Hot Jupiters

Over the past decade, astronomers have discovered evidence of more than 200 planets orbiting stars other than the sun. (A current inventory can be found here ). One of the bigger surprises to emerge from this enterprise is the discovery of some stars that have jupiter-mass planets so close-in that they complete an entire orbit in the course of a few days (for comparison, our own Jupiter takes more than four thousand days to complete an orbit around the Sun). Because of their proximity, these planets are expected to absorb considerably more radiation from their parent star than Jupiter does from the sun. Hence they are often colloquially described as ``Hot Jupiters''.

This new class of planets has raised several fundamental questions about our understanding of planets and their origins. Most germane to our own project is the effect of this enhanced radiation on the structure of the planet and its atmosphere. If the planet is indeed gaseous like Jupiter, we expect it to possess atmospheric powerful winds as a consequence of its rotation. (The proximity to the star means that we expect that tidal forces have synchronised the rotation of the planet so that it rotates with the same period as its orbital period -- much like the Moon does as it circles Earth). However, how these winds interact with the radiation being absorbed from the star is still poorly understood. Indeed, our results suggest that the winds, if they exist, are not capable of carrying much of the absorbed energy around to the side far from the star.

Another question of importance is how these planets came to reside in their present location. There are two ways to explain this. Either the planet was formed where it is now, or it was formed further away from the star and subsequently moved in. Most astronomers believe the latter is the correct explanation, because it is harder to form a planet out of the kind of hot gas one expects to find orbiting close to newly formed stars. However, the mechanism that leads to this "migration" is still somewhat unclear. The proposal most widely accepted at present is the idea that the planet is swept inwards by the gaseous disk out of which it is born. However, there are other proposals such as the scattering of comets or direct planet-planet scattering which are also viable. In particular, there is some evidence that planet-planet scattering may have played a role in the upsilon Andromedae system in particular.

Brad Hansen
Background image by Robert Hurt