Twinkling some 450 light-years away in the constellation Taurus is a complex starry trio known as GG Tau A. Two of the triad’s three stars closely circle one another, like a classic binary pair. But that pair also swings around the third star, creating a turbulent environment that seems like the last place you’d expect to find hints of planet formation.
Except that is exactly what astronomers might be seeing.
Imagine, for a moment, a world with three suns. Shadows would come in triplets, the suns would be continually eclipsing themselves, sunrises and sunsets would be spectacularly variable. We don’t know of any such circumtriple worlds yet, but new observations suggest a planet could be forming around GG Tau A’s three stars.
And what’s more, those observations point to a gassy conveyor belt that’s supplying vital planet-building ingredients to a spot deep within the system.
“If all goes well, in a few million years we may have a circumtriple planet and a circumstellar planet in the same system,” says astronomer William Welsh of San Diego State University, who was not involved in the recent observations. “This system would be off the charts in terms of the coolness factor.”
Astronomers have been observing the GG Tau A system for decades. In 1993, they detected a large, dusty ring encircling the trio (though it wasn’t yet known to be a trio. That was discovered more recently). Made of diffuse gas and dust, the ring itself is about 90 times wider than the Earth-Sun distance (called an astronomical unit), and its inner edge begins about 190 astronomical units from the system’s central stars.
The presence of that ring is not surprising. But recent observations made with the European Southern Observatory’s Atacama Large Millimeter Array reveal a “puzzling” condensation near the outer edge of that ring – something scientists suspect might be a young planet.
“The hot spot we observe may be the signature of an embedded planet, but its nature has to be confirmed and we are still working on it,” says astronomer Anne Dutrey of the University of Bordeaux and France’s National Center for Scientific Research. She and her colleagues report their observations today in Nature.
But finding planets around GG Tau A wasn’t the main goal of the team’s observations. Dutrey and her colleagues were hoping to better understand the longevity of a large disk of planet-building material that lives deep inside the system. Swirling around the singleton of the three stars – called GG Tau Aa – the disk is about 15 astronomical units wide. Normally, planets condense within these disks and form over millions of years, which is what happened in our solar system.
In a close binary system, however, these star-circling disks don’t live long enough to grow planets. They’re sucked into their parent stars in a few thousand years.
Yet GG Tau A is about one million years old, Dutrey says, and the disk is still there. So how is it managing to survive?
What Dutrey and her colleagues found is that there’s a streamer of gas connecting the system’s outer ring with the inner disk, kind of like a lifeline. Material appears to be flowing in from the outer reservoir and replenishing the disk around that single star, keeping it alive long enough to (maybe) grow planets.
The team’s observations are important both for understanding how planets form in complex systems, and for understanding the galactic planet census, Welsh says. He works with NASA’s Kepler spacecraft and has spotted several twin-sunned planets, the first of which landed with a splash in 2011. Now, scientists have confirmed about a dozen of these real-life Tatooines.
“A very sizable fraction of all stars are born in binary systems,” Welsh says. “So a better understanding of planet formation in binaries leads to having a much better grasp about the true planet population in the galaxy.”