Planets in Binary Star Systems Could be Nice and Habitable

The Star Wars world Tatooine is one of the most recognizable planets in the realm of science fiction. It’s a harsh place, and its conditions shaped the hero Luke Skywalker in many ways. In the reality-based Universe, there may not be many worlds like it. That’s because, according to a new study out from Yale researchers, the Universe likes to be more orderly, and that affects planets and their environments.

The study, led by Yale assistant professor Malena Rice and two colleagues, looked at binary star systems with planets. In many of these systems, the planets orbit one of the two stars. “We show, for the first time, that there is an unexpected pile-up of systems where everything is aligned,” said Rice. “The planets orbit precisely in the same direction that the first star rotates, and the second star orbits that system on the same plane as the planets.”

Searching for Binary Star System Planets

To come to that conclusion, Rice’s team looked at data about binary systems in several databases. They also examined triple-star candidates. The Gaia DR3 catalog provided high-precision stellar astrometry. Such accuracy is crucial to determining the separations and distances of binaries and their associated worlds. NASA’s Exoplanet Archive provided planetary system parameters, and the TEPCat catalog of transiting planet properties supplied information about the physical properties in systems where the planetary orbits cross in the line of sight between the star and Earth.

The team used all the data to create 3D geometries mapping the planets in binary star systems. It turns out that nine of the 40 systems they studied had “perfect alignment.” That is, they have joint spin-orbit and orbit-orbit alignment. That means—due to regular, periodic gravitational interactions—the planets and stars move in orbits that have aligned with each other. In other words, everything in the system orbits in the same plane and same direction.

The extrasolar planet TrES-4b orbits a star in Hercules that lies about 1,660 light-years away. There's a binary companion in the system, as well. This is one system studied in the Rice team's research for binary system orbital alignments with planets. Artist's impression, credit: Lowell Observatory.
The extrasolar planet TrES-4b orbits a star in Hercules that lies about 1,660 light-years away. There’s a binary companion in the system, as well. This is one system studied in the Rice team’s research for binary system orbital alignments with planets. Artist’s impression, credit: Lowell Observatory.

That tendency toward some level of synchrony could be one way to look for binary systems that bear life, according to Rice. “It could be an indication that planetary systems like to push toward an orderly configuration,” she said. “This is also good news for life forming in those systems. Stellar companions that are aligned differently can wreak havoc on planetary systems, toppling them over or flash-heating planets over time.”

Misaligned Planet and Star Orbits

In systems that are not aligned, (sometimes termed “misaligned”) the conditions on planets don’t seem promising. Most environments in such worlds would not be at all conducive to life, or the conditions favorable to life. Worlds in such systems might be completely frozen, or completely dry and deserted. Or, their orbits could be irregular as they swing too close or too far from their stars. In that case, their surfaces could be lava—or ice—or change from one extreme to the other.

A misalignment might actually kick planets completely out of the system. In that event, the planet gets ejected to interplanetary space and becomes a rogue planet. Such worlds don’t have the advantage of warmth from a nearby star. The implications for life and habitability on those planets are pretty grim.

Temperate Worlds in Binary Systems

If the study conducted by Rice and her colleagues applies to a lot of binary systems, what kinds of worlds would we expect to see? Certainly, the fictional Tatooine looks forbidding at the same it beckons for exploration. In reality, if the orbital alignments are stable, then it’s possible to think of habitable worlds around those stars. Depending on the conditions, some of them could be quite nice and welcoming.

However, expecting a reasonable, temperate Earth-like climate at such a distant world is a bit of a stretch. It would be quite different from what we experience here at home. That’s because a planet in such a system gets light from two stars. During part of a year, any inhabitants would have continuous daylight from each star lighting up a side of the planet. The temperatures on the surface would vary as a function of stellar distance, however. And, at least some part of a year, one side of the planet would get heated up from both stars, making things pretty toasty for any inhabitants.

With this study in hand, astronomers now have an opportunity to search the databases for more binaries with aligned orbits between stars and planets. In addition, there are interesting implications for studies of young binary systems with protoplanetary disks around each star. For example, astronomers could use the data to understand what dynamical limits there are on disks in multi-star systems. More insight is needed into the masses and viscosities of those disks as their stars evolve, as well as how long those disks survive in the system.

For More Information

A New Beginning: The Search for More Temperate Tatooines
The Orbital Geometries and Stellar Obliquities of Exoplanet-hosting Multistar Systems