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ARIEL: Focus on Exoplanet Atmospheres

Given Centauri Dreams‘s interest in exoplanet studies, it’s no surprise that when I write about the James Webb Space Telescope, it’s usually to fit the observatory into the overall study of other stellar systems. But of course JWST has been conceived to study everything from the earliest stars and galaxies to the ongoing birth of stars out of massive clouds of dust, not to mention objects within our own Solar System. JWST also offers us a real chance to probe exoplanet atmospheres around nearby M-dwarfs, but it is certainly not a dedicated exoplanet mission.

So while we hope for a successful launch in 2020, according to the evolving schedule, and look forward to finding plenty of JWST targets with the upcoming Transiting Exoplanet Survey Satellite (TESS), let’s have a look at a new mission from the European Space Agency with a tight exoplanet focus. The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) has just been selected as an ESA science mission scheduled for launch in 2028. Its reason for existence is to look at how planetary systems form and evolve.

Giovanna Tinetti (University College London) is principal investigator here, which bodes well — I am a great admirer of Dr. Tinetti’s work and see ARIEL as being in excellent hands. Of the mission, Tinetti says this:

“Although we’ve now discovered around 3800 planets orbiting other stars, the nature of these exoplanets remains largely mysterious. ARIEL will study a statistically large sample of exoplanets to give us a truly representative picture of what these planets are like. This will enable us to answer questions about how the chemistry of a planet links to the environment in which it forms, and how its birth and evolution are affected by its parent star.”

Image: Giovanna Tinetti during a lecture at the Flamsteed Astronomy Society. Credit: Andy Sawers.

ARIEL’s targets are to be exoplanets from Jupiter-mass down to super-Earths, with the primary focus on planets in warm and hot orbits close to their parent star. The thinking here is that high temperatures keep different molecular species in circulation throughout the atmosphere, making them more easily detected because they do not as readily sink or form cloud layers that can obscure them. In that sense, a planet at 2000 degrees Celsius becomes a useful laboratory, churning with interesting molecules from the atmosphere and even from the planet’s interior.

Current plans call for a launch from Kourou in French Guiana into an orbit around the L2 Lagrange Point 1.5 million kilometers from Earth, where the Webb instrument is also headed. Here the balance of gravitational forces keeps the spacecraft in a fixed position relative to the Earth and Sun with a minimal expenditure of energy. Both spacecraft will essentially be ‘parked’ at the same region in space, unlike the Spitzer instrument, which is in orbit around the Sun and will eventually lose communications when its orbital path takes it behind Sol.

While JWST will view the universe in infrared, ARIEL’s meter-class primary mirror will also collect visible light, using a spectrometer to study planetary atmospheres through transmission spectroscopy, possible when a planet passes in front of or behind its star. One of its sensors will be capable of detecting the presence of clouds. Tinetti and team hope to observe 1,000 exoplanets, following up on the worlds discovered by upcoming missions like TESS, and paving the way for future European missions like CHEOPS and PLATO. A successful JWST launch will likewise result in new target options for ARIEL. The 1,300 kg spacecraft has a cost cap of 450 million euros, roughly $550 million at current rates.

Image: Artist’s impression of ARIEL on its way to Lagrange Point 2 (L2). Here, the spacecraft is shielded from the Sun and has a clear view of the whole sky. Credit: ESA/STFC RAL Space/UCL/Europlanet-Science Office.

You may have heard about an exoplanet mission called EChO (Exoplanet Characterization Observatory), which was a candidate for ESA’s Cosmic Vision program. ESA did not fund the mission, which led Tinetti and colleagues to design the similar ARIEL observatory around a spacecraft with a lighter, less complex payload. The PLATO mission that supplanted EChO as one of ESA’s medium-class missions is an exoplanet observatory slated for a 2026 launch.


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  • Andrew Palfreyman March 29, 2018, 16:04

    Presumably steps will be taken to sync operations at JWST and ARIEL to avoid them bashing into each other!

  • Michael C. Fidler March 29, 2018, 20:24

    Another in depth article on Ariel.

    The Just-Approved European ARIEL Mission Will Be First Dedicated to Probing Exoplanet Atmospheres.

    Posted on 2018-03-29


  • Nicky March 29, 2018, 22:16

    A space mission dedicated to exoplanet atmosphere characterization is urgent and necessary. One of the fundamental questions in planetary science is the formation of super-Earths. The radius-mass diagram can give us clue on composition, but these exist a region where both massive hydrogen envelope and water-rich envelope can explain the density, so which one is it? Only ARIEL can give us the answer. Water-rich envelope implies that super-Earths formed outside snowline, and pure hydrogen envelope implies super-Earths formed inside snowline.
    In habitable planet context, planets form inside snowline are more habitable, therefore ARIEL will tell us how habitable are the habitable-zone planets by observing their cousins.

