In the last two years, astronomers discovered several exoplanets in binary systems: two stars locked in mutual orbit. These systems come in several types, with the planet orbiting one or both stars. About half the binary systems involve stars that are very far apart: 1000 times the Earth-Sun separation or more. Naively, we might think that those systems are more stable, since the companion star is so far away. However, a new study shows that may not be the case.
Nathan A. Kaib, Sean N. Raymond, and Martin Duncan ran extensive computer simulations to model exoplanets residing in wide binary systems. They found that perturbations from other stars outside the binary system had a profound effect on the shape of the system's orbits. In some cases, planets were ejected from the system entirely or ended up in larger or highly eccentric (elongated) orbits. Based on these results, the researchers argued that some of the observed exoplanet systems with eccentric orbits may actually reside in wide binary systems where we haven't yet detected the companion stars.
A significant fraction of stars in the Milky Way are in binary systems. Some, like the Alpha Centauri system, are tight binaries: the two stars comprising Alpha Centauri are about 18 astronomical units (AU) apart. (1 AU is Earth's average distance to the Sun. For comparison, Neptune's average distance is about 30 AU.) However, some are known as wide binaries, with separations greater than 1000 AU.
In wide binaries, the mutual gravitational attraction between the stars is relatively weak. This allows other, neighboring stars to have a significant influence. As the other stars of the Milky Way gradually shift in their slow orbits around the galactic center, their gravitational influence on the binary fluctuates in time. The effect is to change the shape and size of the wide binary system, altering the mutual orbit over periods of billions of years.
While the process isn't exactly rapid, the new study revealed it can have a profound impact if there are planets orbiting one of the stars in the binary. Alteration of one star's trajectory increased both the size and eccentricity of the planets' orbits. Over a simulation period of about 10 billion years, 30 to 60 percent of systems designed to resemble our Solar System lost one or more planets, leaving the remaining planets in vastly different configurations.
According to the standard models of planetary formation, planets form in regular, circular orbits—ones with nearly zero eccentricity, in other words. However, observations have found many exoplanets are in highly eccentric orbits, and many are much closer to their host star than the naive planet-formation scenario would suggest.
The authors of the study presented their simulations as a possible resolution to some of these problems. If the exoplanets in eccentric orbits actually are in wide binaries—in which the companion star is undetected—then their strange orbits were caused by gravitational perturbations from the natural cycles of the Milky Way. This idea is also in agreement with an earlier paper, which posits that retrograde orbits of some exoplanets—planets orbiting opposite to their star's rotation—could be explained if there once was a binary companion star that is now absent.
These results also suggest that planetary orbits in wide binaries may be less stable over billions of years than they would be in tight binaries. If the two stars are closer together, the authors argued, they are less subject to disturbance from the ebb and flow of external gravitational influence. Of course, any planetary system with three or more interacting objects exhibits complex behavior, given long enough time; even the Solar System cannot be proven to be stable forever. (The fact that it has been relatively stable for 4.5 billion years is no guarantee that state will continue in perpetuity.)
The wide-binary model is certainly testable with further observations. Faint companion stars farther than 1000 AU would be hard to detect or challenging to prove they are gravitationally bound to the exoplanet system. However, if they could be identified for at least some eccentric exoplanet orbits, that would lend a lot of support to the proposed model.
Nature, 2012. DOI: 10.1038/nature11780 (About DOIs).
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