Paul Sutter is a research fellow at the Astronomical Observatory of Trieste and visiting scholar at the Ohio State University's Center for Cosmology and Astro-Particle Physics. Sutter is also host of the podcasts Ask a Spaceman and Realspace, and the YouTube series Space In Your Face. He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
Who wouldn't love to have a delightful little picnic while watching a double sunset, just like the one featured on Tatooine in the early moments of the first "Star Wars" film? But is it even possible for a planet to exist in a multiple-star system, let alone one capable of supporting life as we know it — liquid water, thick but not too-thick atmosphere, stable stellar dynamics, etc.?
Sure! Well, maybe.
So many systems to choose from
Planets can exist in multiple-star systems. Astronomers have even observed them. Take Kepler-47c, a planet five times the mass of the Earth, in a very Earth-like orbit. The only difference is instead of one star in the center of the system, it has two. So contact your local travel agent and you can recreate that iconic film moment — just don't forget a space suit: Kepler-47c isn't exactly capable of supporting life, since it’s most likely a gas giant.
Astronomers don't know for sure just how common planets are around multiple-star systems. To be fair, we don't know for sure just how common planets are anywhere, but they appear to be just this side of quite numerous: a hundred billion planets, with a few billion of them life-friendly, in the Milky Way alone.
But you just can't plop planets down wherever you feel like it, especially when it comes to multiple-star systems. The problem, as usual, is gravity.
Drawn into gravity
We usually think of gravity as easy. Two things attract each other. Done. Drop something, it falls. Launch a rocket, it doesn't. Sure there are tides, and don't get near black holes, but this is the kind of stuff that dead folks with interesting wigs figured out a long time ago.
Gravity is indeed pretty easy, when it's just two things interacting. One planet plus one star? You're golden. It's so easy you can even write down the mathematical solutions of the possible orbits. All sorts of stable configurations. But put in another star? Or a third? The situation gets…tricky. Orbital stability isn't a given. And the math? Just look up a picture of Henri Poincare, one of the first people to try to tackle the problem of multiple objects orbiting together. Look into his eyes, and tell me that isn't the face of a man who has stared into the depths of mathematical hell and barely survived with his sanity intact.
I'm not saying that it's impossible: You may have noticed, if you are observant enough, that our own solar system contains more than two objects. Our system didn't fall apart billions of years ago because there's a hierarchy. In other words, each planet or moon or asteroid or whatever is dominated by one and only one other player.
For example, the Earth cares a lot about the gravitational pull of the sun, but not of Jupiter, and Jupiter feels likewise. The moon cares a lot about the Earth, but not the sun or Ceres. And so on. Every interaction is essentially one-to-one. Thus, all the planets get nice stable little orbits that can last for billions of years. If you broke this hierarchy, say, by shoving Jupiter into the inner system, or inflating it to be 10 times more massive, its gravity would start to compete with the sun's, the hierarchy would be broken, and so would the solar system.
All this gravity business means that planets around multiple star systems have only a few orbital options if they intend to stick around. Most potential orbits are unstable: even the faintest stellar breeze could potentially knock them either out of the system altogether or crashing into another body. To make a system a long-term home, a planet has two choices: either ensure that the gravitational pull of one star completely dominates the other, or that their gravitational effects are equal.
And that's just for a binary star. Don't get me started on multiples.
When two stars dance
Take Kepler-47c: the two stars orbit each other very closely and tightly , and the planet itself is rather far out. Far enough, in fact, that gravitywise the planet doesn't even care that there's two stars — to the planet, the center of the system just looks like a single star with the combined mass of the two suns. Stability achieved.
Another possible configuration is for a planet to only orbit a single star, with the other star far enough away and/or small enough to not matter. Figure-eight patterns around both stars are technically possible, but come on, don't hold your breath looking for one. Truly chaotic orbits — stable, but never repeating the same pattern twice — would be extremely rare, too, as fun as they might be.
So we can get planets in a binary system, although they might be more rare than planets around solitary stars. But there are heaps of binary stars out there, so even if a tiny percentage of them host planets, that still leaves heaps of planets.
Life on around a binary-star system
What does this mean for such a planet's weather and the prospects for life? That's a little more difficult to say, since it's hard to make general, broad-brush statements about the possibility of life anywhere, let alone in these kinds of binary systems. Instead, it's best to examine each system (real or imagined) in detail.
While binary systems certainly have a habitable zone, where liquid water could potentially exist on the surface of a planet, life might find it difficult to gain a foothold. 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.
With that kind of instability, erraticism and irradiation, it's hard to imagine complex life evolving with the kind of regularity it needs. But thankfully, Mother Nature isn't limited by our lack of imagination, so who knows what's out there!
Learn more by listening to the episode "Is Life Possible Around a Binary Star?" on the Ask A Spaceman podcast, available on iTunes (opens in new tab) and on the Web at http://www.askaspaceman.com. Thanks to Adam Diener for the great question! Ask yours on Twitter using #AskASpaceman or by following @PaulMattSutter.
Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com.