Astronomers have found evidence of the first known planet orbiting a pair of stars, it was announced Wednesday.
The search for planets beyond our solar system has yielded results in the past four years. With the count at 20, all of those are thought to be orbiting single stars rather than pairs of stars, called binary stars.
The latest finding represents an advance because the inferred planet is circling a binary star system. Binaries are thought to be more common than single stars -- at least in the neighborhood of our sun.
"There may be binaries orbiting each other once every 100,000 years and each one of those may have planets, but this is the first one where the planet orbits both of the stars," said David Bennett of the University of Notre Dame, who headed up the research along with his colleague Sun Hong Rhie.
Bennett and his team believe they have found evidence of a planet about three times the mass of Jupiter orbiting a star system in Sagittarius about 20,000 light-years away (a light-year is the distance light travels in a year -- about 6 trillion miles).
Astronomers have yet to directly observe the typically dim planets orbiting suns beyond ours, as the presumed planets are lost in the glare of their central stars.
Instead, astronomers have been relying on an inferential technique to find what they presume are extra-solar planets. This approach detects the gravitational tug, or wobble, they exert on their central stars.
Bennett and his team are the first to use a technique called microlensing to successfully detect a planet beyond our solar system.
About two thirds of the stars near our sun are in binary or multiple star systems, said Morris Aizenman of the National Science Foundation's Astronomical Sciences Division. The NSF helped fund the research.
So the latest finding suggests there are more extra-solar planets out there than previously thought, he said.
Gravitational lenses
The approach used by Bennett and his team is based on a phenomenon called gravitational lensing, first theorized by Albert Einstein.
Einstein believed that the light from a distant star could be magnified by a middle object, such as a star or binary system, that passes between the distant object and an observer. When all are aligned, the middle body acts like a lens and makes the distant object appear brighter.
His theory since has been proven correct.
In 1997, Bennett and members of the Microlensing Planet Search project analyzed data from a lensing event called MACHO-97-BLG-41. It was detected by a 74-inch telescope at the Mount Stromlo in Australia, as well as by a 50-inch telescope there. Other observations were made by astronomers at the Wise Observatory in Israel.
During the event, the distant object was too bright to be explained by a lens that contained only one middle star. Using mathematical models, they found that it was best explained by a lens containing two stars, as well as a planet four times the weight of Jupiter.
Microlensing complements wobble detection
Robert Brown, an astronomer at the Space Telescope Science Institute in Baltimore, said microlensing is a promising approach to searching for extra-solar planets.
It will complement the "wobble" detection approach by telling astronomers the frequency of planets of a certain size that exist around stars.
It cannot reveal the number of planets, or their orbits, around a given star because it is based on limited information from one-time observations that are unlikely to occur again in cosmological history -- that is, observations of any temporary alignment that produces a gravitational lens.
But it does reveal information about masses that may not be provided by other approaches, particularly the presence of Earth-sized planets that are less likely to exert a gravitational tug on central stars.
"There is no other technique in practice that can detect the presence of an Earth-mass planet around a star," Brown said.
There are other possible explanations of the brightness that occurred during the MACHO-97-BLG-41 lensing event, Bennett said. The distant star could have a faint companion that enhanced its brightness.
"Our data for this event leaves something to be desired," Bennett said. So far, the Microlensing Planet Search project has only been observing with a telescope in Australia.
They plan to expand operations to telescopes in South Africa and, eventually, South America.
Also, the project will check its results against observations of the distant star made by the Hubble Space Telescope, as well as spectrographic data collected by the Keck Observatory in Hawaii.
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