Extrasolar gas giant planets close to their host stars have likely undergone significant tidal evolution since the time of their formation. Tides probably dominated their orbital evolution once the dust and gas cleared away, and as the orbits evolved there was substantial tidal heating within the planets. The tidal heating history of each gas giant may have contributed significantly to the thermal budget governing the planet’s physical properties, including its radius, which in many cases may be measured by observing transit events. Typically, tidal heating increases as a planet moves inward toward its star and then decreases as its orbit circularizes. In Jackson+ (2008b), my colleagues and I computed tidal heating histories for several planets with measured radii. Several planets, including, for example, HD 209458 b, may have undergone substantial tidal heating during the past billion years, perhaps enough to explain its large measured radius. Our models also show that GJ 876 d may have experienced tremendous heating and is probably not a solid, rocky planet.

Tidal heating of rocky (or terrestrial) extrasolar planets may also span a wide range of values, depending on stellar masses and the planets’ initial orbits. Tidal heating may be sufficiently large (in many cases, in excess of radiogenic heating) and long-lived to drive plate tectonics, similar to the Earth’s, which may enhance the planet’s habitability. In other cases, excessive tidal heating may result in violent volcanism as for Jupiter’s moon Io, probably rendering them unsuitable for life. On water-rich planets, tidal heating may generate subsurface oceans analogous to the ocean in Jupiter’s moon Europa, with similar prospects for habitability. Tidal heating may enhance the outgassing of volatiles, contributing to the formation and replenishment of a planet’s atmosphere. In Jackson+ (2008c), my colleagues and I modeled the tidal heating and evolution of hypothetical extrasolar terrestrial planets to investigate the influence on planetary habitability.

Related Press:

• Tides Have Major Impact on Planet Habitability – U of AZ press release
• Talking About Tides – podcast interview with Simon Mitton about tides and planetary habitability

Related Scientific Publications:

• Jackson+ (2008c). “Tidal Heating of Extrasolar Planets.” ApJ 681, 1631.
• Jackson+ (2008b). “Tidal heating of terrestrial extrasolar planets and implications for their habitability.” MNRAS 391, 237.
• Jackson+ (2008). “Tidal Heating of Extrasolar Terrestrial-scale Planets and Constraints on Habitability.” BAAS 40, 391.