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Exoplanet Climatology: Surprising Find from ESPRESSO

Learning about the climate on exoplanets is not something that the designers of ESPRESSO had in mind. Installed at the European Southern Observatory’s Very Large Telescope at Paranal (Chile), the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations combines the light of the four VLT telescopes, making it a powerful instrument indeed. We have what is in effect a 16-meter telescope that really ramps up the capability of radial velocity methods.

But climate? The case in point is WASP-76b, some 390 light years away in Pisces. So much of the excitement surrounding ESPRESSO has been its ability to drill down to detect small, rocky exoplanets, but this world is somewhere near Jupiter mass, considerably larger in radius, and hellishly close to its star, an F7-class object about 1.5 times as massive as the Sun. The planet orbits the star every 1.8 days at an orbital distance of 0.03 AU and appears to be tidally locked.

The result: Temperatures in the area of 2,400° C, breaking molecules apart and causing metals to evaporate. Throw in a dark-side temperature of about 1,500° C and you’ve got an incubator for extreme winds carrying iron vapor into the nightside, where it precipitates as what we might call an iron rain. This is more than conjecture, as ESPRESSO is powerful enough to detect a strong iron vapor signature at the edge of this planet’s ‘evening,’ with no such signature on the other side.

Says Christope Lovis (University of Geneva):

“Surprisingly… we don’t see iron vapour on the other side of the planet, in the morning. The conclusion is that the iron has condensed during the night. In other words, it rains iron on the night side of this extreme exoplanet.”

Image: WASP-76b’s showers appear to consist of iron raindrops, as seen in this illustration. Credit: M. Kornmesser/ESO.

ESPRESSO is just coming into its own. First light occurred in 2017, and the WASP-76b work draws on the first scientific observations made with the instrument in September of 2018. We’re beginning to see that this spectrograph has unusual potential, as principal investigator Francesco Pepe notes:

“We thought very early on that we could use the instrument not only to discover new planets, but also to characterize those that are already known. However, until 2018, we didn’t realise how powerful ESPRESSO really was in this field.”

Climatology on extreme planets makes that case, and we can assume WASP-76b is only the first such investigation. Meanwhile, we continue to witness the ever increasing power of radial velocity methods, which measure tiny variations in the Doppler signature of stars as they are pulled back and forth by the planetary systems around them.

Transiting worlds have been of crucial assistance as we have built up the exoplanet catalog, but radial velocity can measure planets that do not cross the face of their star as seen from Earth. Moving beyond detection levels of 1 meter per second, the new goal is 10 centimeters per second and below, a radial velocity precision sufficient to detect an Earth-mass planet in the habitable zone of a small star.

The paper is Ehrenreich et al., “Nightside condensation of iron in an ultrahot giant exoplanet,” published online by Nature 11 March 2020 (abstract).

And we should note this, from the journal: “This is an unedited manuscript that has been accepted for publication. Nature Research are providing this early version of the manuscript as a service to our customers. The manuscript will undergo copyediting, typesetting and a proof review before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers apply.”

In the era of easily available preprints, we always have to keep in mind the consequences of authorial and editorial changes later in the process. These can sometimes be significant.

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{ 10 comments… add one }
  • David Hart March 16, 2020, 19:31

    I am wondering how a tidally locked planet can have an ‘evening’ and ‘morning’ side. There is no sunrise or sunset, only a terminator band. I suspect the authors mean that the atmosphere has a preferred rotation direction like Venus.

    • andy March 17, 2020, 19:12

      The bulk of the planet (averaging over all the fluid dynamics that’s doubtless going on) may well be tidally locked but it’s a gas giant so there’s no solid surface to anchor your longitude to. The observed part of the atmospheres of hot Jupiters undergo super-rotation, so terms like “evening” and “morning” are perfectly sensible. If you were in some hypothetical heat-resistant balloon floating in the atmosphere, that’s what you’d experience.

  • wdk March 16, 2020, 20:19

    This might be an extreme case, but applicability to other tidally locked planets. If you have iron evaporating on one side of the planet and raining down as ice on the other, does that effectively change the
    mass properties or principal moments of inertia enough to cause tipping?

    I suppose the material raining down would reheat on the dark side in descent, depending on the overall density of such a world. It might already be down to a metal core and a very strange fluid surrounding.
    If the condensation occured at the far sun side, the rain would be an influence that would cause a prolate form, simply modeled. But what if the rain began shortly past the terminator? Possibly unstable and could cause a torque or even tumbling.

    • Alexander Tolley March 17, 2020, 0:03

      Is it possible to have a planet-sized circulation, with the liquid metal flowing back to the sunward hemisphere to evaporate again and maintain a cycle?

      • Andrei March 17, 2020, 17:17

        I assumed that there would be strong winds with metal rain only on one side of the planet. That heat transport is necessary keeping the metal floating on a planet that will have bound rotation. Then wdk suggested another possibility. If there would be a build up of solid metal on the night side, then the planet would start to turn to get the most mass facing the star. So the planet possibly also a slow rotator, like Venus.

  • wdk March 17, 2020, 10:45

    There must be some kind of circulation going on, but this place is so bizarre, it is hard to assess quickly what the other chemical components might be. The rain might drop down on more refractory materials, whatever they might be – and create lakes or even an ocean atop of it.
    Or it might even solidify, though the pressure and temperature profile is likely extreme as well. But there would be ways all this would spread or circulate back to the solar facing side.

    … You don’t suppose someone set this up for mining?

    • Gary Wilson March 17, 2020, 12:58

      Yes wdk! The USCSS Nostromo is en route as we speak :).

    • andy March 17, 2020, 18:56

      Nice idea but it’s a gas giant so there’d be no surface for lakes or oceans to collect on. Probably the iron vaporises in the depths and some of it gets cycled back to the dayside to begin the cycle again.

  • Joe March 17, 2020, 22:08

    Can you imagine what the magnetic field must be like? All this iron moving about?

  • Ivan Vuletich March 24, 2020, 7:00

    Iron Rain…

    Sounds like the perfect venue for an epic Heavy Metal concert, so loud you could hear it from a neighboring planet!

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