Friday, January 27, 2017

Life Under a Double Star

You've all seen the iconic image: Luke Skywalker on Tatooine, gazing into the sky at two suns. That's George Lucas's way of whacking us over the head with a two-by-four -- "Toto, I've a feeling we're not in Tunisia anymore."

Could life actually exist on a planet orbiting a binary star? Ivan Shevchenko has recently made an extraordinary claim -- he argues, in this paper, that life is actually more likely to develop around binary stars than around a single star like our Sun. In Shevchenko's view, we are the weird ones, while life on planets like Tatooine should be common. 

A planet can orbit a double star in two different ways. In one case, the planet lies close to one of the stars, and the second star is much farther away.  If the second star orbits far enough out, it might have no effect on the planet at all, and the second star would simply appear as a bright object in the night sky.

But the second case is more interesting: the planet orbits both stars -- this happens when the radius of its orbit is much larger than the separation between the two stars. As you can imagine, the gravitational force produced by two stars is a lot more complicated than from just a single star, and no stable planetary orbits are possible if you get too close to the two stars. But far enough away from the stars, it's possible for planets to orbit around the double star system, and a number of these "circumbinary" planets have recently been discovered.

Shevchenko's argument is a bit technical, but he starts by examining some of the conditions that we think are favorable for life to develop: climate stability, the existence of tides, active tectonics, and the delivery of water to the planet after it forms. Shevchenko then argues that these conditions arise naturally in a planet orbiting a double star, whereas they arose on Earth only by accident. So life should be common around double stars, and rare around single stars like our Sun.

So if Shevenko is correct, there's a move theater out there on a distant planet, with multi-tentacled creatures gasping in awe as they see our hero scuttling up an alien landscape to watch a single sun set on the horizon.

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