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Exoplanet Atmospheres: Keeping Up with ARIEL

How is a planet’s composition related to its host star? The European Space Agency’s ARIEL mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is designed to probe the question, examining planetary atmospheres to determine the composition, temperature and chemical processes at work in a large sample of planetary systems.

Transmission spectroscopy is the method, examining spectra as known exoplanets pass in front of, then behind their host stars. Researchers will use light filtering through the atmospheres to unlock the chemical processes within each. ARIEL will survey about 1,000 planetary systems in both visible and infrared wavelengths, probing not just chemistry but the thermal conditions that affect their composition. The mission’s focus is on super-Earths to gas giants, all with temperatures greater than 320 Celsius.

I suspect that principal investigator Giovanna Tinetti (University College London) has been asked about the choice of targets to the point of exhaustion, but one reason for focusing on planets in this size and temperature range is our need to build up a catalog of atmospheres that will inform the entire field, so that when we drill down to small, rocky worlds using future instruments, we’ll have a context in which to place what we see. A high temperature atmosphere is helpful here because it remains in continuous circulation, without the obscuring clouds that make characterization difficult.

Image: Giovanna Tinetti (University College London), principal investigator for ARIEL.

ARIEL will be positioned around L2, the second Sun-Earth Lagrange point, 1.5 million kilometres directly ‘behind’ Earth as viewed from the Sun. A just released ESA report explains the issues the mission will investigate, noting the wide range of planets and stars:

This large and unbiased survey will contribute to answering the first of the four ambitious topics listed in the ESA’s Cosmic Vision: “What are the conditions for planet formation and the emergence of life?”. Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. There is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet’s surface and atmosphere is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth and evolution.

Remember, too, the statistical nature of the inquiry. The sample population is large, so that we move from the small number of atmospheres currently characterized to hundreds. Our understanding of the early stages of planet and atmosphere formation during the first few million years in an infant system as it emerges from the nebular phase should help us relate the chemistry in exoplanets to their other parameters and to the chemical environment of the star.

Image: An example spectrum Ariel could measure from light passing through an exoplanet’s atmosphere. Credit: ESA/STFC RAL Space/UCL/UK Space Agency/ATG Medialab.

Key components — water vapor, carbon dioxide, methane — of planetary atmospheres will be detected, but also more unusual metallic compounds that define the chemical environment within each system studied. For a smaller number of target planets, the spacecraft will perform a survey of cloud systems and atmospheric changes at a seasonal and daily level. Says ARIEL project scientist Theresa Lueftinger: “Our chemical census of hundreds of solar systems will help us understand each planet in context of the chemical environment and composition of the host star, in turn helping us to better understand our own cosmic neighbourhood.”

ESA has just announced that ARIEL has moved from study to implementation phase, the step before negotiations begin with industrial contractors to build the spacecraft, which is scheduled for launch from Kourou (French Guiana) in 2029. Bids on spacecraft hardware will be requested within months, with the prime contractor chosen by the summer of 2021. 50 institutes from 17 European countries are involved, as is NASA, in the payload module, which will have at its heart a one-meter class cryogenic telescope along with associated science instruments.

Three ESA missions with an exoplanet charter are thus framed within a ten-year window, with ARIEL joined by CHEOPS (CHaracterising ExOPlanet Satellite), launched in December 2019, and PLATO (PLAnetary Transits and Oscillations of stars), to be launched in 2026. The latter emphasizes rocky planets in the habitable zone of Sun-like stars.

To keep up with ARIEL, you may want to follow @ArielTelescope on Twitter. You’ll find background information in the recently published ARIEL Definition Study Report, available here.

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{ 11 comments… add one }
  • Ron S. November 18, 2020, 14:56

    That sample spectrum is a little unusual. How is it that they plot chemical emissions (by wavelength) against transit depth? Does anyone have a clear explanation. I can see by the % values on the y-axis that there is a fine grained time resolution at the leading edge of the transit, which is when the atmosphere signal would stand out most clearly. But wavelength vs transit depth?

    • Sam Goodman November 19, 2020, 6:29

      Technically this graph represents an absorption spectrum rather than the related transmission spectrum. The atmosphere is back-lit, so how deep the transit is will vary with molecular absorption. What you see here is the transit depth become greater when you hit a wavelength where a molecule preferentially absorbs light, that’s why the molecule labels are at the peaks in the graph.

  • Geoffrey Hillend November 18, 2020, 15:54

    Ariel will definitely answer some questions about exoplanet atmospheres which are only hypothesis or predictions made on what we know. It is interesting that oxygen is not mentioned. Since oxygen is a biosignature gas, I wonder if they like me don’t expect to see the spectra of oxygen?

    • Sam Goodman November 19, 2020, 6:36

      I’m sure someone with more specific knowledge can correct me, but as far as I know the strongest Oxygen absorption is outside ARIEL’s wavelength range. ARIEL will mostly study hot Jupiters, with a handful of warm super-earths reachable with Ariel’s capabilities. As life is not expected on such worlds I suspect they felt it wasn’t worth the added complexity/cost.

      • Alex Tolley November 19, 2020, 12:33

        It looks like the IR band will detect CO2 and CH4, gases on Earth during the Archaean, and a strong CH4 signature would be a possible indicator of methanogens. N2O absorption is also within its detection range, but whether it would be sensitive enough I have no idea. If it was, N2O might be a possible proxy for an O2 atmosphere as NH3 is oxidized to N2O under oxic conditions and therefore a possible indicator that photosynthesis and life were present.

      • Alex Tolley November 19, 2020, 13:21

        The PDF with the science objectives suggests these molecules are detectable (as well as some metals) and will be part of the characterization:

        H2O, CH4, CO, CO2, NH3

      • Ashley Baldwin November 19, 2020, 13:32

        Yes. ARIELs low end wavelength cut off is 1.95 microns . For a reason . Spectroscopic absorption spectra for shorter wavelengths are incomplete ( as described in the original EChO submission ) .All four 02 peaks are below this at just 0.56, 0.69, 0.76 and 1.27 microns. Raleigh scattering for O3 extends to just 0.76 microns though that is all about temperate terrestrial atmospheres . I’m guessing these were contributing factors determining 1.95 microns as the lower end of ARIEL’s observation range .

  • Ashley Baldwin November 18, 2020, 16:26

    A descoped version of EChO thanks to overspends on previous M class missions . The essential next step in widespread exoplanet atmospheric characterisation nevertheless. It’s just a pity that it’s still EIGHT years away – assuming no COVID related delays. A hiatus ahead of the next big step to characterisation. Frustrating given the technology is there to make that step already. As ever funding the issue . All credit to those who continue make the most of what is on offer to drive forwards and make the small gains that lead to the next steps. ARIEL+ , EChO, came within a whisker of beating PLATO but was sadly a couple of years ahead of its time.

  • Robin Datta November 19, 2020, 0:13

    “What are the conditions for planet formation and the emergence of life?”
    “with temperatures greater than 320° C” if life has emerged, it might be cooking lunch. In which case the question is “What’s for lunch?”

  • andy November 19, 2020, 13:40

    Talking of exoplanet atmospheres, a preprint appeared on arXiv the other day claiming a tentative detection of water on the habitable zone super-Earth LHS 1140 b.

    Edwards et al. (arXiv 2020), “Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b”

    More data (e.g. from JWST) would be required to confirm the result though.

  • Gary Wilson November 20, 2020, 15:19

    LHS 1140 b,c, and possibly d now, look like a great candidates for atmospheric examination by ARIEL if it is sensitive enough to detect them. It’s interesting that LHS 1140 seems to have fewer solar flares than many red dwarf stars (if I have that correct). For those like me that are just interested lay people, LHS 1140b is a super Earth rocky planet in the potential habitable zone, as is c but d, if it exists is outside the zone. The star is only about 39-41 light years away and the planets have been detected by TESS. I think b was first detect by the radial velocity method. I learned a lot from this string of posts. Thank you :)!

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