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Parsing Exoplanet Weather

Although it seems so long ago as to have been in another century (which it actually almost was), the first detection of an exoplanet atmosphere came in the discovery of sodium during a transit of the hot Jupiter HD 209458b in 2002. To achieve it, researchers led by David Charbonneau used the method called transmission spectroscopy, in which they analyzed light from the star as it passed through the atmosphere of the planet. Since then, numerous other compounds have been found in planetary atmospheres, including water, methane and carbon dioxide.

Scientists also expect to find the absorption signatures of metallic compounds in hot Jupiters, and these have been detected in brown dwarfs as well as ultra-hot Jupiters. Now we have new work out of SRON Netherlands Institute for Space Research and the University of Groningen. Led by Marrick Braam, a team of astronomers has found evidence for chromium hydride (CrH) in the atmosphere of the planet WASP-31b, a hot Jupiter with a temperature of about 1200° C in the twilight region where light from the star passes through the atmosphere during a transit.

This is an interesting find because it has implications for weather. The temperature in this region is just where chromium hydride transitions from a liquid to a gas at the corresponding pressure in the atmosphere, making it a potential weather-maker just as water is for Earth. Braam speaks of the possibility of clouds and rain coming out of this, but hastens to add that while his team found chromium hydride with Hubble and Spitzer data, it found none in data from the Very Large Telescope (VLT). That would place the find in the realm of ‘evidence’ rather than proof.

Even so, it gives us something interesting to work with, assuming we do get the James Webb Space Telescope into space later this year. Co-author and SRON Exoplanets program leader Michiel Min notes the significance of the orbital lock hot Jupiters fall into:

“Hot Jupiters, including WASP-31b, always have the same side facing their host star. We therefore expect a day side with chromium hydride in gaseous form and a night side with liquid chromium hydride. According to theoretical models, the large temperature difference creates strong winds. We want to confirm that with observations.”

Image: A hot Jupiter crossing the face of its star as seen from Earth. Credit: ESA/ATG medialab, CC BY-SA 3.0 IGO.

WASP-31b orbits an F star at a distance of 0.047 AU and is one of the lowest density exoplanets yet found, with a mass of 0.478 that of Jupiter and a radius 1.549 times Jupiter’s. Potassium has already been found in its atmosphere as well as evidence for aerosols in the form of clouds and hazes, with some evidence for water vapor and ammonia. Braam and team analyzed previously available transmission data from Hubble and Spitzer using a software retrieval code called TauRex (Tau Retrieval for Exoplanets), reporting “the first statistical evidence for the signatures of CrH in an exoplanet atmosphere.” The paper goes on to note:

The evidence for CrH naturally follows from its presence in brown dwarfs and is expected to be limited to planets with temperatures between 1300 and 2000 K. Cr-bearing species may play a role in the formation of clouds in exoplanet atmospheres, and their detection is also an indication of the accretion of solids during the formation of a planet.

The paper is Braam et al., “Evidence for chromium hydride in the atmosphere of hot Jupiter WASP-31b,” accepted at Astronomy & Astrophysics. Abstract / Full Text.

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{ 15 comments… add one }
  • Michael February 4, 2021, 16:44

    Lava lamps must sell like hot cakes on this world. We could have had these worlds too before been absorbed into the sun early in its youth. These look like survivors in a matter eat matter world.

  • Gary Wilson February 4, 2021, 17:10

    I don’t think I’d want a holiday on WASP-31b. Too much chromium in the air for my taste :).

  • Geoffrey Hillend February 4, 2021, 20:09

    With a wider radius than Jupiter, One might think WASP-31b would have a larger mass than Jupiter, but it’s density is less so it must have a smaller mass. We get the same thing with Uranus which is larger than Neptune but less dense, therefore less massive than Neptune.

    • Ashley Baldwin February 5, 2021, 7:00

      With a semi major axis of less than 0.05 AU from a bright F class star and a temperature of 1200 C I’m not surprised that it is all puffed up, tenuous and bloated. Not a good environment for habitability but a good big transmission spectroscopy target – for both primary and secondary eclipses.

