The simultaneous discovery of two exoplanets slightly smaller than Jupiter and Saturn by an international team of astronomers that includes David Bennett from the University of Notre Dame gives astrophysicists an important clue that solar systems like ours might be quite common.
The report, to be published in the Feb. 15 issue of the journal Science, describes the series of observations that began March 28, 2006, when a collaboration known as Optical Gravitational Microlensing Equipment (OGLE) detected a signal, possibly due to a planet in microlensing event OGLE-2006-BLG-109, that the researchers had discovered and announced two days earlier.
After the OGLE group announced this possible detection of a planetary system via e-mail, other astronomers from the Microlensing Follow-Up Network (MicroFUN), Microlensing Observations in Astrophysics (MOA) and Probing Lensing Anomalies NETwork (PLANET) collaborations also began intensive, round-the-clock observations of this event. The combined data from these four groups revealed a series of brightness variations over the ensuing 11 days that indicated that two planets orbit a star half the mass of the sun located 5,000 light years from Earth. This star, called OGLE-2006-BLG-109L, and its planets were discovered using a technique known as gravitational microlensing.
Early calculations by the reports lead author, Scott Gaudi of Ohio State University, and the MicroFUN group indicated that most of the telltale brightness variations were due to a planet with a mass similar to that of Saturn, but that there was a brief additional brightening observed from Israel and Chile that could only be explained by an additional planet with nearly the mass of Jupiter. However, Gaudis calculations did not provide a perfect fit to the data and involved several approximations.
Subsequently, Bennett performed more sophisticated calculations in his office at Notre Dame using his own advanced computer program that included an important additional feature: the orbital motion of the Saturn-mass planet.
Even though we observed the micolensing effect of the Saturn for less than 0.3 percent of its orbit, the observations simply could not be explained without accounting for the orbit,said Bennett, a research associate professor of astrophysics.
Critical assistance with these calculations was provided by Sergei Nikolaev at Lawrence Livermore National Laboratory, who devoted much supercomputer time to the calculations.
The result was one of the most complicated calculations of a star-planet system using the gravitational microlensing method.
Gravitational microlensing takes advantage of the fact that light is bent as the rays pass close to a massive object, like a star. The gravity from the mass of the intervening object, or lens star, warps surrounding space and acts like a giant magnifying glass. As predicted by Albert Einstein and later confirmed, this phenomena causes an apparent brightening of the light from the backgroundsourcestar. The effect is seen only if the astronomers telescope lies in almost perfect alignment with the source star and the lens star.
Astronomers are then able to detect planets orbiting the lens star if the light from the background star also is warped by the planets.
The discovery of the double planet system was a triumph for astronomers who use this method, which is of such high sensitivity that it can detect planets similar to those in our own solar system, with the exception of Mercury.
These planets could not have been detected without any other technique,Bennett said.
The light curve of this event revealed an unprecedented amount of information about the planetary host star and the planets,he continued.
The effect of the orbital motion of the Earth can be detected in the light curve, and this reveals that the mass of the host star is half the mass of the sun. This mass estimate was confirmed by subsequent observations of the planetary host star with the Keck telescope.
The light curve also reveals the orbital motion of the Saturn-mass planet during the 11 days when the planetary signal was visible.
To date, only 25 multiple planet systems have been observed. A majority are very dissimilar to our solar system and that of OGLE-2006-BLG-109L.
The Jupiter- and Saturn-sized planets orbiting OGLE-2006-BLG-109L are only the fifth and sixth planets that have been detected using the gravitational lensing method. Gaudi and Bennett conclude that if the OGLE-2006-BLG-109L planetary system is typical, then it is possible that they would have similar planets as our own solar system.
_ Contact: David Bennett, research associate professor of astrophysics, 574-631-8298,_ " Bennett@nd.edu ":mailto:Bennett@nd.edu
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