Thursday, July 9, 2015

Deuterium Fusion in the Cores of Inflated Hot-Jupiters

Hot-Jupiters are a class of Jupiter-like exoplanets that reside in very close-in orbits around their parent stars. These planets are strongly irradiated and have intensely hot daysides. Like Jupiter, they are gas giant planets primarily composed of hydrogen and helium. Observations have shown that a large proportion of hot-Jupiters are inflated in size. Their radii appear too large even after accounting for the strong irradiation from the parent star. A number of mechanisms have been proposed to explain the inflated radii of hot-Jupiters. These include tidal heating and Ohmic heating. Basically, the inflated radii of hot-Jupiters require mechanisms that can deposit additional sources of energy within the bulk of the planet.

A study by Ouyed & Jaikumar (2015) suggests yet another mechanism that could account for the inflated radii of hot-Jupiters. Deuterium is an isotope of hydrogen and deuterium-deuterium (DD) fusion in the deep interior of hot-Jupiters can provide an extra source of energy. A problem with this process is that it requires extremely high temperatures (~100,000 K) in a layer of deuterium around the planet’s core. This an order of magnitude larger than the core temperatures typically found in hot-Jupiters. Nevertheless, DD fusion becomes more plausible if it instead occurs in the solid deuterated core of the planet. This is because the likelihood for DD fusion is significantly enhanced in a solid deuterated substrate and the temperatures typically found in the deep interiors of hot-Jupiter (~10,000 K) are sufficient to sustain the fusion process.

In the interior of a hot-Jupiter, core erosion takes place at a temperature of roughly 10,000 K or more. The high surface temperature on a hot-Jupiter induces the conditions needed for core erosion to occur. Erosion frees the deuterium needed to react with the non-eroded part of the core. This process can continue for billions of years, supplying the deuterium needed to sustain DD fusion and maintain the core region at ~10,000 K. The energy produced from DD fusion can keep the hot-Jupiter inflated, and in some causes, even over-inflating the hot-Jupiter.

Ouyed & Jaikumar (2015), “Nuclear Fusion in the Deuterated cores of inflated hot Jupiters”, arXiv:1506.03793 [astro-ph.EP