The European Space Agency (ESA) has formally adopted Ariel, the first mission dedicated to study the nature, formation and evolution of exoplanets.

The mission has passed a rigorous set of reviews which it has been undergoing throughout 2020 to prove the technical feasibility and science case and has now received approval from ESA’s member states, confirming that the team can work towards a launch in 2029.

What is Ariel space mission?

• Ariel, the Atmospheric remote-sensing infrared exoplanet large-survey mission, will survey about 1,000 planets outside our solar system during its lifetime. It will unveil the nature, formation and evolution of a large and assorted sample of planets around different types of stars in our galaxy.

• Ariel is planned for launch on ESA’s new Ariane 6 rocket from Europe’s spaceport in Kourou, French Guiana. It will operate from an orbit around the second Sun-Earth Lagrange point, L2, 1.5 million kilometres directly ‘behind’ Earth as viewed from the Sun, on an initial four-year mission. 

Significance of the mission

• Ariel will be the first mission dedicated to measuring the chemical composition and atmospheric thermal properties of hundreds of transiting exoplanets. 

• Ariel will give us a picture of a diverse range of exoplanets: from extremely hot to temperate, from gaseous to rocky planets orbiting close to their parent stars.

• More than 50 institutes from 17 countries have been working over the past five years to develop the science goals and design the instrumentation which will enable Ariel to survey a diverse sample of around 1,000 planets outside our own solar system.

• By looking specifically at hot planets, scientists are expecting to build an understanding of the formation of planets and their evolution. At hotter temperature, which in some cases it can be more than 2000°C, a greater number of exotic molecules will be visible to Ariel. 

• The instruments will then be able to determine what the atmospheres are made up of and provide scientists a unique insight into the planet’s internal composition and the formation history of the planetary system.

How will it collect data?

• Ariel will have a meter-class telescope primary mirror to collect visible and infrared light from distant star systems. An infrared spectrometer will spread the light into a ‘rainbow’ and extract the chemical fingerprints of gases in the planets’ atmospheres, which become embedded in starlight when a planet passes in front or behind the star. 

• A photometer, a spectrometer and guidance system will capture information on the presence of clouds in the atmospheres of the exoplanets and will allow the spacecraft to point to the target star with high stability and precision. 

• The telescope’s spectrometers will measure the chemical fingerprints of a planet as it crosses in front of – ‘transits’ – its host star, or passes behind it – an ‘occultation’. The measurements will also enable astronomers to observe the dimming of the host star by the planet with a precision of 10–100 parts per million relative to the star.

• The Ariel team is taking a very open approach providing rapid access to data and even encouraging enthusiasts to help select targets and characterise stars. Much of the data will be available to both the science community and general public immediately.

Other exoplanet missions of ESA

• Ariel will be ESA’s third dedicated exoplanet mission to launch within a ten-year period, with each mission tackling a unique aspect of exoplanet science. 

• Cheops, the CHaracterising ExOPlanet Satellite, was launched in December 2019. It is the first mission dedicated to studying bright, nearby stars that are already known to host exoplanets, in order to make high-precision observations of the planet's size as it passes in front of its host star.

• Plato, the PLAnetary Transits and Oscillations of stars mission, will be launched in the 2026 timeframe to find and study extrasolar planetary systems, with a special emphasis on rocky planets around Sun-like stars in the habitable zone – the distance from a star where liquid water can exist on a planet’s surface. 

• Ariel was selected in 2018 as the fourth medium-class science mission in ESA’s Cosmic Vision plan. It was ‘adopted’ by ESA during the Agency’s Science Programme Committee meeting on November 12, paving the way towards construction.

• Ariel, planned to launch in 2029, will focus on warm and hot planets, ranging from super-Earths to gas giants orbiting close to their parent stars, taking advantage of their well-mixed atmospheres to decipher their bulk composition.

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