Good luck pitching a tent on exoplanet WASP-76b, the bloody raindrops here are made out of molten iron The weather is very strange on WASP-76b. Liquid iron rains down on one side of the exoplanet, every night. A paper describing the weird finding was published in Nature on Wednesday. Using the European Southern Observatory's Very Large Telescope in Chile, a large international team of astronomers led by the University of Geneva …
The core of the planet is a molten ball of the elements whose boiling point is lower than the night side temperature. Anything that boils at lower than the night side temperature is part of the 'gas' that makes up the atmosphere.
What is happening is that the day side is hot enough to boil elements out of the core, then strong winds carry these around to the night side, where they cool off enough to precipitate out and return to the core as liquid.
Eventually thermal currents in the liquid will carry this material back through the core to the day side where the process will repeat.
Because that's not what "tidally locked" means. It's certainly possible for a planet (or other body) to do that, but it wouldn't be tidally locked if it did. The planet you're thinking of is Uranus, which is tipped over at nearly 90 degrees compared to pretty much everything else. But it's not tidally locked to the Sun, so different parts of it still get different amounts of light at different times of its year.
As for how it happened, the most recent thinking is that it probably couldn't have been a simple collision with another large body, since a big enough hit to cause the tilt should have outright destroyed it. Something pretty major must have happened to it, but it was likely rather more complicated than previously thought.
Why?
Why "... something pretty major ..."?
Uranus is a looong way out, Sol's influence isn't overwhelmingly huge, perhaps the largest pool of collapsing, whirling stuff in the original cloud out there just happened to be rotating "sideways" and no reat force happened to be around to stop it doing so?
Random chance, a small eddy compressing into a larger on simply because it was the only "force" of any consequence in the vicinity. Uranus's moons would have followed it, of course, as they. too, would have condensed out of the tipped primordial whirly.
And why have no other planets "fallen over" like Uranus? Well, techincally they have, just to a smaller degree. Non of their rotational axes are exactly orthogonal to the Sun's equatorial plane, which is assumed to have been the plane of rotation of the original disc. Uranus is just the outlier.
No magic, no Giant Impactor, no mysterious series of not-so-giant Impactors needed, just simple physics doing its thing.
A soft-merging of two condensation sites co-orbiting explains the Moon and Triton, too. The System is huge, it's been around for a colossal amount of time, it has *room* for things like these. "Giant Impactors" are idiotic.
The "it's a gas giant" bit is correct..
Jupiter ( and for that matter, any gas giant) will have Iron Rain somewhere as well. With enough metal content, the inner temperatures will make it inevitable. The difference with WASP is that one side of the planet gets stoked up hard enough that this iron actually reaches the surface layers where we can detect it, instead of being hidden by several 1000's of miles of boring methane, ammonia, and water ice.
As the article puts it, they "detected a whiff of iron vapour on one side". At these kinds of distances, the star is effectively a dot, even through our best telescopes - how can they tell one side of the planet from another? Typically they look for planets based on changes in the star's brightness as the planet moves in front of it, and estimate the composition by looking at the changes in the emission spectra of the star. While producing some interesting articles like this one, I strongly suspect that much of what we "know" about exoplanets will eventually be disproven, likely by direct observation (i.e. sending a probe there, which obviously isn't happening anytime soon).
"how can they tell one side of the planet from another?"
Doppler shift in Iron emission lines perhaps. It'll presumably be the sum of planetary rotation round its star, and its "daily rotation". It'll (presumably) be slightly different at different points on the planet's orbit (and at different points in the Earth's orbit as well). Can "they" really measure things that finely and sort them out? I haven't the slightest.
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