulum. It orbits its star every 426 days. Located in the constellation Net, this planet has the largest circular orbit of any planet yet detected outside the solar system, notes theorist Alan P. Boss of the Carnegie Institution of Washington (D.C.)
“This is reassuring that we will [eventually] find Jupiter-mass planets in circular, Jupiterlike orbits,” he says. Because such planets reside much farther from their parent star than Earth does from the sun, they would act as a shield protecting inner Earthlike planets from comets and other space debris, Boss says.
The discovery team, which includes Chris Tinney of the Anglo-Australian Observatory, relies on a standard technique to hunt planets. As a planet orbits, it causes its star to move back and forth. Researchers detect that wobble by recording a periodic shift in frequency of the light emitted by the star.
