On March 20, 2018, the European Space Agency (ESA) announced the Atmospheric Remote-Sensing Infrared Exoplanet Large-Survey (ARIEL) mission as the its 4th medium-class mission in the Cosmic Vision program.
ARIEL joins the ranks of the Solar Orbiter, which studies the area near the sun where solar winds are potent, the Planetary Transits and Oscillations of Stars (PLATO), a machine that hunts for new planets like NASA’s Kepler, and Euclid, which analyzes the effect of dark-matter on the early expansion of the universe.
ARIEL, set to launch in 2028, will analyze the atmospheric composition of 1000 known exoplanets. While it will focus on large gas giants unlike the Earth we live on, from the size of super-Earths (planets larger than Earth but much smaller than Neptune) to Jupiter, ARIEL’s data will provide insights into astrobiology.
ARIEL will look at some planets in the “habitable zone,” where the planets receive similar solar flux to Earth. The compositions of those atmospheres could tell us if planets in the habitable zone harbor the right molecules for life similar to Earth.
Concerning astrobiology, ARIEL will also act as a guinea pig for this sort of mass data collection; although this mission is focused on gas giants, the methods learned can be applied to missions that will look at rock-planets like Earth where extraterrestrial life may be.
The reason why ARIEL will look at large planets is that the high temperatures generally keep the molecules in a gas state. Otherwise, the molecules would condense to form clouds or sink to the surface, hiding them from ARIEL and disrupting the spectroscopy.
To analyze the atmospheres of those 1000 planets, ARIEL utilizes spectroscopy.
Spectroscopy analyzes the spectrum of light received after the light runs through translucent matter; different molecules absorb different wavelengths of light, resulting in different emissions spectrums. The spectrums received from the planets hints at the compositions of their atmospheres.
ARIEL will capture this spectrum at two instances in a planet’s orbit: when it passes directly in front of its sun and when it passes behind its sun.
While it is in front of its sun, a phenomenon called a transit, there is a drop in the amount of light received by ARIEL as the planet blocks the sunlight; the difference is the light that passes directly through the atmosphere of the planet. This is termed transmission spectroscopy.
The planet passing behind the sun is called the secondary eclipse, so the information received is called eclipse spectroscopy. Again, there is a drop in light received, since there is no longer the auxiliary reflection of sunlight from the surface of the planet. The difference represents the light from the planet, which is analyzed for its spectrum and the composition of the atmosphere.
ARIEL will also look at phase variations of the planet. While the planet is orbiting, different longitudinal sides are exposed to ARIEL; the different spectrums received by ARIEL create a composite of the varying atmospheric composition of the planet.
On select planets, ARIEL will also be utilized to do deep analyses on cloud systems and seasonal and daily variations in temperature, again lending insight to the atmosphere of the planet.
The general purpose of ARIEL is to understand how our own solar system fits into the universe as a whole, as well as the origin and evolution of planets.
Current data seems to indicate planets form in an orbit, then migrate into a different orbit; during the formation of planets, the composition of the planet-forming disk changes radially from the star. The atmosphere of that planet so many years later will reflect this position from the sun, so ARIEL can confirm or deny the theories of planet migration.
ARIEL was chosen over two other candidates, the Turbulence Heating Observer (THOR), which proposed to analyze the turbulent fluctuations in plasma environments, and the X-ray Imaging Polarimetry Explorer (XIPE), which would use the x-ray wavelength to study an amalgam of physical properties of the universe.
Once again, science is pushing the envelope in leaps no one could have imagined; it’s an ambitious plan to understand the specific composition of 1000 different atmospheres.
As someone whose greatest goal is to confirm the existence of extraterrestrial life, the approval of ARIEL gives me hope that it is a goal that will be realized in my life-time. So, as every good scientist must ask: how far do you think we can go?