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

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.

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.