Rise of the exoplanet explorer
The next generation of alien-world-seeking spacecraft will revolutionise our understanding of distant planets
How the next generation of alienworld hunters will help us to gain an understanding of them and their atmospheres
Almost 30 years ago, two astronomers changed the way we viewed our place within the universe. They had discovered the first extrasolar planets, or exoplanets – planets that orbit stars other than our Sun. Since then we have discovered over 4,300 new and fascinating worlds – planets that orbit two stars, a planet that is so close to its host star that it’s being ripped apart and a planet that’s covered in hot lava. Some even orbit stars like our Sun in the habitable zone where liquid water could exist – and there are hundreds of billions of intriguing new worlds in the Milky Way alone that are yet to be discovered and explored.
To explore these unknown worlds, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, or Ariel, is set to launch at the end of the decade. The mission was selected by the European Space Agency (ESA) as its third dedicated exoplanet hunter to launch within ten years, and the fourth ‘medium-class’ project in the Cosmic Vision program, back in 2018 after beating a mission to study energetic particles around the Earth and an X-ray telescope. For the past five years, scientists have been working on Ariel’s science goals and instruments, which has involved collaboration between more than 50 institutes in 17 countries. NASA will also contribute an instrument to the mission.
Ariel will address one of the key themes of the ESA’s Cosmic Vision program: what are the conditions for planet formation and the emergence of life? “Ariel will study what exoplanets are made of, how they formed and how they evolve by surveying a diverse sample of about 1,000 extrasolar planets in visible and infrared wavelengths simultaneously. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System,” says Professor Günther Hasinger, the ESA’s director of science.
In recent years we have made great progress in the study of exoplanets. However, Ariel is different to the other exoplanet missions that have been launched in the past. “Previous space observatories like Kepler, CoRoT [Convention, Rotation and planetary Transits] and TESS
[the Transiting Exoplanet Survey Satellite] focused on discovering new exoplanets” says Dr Subhajit Sarkar, a research associate at Cardiff University in the UK, who works on the Ariel mission. “Unlike previous missions, Ariel does not aim to discover new planets, but instead will characterise the atmospheres of known exoplanets,” continues Sarkar.
“Ariel will study what exoplanets are made of, how they formed and how they evolve” Günther Hasinger
The Ariel mission will focus on surveying hot and warm giants similar in size to Jupiter and Neptune to super-Earths that orbit close to their host stars, analysing a range of spectral types from hotter Sun-like, white-yellow dwarf stars to red dwarfs. Hot exoplanets, some which reach over 2,000 degrees Celsius (3,632 degrees Fahrenheit), are ideal for studying the chemistry and formation history of the planet because gases continuously circulate throughout the atmosphere, preventing the formation of layers of clouds via condensation.
Exoplanet missions are extremely important and can help us to understand our own origins and the origins of the universe. “Exoplanet studies are one of the most important and appealing fields of contemporary astrophysics,” says Hasinger.
“We study exoplanets for several reasons,” says Sarkar. “Firstly, to understand our own origins. Where did we come from? How did the Solar System form? What led to the formation of the Earth and the origins of life on Earth? Exoplanets allow us to test the theories of planet formation through multiple natural experiments instead of using the example of one we had before”.
Although so far we have found no evidence to suggest that there is life beyond planet Earth, Ariel will rise to the challenge. “Exoplanets are part of the search for life in the universe and the age-old question of ‘are we alone?’ We may be able to detect biosignatures in the spectra of these worlds that reveal the presence of life. Rocky planets that orbit in the ‘habitable zone’ of their star – where liquid water could possibly form on the surface of the planet – are a focus for such a search. This would be one of the greatest discoveries in science, and again place our own origins in context. It may be possible one day to find out how common life is in the universe,” continues Sarkar. Understanding exoplanets can also help us “better understand the processes affecting the Earth” and “may have great relevance for understanding the future evolution of our own planet,” he explains.
The space observatory is currently scheduled to start its search for life in 2029, launching with Comet Interceptor from the ESA’s spaceport in Kourou, French Guiana on the new Ariane 6 rocket, which is currently scheduled to make its maiden launch in 2022. The craft will take roughly six months to reach its orbit and undergo commissioning and science demonstration phases before it can start its primary mission. The space observatory will make its way to Lagrange point 2 (L2), an orbital parking spot roughly 1.5 million kilometres (930,000 miles) from Earth. The initial science mission will last for 3.5 years, with the possibility of a two-year extension.
Once Ariel is in position, the observatory will use its 0.9-metre (three-foot) diameter primary mirror to collect light from faraway stars in the visible and infrared wavelengths. Other instruments,
including an infrared spectrometer and photometer, will perform spectroscopic analyses on the target exoplanets. A spectrum will be obtained for each target exoplanet, and like its predecessors, Ariel will make use of the transit method to study exoplanets, where the dip in starlight is measured as a planet travels in front of its host star. Sarkar explains:
“Each observation will involve measuring the light from the host star for a period of time, during which the planet will transit, producing a ‘light curve’.
The spectrometers allow the light to be split into different wavelengths so that multiple light curves can be obtained as a function of wavelength. The ‘dip’ in the starlight is proportional to the planetstar area ratio and returns a planet radius with wavelength.
“While the bulk of the radius value is due to the planet body and fixed, a small amount of extra absorption occurs due to molecules in the atmosphere of the planet. These molecules absorb differently at different wavelengths, each with its own characteristic spectral ‘fingerprint’. By tracing out the apparent radius variation with wavelength we obtain a ‘transmission spectrum’ of the atmosphere. By analysing this we can find the molecules making up the atmosphere and other properties such as temperature and the presence of clouds and hazes.” Results from Ariel will be made available to the science community and the general public almost immediately. It is hoped that this approach will ensure that the Ariel mission will outperform its mission goals.
In recent years our understanding and knowledge of exoplanets has grown exponentially, and Ariel will surely further our understanding even more, perhaps more than we could ever hope or imagine, and might also help to explain our weird Solar System. “We know that planets are common around all types of stars. However, this planet population is very diverse, covering different sizes, masses, temperatures and orbital parameters. The architectures of most of these planetary systems also do not match that of our Solar System, and there are planet types, such as super-Earths and mini-Neptunes that do not occur in our own Solar System. Therefore the theories that were developed to explain the Solar System planets do not completely explain these new exoplanetary systems. We would like to understand how this complex pattern arises,” explains Sarkar.
The most famous – and arguably the most successful – use of the transit method was by NASA’s Kepler space telescope, which was launched in 2009. In just under ten years the number of known exoplanets rapidly grew from a few dozen to a few thousand. Over the course of its lifetime Kepler observed over 530,000 stars, looking for