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WASP-39b: JWST and Exoplanet Atmospheres

Although I often see the exoplanet WASP-39b referred to as a ‘hot Saturn,’ and sometimes a ‘hot Jupiter,’ the terms don’t really compute. This is a world closer to Saturn than Jupiter in mass, but with a radius somewhat larger than that of Jupiter. Hugging its G-class primary in a seven million kilometer orbit, it completes a circuit every four days. The system is about 700 light years from us in Virgo, and to my mind WASP-39b is a salutary reminder that we can carry analogies to the Solar System only so far.

Because we have nothing in our system that remotely compares to WASP-39b. Let’s celebrate the fact that in this exoplanet we have the opportunity to study a different kind of planet, and remind ourselves of how many worlds we’re finding that are not represented by our own familiar categories. I imagine one day we’ll have more descriptive names for what we now call, by analogy, ‘super-Earths’ and ‘sub-Neptunes’ as well.

I’ve seen WASP-39b referred to in the literature as a ‘highly inflated’ planet, which is an apt description. This is a transiting world, one that has been the subject of numerous studies that have produced insights into giant planet atmospheres under extreme conditions through the spectroscopic study of light from the star as it filters through the planet’s gaseous envelope. Even before the James Webb Space Telescope became available, we had learned a lot about its composition.

Now JWST data take our understanding to an entirely new level. Both the Hubble and Spitzer space observatories have gone to work on WASP-39b in recent years, finding interesting ingredients in an atmosphere that reaches 900 ℃. What we have from JWST, as revealed in a set of five just released scientific papers, is a wide range of atoms and molecules that provides signs of active chemistry and even cloud activity. JWST’s NIRSpec (Near-Infrared Spectrograph), NIRCam (Near Infrared Camera ) and NIRISS (Near Infrared Imager and Slitless Spectrograph) all produced data in this work.

Image: A series of light curves from Webb’s Near-Infrared Spectrograph (NIRSpec) shows the change in brightness of three different wavelengths (colors) of light from the WASP-39 star system over time as the planet transited the star July 10, 2022. Credit: Illustration: NASA, ESA, CSA, and L. Hustak (STScI); Science: The JWST Transiting Exoplanet Community Early Release Science Team.

An international team is behind the set of new papers, with hundreds of scientists analyzing the data. Although carbon dioxide had been found in pre-JWST observations, it appears in the new data at higher resolution, even as sodium, potassium and water vapor detections confirmed earlier findings from other instruments. Methane and hydrogen sulfide do not appear. We also have the first detection in an exoplanet atmosphere of sulfur dioxide, produced from chemical reactions to the star’s light.

Thus we learn of a role for photochemistry in the atmosphere of a star-hugging gas giant. Natalie Batalha is an astronomer at UC-Santa Cruz who was deeply involved in the new WASP-39B observational effort:

“We observed the exoplanet with multiple instruments that, together, provide a broad swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until JWST. Data like these are a game changer.”

The sulfur dioxide detection is worth pausing on, as it may offer insights into planet formation. The passage below is from Shang-Min Tsai et al., one of the five papers now available in preprint form:

The accessibility of sulphur species in exoplanet atmospheres through the aid of photochemistry allows for a new window into planet formation processes, whereas in the Solar System gas giants, the temperature is sufficiently low that sulphur is condensed out as either H2S clouds or together with NH3 as ammonium hydrosulphide (NH4SH) clouds making it more difficult to observe. Sulphur has been detected in protoplanetary discs where it may be primarily in refractory form. As such, sulphur may not undergo the level of processing inherent in the evolution of more volatile species, making it a preferred reference element when tracing the formation history of solar system objects through analysis of elemental ratios. Such efforts for warm giant exoplanets are now a possibility thanks to the observability of photochemically produced SO2. The improved constraints on bulk planetary metallicity provided by the observable SO2 feature further provides information on planet formation histories such as the accretion of solid material.

