Getting Down to Business with JWST

by | Jul 19, 2022 | Exoplanetary Science | 32 comments

So let’s get to work with the James Webb Space Telescope. Those dazzling first images received a gratifying degree of media attention, and even my most space-agnostic neighbors were asking me about what exactly they were looking at. For those of us who track exoplanet research, it’s gratifying to see how quickly JWST has begun to yield results on planets around other stars. Thus WASP-96 b, 1150 light years out in the southern constellation Phoenix, a lightweight puffball planet scorched by its star.

Maybe ‘lightweight’ isn’t the best word. Jupiter is roughly 320 Earth masses, and WASP-96b weighs in at less than half that, but its tight orbit (0.04 AU, or almost ten times closer to its Sun-like star than Mercury) has puffed its diameter up to 1.2 times that of Jupiter. This is a 3.5-day orbit producing temperatures above 800 ?.

As you would imagine, this transiting world is made to order for analysis of its atmosphere. To follow JWST’s future work, we’ll need to start learning new acronyms, the first of them being the telescope’s NIRISS, for Near-Infrared Imager and Slitless Spectrograph. NIRISS was a contribution to the mission from the Canadian Space Agency. The instrument measured light from the WASP-96 system for 6.4 hours on June 21.

Parsing the constituents of an atmosphere involves taking a transmission spectrum, which examines the light of a star as it filters through a transiting planet’s atmosphere. This can then be compared to the light of the star when no transit is occurring. As specific wavelengths of light are absorbed during the transit, atmospheric gasses can be identified. Moreover, scientists can gain information about the atmosphere’s temperature based on the height of peaks in the absorption pattern, while the spectrum’s overall shape can flag the presence of haze and clouds.

These NIRISS observations captured 280 individual spectra detected in a wavelength range from 0.6 microns to 2.8 microns, thus taking us from red into the near infrared. Even with a relatively large object like a gas giant, the actual blockage of starlight is minute, here ranging from 1.36 percent to 1.47 percent. As the image below reveals, the results show the huge promise of the instrument as we move through JWST’s Cycle 1 observations, nearly a quarter of which are to be devoted to exoplanet investigation.

Image: A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves across the star, to the unfiltered starlight detected when the planet is beside the star. Each of the 141 data points (white circles) on this graph represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its atmosphere. The gray lines extending above and below each data point are error bars that show the uncertainty of each measurement, or the reasonable range of actual possible values. For a single observation, the error on these measurements is remarkably small. The blue line is a best-fit model that takes into account the data, the known properties of WASP-96 b and its star (e.g., size, mass, temperature), and assumed characteristics of the atmosphere. Credit: NASA, ESA, CSA, and STScI.

No more detailed infrared transmission spectrum has even been taken of an exoplanet, and this is the first that includes wavelengths longer than 1.6 microns at such resolution, as well as the first to cover the entire frequency range from 0.6 to 2.8 microns simultaneously. Here we can detect water vapor and infer the presence of clouds, as well as finding evidence for haze in the shape of the slope at the left of the spectrum. Peak heights can be used to deduce an atmospheric temperature of about 725 ?.

Moving into wavelengths longer than 1.6 microns gives scientists a part of the spectrum that is made to order for the detection of water, oxygen, methane and carbon dioxide, all of which are expected to be found in other exoplanets observed by the instrument, and a portion of the spectrum not available from predecessor instruments. All this bodes well for what JWST will have to offer as it widens its exoplanet observations.

32 Comments

    • wdk on July 19, 2022 at 16:04

      Schedule displays seem to change quickly. Earlier I had copied down some Trappist-1 data involving slit-less spectroscopy but it has been displaced. There are other interesting “agenda items” though.
      I don’t recall any discussion about these targets: HIP 65426 a and b.
      As the Wikipedia notes, it’s b is a super Jupiter about 14 million years old and no nearby dust disk, discovered about 5 years ago with the European SPHERE observatory. About 92 AUs out from its primary it has several other abnormalities. From the standpoint of astrophysics, the associated articles are circumspect, speaking about prevailing notions of formation. Maybe the overall dust disk model is in error?
      It would seem that the majority of evidence suggests otherwise.

      But then from the standpoint of a group that has so much discussion about what astrophysical discrepancies could imply…. Well. LGM out for a sail?

