A paper published in Nature last week has discovered that planetary systems containing so-called hot Jupiters are likely to have had violent pasts.
The evidence stems from a difference in the way that these planets orbit their parent stars. In our own solar system, the planets orbit the Sun in very similar planes, so that any three-dimensional model of the solar system looks rather like a dinner plate, with all of the planets orbiting in the surface of the plate. In addition, the Sun's own rotation axis is closely aligned with the orbits of the planets.
This alignment is as we would expect from our understanding of how planetary systems form. We think that both stars and planets form from proto-planetary discs -- clouds of gas and dust that collapse under their own gravity, flattening into a swirling disk. The central part of such a disk forms a star, while the outer parts may form into a number of planets around it. Since both the star and the orbits of its planets derive their rotation from the same disk, we would expect them both to be aligned with the disk's original rotation axis.
However, the pattern seen in our own solar system has not been reproduced by other planetary systems that we have observed. These systems often have large Jupiter-sized planets orbiting very close to their parent stars, at a similar distance to that of Mercury from our own Sun. Moreover, the orbital planes of these so-called "hot Jupiters" are almost randomly orientated, showing little correlation with the rotation axes of their parent stars.
It is a puzzle how these systems form, and astronomers think it likely that these Jupiter-sized planets form much further out from their host stars, but later migrate inwards, perhaps as a result of violent gravitational interactions with their neighbours or with nearby stars.
Writing in Nature, Roberto Sanchiz-O'Jeda of MIT and his colleagues present the first observations of the orbits of the planets in a more normal system, as observed by the Kepler Space Telescope. They find that this system, like our own, has the planets orbiting in a common plane which is aligned with the host star's rotation axis.
The implication is that planetary systems do indeed form with their planets orbiting in a common plane, but that violent interactions can upset the orbital alignment. This confirms the theory that hot Jupiters do not form in their present locations, but migrate inward as a result of violent events in their histories.