Exoplanets evolving in a tight binary star system are in a risky environment: US astronomers have developed models that show how these worlds are easily flung into space more often than average. The researchers therefore suspect that in such systems exoplanets are found significantly less frequently.
Previous observations have suggested that in our Milky Way galaxy, binary systems are more common than single stars. Mysteriously, these binary systems have fewer exoplanets than one would expect based on statistical probability. There seems to be something preventing planets from staying longer in these systems, especially in binary systems with an orbital period of less than 7.5 days. David Fleming from the University of Washington has now come up with a solution to this puzzle together with his colleagues.
The researchers developed a number of binary system computer models and examined which forces occur there. Their results show that when two stars circle each other at very close intervals, their originally elliptical orbits become circular over time. In addition, their rotational speed is also reduced.
This development causes the two stars to move farther apart. “A part of the torque from the stellar rotation concerns the orbits of the stars around each other,” explains Fleming. “That makes their orbits larger.” This is not good news for existing exoplanets: with the growth of the orbits of the stars, even the zone in which stable planetary orbits are possible continues to move outward.
The simulations showed that in 87 percent of all close binary star systems at least one exoplanet is flung away – with conservative assumptions. In fact, according to the scientists, the value could even be 99 percent. For Fleming these numbers mean above all that one could exclude some binary systems from the outset in the search for in particular earth-like exoplanets, which will help to save time and resources.