How binary stars' planets are born

A new mathematical simulation shows how gas and dust could swirl into planets in dual-star systems

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Want to make a new planet? All you need is a newborn star and a metric boatload of gas and dust particles. As they orbit the young star, these tiny bits of ice and dust collide, eventually growing up into full-blown planets. 

Seems simple enough. But what about making a planet with two suns? Stars are generally much bigger than planets, and throwing a second one into the mix — as in a binary system — speeds up how fast the pre-planet space dust swirls around thanks to increased gravitational forces. At those speeds, collisions mean destruction and it’s hard to build a planet. But scientists have detected exoplanets orbiting around binary star systems. So how did they get there?

A new mathematical model solves this mystery by simulating the planet formation process in a specific type of binary star system, where the smaller star orbits around the larger star about once a century. The researchers found that, as long as the bits of dust and ice swirl around the main star in a roughly circular orbit, any drag effects from stellar gas become very large in certain parts of the disc. This drag slows down the dust particles to more reasonable, less explosion-y speeds so that they can actually stick together instead of destroying each other. The leading particles in a group are slowed more than the ones behind it, allowing the trailing particles to catch up and join the expanding clump. It's like a cycling road race. Cyclists tend to race in packs because wind drag is reduced behind a teammate. Once larger boulders about 10 kilometer in diameter are formed, they can survive high-speed collisions and are able to grow normally up to planet sizes.

While this particular kind of binary star system is now better understood, the next mystery for the new model to tackle is the formation of "Tatooine"-style planets, which orbit both stars in a binary system instead of just one. NASA’s Kepler Space Telescope has already found some of these deep in space, but we still don’t quite understand how they’re made.