      But I hope that we do not have to wait 12 months to learn what we just did about WASP-96 b. Though the two star systems do not present the same specifics ( overall brightness, distance, spectral peak due to temperature…) I can’t see why it would take that long to establish such signatures, especially since the Trappist 1 system is already closely calibrated and observed. Perhaps if H2O, CH4 and other lines are very residual ( e.g., due to an atmospheric erosion), the 12 months would be to apply the equivalent of a fine-tooth comb?

      • Robin Datta on July 21, 2022 at 23:34

        Tickets to Stockholm could be at stake.

        • wdk on July 22, 2022 at 22:37

          Should we pull down off the dusty shelf from 1994 and re-read “Hubble Wars”? I’d rather not. But I will consider the concerns it raised for a moment:

          I don’t have any proposal riding with JWST. And I suspect that the majority of people interested in Trappist-1, for example, probably do not either, though they can well appreciate the significance/implications of it as an observational target.

          To recall Hubble and its early travails, I can also recall the oft observed comments excerpted from such program histories “engineering failure”….”scientific achievement”, kind of a re-stating of the “per aspera ad astra”.

          For one thing, any instruction on “stars and galaxies” in this country (US) should be poised to talk about imminent discoveries from JWST; students and teachers would want to participate as directly as possible from the bleachers or a room for sidewalk superintendents. Just as the people on this forum wish to do as well, ready to wrack their brains on operation strategy, data and interpretation. Yet to be enticed this far and then come to a security gate like they just drove up to Area 51 is counter-productive. After all, not only did NASA and Congress take on cost over-runs to deliver on this, but so did the public at large. I hope that the Trappist issue is just a brief bump on the road, but it is also possible that exclusivity could get worse than on Hubble rather than better, just as a result of snowballing momentum. And if it does, then there should be reform.

          Public outreach on this sort of thing should be spectral wideband too. Otherwise, in this country, public broadcasting promotion (e.g., Nova) receivers should be re-calibrated for hype.

  2. ljk on July 19, 2022 at 10:34

    James Webb Space Telescope picture shows noticeable damage from micrometeoroid strike

    By Elizabeth Howell published about 24 hours ago

    NASA is unsure how much of an effect space rocks will have on Webb’s lifetime after the event ‘exceeded prelaunch expectations of damage.’

    A small space rock has proven to have a big effect on NASA’s newly operational deep-space telescope.

    A micrometeoroid struck the James Webb Space Telescope between May 22 and 24, impacting one of the observatory’s 18 hexagonal golden mirrors. NASA had disclosed the micrometeoroid strike in June and noted that the debris was more sizeable than pre-launch modeling had accounted for.

    Now, scientists on the mission have shared an image that drives home the severity of the blow in a report(opens in new tab) released July 12 describing what scientists on the mission learned about using the observatory during its first six months in space.

    Happily, in this case the overall effect on Webb was small. That said, the report outlines the investigation and modeling that engineers are undertaking to assess the long-term effects of micrometeroids on Webb.

    Full article here:

    https://www.space.com/james-webb-space-telescope-micrometeoroid-damage

  5. Alex Tolley on July 19, 2022 at 13:07

    I would like more information on the “best-fit” model of the spectrum. It doesn’t look like a best fit by eye by any means, suggesting that the fit is based more on the expected spectrum rather than purely the data.

    • Geoffrey Hillend on July 19, 2022 at 19:31

      Looking at the spectra we look for absorption lines and emission lines. Absorption lines look like troughs and emission lines look like spikes or crests in the wavelength spectrum. Clearly since we are looking at a hot body, so these must be emission lines. The wavelength in microns are on the horizontal axes. With spectroscopy, every atom and molecule has a distinct fingerprint, so it only emits light at a very specific wavelength, the exact same wavelength as it absorbs light. Therefore the expected spectrum is always already known and the data must match it and it does. Water has three different absorption bands and also the same three emission band wavelengths. Also water as a broad absorption spectrum due to the fact it has hydrogen in it. Google, and also https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#:~:text=In%20the%20near-infrared%20range,(10300%20cm%E2%88%921 Consequently, one absorption band is wide across wavelength. Is that what you were concerned with Alex Tolley. Molecules being composed of more than one atom are like that I recall reading.

