Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered a streamer of gas flowing from a massive outer disc toward the inner reaches of a young, low-mass binary star system GG Tau-A.

The never-before-seen feature may be responsible for sustaining a second, smaller disc of planet-forming material that otherwise would have disappeared long ago.

“We have demonstrated that the inner disks can be replenished with fresh material and are thereby potential sites of planet formation,” Emmanuel Di Folco, co-author of the study from the Laboratory of Astrophysics of Bordeaux, France told me. "Our finding with ALMA is that there is a large amount of cold material that flows into the cavity (from the outer ring) towards the inner disk(s) and stars. The infalling material can nurture the inner disk(s) and extend their lifetime on timescales long enough to sustain planet formation therein.”

The researchers detailed their findings in a paper to be published in the journal Nature on Oct. 30, 2014. 

The team led by Anne Dutrey from the Laboratory of Astrophysics of Bordeaux used ALMA to observe the distribution of dust and gas in GG Tau-A, which is part of a more complex multiple-star system called GG Tauri. The object is only a few million years old and lies about 450 light-years from Earth in the constellation of Taurus (The Bull).

Like biblical Ezekiel’s “wheel in a wheel” GG Tau-A contains a large, outer disc encircling the entire system as well as an inner disc around the main central star. This second inner disc has a mass roughly equivalent to that of Jupiter. Its presence has been an intriguing mystery for astronomers since it is losing material to its central star at a rate that should have depleted it long ago.

While observing these structures with ALMA, the team made the exciting discovery of gas clumps in the region between the two discs. The new observations suggest that material is being transferred from the outer to the inner disc, creating a sustaining lifeline between the two.

“Our findings demonstrate that material can flow (as predicted by numerical simulations, but it is observed here for the first time) from the outer disk towards the inner disks around the individual stars. This mechanism is of prime importance to sustain planet formation (which may take place on timescales of few million years).” Di Folco said. 

“Material flowing through the cavity was predicted by computer simulations but has not been imaged before. Detecting these clumps indicates that material is moving between the discs, allowing one to feed off the other,” Dutrey explained. “These observations demonstrate that material from the outer disc can sustain the inner disc for a long time. This has major consequences for potential planet formation.” 

Planets are born from the material left over from star birth. This is a slow process, meaning that an enduring disc is a prerequisite for planet formation. If the feeding process into the inner disc now seen with ALMA occurs in other multiple-star systems the findings introduce a vast number of new potential locations to find exoplanets in the future. 

“We have found a hint that a young planet might be forming at the outer edge of the large ring of dust and gas that encircles the three stars,” Di Folco noted. “The bright "blob" (or spot) appears to be twice hotter than its close environment and we think it may be related to accretion onto a young proto-planet (not the emission from the planet itself).” 

The first phase of exoplanet searches was directed at single-host stars like the Sun. More recently it has been shown that a large fraction of giant planets orbit binary-star systems. Now, researchers have begun to take an even closer look and investigate the possibility of planets orbiting the individual stars of multiple-star systems. The new discovery supports the possible existence of such planets, giving exoplanet discoverers new happy hunting grounds. 

Half of Sun-like stars are born in binary systems, meaning that these findings will have major consequences for the hunt for exoplanets. Di Folco revealed that: “This means that we have found a mechanism to sustain planet formation that applies to a significant number of stars in the Milky Way. Our observations are a big step forward in truly understanding planet formation.” 

Asked about future observations of the GG Tau-A system, he noticed that they already have new observations of this system with ALMA to investigate more in depth the physical conditions in the inner and outer disks and to understand if these conditions are favorable to planet formation. “We also have Hubble Space Telescope observations in collaboration with our american colleagues to study the accretion shocks of the infalling hot gas when it reaches the inner system (the stars and their disks).” 

The research is presented in a paper entitled “Planet formation in the young, low-mass multiple stellar system GG Tau-A” by A. Dutrey et al., to appear in the journal Nature. 

The team is composed of Anne Dutrey (University Bordeaux/CNRS, France), Emmanuel Di Folco (University Bordeaux/CNRS), Stephane Guilloteau (University Bordeaux/CNRS), Yann Boehler (University of Mexico, Michoacan, Mexico), Jeff Bary (Colgate University, Hamilton, USA), Tracy Beck (Space Telescope Science Institute, Baltimore, USA), Hervé Beust (IPAG, Grenoble, France), Edwige Chapillon (University Bordeaux/IRAM, France), Fredéric Gueth (IRAM, Saint Martin d’Hères, France), Jean-Marc Huré (University Bordeaux/CNRS), Arnaud Pierens (University Bordeaux/CNRS), Vincent Piétu (IRAM), Michal Simon (Stony Brook University, USA) and Ya-Wen Tang (Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan).