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The Value of LHS 475b

LHS 475b, a planet whose diameter is all but identical to Earth’s, makes news not so much because of what it is but because of what it tells us about studying the atmospheres of small rocky worlds. Credit for the confirmation of this planet goes to the NIRSpec (Near-Infrared Spectrograph) instrument aboard the James Webb Space Telescope, and LHS 475b marks the telescope’s first exoplanet catch. Data from the Transiting Exoplanet Survey Satellite (TESS) were sufficient to point scientists toward this system for a closer look. JWST confirmed the planet after only two transits.

Based on this detection, the Webb telescope is going to live up to expectations about its capabilities in exoplanet work. NIRSpec is a European Space Agency contribution to the JWST mission, and a major one, as the instrument’s multi-object spectroscopy mode is able to obtain spectra of up to 100 objects simultaneously, a capability that maximizes JWST observing time. No other spectrograph in space can do this, but NIRSpec deploys a so-called ‘micro-shutter’ subsystem developed for the instrument by NASA GSFC. Think of tiny windows with shutters, each measuring 100 by 200 microns.

The shutters operate as a magnetic field is applied and can be controlled individually. Murzy Jhabvala is chief engineer of Goddard’s Instrument Technology and Systems Division:

“To build a telescope that can peer farther than Hubble can, we needed brand new technology. We’ve worked on this design for over six years, opening and closing the tiny shutters tens of thousands of times in order to perfect the technology.”

LHS 475b is a rocky world that orbits a red dwarf some 40 light years out in Octans. The data from NIRSpec, taken on 31 August 2022, could not be clearer, as the image below shows.

Image: This graphic shows the change in relative brightness of the star-planet system at LHS 475 spanning three hours. The spectrum shows that the brightness of the system remains steady until the planet begins to transit the star. It then decreases, representing when the planet is directly in front of the star. The brightness increases again when the planet is no longer blocking the star, at which point it levels out.] Credit: NASA, ESA, CSA, L. Hustak (STScI), K. Stevenson, J. Lustig-Yaeger, E. May (Johns Hopkins University Applied Physics Laboratory), G. Fu (Johns Hopkins University), and S. Moran (University of Arizona).

Transmission spectroscopy – applying the NIRSpec capabilities to the spectrum of starlight passing through the planet’s atmosphere during ingress and egress from a transit – should give us the opportunity to analyze the components of that atmosphere, assuming one is present. That work is ongoing but promising, as the scientists in a team led by Kevin Stevenson and Jacob Lustig-Yaeger (Johns Hopkins University Applied Physics Laboratory) pursue their investigation. JWST’s sensitivity to a range of molecules is clear, and the researchers have been able to rule out a methane-dominated atmosphere, while a compact envelope made up entirely of carbon dioxide remains a possibility. Additional spectra will be taken this summer.

Image: The graphic shows the transmission spectrum of the rocky exoplanet LHS 475b. The data points are plotted as white circles with grey error bars on a graph of the amount of light blocked in percent on the vertical axis versus wavelength of light in microns on the horizontal axis. A straight green line represents a best-fit model. A curvy red line represents a methane model, and a slightly less curvy purple line represents a carbon dioxide model.] Credit: NASA, ESA, CSA, L. Hustak (STScI), K. Stevenson, J. Lustig-Yaeger, E. May (Johns Hopkins University Applied Physics Laboratory), G. Fu (Johns Hopkins University), and S. Moran (University of Arizona).

Given that LHS 475b orbits its star in two days, it’s no surprise to learn that the planet is considerably warmer than Earth even though it orbits an M-dwarf. We may well be looking at a Venus analogue, if an atmosphere does turn out to be present.

Bear in mind as JWST pushes into this area that M-dwarfs are prone to flares, especially in their earlier stages of development, and thus raise the question of whether a thick and detectable atmosphere can survive. LHS 475b may help us find out. The signs are promising, as the paper notes:

…our non-detection of starspot crossings during transit and the lack of stellar contamination in the transmission spectrum are promising signs in this initial reconnaissance of LHS 475b. These findings indicate that additional transit observations of LHS 475b with JWST are likely to tighten the constraints on a possible atmosphere. A third transit of LHS 475b is scheduled as part of this program (GO 1981) in 2023. An alternative path to break the degeneracy between a cloudy planet and an airless body is to obtain thermal emission measurements of LHS 475b during secondary eclipse because an airless body is expected to be several hundred Kelvin hotter than a cloudy world and will therefore produce large and detectable eclipse depths at JWST’s MIRI wavelengths… Our findings only skim the surface of what is possible with JWST.

The paper is Lustig-Yaeger et al., “A JWST transmission spectrum of a nearby Earth-sized exoplanet,” in process at Nature Astronomy (preprint).

{ 10 comments… add one }
  • Michael Fidler January 13, 2023, 16:39

    One point that may come out from the very sharp LHS 475b transit light curves is any exomoons. It looks like the transit above is just one but wonder if it may be stacked? These two current techniques may help bring out the detail needed.

    Exomoons: A New Technique for Detection.

    A New Way to Search for Exomoons.
    SEPTEMBER 13, 2021

  • William R. Alschuler January 14, 2023, 14:00

    There is something odd about the spectrum image caption. To my eyes the straight line is yellow-brown, not green, the curvy line (methane) is green, not red, and the less curvy line (CO2) is purple. The error bars are just barely visible and only longwards of 4 microns and their size is large, not useful yet to separate the possibilities.

    • Geoffrey Hillend January 14, 2023, 19:44

      I agree. There has to be a lot more data emission and absorption points there in order to make a useful spectrum, i.e, to make the crests and troughs of a differentiated wave spectrum in in a region, so I guess there is no methane. I agree that we should have to see a longer range of wavelength spectrum to see more gases in that are in a planetary atmosphere since carbon dioxide absorption which is at the peak intensity is at 15 microns, etc. We do see a little of it at the lower intensity absorption band for carbon dioxide 4.3 microns, the near infra red in the data. We should see a lot of absorption and emission data points at 15 microns, the mid infra red if the planet more closely resembles at runaway greenhouse effect and an atmosphere of mostly carbon dioxide like Venus.

      Carbon Dioxide is a heavy gas, so it takes more heat for it to have the kinetic energy to reach escape velocity, the jeans escape so we shouldn’t be too surprised to see it dominate Earth sized exoplanets in a region which is outside the life belt too close to the star.

    • Alex Tolley January 15, 2023, 18:33

      The images in the post are not the ones from the preprint paper. The chart in question does use some data from figure 2 in the paper. IDK what the sources are for the images in the post.

  • Michael Fidler January 15, 2023, 21:21

    What if LHS 475b is an earth size IO? Both have an orbit of around 2 days. Being this close to a massive object, weighing in at more the 1/4 the mass of our Sun. Less the 2 million miles from the M3.5V red dwarf (V is for Variable), interactions between the planet and star should be very common since the M dwarf is very active. So are we looking at a high mass atmosphere with all the lighter elements striped from the planets exploding volcanos? What combinations of elements could make up such a violent atmosphere? What of the ion/electron flux tube that Io, in its motion through the Io plasma torus at Jupiter, continuously generates an Alfvén wing that carries two billion kilowatts of power into the jovian ionosphere. This could be a billion times more powerful on LHS 475b…

  • Michael Laird January 18, 2023, 15:25

    My understanding was that any atmosphere that we might consider promising for life near a red dwarf would be in a bit of a race to see if flares could disperse it into space before the planet became tidally locked and what was left of it froze and fell in a large heap on the dark side of the planet.

    Of course it makes some sense that a Venus-like atmosphere, especially containing a great deal of carbon dioxide would take a lot of flares to get rid of it, and a heavy atmosphere receiving huge amounts of energy (especially one well inside the so-called Goldilocks zone, might also ensure things are too hot, even on the dark side, for it to freeze and precipitate.

    • Michael Fidler January 18, 2023, 23:18

      The details on this planet would indicate a much larger influx of energy being bombarded into it and a much higher internal heater thru induction heating. Compared to the Trappist 1 system which has a M8V red dwarf with one third the mass and less the half the diameter, the interaction of the M3.5V red dwarf with the planet LHS 475b would cause a mostly liquid ball of high energy eruptions and magma. One possible outcome of this could be a football shaped planet rotating on its side.
      Red dwarfs that have planets further away like Trappists 1 may have their volatile elements replenished by the large reservoir of comets left from the formation since the gas and ice giants do not seem to form in these systems. Take a look at this video of the new theory on the formation of Rocky Super-Earths. At about 8 seconds there is a long list of Kepler multi planet systems that show this without any giant planets.


      Formation of Rocky Super-Earths From A Narrow Ring of Planetesimals.

      In the small red dwarf systems comet impacts would be much more common such as in Trappist 1d, a small low density water world that has a year 100 times faster then ours. Tight and fast orbits cause more comet impacts then a large open slow moving planetary system like ours…

      • Alex Tolley January 19, 2023, 13:05

        Tight and fast orbits cause more comet impacts then a large open slow moving planetary system like ours…

        Isn’t that like saying you get wetter running in the rain?

        Perhaps you can explain why this is the case with comet impacts?

        • Michael Fidler January 20, 2023, 7:04

          In the case of LHS 475b the orbital speed is around 125,000 MPH which is close Mercury’s 107,000 MPH but it completes its orbit in 2 days instead of 88 days. This in a space 15 times closer to the red dwarf.
          The normal red dwarfs do not have large giant planets beyond the small rocky earths and super earths, so the gases that form them will create comets. This could cause a large reservoir of comets to develop but they will be on a much smaller scale with orbital periods for Halley’s comet of just 15 years and the million year Oort cloud comets taking just 200,00 years.
          Taking into account these differences and the higher speed of the comets in the smaller tighter space around the star that LHS 475b orbits in, I see a 125 times the impact rate then on our planet.
          So instead of 65 million years since the last major earth shattering comet impact, it would be only 500,000 years.

          Now this is for a planet that may be a giant lava ball so no telling what could be the result, but in a more relaxed system like Trappist 1 is a different story may be that two inner planets may of been rich ocean worlds early on. The end result being the oceans would be destroyed both from interaction with the M8V red dwarf activity and similar effects like Io. This may have resulted in Trappist 1d becoming a water world from the clouds of H2O and Hydrogen and Oxygen from the two inner planets desiccation!

          As for running in the rain this would be more like flying near the center of a TYPHOON with the calm center being the red dwarf.

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