It’s an iconic shot: Luke Skywalker watches double sunsets as evening falls on Tattooine in Star Wars: A New Hope. Science fiction films like Star Wars, and novels like Brian Aldiss’ The Dark Light Years and Fire Time by Poul Anderson, have often featured habitable planets with binary or even trinary stars. But could humans–or any complex life–survive on a planet with two suns?
What is a Binary Star System? Binaries are star systems with two suns. Trinary systems are also possible, but rarer than binaries. In fact, binary systems may be more common than our own single-star system: four-fifths of the stars in the night sky visible as single points of light are actually binary pairs.
Our sun may even have been part of a binary system. The other star likely formed in the same cloud of gas and dust as our own sun, but broke off and joined the Milky Way’s stellar population billions of years ago.
Binaries are classified based on their distance from each other (as either wide or close binaries), as well as how they look when observed from Earth (whether they eclipse each other in their orbit, for example). In a binary pair, one star may orbit the other, or the two stars may orbit around a common center of mass called a barycenter.
Life in a Binary Star System? One thing is certain: binary systems make it even dicier for life to evolve. Double-star systems introduce more variations—sometimes wild fluctuations— in the amounts of heat and light (as UV radiation) received by any planet orbiting them. Planets with climates that regularly go from freezing to boiling aren’t likely to support Earthlike life, which depends on liquid water and a relatively narrow temperature range.
Another factor is that binaries may make it more difficult for planets to form at all. The competing gravitational pull of two stars on a protoplanetary disk of dust and rocky fragments may disrupt the formation of those rocky grains into protoplanets. On the other hand, that same gravitational disruption may actually make it easier for protoplanets to form by “stirring up” the protoplanetary disk.
However, scientists think that both non-circumbinary planets (planets that orbit one member of a binary) and circumbinary planets (planets that orbit both stars) could harbor life if they are located within both the system’s habitable zone and orbital stability zone.
Night and Day, Fire and Ice: Astronomers don’t like speculating too much about squishy biology, but we can make some guesses based on what they know. For instance, Paul Sutter says,
“Orbiting two stars at once, as our friend Kepler-47c does, makes life very elliptical, occasionally bringing the planet out of the zone. Life doesn’t take too kindly to frequently freezing over.
Orbiting just one star in a binary system? Well, sometimes you’ll have two stars in your sky at once, which can be a tad toasty. And sometimes you’ll have a star on each face of the planet, ruining the night. And don’t forget the double-doses of UV radiation and solar flares.”
So life that evolves on a world that dips in and out of the habitable zone might have evolved to withstand periodic freezing — by hibernation, or clustering around the lifegiving heat of deep-sea ocean vents. Periodic extreme heat might result in life that escapes the worst of it by burrowing, growing a calcified shell, or migrating to the cooler regions if the planet’s axis has a tilt like Earth’s does. We may also find extreme-heat-dependent adaptations, such as seeds that only germinate after forest fires. A montane plant called fireweed already uses this strategy on Earth.
Irregular day-night cycles might result in life with bio-rhythms attuned to two or more day-night periods, or to shorter active and rest periods. For instance, some Earth animals like rabbits, deer, and fireflies are crepuscular, or most active at dawn and dusk. On a planet with potentially multiple dawns and dusks in a 24-hour period, crepuscular cycles could be much more common!
Finally, exposure to higher levels of radiation and UV rays could have forced lifeforms to evolve powerful anti-oxidant and UV-blocking pigments to protect themselves from cancers and DNA damage. It’s even possible that their DNA (or equivalent hereditary molecule) could have evolved a more stable structure more resistant to damage.
Current estimates suggest 50-60% of known binary systems are capable of supporting life–that is, their planets, if they have any, would be both in habitable and orbitally stable zones. I don’t know about you, but I like those odds!
I drew on real science about binary star systems and how life might evolve on them to create the binary system in my novel, Absence of Blade. What’s the coolest fictional example of a binary star system you can think of? Tell me in the comments!