The European Space Agency has selected the ARIEL mission to analyze the atmospheric composition and true nature of distant worlds.

ARIEL Mission
An artist's conception of ARIEL in space.

It's amazing to think that we now live in a era in which we  know of thousands of worlds beyond our solar system. Almost monthly, we hear news of the fastest, hottest, or potentially Earth-like worlds . . . but what are distant exoplanets really like?

The European Space Agency (ESA) has selected the fourth medium-class mission for its Cosmic Visions program to help answer this key question: ARIEL, the Atmospheric Remote-sensing Exoplanet Large-survey.

ARIEL will launch in 2028 time-frame on an Ariane 6-2 rocket, a new launch vehicle that will see its maiden flight in 2020. Planned as a four-year mission, ARIEL will join the James Webb Space Telescope (now launching in 2020) in a Lissajous or halo orbit around the anti-sunward L2 Lagrange Point almost 1 million miles (1.5 million kilometers) from Earth.

ARIEL's orbit
ARIEL's orbit around the L2 Lagrange Point.

Analyzing Alien Worlds

ARIEL won't discover new exoplanets on its own. Instead, ARIEL will perform followup observations of at least 1,000 known transiting exoplanets, collecting their spectra to define and characterize atmospheric composition. ARIEL will come equipped with a 1-meter (3.1-foot) primary mirror and a near-infrared spectrometer to accomplish these observations. ARIEL will also seek to document the presence of cloud cover, seasonal variations, and brightness changes on distant worlds. ARIEL's instruments will target a narrow range of visible and infrared wavelengths (0.5-7.8 microns), allowing for a simple, lightweight science package.

“The essential nature of exoplanets is still something of a mystery to us,” says Giovanna Tinetti (University College London) in a recent press release. “If we are going to answer questions, such as how is the chemistry of a planet linked to the environment in which it forms, or is its birth and evolution driven by its host star, we need to study a statistically large sample of exoplanets.”

Not only will ARIEL contrast the composition of a target exoplanet against that of its host star, but it could also reveal if any interesting chemistry is afoot. For example, a “red edge” at 0.7 μm could betray the presence of the chemical compound chlorophyll on a distant world — on Earth the only known producer of chlorophyll is plant life.

The UK Space Agency will take the lead on the 450 million-euro ($555 million) mission, along with 11 European countries. There might also be a NASA component to the mission. Construction and testing of the science payloads for the ARIEL mission will be carried out at the Rutherford Appleton Laboratory in Hartwell, Oxfordshire in the UK.

ARIEL was selected over the space plasma physics mission THOR (the Turbulence Heating Observer) and the X-ray observatory XIPE (the X-ray Imaging Polarimetry Explorer) during this fourth selection round. Other medium-class ESA missions in the pipeline are the Solar Orbiter solar physics mission set for 2019, the Euclid dark energy and dark matter mission set for 2020, and PLATO (the Planetary Transits and Oscillations mission) which will characterize the density, size, and mass of selected exoplanets.

The ARIEL proposal built on EchO (the Exoplanet Characterization Observatory), which was passed over last selection round in favor of PLATO.

The current tally for known exoplanets stands at 3,757. With Kepler most likely coming to an end this year and TESS set for a launch that's coming right up next week on April 16th, ARIEL will have lots of targets to choose from. It's important to note that the transit method introduces a selection bias weighted toward worlds in short orbits close to their host stars. There are more than likely lots of worlds in even nearby star systems that aren't noticeable using the transit technique.

Still, findings from the ARIEL mission will go a long way toward making real statistical models of just what these worlds might actually be like, and how the local environment influences the development of a given world.

It's exciting to think: just over a quarter of a century ago, no exoplanets were known. Now, we're getting enough examples to populate a new field of exoplanet science, with the ability to say just how common (or rare) a solar system like our own really is.



Image of Andrew James

Andrew James

April 13, 2018 at 9:38 pm

Saying "Planned as a four-year mission, ARIEL will join the James Webb Space Telescope (now launching in 2020)...." & "There might also be a NASA component to the mission." is plainly misleading and seems to imply this ESA mission is being propped up, prposed or supported by NASA. It is not.

NASA via JPL will as "A Partner Mission of Opportunity (PMO) has been conditionally selected to provide detectors for the Fine Guidance Sensor assembly of the Atmospheric Remote Sensing Infrared Exoplanet Large-Survey (ARIEL) mission -"[1] This will be paid for by the ESA not the American taxpayer. It will not be gifted.

Similar misgiving statements also appear elsewhere: "Contribution to ARIEL Spectroscopy of Exoplanets (CASE). This would provide fine-guidance detectors for the proposed Atmospheric Remote Sensing Infrared Exoplanet Large-Survey (ARIEL) mission, which is led by the European Space Agency. ARIEL's main goal is to look at the wavelengths of light emitted by exoplanets, ranging from super-Earths to gas giants. CASE can proceed only if NASA selects it and if ESA decides to go ahead with ARIEL."[3]</a

Also :"The ARIEL mission concept has been developed by a consortium of more than 50 institutes from 12 countries, which include UK, France, Italy, Germany, the Netherlands, Poland, Spain, Belgium, Austria, Denmark, Ireland and Portugal. "[3]

Günther Hasinger, ESA Director of Science, explains "ARIEL will allow European scientists to maintain competitiveness in this dynamic field. It will build on the experiences and knowledge gained from previous exoplanet missions."[4]

Singing the praises of non-North American science is good, but trying to imply it is underpinning the skirt-tails just on the basis of jingoism or nationalism is totally unacceptable.

Note: NASA's own possible planned mission is the Fast INfrared Exoplanet Spectroscopy Survey Explorer (FINESSE), which is still being considered. Under the current priorities of NASA, privatisation goals, and the economic constraints stated by the US Government, this mission seems to be a low priority. Motive here?

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Image of alphapsa


April 20, 2018 at 12:50 pm

"PLATO (the Planetary Transits and Oscillations mission) which will characterize the density, size, and mass of selected exoplanets."

PLATO, to be launched in 2026, is a survey mission much like Kepler and TESS. Successful as Kepler was, it failed with its primary goal of detecting an Earth analogue (Earth-like planet in an Earth-like orbit, around a solar-like star). TESS will solve another problem with the Kepler planets, that they are distant and hard to follow up with radial velocity measurements, but will be focussed on short-period planets (~weeks). PLATO is designed to find plenty of those elusive Earth analogues (among other things) and will, together with TESS, provide targets for ARIEL.

Incidently, there is a targeted small ESA mission designed to " characterize the density, size, and mass of selected exoplanets"; it is called CHEOPS and has a launch window between Dec this year and April 2019.

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