Image: New observations of WASP-39b with the JWST have provided a clearer picture of the exoplanet, showing the presence of sodium, potassium, water, carbon dioxide, carbon monoxide and sulfur dioxide in the planet’s atmosphere. This artist’s illustration also displays newly detected patches of clouds scattered across the planet. Credit: Melissa Weiss/Center for Astrophysics | Harvard & Smithsonian.

All this is excellent news as we ponder the implications for JWST’s capabilities in relation to small, terrestrial worlds. The work is part of a program called Director’s Discretionary-Early Release Science (DD-ERS), which is designed to help the scientific community quickly get up to speed with the capabilities of the new instrument. Judging from these results, we can expect a string of new insights into nearby exoplanetary systems. Just what will JWST uncover, for example, in the TRAPPIST-1 system?

The two papers among the five that I’ve had the chance to go through are Tsai et al., “Direct Evidence of Photochemistry in an Exoplanet Atmosphere” (preprint) and Alderson et al., ”Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H” (preprint). The other three papers are: Feinstein et al., “Early Release Science of the exoplanet WASP-39b with JWST NIRISS (preprint); Ahrer et al., “Early Release Science of the exoplanet WASP-39b with JWST NIRCam (preprint); and Rustamkulov et al., “Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM” (abstract).

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{ 11 comments… add one }
  • Edwin November 30, 2022, 5:46

    Hi Paul

    Yes its hard to pin all these planet types down, but articles like this one and others posted here really do help. I still need to read and study the post about Ocean/Sub Neptune planets in more detail.

    “Hot Jupiter’s, Hot Saturns, Neptunes Super Earths” I take to mean the mass and size of these planets and the hot reference means they are orbiting close to there stars and many are probably evaporating away too.

    WASP-39b is clearly a Saturn mass and sized planet very close to its star. So saying its a “Hot Saturn” gives the reader something to understand about this planet.

    Cheers Edwin

  • Kamal M Ali December 1, 2022, 1:41

    OT: When will Webb observe Trappist I ? (Or when will those results become public?)

  • Mike Serfas December 1, 2022, 10:08

    Just because hot Jupiters aren’t in a “habitable zone” doesn’t mean they couldn’t offer some interesting possibilities. According to Barstow, 2017 ( https://arxiv.org/pdf/1610.01841.pdf ), planets with equilibrium temperature 900 to 2100 K can still have cloud layers, and these are most likely water vapor clouds. The pressure at these cloud layers can range from 1 bar to as low as 0.01 mbar. Now I know there is no liquid water at a lower pressure than its triple point (6 mbar), and I found a paper ( https://arxiv.org/abs/2204.11729 ) saying Jupiters over 1800 K may have a temperature inversion from metal oxides/hydrides, which I suppose could be a problem. In any case, these are planets with often less water than the Sun, which may not have much to offer. Nonetheless… if astronomers can detect hot Jupiters with clouds with liquid water during a transit, is there a chance they could demonstrate traces of organisms living within one of them?

    • Alex Tolley December 1, 2022, 15:40

      Not unlike the argument for life in the temperate clouds on Venus (although they are rather acidic too!).

      Which segues to what do we mean/imply by life? It has often been proposed that we could build floating cities in Venus’ clouds where the temperature and pressure are terrestrial. Would an ETI detection of such a bio/technosignature imply humans evolved on Venus and migrated to the clouds as the runaway greenhouse wrecked the surface, much as the argument goes for life in the Venusian clouds if we find it?

      IIRC, Charlie Stross has humans living in the clouds on Saturn at the end of his novel Accelerando. The same issue, although the technosignatures everywhere else in the solar system would give ETI pause as to origin.

      If machine “life” becomes the dominant intelligent agency in the galaxy, what might such a civilization need from a hot Jupiter? Would metals in the atmosphere hide the presence of technological artifacts in the noise?

      • Mike Serfas December 2, 2022, 12:23

        I was hoping for absorption bands from pigments or genetic material, but floating cities wouldn’t be unwelcome. (To be honest, I don’t know how feasible even that is: the second paper says “CO, H2O, OCS, and our modeled cloud deck all have overlapping opacity … we were unable to unambiguously identify the individual contributions from CO and other molecules over this wavelength region at the resolution presented in this work.” How much can they improve the resolution, or are there other better instruments?)

