Binary Stars Host Potential Planet-forming Disk

Giant planets, comets and Pluto-like bodies may form around binary star systems, not just single stars, a new study suggests.

The evidence comes via a rotating, molecular disk found orbiting the young binary star system V4046 Sagittarii with the Submillimeter Array's radio telescope system atop Mauna Kea, Hawaii. This finding was announced last week by a team of astronomers at the 214th meeting of the American Astronomical Society in Pasadena, Calif.

The discovery suggests that other binary systems could host as-yet undetected planets. Planets around binary stars have traditionally been difficult to detect because the added star complicates traditional planet-searching techniques. Extrasolar planets have been found orbiting around a few other binaries, but the stars in these systems are mostly much further apart than the V4046 Sagittarii pair.

"In this case the stars are so close together, and the profile of the gas ? in terms of the types of molecules that are there ? is so much like the types of gaseous disks that we see around single stars, that we now have a direct link between planets forming around single stars and planets forming around double stars," said team member Joel Kastner of Rochester Institute of Technology in New York.

The molecular disk ? a noxious cloud of carbon monoxide and hydrogen cyanide ? around V4046 Sagittarii extends from within what would be the orbit of Neptune to about 10 times beyond that orbit. The region is analogous to the zone in our solar system that encompasses the gas giant planets and the Kuiper belt objects.

"We believe that V4046 Sagittarii provides one of the clearest examples yet discovered of a Keplerian, planet-forming disk orbiting a young star system," said team member David Wilner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "This particular system is made that much more remarkable by the fact that it consists of a pair of roughly solar-mass stars that are approximately 12 million years old and are separated by a mere five solar diameters."

The average diameter of the sun is about 109 Earths across, and it is thought to be about 4.5 billion years old.

"This could be the oldest known orbiting protoplanetary disk and it shows that, at least for some stars, formation of Jovian-mass planets may continue well after the few million years which astronomers have deduced is characteristic of the formation time for most such planets," said team member Ben Zuckerman of UCLA.

Why these stars seem to hang on to their disks longer than others isn't known.

"It remains an astronomical mystery as to why a tiny percentage of T Tauri stars (young sun-like stars) can retain large amounts of orbiting dust and gas for a period of 10 [million years] or more, since the dust and gas dissipates around most such stars in only a few [million years]," Zuckerman told SPACE.com in an email. "The longer the gas remains, the longer a star has to form gas giant planets like Jupiter and Saturn."

More than 300 extrasolar planets have been found orbiting other stars in the galaxy, but many of these are much further away than V4046 Sagittarii.

"At a distance of only 240 light-years from the solar system, the V4046 Sagittarii binary is at least two times closer to Earth than almost all known planet-forming star systems, which gives us a good shot at imaging any planets that have already formed and are now orbiting the stars," said team member David Rodriguez, also of UCLA.

The team plans to do further observations of the system to learn more about its peculiar nature.

"The immediate next step is to use computer models determine some of the properties of this system, such as the inclination of the disk," Rodriguez said in an email. He and his colleagues will also probe the system to see what other molecules are present in the disk.

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