For decades, astronomers have assumed that Earth-like planets cannot form around binary stars on account of wacky gravitational effects. Which, for Star Wars fans, was a total downer. But a new study suggests that not only is the formation of Tatooine-like planets very much possible, they may actually be quite common.
Unlike the accretion disk surrounding a young solitary star, the planet-forming environment around a binary system is subject to seriously disruptive gravitational ebbs and flows. In turn, many astronomers believed that the formation of rocky planets around binary stars is either very difficult or outright impossible. A new study by Ben Bromley from the University of Utah and Scott Kenyon of the Smithsonian Astrophysical Observatory now suggests otherwise.
"Tatooine in Star Wars was inspirational to many of us," Bromley told io9. "It was disappointing when earlier theories predicted that making Earth-like planets would be problematic or impossible. Our work shows that nature provides a clear path for making planets around binary stars, following the same recipe as for Earth."
So, along with the discovery that terrestrial planets are common, so too are Tatooines, say the researchers.
"We started our study to understand Pluto, its large moon Charon, and their family of satellites," says Bromley. "That work led us to Tatooine, an example of how learning about our own neighborhood can teach us about the Universe at large."
That Universe at large happens to contain an abundance of binary star systems. So many, in fact, that multiple star systems account for roughly half of all star systems in the galaxy. It's crucial, therefore, that astronomers understand the planetary architectures and gravitational topologies that typify these systems, especially if we're to look for life within them.
The primary finding of the new study is that, outside a region near the binary where orbits are typically unstable, planet formation — whether it be a gas giant or a terrestrial Earth-like planet — can unfold in pretty much the same way as it does in a system with a single star.
"In our scenario, planets are as prevalent around binaries as around single stars," the researchers noted in a University of Utah statement.
That's a surprising conclusion given what we thought we knew about binary stars.
What makes this possible is the presence of what astronomers call "most circular orbits."
During the protoplanetary phase of development, binary stars suck up the bits of gas and dust that would normally come together to form planets. That is unless, as the researchers point out, this debris is in the right orbit — one that happens to be "most circular."
Artistic impression of Kepler-47, the first transiting circumbinary system — multiple planets orbiting two suns. (Credit: NASA/JPL-Caltech/T. Pyle)
To be fair, these orbits are not exactly circular. More like oval-shaped orbits containing numerous smaller waves within them.
"Circular orbits are special around a single star because growing planets can settle on them, and gently collect material around them," Bromley explained to io9.
But because binary stars exert gravity differently than single stars, they generate eccentric orbits that carve out paths were growing planets can settle. These orbits can get so tangled that they cross each other's paths at high speeds, resulting in a regular onslaught of planetesimal collisions that prohibit further growth.
"These paths are distorted from a circle so that planets and neighboring particles can ebb and flow together in response to the binary's gravity, like seabirds floating on waves in a rough sea," he says. "If the binary stars are on elliptical orbits themselves, these most circular paths turns out to be elliptical, too."
But as Bromley points out, if the planetesimals are in an oval-shaped orbit instead of a circle, their orbits can be nested and they won't bash into each other.
"They can find orbits where planets can form," says Bromley.
Prior to this study, computer models developed by astrophysicist Zoe Leinhardt of the University of Bristol showed that it was possible for Tatooines to form. But her work focused exclusively on models of the Kepler-34 binary system and its gas giant Kepler-34(AB)b. Bromley and Kenyon took a more expansive approach, using complex mathematical formulas to describe how binary stars can be orbited by gas giants as well as planetesimals.
To date, NASA's Kepler space telescope has helped astronomers discover more than 1,000 extrasolar planets. Seven of these orbit either within or near the habitable zone of binary stars — but not one of them is a rocky, Earth-like planet. They're all Neptune- or Jupiter-sized giants.
That doesn't mean Tatooine-like planets aren't out there. This may be an observational selection effect whereby gas giants are simply easier to detect with the current generation of telescopic technologies. If the Bromley and Kenyon paper is of any indication, future planet-hunting missions may find these systems in abundance.
This study has been submitted to the Astrophysical Journal for review. For now, you can read it at the preprint arXiv: "Planet formation around binary stars: Tatooine made easy".