Monday, June 3, 2013

Radiation-Blasted Exoplanets

Figure 1: Artist’s impression of CoRoT-2b - a hot-Jupiter being blasted by intense radiation from its parent star. Credit: NASA/CXC/M.Weiss.

Hot-Jupiters are a class of Jupiter-sized exoplanets that orbit very close to their parent stars and receive intense amounts of stellar radiation. A typical hot-Jupiter orbits its parent star at ~1/20th the Sun-Earth distance and receives ~10,000 times more radiation that Jupiter does from the Sun. Being so close to its parent star, a hot-Jupiter is expected to be tidally-locked whereby one side continuously faces it parent star while the other side points away into the darkness of space. A study done by Daniel Fabrycky (2008) shows that reradiated thermal radiation from a hot-Jupiter carries away momentum and this can gradually change the star-planet separation distance by ~1 percent over the planet’s lifetime. For more extreme cases, this change can be as much as a few percent.

Radiation is comprised of photons which carry momentum. As a result, a force is exerted on a planet when radiation is reflected, absorbed or reradiated by the planet. Radiation that is reflected or absorbed by a hot-Jupiter simply pushes it away from its parent star. However, the direction of force exerted on a hot-Jupiter by reradiated thermal radiation is more complicated. Three-dimensional models suggest that the hottest region on a hot-Jupiter is unlikely to lie at the planet’s substellar point. Instead, the hottest region is displaced eastward by a superrotating wind. This causes reradiated thermal radiation from a hot-Jupiter to be emitted in a preferential direction and thus acts as a radiative thruster which adds angular momentum to the planet, pushing it into a wider orbit around its parent star.

Figure 2: The force vectors due to radiation asymmetry as a function of orbital position for the HAT-P-2b - a hot-Jupiter in an eccentric 5.63-day orbit around a star that is slightly bigger and hotter than the Sun. Credit: Daniel Fabrycky (2008).

The radiative thruster effect is amplified for hot-Jupiters that orbit closer than typical to their parent stars or for hot-Jupiters with inflated cross-sectional areas. For example, OGLE-TR-56b is a hot-Jupiter which orbits in a very tight 1.21-day orbit around its parent star. As a consequence, the stupendous amount of stellar radiation received by OGLE-TR-56b amplifies the radiative thruster effect. The result is an estimated increase in the star-planet separation distance of OGLE-TR-56b by ~5 percent over the planet’s lifetime.

This study shows that the intense radiation received and reradiated by a close-in exoplanet can change the planet’s orbit over its lifetime. Although the study done by Daniel Fabrycky (2008) considered only hot-Jupiters, the radiative thruster effect is expected to be more pronounced for Earth-sized rocky planets in close-in orbits around their parent stars. A number of such exoplanets have already been discovered, and they include planets such as Kepler-10b and KIC 8435766b. These terrestrial-sized planets are likely to have a smaller cross-section per unit mass than hot-Jupiters and are likely to be more influenced by the radiative thruster effect.

Daniel Fabrycky (2008), “Radiative Thrusters on Close-in Extrasolar Planets”, arXiv:0803.1839 [astro-ph]