The Jupiter mass planet WASP76b orbits its 1.5*Sunmass star at 0.03AU and is tidally locked. The day side heats to 2400C which vaporizes metals, including iron observed in its atmosphere. The balmy night side is only 1500C causing ferocious winds which carry the iron vapor from hot to cool side. There it condenses, falling as rain.

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Could this redistribution of mass change how the planet faces its star, and what could make that happen? For example, if the winds blew only one direction (I don't know why this would occur) might that act as a small spin that tidal locking forces might try to neutralize?

This is a refugee question from Space Exploration.

  • $\begingroup$ @uhoh Someone who understands tidal locking ought to be able to explain whether a redistribution of mass would change what face a planet presented, whether this particular planet or not. This is physics in space so I considered Astronomy. I can move it if that is the right thing. $\endgroup$
    – Pilothead
    Commented Mar 12, 2020 at 4:21
  • $\begingroup$ I've changed my mind about "primarily opinion-based" and deleted my comments because probably conservation of angular momentum is all that's needed to generate a plausible answer; there's much known (or at least theorized) information about this particular planet. $\endgroup$
    – uhoh
    Commented Mar 12, 2020 at 4:40
  • $\begingroup$ at those temperatures I'd expect active tectonic currents (like on earth) in the planet's mantle, carrying the materials back to the cooler side and offsetting any mass change. $\endgroup$
    – jwenting
    Commented Mar 12, 2020 at 5:01
  • $\begingroup$ If the iron rained out as soon as the vapor entered the dark side, an oblate spheroid would be formed, which might (I haven't done the math) be flipped 90 degrees by tidal forces. Also, if you make assumptions about the interior structure, something like this could happen: Great tilt gave Mars a new face. $\endgroup$ Commented Mar 18, 2020 at 4:13

1 Answer 1


The important thing to bear in mind is that WASP-76b is a gas giant (at the time of writing, the NASA Exoplanet Archive gives values of 0.92 Jupiter masses and 1.83 Jupiter radii, so this planet is much less dense than Jupiter is): there's no solid surface for the iron to build up on. Instead what would happen is that the iron would rain down until it reached depths at which the temperatures are sufficiently high for it to vaporise again. This would then allow it to be carried back to the dayside as iron vapour. Once it is back on the day side of the planet, the increased temperature would allow it to rise back up to higher altitudes, where the winds would then carry it back to the night side in an "iron cycle".

Superrotation is predicted to be a general feature of hot Jupiter atmospheres, though the detection of westward displacements of hotspots on planets like CoRoT-2b suggests that things may be more complex: see for example Debras et al. (2019) "Acceleration of superrotation in simulated hot Jupiter atmospheres" for some discussion of how this might occur.

There's also the question of the so-called "thermal tide", where the atmospheric heating affects the rotation. This effect is thought to be responsible for Venus avoiding tidal lock. On a gas giant there isn't a solid surface for the atmosphere to couple to, but in some cases this effect may contribute to at least the outer radiative zone of the planet being in non-synchronous rotation, see Auclair-Desrotour & Leconte (2018) "Semidiurnal thermal tides in asynchronously rotating hot Jupiters" for details.

  • 3
    $\begingroup$ What doesn't make sense about a gas giant being tidally locked? Gas giants do have rotation periods: in particular the deep interior rotates as a rigid body. $\endgroup$
    – user24157
    Commented Mar 20, 2020 at 19:53
  • 2
    $\begingroup$ There is no known terrestrial planet of Jupiter mass. It's unlikely that such could even exist and no known way to create it. $\endgroup$ Commented Mar 21, 2020 at 1:01

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