  • Ashley Baldwin March 30, 2018, 9:51

    Great piece Paul. Sorry I couldn’t have contributed more directly,

    The down election of ARIEL for the ESA M4 programme is great news. No two ways about it. Also it’s somewhat bittersweet too as due to previous M class overspend the M4 round has a budget of just 450 million instead of the previous 550 euros . That’s a big drop and the main reason why ARIEL is a meter class telescope instead of EChO’s 1.5m. ( the arxiv article on this is still a great overview of how ARIEL will operate too and I would recommend it strongly ) This may not sound a lot but it effectively means it has less than 50% of the latter’s light gathering capacity and thus requires more planetary transits to stick together (“bin”) for atmospheric categorisation . Which means it has to target larger planets closer into their stars that offer plentiful and deep transits but at the cost of being hot and less likely “terrestrial ” in nature. That said four years , and hopefully longer should offer the potential to assess some temperate terrestrial a planets around cooler nearby and brighter late M dwarfs . Just the sort of planets TESS has been designed to find – so good dovetailing. Transit spectroscopy us still a technique in its infancy , certainly in space – so ARIEL will dramatically refine it as a science whilst finally building up a large and much neede database base to take explanetary characterisation to a higher level than just, size, mass and orbital parameters .

    To be fair the unsuccessful ECHO M3 bid ( which ironically lost out to PLATO) was the right idea at the wrong time. The infrared sensors required for its work weren’t avilable or economically viable in Europe ( or even the more advance US) till recently. They are still eye wateringly costly versus CCDs though (which only work from 0.4-1microns wavelength ) which is part of the reason ARIEL’s spectroscopic bandwidth only extends from 1.95 to 7.8microns as opposed to EChO’s 16 microns ( which brings the extra hassle and cost of active sensor cooling versus ARIEL’s easier passive ) . Why 7.8 microns ? Because the strongest absorption peak of all important atmospheric methane occurs at 7.7 microns ( close to AKON important “biosignature” gases sulphur dioxide and nitrous oxide too)

    It should be noted that ARIEL won’t operate in isolation. By launch there should be preexisting data built up on many TESS targets from Hubble and Spitzer, both of which could operate into the next decade .

    Irrespective of JWST though one ( minimal admittedly ) benefit of its ongoing delay is that it could operate in tandem with ARIEL. The NASA medium Explorer concept FINESSE a closely reacted 0.7m transit spectroscopy telescope awaits hopeful selection in June and if successful will launch circa 2023 to hopefully supplement and work with ARIEL too, it’s bad width is 0.4-5 microns , not coincidentally designed to dovetail perfectly with ARIEL( it’s PI, Mark Swain is also a co investigator for ARIEL and a long tern collaborator of Giovanna Tinetti)
    Tinetti is also PI for the 0.5m private transit spectroscopy telescope, Twinkle. Taken together , ARIEL, FINESSE and Teinkle have about 80% of EChO’s light gathering and about half its bandwidth – the important bit and most suited for temperate terrestrial assessment.

    So two steps of the transit spectroscopy revolution are in place with FINESSE to follow hopefully in June . Add in the METIS NIR imaging spectrograph on the E-ELT from 2025 as well and there is plenty of exciting exoplanet science to come next decade .

  • Michael C. Fidler March 30, 2018, 10:41

    Professor Giovanna Tinetti is also the Science Lead for the Twinkle mission. Construction of the satellite will begin in the spring of 2018 for a launch in late 2020.

    Overview of current Twinkle design:

    SSTL-300 platform
    Low-Earth Orbit (Sun-synchronous Polar Orbit)
    Altitude: 650-700km
    Payload mass: <100kg
    Payload power consumption: <100W
    Primary mirror aperture: 45cm
    Wavelength range: 0.4 – 4.5µm
    Spectral resolution: R=300 maximum


    SPIE: Twinkle – A Low Earth Orbit Visible and Infrared Exoplanet Spectroscopy Observatory; G. Savini and the Twinkle Payload Consortium. Preprint (SPIE astronomical telescopes + instrumentation 2016 paper 9904—175) – Proceedings of the SPIE (2016)


    SPIE: The infrared spectrometer for Twinkle; M. Wells. Preprint (SPIE astronomical telescopes + instrumentation 2016 paper 9904—176) – Proceedings of the SPIE (2016)


    4S Symposium: Twinkle: A new idea for commercial astrophysics missions.


