Planets may form in multi-star systems more often than we realised and indeed we might even have caught one in the act.
If two stars are widely spaced, it would be possible for a planet to survive orbiting one without being greatly affected by the other, but could it form that way? What about a Tatooine situation where a planet orbits both stars at once?
These questions discouraged astronomers from even looking for planets in binary systems. A planet was found in 1993 orbiting PSR B1620-26, which was a pulsar/white dwarf combination but aside from this case, most early planetary discoveries were around single stars. Now, however, new configurations are being revealed. It is clear that circumbinary planets are not just isolated freaks, but there is still a lot we don't know about how they come to be.
All of this makes the images from the Atacama Large Millimeter/submillimeter Array (ALMA) of the GG Tauri system exceptionally exciting. Located 460 light years away in the Taurus Dark Cloud, GG Tauri is a set of five stars 1 to 5 million years old; so young they are still surrounded by a protoplanetary disk. Two of the stars are so distant they would not affect planetary formation, but the area known as GG Tauri A contains three stars with distances less than that between the Sun and Neptune.
On closer examination there is a major disk hundreds of astronomical units wide encircling the brightest star (GG Tauri-Aa) and its two tightly locked companions, along with an inner disk 15 AU wide surrounding GG Tauri-Aa alone.
The mass of the inner disk is estimated to roughly equal that of Jupiter, although whether it will eventually become one large planet or several smaller ones remains to be seen. The most significant aspect of the findings however, is the connection between the inner and outer material.
In Nature, Dr. Anne Dutrey of the French National Center for Scientific Research (CNRS) reports that material is streaming between the two, and refreshing the inner disk, which otherwise would be depleted within a few thousand years as material became captured by the stars in their orbits.
"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."
Planets take a millions year to coalesce, so if the inner disk could not survive for that time they would never get the chance to form. The umbilical cord of gas and dust running between the two disks could be enough to eventually birth a new world, indicating the hunt for extrasolar planets should widen its focus to include locations previously written off as unlikely.
There are signs that a planet may be starting to form at the outer edge of the inner disk, but Dutrey says more observations are needed to confirm this.
University of Hawaii. A wider view of the GG Tauri system, with the stars themselves and masked.