WASP-12 b shows how tides and rotation reshape atmospheric readings

The pull of a nearby star and a planet’s rapid rotation can change the effective gravity across a close-orbiting planet’s atmosphere. Models for WASP-12 b and WASP-121 b show that accounting for these effects changes predicted transit signals and the molecular abundances inferred from them. The effect is larger for WASP-12 b, while tests find smaller shifts for more complex atmospheric mixtures.

Star system: WASP 12 | Distance from the Sun: 1347 light years

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WASP-12 b shows how tides and rotation reshape atmospheric readings

Star system: WASP 12 | Distance from the Sun: 1347 light years

A host star’s tidal pull and a planet’s rapid rotation can change the effective gravity in the planet’s atmosphere. For close-orbiting worlds, this can alter how spread out the atmosphere is and affect estimates of its molecular composition. Models of WASP-12 b and WASP-121 b show that including these effects changes predicted transit signals and the abundances inferred from them.

The calculations account for tidal and centrifugal effects, the outward influence associated with rotation, in models of the planets’ atmospheres. WASP-12 b was observed with the Hubble Space Telescope, and WASP-121 b with the James Webb Space Telescope. In the model calculations, the reduced effective gravity has a stronger effect on WASP-12 b: predicted transit depths for major molecules rise by 300 to 600 parts per million. The changes for WASP-121 b are 60 to 140 parts per million.

Atmospheric retrievals, which use a planet’s observed light to estimate the gases in its atmosphere, produce different molecular abundance estimates when they include the gravity corrections. Synthetic tests, which add modelled signals to data and check whether the analysis recovers them, find statistically significant shifts for simple atmospheric mixtures. For more complex mixtures, the shifts are smaller because multiple possible atmospheric explanations make the results less distinct. Tests with changing temperatures at different atmospheric heights and with cloud models retain the same broad trends.

Key findings
Tidal gravity and planetary rotation can change effective gravity, atmospheric scale height and inferred molecular abundances.
Modelled transit depths for major molecules increase by 300 to 600 parts per million for WASP-12 b, compared with 60 to 140 parts per million for WASP-121 b.
Gravity corrections change the retrieved molecular abundances for both planets.
Synthetic tests find statistically significant shifts for simple atmospheric mixtures, while more complex mixtures show smaller changes.
The broad trends remain consistent in models with changing temperatures across the atmosphere and with clouds.

Scientific study

Effect of tidal gravity and planetary rotation on the retrieved atmospheric abundances of close-in exoplanets (external link)