Green light for the ARIEL mission...

...which should help us understand the nature of exoplanets better

The European Space Agency’s (ESA) Programme Committee has just selected the ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission for a launch scheduled in 2028. This space telescope will be dedicated to observing the infra-red light from exoplanets’ atmospheres. It is very satisfying for the scientific teams involved, including the Institute of Space Astrophysics (IAS-UPSud/CNRS).

The ESA’s Programme Committee has just selected ARIEL as the 4th medium-class mission in the “Cosmic Vision” programme. The purpose of this probe is to observe several hundred exoplanets whose size varies between that of Jupiter and Earth.

A key to deciphering the properties of the atmosphere of other worlds

Although we have now discovered nearly 4,000 planets orbiting around other stars, the nature of these exoplanets remains largely mysterious. Scientists still do not know how much a planet’s chemistry is related to its environment on formation, including the type of host star, or whether the type of host star determines the physics and chemistry of the planet’s birth and development.

The ARIEL mission aims to provide answers to these questions by systematically probing the atmospheres of about a 1,000 extrasolar planets, from gas giants to rocky planets, whether they are hot or temperate, around different types of stars.

Expertise of French research laboratories

The project originated in studies performed in the last 20 years. The first results obtained by the CoRoT and Kepler satellites for the detection of planets, and then Spitzer and Hubble for the spectral analysis of atmospheres, underlined the difficulty in measuring these and the immense diversity of the exoplanets’ atmospheres. Dedicated and optimised instruments were required to compensate for these difficulties. So, a group of scientists started studies in 2008.

From 2015, drawing on their accumulated expertise, teams from five French laboratories, including IAS (IAS-UPSud/CNRS)  took on a very significant role in the ARIEL studies. Their involvement was critical in drawing up the scientific proposal, the mission scenario and feasibility studies for the infra-red spectrometer at the centre of the satellite. This led to French teams taking responsibility for designing and producing the infra-red spectrometer. This will be built by CEA-Irfu (Institute of Research into the Fundamental Laws of the Universe at the Atomic Energy Commission) under the supervision of CNES (National Centre for Space Studies) with important contributions by IAS and LESIA (the Laboratory for Space Studies and Instrumentation in Astrophysics).

Finally, Ariel was put forward by a consortium of more than 60 institutes in 15 European countries  in response to the ESA’s call for proposal of missions in August 2014. The scientific and technical co-ordination will be provided by the United Kingdom. A contribution by the United States is currently under consideration.

Opening a new page in the history of studying exoplanets

ARIEL will be made up of a telescope of about a metre in diameter to collect the visible and infra-red light from the stars which an exoplanet orbits around. A spectrometer will spread this light into a ‘rainbow’, and when the exoplanet transits in front of its star, the spectrum obtained will have absorption lines of the star’s light taken in by molecules in the exoplanet’s atmosphere.

“The observation is particularly difficult, as information about the planet has to be separated from the light of the star, which is 1,000 to 100,000 times brighter than the planet. The highest performance is obtained due to the instrument’s intrinsic photometric stability and a very rigorous knowledge of its operation,” says Marc Ollivier, an astronomer and director at IAS.

The ARIEL mission will thus address the fundamental questions of the formation and evolution of exoplanets in their planetary systems, by examining the atmospheres of several hundred planets orbiting around different types of stars. The statistical approach is critical and it will allow a new page to be opened in the history of the study of exoplanets. ARIEL will be launched from the Kourou base in French Guiana in May 2028 and will be placed in orbit at the Lagrange L2 site, located 1.5 million kilometres from Earth.

Marc Ollivier, Institut d’Astrophysique Spatiale (IAS), CNRS / Université Paris-Sud / Université Paris-Saclay (marc.ollivier @