Earth-like planets that orbit twin stars may be quite common, a new paper proposes, providing hope of sunsets glorious beyond anything Earth can provide.
When the original "Star Wars" came out, the dual sunset scene on Tatooine was viewed as something almost as improbable as lightsabers or a spacecraft that banks in a vacuum. Binary stars are common (in this and galaxies far, far away) and it makes sense for planets to be able to form around one member of a wide pair. In that case, however, the companion star would be too distant for much of a show.
Astrophysicists were far more skeptical about the prospects of planets orbiting both stars at once. But ever since the Kepler Space Telescope revealed seven gas giants in orbits once considered improbable, speculation has increased about whether the same could apply for rocky, Earth-like planets. If so, could these worlds remain habitable through the inevitable warming and cooling as the planet approaches and recedes from the stars in turn?
A paper submitted to the Astrophysical Journal, and pre-published on ArXiv.org, argues that the answer to both questions is yes. The paper, titled "Planet formation around binary stars: Tatooine made easy," considers the orbits of small rocks known as planetesimals and investigates whether they could aggregate into planets while circling two stars.
"We took our sweet numerical time to show that the ride around a pair of stars can be just as smooth as around one,” says lead author Professor Ben Bromley of the University of Utah. Consequently, he concludes, “We think Tatooines may be common in the universe.”
Bromley was inspired by the way the four small moons of Pluto and Charon survive in orbit around the pair.
Co-author Dr. Scott Kenyon of the Smithsonian Astrophysical Observatory says, "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."
Confusingly, the orbit in which planets can form is referred to as the “most circular orbit” even though it may not be circular at all. “It's an oval with nipples,” Kenyon says.
“Planetesimals need to merge gently together to grow,” Kenyon says. “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." This occurs even under the influence of a larger planet's gravity. However, previous research had noted that around binary stars planetesimals cross each other's paths at high speed, producing collisions that lead to smaller objects, not bigger ones.
Bromley and Kenyon conclude that planetesimals seldom move into these demolition derby orbits. Instead, they fall into oval orbits. “If the planetesimals are in an oval-shaped orbit instead of a circle,” Kenyon says, “Their orbits can be nested and they won't bash into each other. They can find orbits where planets can form."