The world’s first space telescope dedicated to studying the atmospheres of exoplanets is set to launch in 2028, after the European Space Agency (ESA) selected the mission for development on 20 March.
The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) — to be built with a budget of €450 million (US$552 million) — will chart more than 1,000 known planetary systems outside our own. Its mission is to understand the links between a planet’s chemistry and its environment, says Giovanna Tinetti, a planetary scientist at University College London and principal investigator for the mission. So far, exoplanet probes have mainly focused on finding new planets and measuring their sizes, masses and orbits. “ARIEL can really give us a full picture of what exoplanets are made of, how they form and how they evolve,” says Tinetti.
A spectrograph aboard the observatory will study the light that filters through a planet’s atmosphere as it passes — or transits — across the face of its host star, revealing chemical fingerprints of gases that shroud the body. The instrument will also try to refine estimates of a planet’s temperature by teasing out how light from its star changes when the body moves behind it, revealing details about a planet’s overall radiation. ARIEL will monitor the worlds at snapshots in time, as well as measure how the chemistry and atmospheres of planets vary across their surfaces and with the seasons, says Tinetti.
ESA’s science-planning committee picked ARIEL at a meeting in Paris, selecting it over two other shortlisted medium-class missions: an X-ray telescope designed to study the polarization of light from energetic phenomena such as black holes, and a detector to probe the behaviour of plasma in near-Earth space. Missions in this class generally have budgets of around €550 million, but overrunning costs elsewhere in ESA’s science budget mean that ARIEL will have to be built for around €100 million less than previous medium-class projects.
ARIEL is one of several exoplanet missions in the works around the world, but its focus on planetary atmospheres is rare. NASA’s planet-hunting Transiting Exoplanet Survey Satellite and ESA’s Characterizing Exoplanet Satellite mission, designed to measure planet size and density, are both scheduled to launch this year. And Plato, the previous medium-sized project selected by ESA in 2014, will study the size and mass of Earth-like exoplanets after it launches in 2026. (NASA’s long-awaited James Webb Space Telescope (JWST), the Hubble successor set to launch in 2019, will also be able to study planetary atmospheres. But because it is a general-purpose instrument, time for such studies will be limited.)
Together, this generation of telescopes will allow scientists to test models of planetary formation and composition, says Elizabeth Tasker, an astronomer and exoplanet researcher at the Japan Aerospace Exploration Agency’s Institute of Space and Astronautical Science in Sagamihara. Although ground-based telescopes have been peering at planetary atmospheres for almost two decades, the range of sky they can cover is limited, and Earth’s atmosphere obscures some wavelengths of light. ARIEL, from a vantage point between Earth and the Sun, will be able to scour the whole sky and see a wider range of wavelengths than are visible from our planet, says Tinetti. And unlike the JWST, ARIEL will be able to simultaneously collect data in the visible and infrared parts of the spectrum. That means it will be able take relevant measurements from a single transit, avoiding uncertainties that might arise from changes in a host star’s brightness between observations.
The bigger picture
Since the first exoplanet discoveries in the early 1990s, some 3,700 worlds have been confirmed in 2,800 systems. ARIEL will focus on planets that have an estimated temperature of more than 350 ºC — and are thus unlikely to host life as we know it — and a range of masses, in particular those heavier than a few Earth masses. An advantage of studying warm planets is that their atmospheres usually reflect the body’s overall composition, whereas cooler planets’ chemicals can condense into clouds or solid layers and remain hidden, says Tinetti.
The probe’s ability to chart huge numbers of planets will be crucial to building up a picture of how a planet’s atmosphere relates to its formation and evolution, says Yuka Fujii, a planetary scientist at the Tokyo Institute of Technology.
Scientists often focus on planets that could harbour life. But the diversity of exoplanet types and systems revealed by studies so far — and the growing realization that our own system may be atypical — make understanding the bigger picture more important, says Tinetti. The observations will build a ‘standard model’ of how a planet’s chemistry depends on its star and the condition of its birth, she says. Comparing a planet with this model will become an essential first step before asking whether it is habitable, she says. Otherwise, researchers can over-interpret signatures and be misled about the likely existence of life on a planet, she adds.
Without the ability to study planetary atmospheres, astronomers have stalled in their efforts to make sense of the diversity of mysterious planets found so far, says Sara Seager, a planetary scientist at the Massachusetts Institute of Technology in Cambridge. Researchers were once extremely sceptical about whether the technique ARIEL will use, which Seager developed in the late 1990s, would ever bear fruit, she says. “Now there is a whole mission about it. That’s absolutely wild.”
Nature 555, 571 (2018)