Article Summary #12

An Orbital Period of 0.94 days for the Hot Jupiter Planet WASP-18b

C. Hellier, D. R. Anderson, A. Collier Cameron, et al. 2009, Nature, Vol 460, 1098
http://www.nature.com/nature/journal/v460/n7259/full/nature08245.html

“Hot Jupiters” are exoplanets that initially form at the edge of their stellar system, far from their host stars, but due to the interactions with the proto-planetary disk (the disk around a young star consisting of highly dense gas that rotates around it) or by interaction with a massive object (i.e. massive planet), they end up in the star’s proximity (~0.02 astronomical units). Due to this short distance, the radiation from the star increases the planet’s temperature.

Hellier et al. 2009 look at the planet WASP-18b (first exoplanet that was confirmed to have a period smaller than a day) and analyze is tidal interactions with its host star. They found the stellar mass to be close to that of our Sun and the stellar age of 630 Myr, this making it one of the youngest known planet-hosting stars.

Tidal interaction theory states that when a massive planet is close enough to its host star it causes the formation of a “tidal bulge” on that star (similarly to the how the moon causes the formation of water tides on Earth). That bulge exerts a torque on the planet, decreasing the angular momentum of the planet and, thus, the latter starts spiraling inwards towards the star. The time required for the planet to spiral onto the star depends on several characteristics of the planet: mass, orbit, distance, and a tidal dissipation parameter Q (a quality factor that describes the ratio of the available energy to the dissipated energy during each period). This paper discusses how the value of Q affects the planet-star interaction.

The result they found was that if the Q calculated for the Solar System bodies would apply to WASP-18b, then the star would be spiraling inwards onto the host star in a very short time (less than 1% of the lifetime of its host star). Thus, it is assumed that this planet is an exception from what has been seen so far, or that the Q value does not apply to all systems in the same way.s