Binary Stars Host Potential Planet-forming Disk

Giantplanets, comets and Pluto-like bodies may form around binary star systems, notjust single stars, a new study suggests.

Theevidence comes via a rotating, moleculardisk found orbiting the young binary star system V4046 Sagittarii with theSubmillimeter Array's radio telescope system atop Mauna Kea, Hawaii. Thisfinding was announced last week by a team of astronomers at the 214th meetingof the American Astronomical Society in Pasadena, Calif.

Thediscovery suggests that otherbinary systems could host as-yet undetected planets. Planets around binarystars have traditionally been difficult to detect because the added starcomplicates traditional planet-searching techniques. Extrasolar planets havebeen found orbiting around a few other binaries, but the stars in these systemsare mostly much further apart than the V4046 Sagittarii pair.

"Inthis case the stars are so close together, and the profile of the gas ? interms of the types of molecules that are there ? is so much like the types ofgaseous disks that we see around single stars, that we now have a direct linkbetween planets forming around single stars and planets forming around doublestars," said team member Joel Kastner of Rochester Institute of Technologyin New York.

Themolecular 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 solarsystem that encompasses the gas giant planets and the Kuiper belt objects.

"Webelieve that V4046 Sagittarii provides one of the clearest examples yetdiscovered of a Keplerian, planet-forming disk orbiting a young starsystem," said team member David Wilner of the Harvard-Smithsonian Centerfor Astrophysics in Cambridge, Mass. "This particular system is made thatmuch more remarkable by the fact that it consists of a pair of roughlysolar-mass stars that are approximately 12 million years old and are separatedby a mere five solar diameters."

The averagediameter of the sun is about 109 Earths across, and it is thought to be about4.5 billion years old.

"Thiscould be the oldest known orbiting protoplanetary disk and it shows that, atleast for some stars, formation of Jovian-mass planets may continue well afterthe few million years which astronomers have deduced is characteristic of theformation time for most such planets," said team member Ben Zuckerman ofUCLA.

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

"Itremains 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 aperiod of 10 [million years] or more, since the dust and gas dissipates aroundmost such stars in only a few [million years]," Zuckerman told SPACE.comin an email. "The longer the gas remains, the longer a star has to formgas giant planets like Jupiter and Saturn."

More than 300extrasolarplanets have been found orbiting other stars in the galaxy, but many ofthese are much further away than V4046 Sagittarii.

"At adistance of only 240 light-years from the solar system, the V4046 Sagittariibinary is at least two times closer to Earth than almost all knownplanet-forming star systems, which gives us a good shot at imaging any planetsthat have already formed and are now orbiting the stars," said team memberDavid Rodriguez, also of UCLA.

The teamplans to do further observations of the system to learn more about its peculiarnature.

"Theimmediate next step is to use computer models determine some of the propertiesof this system, such as the inclination of the disk," Rodriguez said in anemail. He and his colleagues will also probe the system to see what othermolecules are present in the disk.

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