Misaligned planets left behind when a parent evaporates

Ars Technica » Scientific Method 2012-11-14

Artist's impression of a "hot Jupiter," a massive planet orbiting very close to its host star.

The Solar System is a remarkably orderly place. The eight planets orbit in the same direction the Sun rotates and in nearly the same plane as the Sun's equator. Many smaller bodies—asteroids, comets, etc.—do the same. However, exoplanets don't always play as nicely, and a noticeable fraction even have orbits opposite their star's rotation. The widely accepted models of planet formation can't describe these misaligned exoplanetary systems.

A new idea proposed by Konstantin Batygin may help resolve the problem. 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.

Exoplanets are often detected using transits: when the planet passes in front of its host star, it produces a small eclipse. This is most prominently used by the Kepler observatory. In addition to indicating the presence of a planet, these transits create an unexpected optical illusion: they make the star look like it's moving (or, at least, moving more than it already is).

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