Ohmic Dissipation in Mini-Neptunes

• Published in: The Astrophysical journal 2017-09, Vol.846 (1), p.47
• Main Authors: Pu, Bonan, Valencia, Diana
• Format: Article
• Language: English
• Subjects: Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Atmospheric models; ENERGY ABSORPTION; ENERGY LOSSES; Evaporation; Heating; INSOLATION; IONIZATION; MAGNETIC FIELDS; Magnetic induction; MASS; Ohmic dissipation; Planetary composition; Planetary magnetic fields; Planetary mass; Planets; planets and satellites: composition; planets and satellites: interiors; planets and satellites: physical evolution; SATELLITES; Thermal evolution
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa826f

In the presence of a magnetic field and weakly ionizing winds, ohmic dissipation is expected to take place in the envelopes of Jovian and lower-mass planets alike. While the process has been investigated on the former, there have been no studies done on mini-Neptunes so far. From structure and thermal evolution models, we determine that the required energy deposition for halting the contraction of mini-Neptunes increases with planetary mass and envelope fraction. Scaled to the insolation power, the ohmic heating needed is small: ∼ 10 − 5 orders of magnitude lower than for exo-Jupiters ∼ 10 − 2 . Conversely, from solving the magnetic induction equation, we find that ohmic energy is dissipated more readily for lower-mass planets and those with larger envelope fractions. Combining these two trends, we find that ohmic dissipation in hot mini-Neptunes is strong enough to inflate their radii ( ∼ 10 15 W for T eq = 1400 K ). The implication is that the radii of hot mini-Neptunes may be attributed in part to ohmic heating. Thus, there is a trade-off between ohmic dissipation and H/He content for hot mini-Neptunes, adding a new degeneracy for the interpretation of the composition of such planets. In addition, ohmic dissipation would make mini-Neptunes more vulnerable to atmospheric evaporation.