        I’ve daydreamed here of life on Saturn, but life on a hot Jupiter might have weird, powerful advantages if it can exist at all. Imagine something that lives very close to the triple point of water. One single cell can enzymatically control the nucleation of gas bubbles and ice crystals at close to the same temperature! So far as I know it is easier to freeze a supercooled solution than to melt a frozen object to a supercooled state – there’s a strange hysteresis that seems to hint that vast amounts of free energy could be extracted from slight fluctuations in temperature. The freezing of a small region of fluid, linked to a biochemical reaction, should make it “irreversible” (like using up ATP), but once the frozen structure melts you can do it again. I suppose you can’t beat Carnot efficiency, but I can’t think of an Earthly organism known to work as a heat engine at all. With intracellular droplets of gas and solid water containing varying levels of impurities and guarded by well-regulated antifreeze/antiboiling proteins, these organisms should maintain their liquid cytoplasm over a wider range of conditions than the thermometer suggests.

  • Thomas R Mazanec December 1, 2022, 12:19

    Because we have nothing in our system that remotely compares to WASP-39b. Let’s celebrate the fact that in this exoplanet we have the opportunity to study a different kind of planet, and remind ourselves of how many worlds we’re finding that are not represented by our own familiar categories. I imagine one day we’ll have more descriptive names for what we now call, by analogy, ‘super-Earths’ and ‘sub-Neptunes’ as well.

    You mean something like this?
    https://www.orionsarm.com/eg-article/491c78b89879b#:~:text=SUPER%2DTERRESTRIAL%20CLASS%3A%20worlds%20that,Terrestrial%20worlds%20and%20Neptunian%20worlds

  • Brian Altmeyer December 1, 2022, 23:04

    Pretending that the planets of our system are somehow archetypes for the wider universe never made any kind of sense. That was obvious almost from the get-go, when we started finding huge gas giants in tight orbits, and was only reinforced when we started finding large rocky and small gaseous worlds nearly overlapping in mass.

    What we really learned is that the different features of a planet aren’t simply related. It also reminded us that the environments we care about only exist in very narrow strips on a surface, since to a distant observer, Earth, Mars, and Venus are basically the same planet. But obviously they aren’t, so we have a lot of learning to do…

  • Geoffrey Hillend December 2, 2022, 17:54

    Quote by Brian Altmeyer: “Pretending that the planets of our system are somehow archetypes for the wider universe never made any kind of sense. ” Being the expert in Jungian psychology, I completely agree. Jung would never had said that. He would say something like archetypes are universal ideas that always apply to the general principles of our universe. Consequently, the myths associated with planets receive only a small fraction of the potential projected meaning contained in an archetype. For example: Jupiter is the king of the gods in myth because it is the largest planet which would apply to the largest planet in any solar system. It also applies to anything large, bigger than life, and greatness including physicists like Albert Einstein, and the principles themselves like general relativity. etc.

    In higher mind thinking or philosophy and depth psychology we have dual meanings so the similarities don’t cancel out the differences. They call this the intuition and seeing things in terms of the whole or larger, broader picture. There’s that archetype again.

    • Brian Altmeyer December 8, 2022, 6:13

      I wasn’t talking about psychology, actually. I just meant our observational bias of seeing one type over another due to instrumentation, with radial velocity vs. transits and whatnot.

  • Ron S. December 2, 2022, 22:31

    ” Jupiter is the king of the gods in myth because it is the largest planet which would apply to the largest planet in any solar system.”

    I’m reasonably certain that the ancients that composed the mythology had little data on the size of any of the planets other than Earth.

  • Geoffrey Hillend December 3, 2022, 15:45

    Absolutely since there was more unconsciousness or what is unknown such as the knowledge about the physical universe, the unconscious was more active. The ancients also did not have a knowledge of depth psychology, but a theory of knowledge like Tao philosophy and Plato’s ideas were it’s roots or early beginnings.

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