  • ljk March 30, 2018, 13:07

    Speaking of alien atmospheres, there is one much closer to home that might have life in it…


    To quote:

    Limaye, who conducts his research as a NASA participating scientist in the Japan Aerospace Exploration Agency’s Akatsuki mission to Venus, was eager to revisit the idea of exploring the planet’s atmosphere after a chance meeting at a teachers’ workshop with paper co-author Grzegorz Słowik of Poland’s University of Zielona Góra. Slowik made him aware of bacteria on Earth with light-absorbing properties similar to those of unidentified particles that make up unexplained dark patches observed in the clouds of Venus. Spectroscopic observations, particularly in the ultraviolet, show that the dark patches are composed of concentrated sulfuric acid and other unknown light-absorbing particles.

    Those dark patches have been a mystery since they were first observed by ground-based telescopes nearly a century ago, says Limaye. They were studied in more detail by subsequent probes to the planet.

    “Venus shows some episodic dark, sulfuric rich patches, with contrasts up to 30–40 percent in the ultraviolet, and muted in longer wavelengths. These patches persist for days, changing their shape and contrasts continuously and appear to be scale dependent,” says Limaye.

    “Venus has had plenty of time to evolve life on its own,” explains Limaye, noting that some models suggest Venus once had a habitable climate with liquid water on its surface for as long as 2 billion years. “That’s much longer than is believed to have occurred on Mars.”

    The particles that make up the dark patches have almost the same dimensions as some bacteria on Earth, although the instruments that have sampled Venus’ atmosphere to date are incapable of distinguishing between materials of an organic or inorganic nature.

    The patches could be something akin to the algae blooms that occur routinely in the lakes and oceans of Earth, according to Limaye and Mogul — only these would need to be sustained in the Venusian atmosphere.

    Limaye, who has spent his career studying planetary atmospheres, was further inspired to revisit the idea of microbial life in the clouds of Venus by a visit to Tso Kar, a high-altitude salt lake in northern India where he observed the powdery residue of sulfur-fixing bacteria concentrated on decaying grass at the edge of the lake being wafted into the atmosphere.

    • Harry R Ray April 2, 2018, 10:09

      GOOD NEWS: Northrop Grumman is ALREADY developing VAMP(Venus Atmospleric Maneuverable platform)which would fly like a plane and float like a blimp! It could stay aloft for up to a year. VERY GOOD NEWS: This craft could carry a MICROSCOPE capable of making an UNIMPEACHABLE detection of microbiotic life with ABSOLUTELY NO NEED of a sample return mission!!! Considering the current state of NASA, I URGE Uri Geller and Breakthrough Initiatives to BYPASS NASA and negotiate DIRECTLY with Northrop Grumman to build it ASAP!!! Perhaps(VERY wishful thinking here)Falcon Heavy could take it to Venus in as little as FIVE years!!!

    • Harry R Ray April 2, 2018, 10:19

      Sorry, I meant Yuri Milner. ALSO: It is now speculated that living organisms are responsible for PRODUCING all of that sulphuric acid IN THE FIRST PLACE!

      • ljk April 2, 2018, 11:38

        As opposed to all that volcanic activity on Venus?

        I am not saying it would be impossible, just that Sol 2 has lots of volcanoes and many of them seem to be active.

        • Harry R Ray April 3, 2018, 9:45

          Initially, you are most likely correct, BUT; to REPLENISH the H2SO4 on a CONTINUAL basis AFTER the dense high pressure CO2 troposphere has been established, I don’t think so. Modern day volcanic eruptions with an EXTENSIVE H2SO4 component are just not ENERGETIC enough to push the HEAVIER H2SO4 up through SO MUCH CO2 to get to the cloud tops, so the “finished product” H2SO4 is most likely doomed to react with the surface, instead. However, this poses ANOTHER problem; How do the putative H2SO4 PRODUCING micro-organsims get the materials necessary to do their thing. I envision transport via volcanic activity of the INDIVIDUAL COMPONENTS of the H2SO4, each of which is LIGHTER than the H2SO4 ITSELF(and or less heavy acids)which the putative organisms then ingest and metabolize and release H2SO4 as a wast product. However, an alternate solution would be the GLOBAL hypervolcanic “event” that occurred a billion or so years ago. It ONE: may have been powerful enough to create a global temperature INCREASE that would EXPAND the CO2 troposphere ENOUGH to let the H2SO4 through but would not be TOO HOT to reduce the H2SO4 to its individual components. This scenario ONLY works if ENOUGH H2SO4 were transported AT THAT TIME to insure that photolysis of H2SO4 at the cloud tops could NOT REMOVE ALL OF IT over a period of one billion years,

  • ole burde March 31, 2018, 7:03

    Just imagine what could have been done with the billions invested in the JWST , if invested in a well planned SERIES of instruments ”with a tight exoplanet focus ….and perhaps the resulting increased pace of dicovery could have brought even better funding to the area …

  • Gideon Marcus April 3, 2018, 23:30

    Thank you for this, Paul. I can’t wait for TESS and ARIEL!

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