Sunday, August 3, 2014

Estimating the Mass-Loss for a Super-Earth

Figure 1: Artist’s impression of a super-Earth or mini-Neptune with a hydrogen-helium envelope.

GJ 1214 is a red dwarf star located at a distance of approximately 47 light years. In 2009, an exoplanet identified as GJ 1214 b was reported around the red dwarf star. GJ 1214 b has 6.55 ± 0.98 times the mass of Earth, 2.68 ± 0.13 times the radius of Earth and an orbital period of 1.58 days. Its short orbital period indicates it is very close to its host star. The mass and radius of GJ 1214 b suggests it has a density of only 1.9 ± 0.4 g/cm³. For comparison, the mean density of Earth is 5.515 g/cm³. This indicates GJ 1214 b is not dense enough to be a rocky planet. Instead, the bulk composition of GJ 1214 b is consistent with one of three different models: (1) a mini-Neptune, (2) a hot ocean-planet, or (3) a rocky core with an extended hydrogen-helium envelope.

A planet that orbits very close to its parent star receives high levels of stellar insolation and large amounts of high energy radiation from stellar activity. This can heat up the planet sufficiently to drive planetary mass-loss. In fact, ongoing mass-loss has been observed for a number of hot-Jupiters. Using the XMM-Newton, a space-based X-ray telescope, Lalitha et al. (2014) observed the host star of GJ 1214 b in X-rays and show that it is a mildly active star with an X-ray luminosity of 7.4×10^25 erg/s. An “erg” is a unit of energy equal to 100 nanojoule. Planetary mass-loss is primarily driven by X-ray and extreme UV radiation. In the case of the host star of GJ 1214 b, based on its observed X-ray luminosity and a computed extreme UV luminosity of 1.23×10^27 erg/s, the estimated mass-loss rate of GJ 1214 b is ~1.3×10^10 g/s, or ~13,000 metric tons per second. Given that the host star of GJ 1214 b has an age of between 5 to 10 billion years, GJ 1214 b is estimated to have lost a total of 2 to 5.6 Earth-masses.

Figure 2: Estimated total planetary mass-loss for GJ 1214 b. An age of 5 to 10 billion years leads to a total mass-loss of between 2 to 5.6 Earth-masses. Lalitha et al. (2014).

Lalitha et al. (2014), “X-ray emission from the super-Earth host GJ 1214”, arXiv:1407.2741 [astro-ph.SR]