The Mystery of Tatooine-Type Planet Formation

Tatooine is the desert-like home planet of Luke Skywalker in the Star Wars movies. Famously, it orbits a binary star system so that two suns rise above the horizon each morning.

Tatooine is a work of fiction. But in recent years astronomers have found a number of real Tatooine-like exoplanets that orbit binary star systems–for example Kepler 16b, 34b and 35b

But as Sijme-Jan Paardekooper at the University of Cambridge,UK, and a few pals point out: “The existence of planets in these systems baffes planet formation theory.”

And that raises an interesting problem: how did these planets form?

Today, Paardekooper and co provide an answer. These guys simulate planet formation in a binary system at the distance these planets orbit–for 16b that’s just 0.7 AU, about the same distance as Venus from the Sun.

Previous studies have shown that the changing gravitational forces in a binary system stir up any cloud of gas and dust, ensuring a high encounter velocity between rocks. That would make it difficult for them to aggregate and stick together.

However, some astrophysicists have suggested that gas drag–the natural viscosity of gas clouds–might dampen this effect, making it possible for Tatooine-like planets to form after all.

Paardekooper and co put this theory firmly to the sword. They say their simulations show numerous perturbations that gas drag cannot prevent. “This makes the current location of the planets Kepler 16b, 34b and 35b very hostile for planetesimal accretion,” they say. 

Instead, the most likely way Tatooine-like planets form is much further away from their suns where gravitational conditions are much calmer. These planets must then have migrated to their current position over many millions of years, probably due to the dynamics of gas clouds which are known to generate pressure gradients that force planets inwards. 

Paardekooper and co make an interesting prediction, however. They say that whatever the migration mechanism of these planets, they lie at the same distance as what must have been the inner edge of their parent gas clouds. So they are about as close to their suns as is theoretically possible with this mechanism.

The implication is that we’re not likely to find exoplanets closer than this around binary stars. 

Interestingly, this distance is about the same as the habitable zone, at least for 16b. This lies at the outer edge of the zone and is thought to be a gas giant with a surface temperature of about -70 degrees C. That means its exomoons, if it has any, could be interesting. 

And if Tatooine-like planets stars tend to accumulate at this distance, perhaps the possibility of alien life forms watching twin suns rise on their home planet isn’t so unlikely after all. 

Ref: arxiv.org/abs/1206.3484: How Not To Build Tatooine: The Difficulty Of In Situ Formation of Circumbinary Planets Kepler 16b, Kepler 34b and Kepler 35b