      Even ignoring complicating clouds, it’s going to take a big telescope a long time to conduct analyses of cooler and much shallower terrestrial like atmospheres . Even Trappist-1 D and E prime targets are going to stretch JWST’s capabilities to identify all but CO2 within a realistic observation run. With even that – if present – taking the telescope and payload to its operational limits.

  • Andrew Palfreyman February 4, 2021, 23:12

    Still no oxygen amd chlorophyll. I’ll wait.

    • Ashley Baldwin February 5, 2021, 11:39

      Transmission spectroscopy has and will continue to revolutionise our characterisation and understanding of planetary atmospheres . But it has numerous significant limitations. Primary eclipse (night side/absorption ) spectroscopy – whereby starlight passes from behind and through the terminator of a transiting planet – is photon starved because of the small transmission area involved . Even for large , close in planets with heat inflated atmospheres as here. Much smaller terrestrial planets , with shallow terrestrial style atmospheres, will thus be even more photon starved.

      The likely presence of clouds ( consider Earth’s consistently high average cloud cover ) will make this even more pronounced. So a big telescope or/and long observation runs required.

      Secondary eclipses , whereby the star transits in front of the planet – for spectroscopy of the ‘day side’/ emission spectrum of the planet ( via subtraction of the ‘during eclipse’ spectrum from the ‘immediately before eclipse’ spectrum ) – is even more photon starved. Manageable for bigger, hotter gas giant planets around smaller stars but not yet possessing the sensitivity for temperate terrestrial style planets .

      A further problem is atmospheric refraction of transiting light. Especially shorter wavelengths including visible. Refracted away from the transiting planet . So resultant spectra are only for the outer layers of atmospheres. Not necessarily an issue for the deep atmosphere envelopes of gas giants but a big problem for shallow terrestrial atmospheres. The sort where we would hope to find oxygen and other bio signature gases like methane. Essentially because these gases would only exist in the troposphere of terrestrial atmospheres – within 10-20kms of the planetary surface . The bit where life – if it exists – and weather, might lurk. Thanks to refraction out of reach of all but longer near infra red wavelengths. Only M2 and later red dwarfs emit sufficient quantities of these wavelengths to theoretically allow meaningful transmission spectra to be measured – just. Even then only for the outer regions of a troposphere of any erstwhile habitable planet.

      I’m not sure that transmission spectroscopy of any type would collect enough photons to pick up the absorption spectrum of chlorophyll – ‘red edge’ or whatever – which will likely require direct imaging . Being demanding, even then only for the larger LUVOIR telescopes of the distant future with months long observation runs.

      No time soon then.

  • Michael Sloboda February 5, 2021, 9:15

    I suppose CrH as a set of prominent spectral lines so we can see it in a hot atmosphere. How did Cr get into the atmosphere? Wouldn’t all the Cr sink to the core along with other dense metals? What does that say about convection currents deep into the hot mini-Jupiter

  • Geoffrey Hillend February 5, 2021, 18:24

    I hope we make a star shade for the JWST, one with propulsion that can move and extend the range of direct imaging.

    • Ashley Baldwin February 6, 2021, 8:28

      Not going to happen I’m afraid . JWST would need to be adapted to operate in conjunction with a star-shade. Not an ‘excessive’ cost at few extra tens of millions of dollars but for a scope that is already monstrously over budget at around ten billion dollars – along with additional cost from a further launch delay – not going to happen.

      Star-shade technology is still a fair way from the kind of technological maturity ( especially vulnerability to micro meteor damage and refraction around its edges – ‘glint’ ) required for operations .

      There is a better chance of a star-shade rendezvous mission with the Nancy Roman telescope – a thirty something metre shade was explored as part of the probe class Exo-S concept . Though it would require a billion additional dollars to fly. Exo-S ( a report well worth reading to understand star-shades ) explored this as a practical option although its primary concept only utilised a 1.1m ‘ off the shelf ‘ NextView telescope. So the 2.4m Roman telescope would be much more capable – with the potential to image habitable terrestrial planets with the superior performance of a shade over its demonstrator coronagraph . The two used in concert might even allow ( with a suitable software upgrade) imaging of promising nearby binary systems like alpha Centauri , 61 Cygni , Eta Cassiopeia , 36 Ophiuchi and 70 Ophiuchi . None of which could be imaged with Roman’s coronagraph alone. None of which are on the search list of even HabEX !

