Theory of Relativity vs. Tatooine: Close exoplanets around binary stars unstable
Two researchers have found an explanation for why we find almost no exoplanets orbiting binary stars. According to them, Einstein is to blame again.
Artistic representation of an exoplanet in a binary star system
(Image: NASA/Goddard Space Flight Center)
Effects predicted in Albert Einstein's theory of relativity mean that there are practically no exoplanets orbiting two stars simultaneously in a stable orbit. An international research group has discovered this and summarizes their findings with a reference to Star Wars. Because this is precisely why we find almost no equivalent to the desert planet Tatooine. The cause is the so-called precession, which has different causes for the two stars and the planet orbiting them. Therefore, stable orbits cannot develop, and a possibly existing planet would either be pulled into the stars or flung out of the system.
Einstein Prevents Tatooine
The study authors from the University of California, Berkeley, and the American University of Beirut have calculated that the interplay of the two stars in a binary system, as a consequence of the theory of relativity, inevitably leads to their orbits around each other shrinking. At the same time, their precession, i.e., the change in direction of their rotation axes, increases, while that of a possible planet decreases precisely due to this process. As soon as both come into resonance, the planet is pushed into an extreme orbit that takes it much closer to the stars and then much further away. Then it is only a matter of time until it is either destroyed internally or flung out: “In either case, you get rid of the planet,” says lead author Mohammad Farhat laconically.
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The calculations presented in The Astrophysical Journal Letters do not mean that no exoplanets can orbit binary stars at all. However, stable orbits are only possible if the celestial bodies are so far out that they are very difficult to detect with current technology. This explains why, instead of the 300 examples that would be expected based on our models, we have found only 14 such exoplanets. None of them orbit closely around two stars, and twelve are just outside the instability zone.
(mho)
