How a planet could form within a system where two stars orbit each other has been mapped out, explaining how planets exist where previous models said they shouldn't. These close binary systems are so common that this greatly expands the potential places we can look for planets – and therefore life.
Astronomers poked fun at many aspects of the original Star Wars. The possibility of a planet like Tatooine with two large suns wasn't considered such a laughing matter, but was still considered very unlikely. If planets existed at all in binary star systems, it was thought, it was probably only where the stars were so far apart the gravity of one star would barely affect planets around the other – yet the first confirmed planet beyond our solar system lies in a binary system. The Kepler Telescope has found a dozen “Tatooine planets” and TESS has shown we're missing many more.
So many varieties of binary systems exist that, depending on the relative sizes of the stars and their distances apart, it's impossible to model all scenarios. Researchers focused on one of considerable significance because it closely resembles Alpha Centauri. Although we have yet to discover a planet around our nearest bright neighbor, the findings reveal grounds for hope in a paper published in Astronomy and Astrophysics.
Alpha Centauri A and B orbit each other every 80 years. Dr Roman Rafikov of Cambridge University and Dr Kedron Silsbee of the Max Plank Institute for Extra-terrestrial Physics rounded up and modeled a pair locked in a 100-year dance.
"A system like this would be the equivalent of a second Sun where Uranus is, which would have made our own solar system look very different," Rafikov said in a statement.
“Planet formation in binary systems is more complicated, because the companion star acts like a giant eggbeater, dynamically exciting the protoplanetary disc," Rafikov added. This speeds up the motion of particles in the disk of gas, dust, and ice that surrounds a newly formed star, making it less likely they will stick together – instead, having collisions that break each other apart.
So how have any planets been found in binaries at all? The authors found that planets can still form under these higher energy conditions, but the disk from which they condense needs to be almost circular, rather than stretched in one direction. When this occurs, the drag effect of gas in the system and the disk's gravity slow the planetesimals down enough they stick rather than shatter on encounters. The authors had modeled each of these slowing effects on their own before – but found the combination of both can be quite potent.
A circular disk is not enough on its own, the paper concludes; planetesimals need to start out with a minimum width of 1-10 km (0.6-6 miles) across. The process by which material in the disk could quickly aggregate to this size could also help us understand planetary formation around single stars like our Sun.
The authors acknowledge the true situation is even more complex than what they have modeled. The material in the disk slowly evaporates, causing the locations where there is sufficient drag to allow planetary formation to occur to change with time. “Over the disk lifetime, planetesimal growth may be promoted in many parts of the disk,” the paper notes, but so far the modeling has been done as if these crucial locations were constant.