Comet Enthusiast

Written by Steve Nerlich

A detailed literature review by Showman et al available on arXiv, outlines the scant observational data available on exoplanet atmospheres and then explores in detail what exoplanet atmospheres might be like, based on solar system examples and associated theory.

Key specifications such as an exoplanet's solar flux, spin and size are all potentially measurable with current or foreseeable technologies (e.g. the James Webb Space Telescope). This allows some robust estimates of the behaviour of an exoplanet's outer atmosphere – which we can expect to confirm through observation, as technologies continue to improve.

Trying to determine the behaviour of the atmosphere of a hot Jupiter – a gas giant so close to its star that it is either tidally locked or caught in a slow orbital resonance – is tricky, given that we have no precedents here in our solar system.

But, there is Venus – which, although not tidally locked, has such a slow rotation (once every 243 Earth days) that its dynamics virtually match those of a tidally locked planet.

Interestingly, Venus' upper atmosphere super-rotates, meaning it circulates in the same direction as the planet's rotation but much faster – in Venus' case, at sixty times the speed of the planet's rotation. It’s likely that these winds are driven by the large temperature gradient that exists between the day and night sides of the planet.

Conversely Earth, with its rapid rotation, has much less potential difference between its day and night side temperatures – so that its weather systems are more strongly influenced by the actual rotation of the planet and also by the temperature gradient between equator and pole. The nett result is lots of circular weather systems with their direction determined by the Coriolis effect – counter-clockwise in the northern hemisphere and clockwise in the southern.

And of course we do have gas giants, even if they aren't hot. Being so far from the Sun, dayside-nightside and equator-pole temperature gradients have little influence on our gas giants' atmospheric circulation. The most significant issues are each planet's rotation speed and each planet's size.

Jupiter and Saturn's larger radius exceeds their Rhines scale forcing the bulk flow of their atmospheres to break up into distinct bands with turbulent eddies between them. However, the smaller radius of Uranus and Neptune allows the bulk of the atmosphere to circulate as an unbroken whole, only breaking into two smaller bands at each pole.

The 'Rhines Scale' applied to solar system gas giants predicts that atmospheric circulation on large radius planets (Jupiter and Saturn) fragments into distinct bands, but doesn't on smaller radius planets (Uranus and Neptune). Credit: Showman et al 2010.

Partly because it's cooler, but mostly because it's smaller, Neptune's atmosphere has much less turbulent flow than Jupiter – which goes some way to explaining why it has the fastest stratospheric wind speeds in the solar system.

All these factors are useful in trying to determine how the atmosphere of a hot Jupiter might behave. Being so close to their star, it's likely these planets will be partly or fully tidally locked – so the main driver for atmospheric circulation will be, like Venus, the dayside-nightside temperature gradient . So a super-rotating stratosphere, circulating many times faster than the inner parts of the planet, is plausible.

From there, modelling suggests that the combination of fast wind speed and slow rotation means the Rhines scale will become bigger than a Jupiter-sized planetary radius , so there will be less turbulent flow and the upper atmosphere might circulate as one, without breaking up into the multiple bands we see on Jupiter.

Anyway, that’s my take on an interesting 50 page article with lots of (to me) bewildering formulae, but also lots of comprehensible narrative and diagrams. The article consolidates current thinking and lays a sound foundation for making sense of future observational data – both hallmarks of a nicely crafted ‘lit review’.

Filed under: Extrasolar Planets

Tags: Exoplanet atmospheres