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Planet-forming 'wheel in a wheel' found in a Binary Star System

Jenny Winder, News Writer
Oct 31, 2014, 17:07 UTC

Sen—Scientists studying a double star, known as a binary system, have detected a streamer of dust and gas that may allow planets to form in that gravitationally perturbed environment.

Half of Sun-like stars are born in binary systems, meaning that these findings will have major consequences for the hunt for exoplanets.

This never-before-seen feature, was found in a binary called GG Tau-A, using the Atacama Large Millimeter/submillimeter Array (ALMA). It was recently discovered that one of GG Tau-A’s components is itself a double star.

The system is only a few million years old and lies approximately 460 light-years from Earth in the constellation Taurus. The feature may be responsible for sustaining a second, smaller disk of planet-forming material that otherwise would have disappeared long ago.


The stunning Milky Way above the antennas at the ALMA Observatory. Image credit: Y. Beletsky (LCO) ESO

Leader of the research team, Anne Dutrey from the Laboratory of Astrophysics of Bordeaux and the National Centre for Scientific Research (CNRS) told Sen: "In the early nineties, we started to observe GG Tau A because it was a very bright young binary system at millimeter wavelengths. This suggest that the amount of material around the stellar system was large.

"At that time, we were starting to observe these kind of young stars similar to the Sun when it was about one million year old. Hence, observing first the brighter objects was important."

GG Tau-A contains an outer disk encircling the entire system as well as an inner disk around the main central star. This second inner disk has a mass roughly equivalent to that of Jupiter. Its presence is intriguing as it is losing material to its central star at a rate that should have depleted it long ago.

The discovery of gas clumps in the region between the two disks suggest that material is being transferred from the outer disk to the inner disk in a streamer, creating a sustaining lifeline between the two.

"Material flowing through the cavity was predicted by computer simulations but never imaged before. Detecting these clumps indicates that material is moving between the disks, allowing one to feed off the other," says Dutrey "These observations demonstrate that material from the outer disk can sustain the inner disk for a long time. This has major consequences for potential planet formation."

An enduring disk is a prerequisite for the slow process of planet formation. If the feeding process into the inner disk occurs in other multiple-star systems the findings introduce a vast number of new potential locations to find exoplanets.

A large fraction of giant planets orbit binary-star systems. Now, researchers are investigating planets orbiting the individual stars of multiple-star systems. The new discovery supports the possible existence of such planets.

Jeffrey Bary, an astronomer at Colgate University in Hamilton and co-author of the paper, told Sen: "The unprecedented combination of ALMA's sensitivity and resolution is what enabled us to clearly see the gas moving between the large circumsystem ring/disk and the inner circumstellar disk surrounding one of the central stars.  

"Prior to this observation, I do not think that many astronomers would have doubted that this lifeline of dust and gas existed, we simply could not directly detect it until we pointed ALMA at the system." 

The astronomers are following up on their discovery. Dutrey told Sen: "We are already working on new ALMA observations to better quantify how the mass is transferred from the outer ring onto the circumstellar disk orbiting around GG Tau Aa. These new data will also allow us to constrain the physical conditions inside the outer ring."