      Cry havoc and let slip the dogs of war.

  • Henry Cordova February 5, 2021, 20:44

    It may seem obvious, but we should still make it a point to remind ourselves that the angle on the celestial sphere in which a distant astronomer must reside in order to see a transiting planet is equal to the angle on the celestial sphere which the star subtends on the transiting planet’s celestial sphere. So for example, if our sun subtends an angle of 30′ of arc as seen from earth, then only ET astronomers residing in a band 30′ wide centered on our ecliptic will be able to see the earth transit the sun–once a year!. Its easy to prove (hint: use similar triangles).

    Clearly, this geometry favors very close-orbiting planets. Jupiter, at about 5AU from the Sun, would only be visible in transit in a band 6′ of arc wide. Only ET astronomers on worlds inside a narrow strip 1′ wide would be able to catch Neptune in transit across the solar disc. Not only that, eclipses of near-orbiting planets would occur much more frequently to a distant observer. Jupiter transits would occur about 12 years apart, Mercury transits every 88 days.

    There may be a lot of planets out there we’re missing, and close-orbiters may be the exception, not the rule.

    • Ashley Baldwin February 8, 2021, 14:30

      Transit photometry , Doppler spectroscopy , transit timing variation and transit duration variation all favour close in and larger planets . A big source of observation bias – not that it has stopped a whole load of exoplanet astrophysical extrapolation and ‘characterisation’. Even direct imaging will favour closer , brighter planets – bar remote and nascent super Jupiters still radiating with their heat of formation.

      Microlensing does cover most sized planets ( down to Mars mass) in most orbits – but only as one off discoveries .

      Astrometry alone favours detailed characterisation and lasting discovery of wider orbit planets ( with baseline determined by observation duration ) . Though even here favouring larger mass. The sooner the technological barriers to the high precision ( such as sensor array dark current ) required for smaller planets are overcome the better. Sub micro-arc sensitivity for terrestrial mass bodies , though Gaia with its 10 micro-arc performance is expected to find tens of thousands of Jupiter mass bodies – including multiple ‘ true Jupiter ’ analogues.

      • Geoffrey Hillend February 16, 2021, 17:11

        There is also transit spectroscopy for it to work there is to be a fortuitous alignment of the orbit of the exoplanet orbits with the star, so the exoplanets have both primary and secondary transits, and the planet has to pass out of sight behind the star and pass in front of the star. Only the star light is seen when the planet passes behind the star and both planet and starlight are seen when the planet is not behind the star or not in secondary transit. The spectroscopy of the starlight alone is subtracted from starlight plus planet, but we can’t do that without the alignment so there can be any transmission spectroscopy if the planet does not cross in front of the star or we subtract to get the spectroscopy to differentiate the star light from the planet.

  • Geoffrey Hillend February 15, 2021, 17:31

    I didn’t think about the micro meteors and damage to the star shade. I agree with you, Since transmission spectroscopy is light that passes through the atmosphere during primary transit when the star is in front of the planet, I agree that we most likely won’t see any light at all from plants and absorption spectrum of chlorophyll because the light has to reflect off of the surface of the plants to get an absorption spectrum which can only happen when the planet is in a different phase in it’s orbit or not in front of the star but on either side of the star. If we look at the whole spectrum, all phase angles in time in throughout entire orbit maybe we might pick up something, but it is clear that we would have find an oxygen spectrum first which will be found throughout most of the atmosphere i.e., the oxygen will be easier to spot and if there is no oxygen, then we won’t need to look for any absorption spectrum of chlorophyll which won’t be there without oxygen.

  • Geoffrey Hillend February 15, 2021, 17:35

    Excuse me, the primary transit is when the planet is directly in front of the star, so the light passes through the atmosphere which causes an absorption spectrum. I think there is a small amount of emission. It can’t cause a reflection of the surface in transmission spectroscopy since the light is only going through the atmosphere.

  • Geoffrey Hillend February 15, 2021, 17:41

    There may be reflection off plants during primary transit, but it would be too small to observe as Ashley Baldwin has mentioned.

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