      • Alex Tolley on July 19, 2022 at 21:07

        So why does the blue best-fit line not look like the best fit for the data? I understand that using an expected spectrum can be fit to the data, but then the data is not really being analyzed, but rather just being used to show that there is an approximate match to the theoretical spectrum. Are the differences due to measurement error or showing data for other molecules?

        What I was asking for is the details of how this best-fit was done.

        • Joshua on July 20, 2022 at 9:13

          Usually the way these fits are done is to generate a predicted, theoretical spectrum based on what we know of the temperature, scale height, and composition of the atmosphere, and then adjust the free parameters of that theoretical model to best fit the data. So you would not expect the model to be a best fit to the data in the mathematical sense, since you are still constrained by the physics of atmospheres and radiative transfer, and the actual spectrum of the star.

          But there are always unknowns that your model can’t capture, like the abundance of various trace elements, the presence, altitude, and composition of clouds and hazes (really hard to know ahead of time), and other second-order effects. So it’s not surprising that the model is not a perfect fit. We’re still in the very early stages of understanding these atmospheres, so results like this are still very exciting and informative.

          • Alex Tolley on July 20, 2022 at 15:51

            Thank you. That is what I surmised. Given that a Jupiter class planet would have an atmosphere that includes methane, ammonia, hydrogen sulfide, etc, the spectra of these compounds were ruled out as not fitting with the data. Where the best fit curve of water seems to depart from the data ( 2.1 um) would that be due to other compounds, or physical effects like haze, etc?

            • Geoffrey Hillend on July 20, 2022 at 17:35

              I don’t think that NASA is ruling anything out of the WASP 96b spectra but only labeling water. The spectra of methane, ammonia, hydrogen sulfide are at a different wavelength than water and are not shown. Also it is o.k. for water to be wide across the wavelength spectrum because it is a molecule. It is quantum mechanical, the quantum jumps in the electrons.. The spectra of water vapor is broadened across wavelength because we see the photons from the exited states as well as the ground state since it is a hot body.

            • Geoffrey Hillend on July 20, 2022 at 17:48

              Excuse me for the typo. I meant excited states which are also a unique fingerprint to the atom or molecule like the ground state or rest state which is where absorption occurs which looks like a trough. Each excited state can emit a photon of a a slightly different wavelength, but are exact for their level like level one, level two, level three, it takes an exact amount of energy to make each quantum jump and are seen in spectral emission lines and looks like a spike.

            • Geoffrey Hillend on July 20, 2022 at 17:50

              It is the excited electron that emits the photons which are light absorbed from the star.

            • Geoffrey Hillend on July 20, 2022 at 18:03

              The above graph of WASP 96 b is only 2 microns wide from .75 to 2.76 which is in the near infra red spectrum. Methane is 3.3 to 7.7 um, Ammonia 20 to 35 microns, hydrogen sulfide 2.6 microns. These are outside the wave spectrum of the graph above.

              • Alex Tolley on July 21, 2022 at 8:51

                Note that the JWST has both near and mid IR cameras that range from 0.6 to 28 um wavelengths.

                Webb Infrared detectors

                • Geoffrey Hillend on July 21, 2022 at 16:53

                  That is interesting, Alex Tolley I knew the spectral range of JWST’s instruments, but I have not seen what they look like or how they work. I also like to understand the data of NASA’s WASP 96 b spectra and I got the idea that emission lines look like crests or spikes from Sara Seager’s book Exoplanet Atmospheres. P. 106. This NASA web page titled
                  Exoplanet WASP 96 b Atmospheric Characteristics explains the WASP 96 b spectra with more detail with more physics which is what I like. https://exoplanets.nasa.gov/resources/2326/exoplanet-wasp-96-b-atmospheric-characteristics/ Quote from that page: “The gray lines extending above and below each data point are error bars that show the uncertainty of each measurement, or the reasonable range of actual possible values. For a single observation, the error on these measurements is remarkably small. ”
                  We can see that the reasonable range is still in the wavelength spectrum that we would find water, so I don’t think there is any ambiguity in the data. Jupiter also has water vapor in it’s atmosphere and is also a gas giant like WASP 96 b. Also the “bumps” or crests are emission lines and I am wondering if the troughs are absorption lines. It would be nice to see all of the physics in the diagram explained although I trust the data.

