The single best known planet in the Star Wars films is Tatooine, a hot desert planet with two suns. It is the home planet of Luke and Anakin Skywalker (a.k.a. Darth Vader) and is one of the settings for the seventh film in the series which is due out in December.
For the last decade fans of the films have been a bit disappointed because, according to physicists, the planet was impossible. Theoretically, only gas giants were capable of forming in binary star systems and the data from NASA’s Kepler Space Telescope seemed to support that idea. To date only seven planets have been found in such systems and all of them have been Neptune or Jupiter sized gas giants.
However, a new mathematical analysis argues that Tatooine like planets are not only possible but could be quite common.
“Tatooine sunsets may be common after all. Our main result is that outside a small region near a binary star, [either rocky or gas-giant] planet formation can proceed in much the same way as around a single star. Our main result is that outside a small region near a binary star, [either rocky or gas-giant] planet formation can proceed in much the same way as around a single star. In our scenario, planets are as prevalent around binaries as around single stars,” says the study by astrophysicists Ben Bromley of the University of Utah and Scott Kenyon of the Smithsonian Astrophysical Observatory.
The NASA funded study, titled “Planet formation around binary stars: Tatooine made easy,” has been submitted to the journal Astrophysical Journal and an unreviewed copy has been published on the ArXiv website.
The study was a spinoff of earlier work by the two researchers that showed that Pluto and its largest moon Charon act like a binary system, with four smaller moons orbiting both.
According to the researchers, the problem with previous studies is that they focused on circular, ‘Earth-like’ orbits.
“Planets form like dust bunnies under your bed, glomming together to make larger and larger objects. When planets form around a binary, the binary scrambles up the dust bunnies unless they are on just the right orbit,” says Kenyon, whose observatory is part of the Harvard-Smithsonian Center for Astrophysics in a statement.
However, according to the research, “just the right orbit” is not particularly difficult to attain. The orbit of a planet like Tatooine would be an oval, with ripples caused by the cyclic tugging of the two suns orbiting one another.
“For over a decade, astrophysicists believed that planets like Earth could not form around most binary stars, at least not close enough to support life. The problem is that planetesimals need to merge gently together to grow. Around a single star, planetesimals tend to follow circular paths – concentric rings that do not cross. If planetesimals do approach each other, they can merge together gently,” said Kenyon.
Bromley adds that their formula shows that planetesimals will more likely achieve oval orbits rather than circular ones when they are young.
“Planets, when they are small, will naturally seek these oval orbits and never start off on circular ones. … If the planetesimals are in an oval-shaped orbit instead of a circle, their orbits can be nested and they won’t bash into each other. They can find orbits where planets can form,” he said.
The research did not simulate the billions of years that it could take for a planet like Earth to form and develop advanced life, however they did show that planetesimals could survive without violent collisions for tens of thousands of years in oval orbits around binary stars.
“We are saying you can set the stage to make these things. It is just as easy to make an Earthlike planet around a binary star as it is around a single star like our sun. So we think that Tatooines may be common in the universe,” said Bromley.
As for the Kepler data, the researchers urge patience. Most of the planets found to date, in any system, have been gas giants or large, rocky worlds known as “super-Earth’s”. The smaller and more distant a planet is, the less visible it is using current technology. As more powerful telescopes come online over the next decade, there is little doubt that many hundreds of thousands or possibly millions of additional planets will be discovered.
