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May 1, 1997|
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The new planet is the ninth one found orbiting a Sun-like star--the same number of planets as in our own solar system. The discovery will be reported in a future issue of Astrophysical Journal Letters.
The planet fits the description of a "hot Jupiter," says Brown. Ten percent larger than our own Jupiter, it sizzles at 300 degrees Celsius, or more than 500 degrees Fahrenheit--too hot for water or life. All that heat is the result of a snug orbit; the planet circles its star even more closely than Mercury does the Sun. Five other exoplanets (those outside our solar system) fit this same pattern of large size and close-in orbit.
The discovery of so many hot Jupiters suggests there may also be Earth-size planets hiding in space. According to Brown, "A big planet close to its star is the easiest kind to find," because it exerts a large gravitational force on its star that can be detected from Earth. Besides moving through space as all stars do, one with a planet also wobbles regularly over time in synch with tugs from its orbiting planet. To an observer on Earth, this means that the star's speed through the universe relative to that of our own solar system appears to change with a regular pattern, detected in the starlight.
Once we notice a periodic change in the starlight, says Brown, "then we ask, is this evidence of an orbiting planet, or is it something else?" To find the answer in this case, the team began observing Rho CrB more intensively about six months ago, using the NCAR/SAO advanced fiber-optic Echelle spectrograph (AFOE). The instrument is attached to the 1.5-meter telescope operated by the SAO at Mt. Hopkins.
Because the AFOE shares its telescope with other instruments, the observing team gets only about a week of observing time each month. Still, by the end of February, Brown, Kennelly, and the others had collected enough observations to narrow down the planet's possible orbits to two. Eager to find which was correct, the astronomers traded some of their future telescope time to get 12 consecutive nights of viewing in March. Those observations exactly matched the pattern of one of the two predicted orbits: a circular orbit with a 40-day period.
One remaining concern was that the star's speed variation might result from pulsations from within the star rather than from the gravitational pull of a planet. This had recently been suggested by David Gray of the University of Western Ontario to explain wobbles seen in the motion of 51 Pegasi, the star that the first exoplanet in the recent chain of discoveries was believed to orbit. However, for Rho CrB, this turned out not to be a worry. "Because the 40-day period of Rho Crb is so long," says Brown, "we concluded that an orbiting planet rather than star pulsations are causing the regular pattern of speed variation in this star."
Although scientists have been searching for exoplanets for a decade or more using the spectrographic technique, all nine extrasolar planets orbiting Sun-like stars 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." Scientists were expecting star systems to look like our own solar system, where Jupiter's great distance from the sun allowed the rapid formation of its icy core. At the tiny orbital distances of these new planets, the reasoning went, the frozen building blocks for such a core would have vaporized, preventing such a large planet from forming.
"It was solar chauvinism, " concludes Brown. "As soon as the first hot Jupiter was discovered orbiting 51 Pegasi, we knew what to look for." Last year the NCAR/SAO team was the second to confirm that a planet had been discovered outside the solar system.
NCAR's portion of this research was funded by the National Science Foundation (NSF). NCAR is managed by the University Corportion for Atmospheric Research under NSF sponsorship. UCAR is a consortium of 62 North American universities offering the Ph.D. in atmospheric or related sciences.
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