                  Great idea by NASA! I hope it works. https://www.nasa.gov/feature/nasa-seeks-public-s-designs-to-throw-shade-in-space

            • Geoffrey Hillend on July 20, 2022 at 18:06

              Maybe hydrogen sulfide 2.6 microns is shown in the graph above, but not labeled.

        • Mike Serfas on July 20, 2022 at 18:11

          It looks like the “assumed characteristics of the atmosphere” must not be quite right – I doubt there’s a preprint yet? I notice the wavelengths run from 0.00007 cm to 0.00028 cm = 14000 cm-1 to 3600 cm-1. Infrared light can have wavelengths up to a millimeter, and everything under 2.5 microns is right off the end of the usual IR spectrum from undergrad chemistry because it’s not so simple to interpret. Rather, this is near-IR spectroscopy, more obscure to many of us ( see https://www.impopen.com
          /introduction-near-infrared-nir-spectroscopy ). I have no idea how you even look up what might be absorbing / reflecting at the departures from the model we see. Or rather calculate – there is a strange assortment of combination bands and overtones that go into a NIR spectrum. I imagine there are some folks at NASA who are _very_ clever with this, and who will nonetheless get a lot better over the next few years after having to present a model like that to the world.

  6. Geoffrey Hillend on July 19, 2022 at 17:53

    Impressive. Hopefully soon we will also get the TRAPPIST-1
    spectroscopy as promised by NASA.

  8. Geoffrey Hillend on July 19, 2022 at 19:46

    I assume the light being blocked is the light being absorbed by the gases in the atmosphere of WASP 96 b in the diagram above.

  9. wdk on July 19, 2022 at 21:34

    From the discussion above, thought I would check to see what WASP 96 b previous observations looked like. E.g., was there any case for water vapor previously? The answer did not come up directly. Rather, it looked like the attraction of 96 b was a clear upper atmosphere judging from the sharp sodium (Na) absorption signature. Water vapor appeared to be addressed in the same ground based (Magellan &VLT) and Hubble observation reports. Though I suspect that the sodium line calibrations were in the visual range, giving validation to patterns to be discerned in the IR.

    In a transit (planet in front of the star), I would presume that the presence of water vapor absorption lines in the “common” spectrum would be on account of the presence in the planetary atmosphere.
    For example, with a G star similar to the sun, water vapor would be dissociated in the stellar upper atmosphere of 5800 K or so. But then atomic sodium could be present in a hot Jupiter or a number of stellar types in the visual spectrum. So, during a transit, monitoring a sodium absorption feature, one would expect some alteration (magnitude?) but not much doppler shift unless it were related to thermal or pressure effects of the two atmospheres (broadening or narrowing?). As to which, that what spectroscopic specialists are paid for.

  10. Edwin on July 20, 2022 at 6:27

    Hi Paul and readers.

    An excellent post and comments too, Its a shame about the small strike on the mirror, but I’m sure looking forward to the results on nearby exoplanets, but a years wait?

    I’m looking forward your posts Paul when the results to arrive in.

    Thanks Edwin

  11. Geoffrey Hillend on July 20, 2022 at 15:00

    The idea of how spectroscopy works is simple. The light is blocked because the molecules of a particular gas like water vapor absorb the star light at a specific wavelength unique to it, so that absorbed light is missing from the spectrum. In the visible spectrum these absorption lines are black lines in rainbow color of the spectrum of the prism’s colors reflected on the wall where the light of an atom or molecule absorbs, so it is missing from the spectrum, the same principle.. Here it is the near infra red. Our atmosphere is opaque to certain wavelength like the mid infra red and some of the near infra red. Consequently, we can’t see from the ground what the JWST can see at some wavelengths which are blocked by Earth’s atmosphere

    From what I recall NASA saying is that the meteor strike on the JWST did not move the mirror segment enough to effect the performance of the JWST which is still optimal. Further strikes won’t be good though. Hopefully they are rare.

    • Robin Datta on July 21, 2022 at 23:25

      I don’t know if med school textbooks count as science, but I retired from Emergency Medicine over a decade ago, and the swamps of hard-science mathematics is just not my milieu.

      • ljk on July 22, 2022 at 11:33

        These days I would settle for people reading mainstream/popular level science works, or the equivalent documentaries and science programs.