ESA’s green light for the ARIEL mission

ARIEL will be launched from the Kourou space center in Guyana, in May 2028, and will be injected into the L2 Lagrange point, 1.5 million km from the Earth.

Its objective will be to study systematically the atmospheres of on the order of one thousand extra-solar planets, gas giants with rocky planets, be they hot or temperate, around stars of different kinds.

ARIEL will measure the composition and structure of the planetary atmospheres, will constrain the nature of the planetary cores, will detect the presence of clouds and will study the interactions with the host stars.

The selection procedure will involve a number of steps :

• Call for tender for the mission, launched by ESA in August 2014 ;
• Pre-selection in June 2015 of 3 missions out of the 27 for detailed studies ;
• Evaluation of the results of these studies by a committee of experts, and the recommendation of one mission in November 2017 ;
• Formal vote by ESA’s Program Committee, followed by a public announcement in March 2018.

At this stage, Ariel will become officially the 4th intermediate class mission of the « Cosmic Vision » program, with a budgetary envelope of 450 millions euros.

The critical role of French laboratories

Ariel was proposed by a consortium of 60 institutes from 15 European countries [1].

The scientific and technical coordination is under the responsibility of the United Kingdom. An american contribution is currently being studied.

ARIEL will be composed of a roughly 1 metre diameter to collect the visible and infra-red radiation of stars around which an exoplanet is in orbit. A spectrometer will break up this light into a "rainbow", and when an exoplanet transits its star, the resulting stellar spectrum will display absorption bands of the molecules in the exoplanet’s atmosphere.

ARIEL is born from work done during the past ten years. In 2007, water vapor, followed by carbon dioxide and carbon monoxide, were detected in the atmosphere of the hot Jupiter HD189733b thinks to the Spitzer and Hubble telescopes.

These first results emphasized the difficulties inherent in the measurement, as well as the enormous diversity of exoplanetary atmosphers. In order to realize such difficult measurements for a large number of planets, its is necessary to have a dedicated optimized instrument.

A scientific team was thus set up starting in 2008

Starting in 2015, benefiting from their accumulated expertise, five French laboratories (IAP, DAp/AIM, IAS, LESIA, LAB) took a very important part in the ARIEL studies. Their contributions for the development of the scientific proposal, for the mission scenario and for the feasibility study of the infra-red spectrometer within the satellite, were vital.

As a consequence, the French teams have taken on the responsibility for the design and the delivery of the infra-red spectrometer. These latter elements will be built by the CEA-Irfu with important contributions from the IAS and LESIA, under the control of the CNES.

« The study of the ARIEL mission was based on the EChO project, a candidate for ESA’s M3 Mission in 2014. The evolution of exoplanetary research has clearly identified spectroscopy as a major activity for the coming decade in the study of their atmosphers », emphasized Pierre Drossart, the LESIA director.