EVERY DAY, IN OUR OWN GALAXY, there are planets circling pairs of stars. Like the planet Tatooine in Star Wars, these planets might enjoy spectacular binary sunsets — if the geometry of the planet and stars’ orbits are right.
Of course, whether there’s anyone around to appreciate those sunsets is another question. We’re still a ways off from sending robots to the nearest exoplanetary system, let alone people. So for now, we’ll have to be content with figuring out what conditions on these planets are like — and whether alien life could ever arise there.
HERE’S THE BACKGROUND — Since it was launched in 2009, the Kepler Space Telescope has helped find thousands of exoplanets orbiting other stars in our galaxy. Of these planets, at least nine are known to be circumbinary planets — that is, they orbit two or more stars in a binary star system.
These planets are all giant planets the size of Neptune or larger, but that is in part due to sample bias — larger planets are easier for Kepler to find, so this doesn’t mean smaller, Earth-size planets aren’t also in orbit around binary star systems. But the extra gravitational effects of a giant planet could make for a precarious situation. At least in some cases, interactions between the two stars and the giant planet would probably swing a smaller rocky planet around wildly or kick it out of the system entirely.
In our solar system, the gas giant Jupiter is the biggest gravitational body after the Sun. If our Sun were two stars instead of one, interactions between the stars and Jupiter might mess with the other solar system planets’ orbits over time.
A new study published April 15 in Frontiers in Astronomy in Space Science found that a giant planet neighbor does not necessarily spell doom for potential life on a Tatooine-like planet.
WHAT’S NEW — Researchers have previously determined that binary star systems can have a so-called “habitable zone,” a region within which a rocky planet could exist with surface temperatures that allow for liquid water. This is considered one of the key factors in what would be the ripe conditions for life to flourish.
In the new study, researchers found that rocky planets could theoretically orbit in the habitable zone around some binary stars without getting kicked out as a result of gravitational interactions — even if there was also a giant planet in the system.
Of nine known circumbinary planetary systems found by Kepler, the researchers suggest most of them could safely harbor a rocky planet in the habitable zone. Three systems in particular — Kepler-35, Kepler-38, and Kepler-64 — are the most promising.
HOW THEY DID IT — To determine how promising each of the Kepler systems might be for hosting potentially habitable Earth-like planets, the researchers modeled gravitational interactions between a binary star system and a rocky planet in the habitable zone. By running different scenarios through a computer model, they could identify where such a planet could exist in a stable orbit. Then, they added a giant planet into the mix to see if the rocky planet’s orbit remained stable.
“When you add another body, a fourth one, then things are even more complicated,” Nikolaos Georgakarakos, a researcher at New York University Abu Dhabi in the UAE and lead author of the new study, tells Inverse. Beyond two-body systems, each additional gravitational body adds many more gravitational effects and interactions.
The models ruled out two of the nine systems, Kepler-16 and Kepler-1647, because adding in a giant planet essentially created chaos within the habitable zones — no rocky planet could possibly exist there given what we know about star systems. Of the remaining seven systems, the researchers looked at how widely the orbits of rocky planets could vary within each system while remaining within the space where incoming heat from the stars would allow liquid water to exist at their surface.
Based on this measure, three systems, Kepler-35, Kepler-38, and Kepler-64, have the most potential for stable, rocky planets that could have liquid water — and potentially life.
WHY IT MATTERS — Binary star systems may seem more sci-fi than reality, but in truth, they are very common in our galaxy — roughly half of Sun-like stars in our neighborhood are in binary systems. Knowing whether these systems are hospitable to life is a major component in our search for life in the universe beyond our own planet.
“It's very interesting to know that around the stellar binaries — even with the influence of the gas giants, many terrestrial planets would still be quite stable,” Gongjie Li tells Inverse. Li is an astrophysicist at the Georgia Institute of Technology who was not involved in the study.
WHAT’S NEXT — There’s a lot more researchers could do to expand on this model, Georgakarakos says. For example, this model looked at planets with orbits that were mostly aligned with the plane of the binary star system but didn’t account for the possibility of planets with orbits that are tilted.
Georgakarakos notes that the new finding is still just one step toward finding out whether a rocky circumbinary planet may actually be habitable. Many factors, like a planet’s atmosphere and the kinds of radiation given off by the binary stars, play into whether a planet might be hospitable to life as we know it.
Li agrees. “The way we understand habitability of planets is still limited,” she says. But, she says, the new findings are the first step toward future investigations of the potential habitability of rocky planets in these systems.
Abstract: Determining habitable zones in binary star systems can be a challenging task due to the combination of perturbed planetary orbits and varying stellar irradiation conditions. The concept of “dynamically informed habitable zones” allows us, nevertheless, to make predictions on where to look for habitable worlds in such complex environments. Dynamically informed habitable zones have been used in the past to investigate the habitability of circumstellar planets in binary systems and Earth-like analogs in systems with giant planets. Here, we extend the concept to potentially habitable worlds on circumbinary orbits. We show that habitable zone borders can be found analytically even when another giant planet is present in the system. By applying this methodology to Kepler-16, Kepler-34, Kepler-35, Kepler-38, Kepler-64, Kepler-413, Kepler-453, Kepler-1647, and Kepler-1661 we demonstrate that the presence of the known giant planets in the majority of those systems does not preclude the existence of potentially habitable worlds. Among the investigated systems Kepler-35, Kepler-38, and Kepler-64 currently seem to offer the most benign environment. In contrast, Kepler-16 and Kepler-1647 are unlikely to host habitable worlds.