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Summer 1997

New Planet Is Ninth outside Solar System

With the discovery of a new planet in the system of Rho Coronae Borealis, a Sun-like star about 50 light years from our solar system, the number of planets that have been discovered orbiting around other stars like ours now equals the number in our own solar system: nine.

The newly discovered planet's characteristics are those of a "hot Jupiter," says NCAR's Tim Brown (High Altitude Observatory), one of a group of astronomers who made the discovery. The other team members are Sylvain Korzennik, Martin Krockenberger, Peter Nisenson, and Robert Noyes of the Smithsonian Astrophysical Observatory (SAO); Harvard University graduate student Saurabh Jha; Edward Kennelly of NCAR's HAO; and Scott Horner of Pennsylvania State University. The new planet is perhaps 10% larger than Jupiter, but it orbits closer to its star than our planet Mercury does to the Sun. This proximity suggests a surface temperature of about 300 degrees C (570 degrees F), making the planet an unlikely place for life to form.

The nine extrasolar planets in comparison with the inner planets of our solar system (top). The name of the star is given in the center of each line, and the planet is shown at the correct distance from its star. Sizes are given in relation to the mass of Jupiter (Mjup). The new planet is shown at bottom. (Illustration courtesy of Paul Butler and Geoffrey Marcy.)

Planetary discoveries rely on evidence considerably more arcane than observations by telescope--even by a high-powered, spaceborne telescope like the Hubble. The stars are simply too far away. Scientists deduce all kinds of stellar phenomena, including possible planets, by analyzing minuscule changes in the electromagnetic spectra of the stars' light.

A planet the size of Jupiter, or bigger, is the only kind that astronomers are yet able to find, because it exerts considerable gravitational force on its star. All stars move through space, but a star with a big, close planet has an additional motion: a wobble that occurs because the orbiting planet tugs the star out of its path. To an observer on earth, the star's light appears to move back and forth--but not by much. The variation of Rho Coronae Borealis is only 67 meters per second (about 150 miles per hour) in 40 days.

Once an astronomer has spotted a star whose light changes periodically, says Brown, "then the question is, is the variation evidence of an orbiting planet, or is it something else?" To answer that question for Rho Coronae Borealis, the astronomers began observing it more intensively about six months ago. Their observations were made on the advanced fiber-optic Echelle (AFOE) spectrograph, built by HAO and the SAO. The instrument is attached to a telescope at the Whipple Observatory in Mt. Hopkins, Arizona, operated by the SAO; it analyzes spectra of the light collected by the telescope.

Because the AFOE shares its telescope with other instruments, the observing team only gets about a week of observing time each month. Still, by the end of February, they had enough observations to narrow down the possible orbits to two. The astronomers traded some future telescope time to get 12 consecutive nights of viewing in March. Those observations exactly matched the pattern predicted by one of the two orbits: a circular orbit with a 40-day period.

A circular orbit suggests that the planet was formed like those in our own solar system, that is, through the slow coalescence of dust and gas from the circularly rotating disk that is thought to surround all newborn stars. A more eccentric, or highly elliptical, orbit would imply that the companion object was a failed star, the unsuccessful second partner in a potential binary star system.

A lingering worry was that the star's variation in speed might result from its own pulsations, rather than from the gravitational attraction of a planet. This explanation was suggested by David Gray (University of Western Ontario) in Nature for similar evidence used to deduce the first extrasolar planet, orbiting 51 Pegasi. The 40-day period of Rho Coronae Borealis is so long, however, that the team concludes that pulsations are unlikely to be the cause.

Although searches for planets have been conducted by spectrograph for a decade or more, all nine extrasolar planets have been discovered in the last year and a half. Brown explains, "Nobody ever thought that there could be planets this big with orbits this small." Hampered by "solar chauvinism," astronomers were expecting planets around other Sun-like stars to look pretty much like those in our own solar system. Jupiter has an icy core; at the tiny orbital distances where most new planets have been found, the reasoning went, the frozen building blocks for such a core should have vaporized and prevented a giant planet from forming. As soon as 51 Pegasi's hot Jupiter was discovered, Brown says, "We knew what to look for."

For further details, contact Brown (303-497-1561 or timbrown@ucar.edu). General information on the search for extrasolar planets can be found at the San Francisco State University Planet Search Project's Web site.

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Last revised: Tue Apr 4 13:50:42 MDT 2000