"AFOE Observations of 51 Pegasi: Evidence for Planets?" is being presented at the winter meeting of the American Astronomical Society in San Antonio, Texas, 14-18 January. Coauthors include Scott Horner (Pennsylvania State University); Sylvain Korzennik, Martin Krockenberger, Peter Nisenson, and Robert Noyes (Harvard-Smithsonian Center for Astrophysics); and Carrie Rowland (an undergraduate at Stetson University employed at NCAR last summer).
The scientists base their findings on observations of variations over time in 51 Pegasi's radial velocity (the star's speed relative to our solar system). They used the advanced fiber-optic Echelle spectrograph (AFOE) built by NCAR's High Altitude Observatory and the Harvard-Smithsonian Center. The AFOE is sited at the Whipple Observatory at Mt. Hopkins, Arizona, operated by the Harvard-Smithsonian Center.
When a star has one or more planets orbiting around it, the planets induce a compensating motion in the star so that the bodies are all orbiting around a single center of mass (normally located very close to the star). The resulting stellar wobble can be detected to a long-term precision of 20-30 meters per second using the AFOE.
Beginning in 1993, the AFOE team reguarly observed of a set of stars chosen by colleague David Latham (Harvard University) as being likely to have variations in radial velocity that would suggest the presence of one or more planets. One of the stars observed by the NCAR team was 51 Pegasi, about 45 light years from the Earth. "It is a nearby star as these things go," says Brown, "and it is a very close analog to the Sun in size and age."
While observing in October, Brown and Kennelly received a call from Noyes notifying them that a planet had been discovered near 51 Pegasi. The Swiss team behind the discovery had found a radial velocity variation of around 60 m/s with a period of 4.2 days. The data implied a planet half the mass of Jupiter that took only four days to orbit 51 Pegasi, at a distance of five million miles (5% of the Earth-Sun distance). Skeptical yet intrigued by the Swiss findings, Kennelly and Brown stepped up their observations of 51 Pegasi to twice nightly. The data convinced them of the new planet's presence. "We were the second group, behind a team at San Francisco State University, to confirm the results of Mayor and Queloz. The trail of planet detection is littered with corpses, so the fact that three independent groups have confirmed this planet's existence is encouraging."
What about the potential for other planets? "There's been speculation that there may be a second planet in the system. That would be strong evidence that the first object was indeed a planet," says Brown. Because the AFOE data on 51 Pegasi extend back to 1993--further back than the Swiss data--the AFOE team was able to analyze nearly 100 separate observations to separate out the effects of the newly discovered planet and see if other, longer-term signals were present from a second planet.
Based on their analyses to date, the NCAR scientists conclude that "there is probably not an object larger than ten Jupiter masses closer to 51 Pegasi than two astronomical units [roughly the distance between the Sun and Mars]," says Brown. Continued analysis will narrow the range of uncertainty, he adds.
NCAR is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.