Many star systems seem to resemble our own Solar System: the planets orbit their host star in the same direction that the star spins. Admittedly, the data is still sparse: it’s not always possible to get that measurement. The brief version: you need the planet to transit or briefly eclipse its host star, and you need to be able to measure the small change in the star’s spectrum. This is most easily done when the planet is close in and relatively large—a class of exoplanets known as hot Jupiters, since they are big and close enough to experience extreme temperatures. Surprisingly, some of these hot Jupiters orbit their stars the wrong way, and this misalignment is difficult to explain within the standard theory of planet formation. However, a new model suggests that if the original star system had two stars, it would mess up the protoplanetary disk, leaving orbits askew.

In this revised model, strongly misaligned orbits are the result of another factor that influenced planet formation: a second star in the system. The gravitational influence of the companion star twisted the orbit of the exoplanet, pulling it out of alignment. And, in many cases, the star would leave little trace beyond the altered orbits: Sun-like stars often form in pairs or larger assemblies, but some of them evaporate over time. [Read more….]