twin suns: better than one?


[NOTE: This post originally appeared at our new home at Scientific American.]

Yesterday Greg Laden posted a tantalizing snippet from a news release announcing a NASA press conference (for 2 PM EST today, at which point the embargo was lifted), heralding a new discovery by the Kepler mission. What could it be? Perhaps….. ALIENS??? Or something even weirder? Here's a clue: "A representative from Industrial Light & Magic (ILM), a division of Lucasfilm Ltd., will join a panel of scientists to discuss the discovery."

One of the more evocative scenes in the original Star Wars film shows a disaffected Luke Skywalker staring glumly at two setting suns in the sky on his desert-like home planet of Tatooine. These "suns" are G-type and K-type twin stars (known as Tatoo I and Tatoo II — I looked it up, okay?). The planet's surface is so hot and dry that the human inhabitants live underground and "farm" any moisture they can scrounge up. It's a pretty grim existence. No wonder young Skywalker is so anxious to leave.

It's not the only science fiction planet with two suns, either. Everyone's favorite vampire with a soul, Angel, had a lengthy story arc in Season 2 set in a seemingly bucolic dimensional world known as Pylea, which also had two suns — and apparently a different kind of sunlight, since Angel is delighted to discover that he doesn’t spontaneously combust when hit with sunlight on Pylea, exulting, “Did everybody notice how much fire I’m not on?” On Pylea, humans are slaves and the dominant species of horned demons have no concept of music. Which is why everyone's favorite karaoke-loving empath demon, Lorne (a.k.a. Krevlornswath of the Deathwok clan) left Pylea in the first place. His love of music made him a freak and an outcast.

Ahem. The point is, this whole "world with two suns" thing is a common trope. So fans of science fiction and fantasy should be thrilled at the reason for NASA's special press conference: a spanking new paper by Kepler scientists in the September 15 issue of Science, reporting the discovery of a planet roughly the size of Saturn that has "two suns" — that is, it orbits both stars in a binary system. Squee! Let us pause a moment to do the Dance of Joy!

Seriously, this is terrific news. Can discovery of the womp rat, bantha, Sarlacc, or Krayt Dragon be far behind? Well, maybe. Let's take a quick look at what we'd need for such a planet to (a) form in the first place, and (b) stick around long enough to foster conditions conducive to life — any life, not necessarily of the exiled Jedi knight variety, or the horned green-skinned members of the Deathwok clan.

Binary star systems are quite common in our universe. But can they form planets? The good news: yes, they can! Astronomers didn't used to think this was so. In a single-star system, like our own solar system, a young star is surrounded by an accretion disk of dust and gas, which gradually starts to clump together over millions of years, and as it cools, those clumps turn into rocks and form the cores of fledgling protoplanets. Those protoplanets pull in more and more debris (via gravity) until voila! You've got yourself a full-sized planet.

Things get a bit more complicated in a binary system, because you've got competing gravitational pulls, which can interfere with this accretionary process. The dust and gas that would otherwise form planets could be ejected from the system before said planets could fully form.

But it turns out that's not the only possible model for forming planets. Last year astronomers at Tennessee State University discovered a Jupiter-like planet orbiting one of a pair of binary stars, which they dubbed Inrakluk. It prompted them to propose an alternative "gravitational collapse" model for planets in binary star systems. Yes, it creates a turbulent environment for planet formation, but it can also lead to "overdense" regions of dust and gas, dense enough that the gravity of the cloud could rapidly collapse and form planets in just a few thousand years — big ones, too, on a par with the gassy giant Jupiter. So yay! Planets can form in binary star systems. The next question is whether such planets have stable orbits, and here's where we get a bit of a reality check in our dream of a real-world Tatooine.

Most planets in binary star systems have unstable orbits, thanks to the dueling gravitational pulls of the two stars. Generally, one of two things happens: the newborn planet collides with one of its parent stars, or it is ejected out of the system. But there are some scenarios where a binary star system could have a stable orbit. For instance, if the two stars were very close together, but the planet in question was further away, its orbit would be a bit wobbly, but otherwise stable. Alternatively if the stars were far apart from each other, and the planet orbited just one of them, the orbit would be fairly stable, because the gravitational tug-of-war would be diminished.

That's what so unusual about this latest Kepler discovery: the planet is orbiting both members of the dual star system — behavior that astronomers have suspected might occur, but haven't been able to observe such a planetary transit until now. (A planetary transit is what happens when a star's light dims as a planet passes in front of it — one of the methods astronomers use to locate exoplanets.)

The team of researchers, led by the SETI Institute's Laurence Doyle, used data collected by NASA's Kepler Space Telescope, which tracks the brightness of over 150,000 stars, to find this unlikely planet. Specifically, they noticed a binary star system with two eclipses: one when the smaller star of the pair partially blocks its larger sibling, and another (secondary) eclipse in which the larger star blocks out the smaller one.

Naturally, these eclipses caused corresponding drops in brightness, which Kepler duly measured. But then Doyle et al. noticed other drops in brightness — third and fourth eclipses, if you will, that could not be attributed to the stars themselves being in an eclipse position. This strongly suggested the presence of a planet or other orbiting object.

It was also clear from the data that this object was orbiting both of the stars, because the extra "dimming events" (eclipses) occurred at irregular intervals, so the two stars had to be in different positions in their orbit each time the third body passed by. Doyle's team measured all the variations in timing for each of the four eclipses and calculated the gravitational interactions between all three bodies, concluding that this mysterious object was, indeed, a planet, roughly the size of Saturn, orbiting its parent stars every 229 days. (A separate analysis revealed the twin stars orbit each other every 41 days.)

It's nothing like the ravaged desert setting of Tatooine (and certainly unlike the pastoral landscape of Pylea). Dubbed Kepler 16b — could someone from LucasFilm help NASA come up with a catchier name? — the newly discovered planet is really, really cold (-100 degrees Fahrenheit, at least, suitable only for a mythical Frost Giant), and probably does not foster life. That's because Kepler 16b's two suns are smaller and cooler than our own sun.

This has been a banner year for research on planets with twin suns. In April, Jack O'Malley James, an astronomer with the University of St. Andrews in Scotland, co-authored a paper based on computer simulations hypothesizing that Earth-like alien planets with multiple suns might also boast trees and shrubs that are black or gray instead of green:

Photosynthesis — converting sunlight into energy — is the basis for the majority of life on Earth. It is the energy source for plants and, hence, animals higher up the food chain. With multiple light sources, life may have adapted to use all suns, or different forms may develop that choose to use one specific sun. This may be the more likely option for planets on which parts of the surface are illuminated by only one sun for long periods of time.

"If a planet were found in a system with two or more stars, there would potentially be multiple sources of energy available to drive photosynthesis," he said in a statement at the time. "The temperature of a star determines its color and, hence, the color of light used for photosynthesis. Depending on the colors of their starlight, plants would evolve very differently." Assuming, of course, that the planet wasn't freezing cold, thereby making it impossible for plants to grow at all on its surface.

So science fiction is fast becoming science fact, it seems. Not only do we have experimental confirmation of the existence of exoplanets with twin suns, it's possible those suns would give off a different kind of light (energy), with implications for the kinds of flora (and fauna) it could sustain. We eagerly await a scientific paper on whether such sunlight would be particularly hospitable to vampires. And if so? Then we will dance!

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