JWST | Understanding the diversity of unusual worlds

The James Webb Space Telescope will allow the observation of planets and other bodies in our solar system in order to understand their origin and evolution, but it will also bstudy exoplanets, planets that orbit stars other than our sun. Among these targets, the telescope will be able to focus its observations on rocky exoplanets, present in the habitable zone of their system - i.e. not too close nor too far from their star for the possible presence of surface liquid water . However, being at the right distance is not enough for determining their habitability (possibility for life to develop on a planet or moon), it is also necessary to know if these rocky planets have an atmosphere, if they have liquid water on the surface, etc.   

Once rocky exoplanets are detected, Webb will allow the analysis of their atmosphere thanks to a technique of transmission spectroscopy which will examine the light of the star filtered through the atmosphere of the planets, and thus willprovide information on their chemical composition.  JWST will enable this exploration of the diversity of exoplanets and the diversity of their atmospheres, a search that will take years and is essential to establish a thorough baseline knowledge, a context in which we can then search for anomalies that could be possibly explained by biological rather than chemical or physical processes. This adventure is therefore multidisciplinary, but biology must be patient before we can speak of a greater or lesser probability of the presence of life on certain exoplanets. There are still many unknowns in astrophysics, planetary geology, atmospheric chemistry, as well as in biology, but these challenges are exciting and JWST is a great tool to advance knowledge! 

We are involved in the analysis of the JWST data through our PORTAL (PhOtotRophy on habTtAbLes rocky pLanets) project which studies the habitability of temperate rocky planets orbiting very small mass stars and the possibility of life on such planets. The objectives of this project are to determine the characteristics of the physical and irradiation conditions on the surface of planets located in the habitable zone of TRAPPIST-1 from observational data and theoretical modeling. Under these conditions, we will investigate the possibility of phototrophy (living organisms that derive their energy from light, by photosynthesis or proteins, using pigments) in the infrared, and the detectability of its signatures in different types of samples: from the Precambrian Earth and modern extreme environments, in simulated exoplanetary conditions in a new "TRAPPIST biodome", and on planets orbiting TRAPPIST-1. Our PORTAL project will thus contribute to the understanding of phototrophy on Earth and its evolution, and to the possibility of phototrophy on habitable rocky exoplanets orbiting another type of star (ultra-cold red dwarf) than the sun but very common in the universe. The project requires multidisciplinary expertise in astrophysics, geophysics, biology (microbiology, paleobiology, geobiology, photosynthesis), physics and chemistry of the atmosphere and modeling, which is why it brings together many researchers from Belgian universities and Royal Observatory but also from the University of Berlin, the University of Paris 6, and the Bordeaux Observatory.  

Astrobiology is the study of the origin, evolution, distribution, and future of life in the Universe, starting with the Earth, the only biological planet known to date. How did life appear? How did it evolve? What are its limits? What is its frequency in the Universe? What traces of life or possible biosignatures should we look for to reconstruct its history on Earth, or to detect it elsewhere in the solar system or beyond? With which instruments ? What space missions ?  Under what conditions is it possible elsewhere? The question of origins is central in astrobiology: origin of rocky (exo)planets, origins of life, of biological innovations, of complex life (unicellular and multicellular), of metabolisms such as photosynthesis, ... Astrobiology is, in essence, multidisciplinary, ranging from astrophysics to biology, through geology, chemistry, planetary sciences, engineering, history of science and philosophy, to taddress these fundamental questions that challenge and fascinate all of us.

A trace of life can be morphological (organic or mineral fossils, rocks or sediments built or modified by life such as stromatolites, microbial mats , ...), chemical (complex molecules such as some lipids or pigments; isotopic fractionation of elements used by life (C, N, S, Fe, ...), a non-random patterns in organic molecules such as the chirality of amino acids or the exclusive presence of even or odd numbers of carbon, ..), or spectroscopic (desequilibrium between co-occurring atmospheric gases).  However, for all these potential traces, there are natural abiotic processes that can produce similar structures, molecules or gases. It is therefore crucial to understand the context of formation and preservation of putative traces and the natural diversity of chemical and physical processes in the absence of life, before being able to conclude with a greater or lesser probability and confidence that the observations are of biological origin. 

The search for traces of life on other planets is very different depending on whether we target the solar system, with the possibility of studying and taking samples on site or samples brought back to Earth, or whether we target exoplanetary systems where we will never be able to land. In both cases, the first step is to determine if the planets are or have been habitable. Then, the search for traces of life on exoplanets focuses on the detection and composition of the atmosphere.   

Future technological developments may allow us to analyze the surface of planets, which could suggest the likelihood of the presence of life, for example via the absorption or reflection of light (fluorescence) by pigments as is done by satellite over the Earth's oceans to map, identify and quantify phytoplankton (important phototrophic microorganisms that are primary producers at the base of trophic chains and play a major role in the carbon cycle and oxygenation of the atmosphere). The European Extremely large telescope (E-ELT) which should be inaugurated in 2025 will be able to analyze the polarized light reflected by a planet and to differentiate it from the non-polarized light of its star. With the VLT (Very Large Telescope), it is already possible to analyze the light from the Earth illuminated by the Sun and reflected on the Moon which acts as a mirror (Earthshine), and to see the presence of water and vegetation. But we are still very far from that for exoplanets!  

In the framework of the PORTAL project - which we have developed with several colleagues and which will be fed by data transmitted by the JWST - we started from the idea that stellar radiation represents an efficient and unlimited source of energy, abundantly used by life and at the basis of most food webs on Earth. Life also developed strategies (such as sun-screen pigments or shaded habitats) to protect itself against harmful UV radiations. It might therefore be possible that, if other  microbial lifeforms  exist elsewhere in the universe,they would use light from other stars . Among the thousands of exoplanets discovered to date, a few dozen are potentially habitable, and their atmospheric compositions will soon be described thanks to new telescopes, notably JWST. A thorough assessment of the habitability of planetary systems around low-mass dwarf stars is therefore essential for understanding the universality and limits of life. Because the luminosity of these cool stars is much lower than the sun, their rocky planets must orbit very close to them to be habitable. Since the light spectrum of these stars emits mostly in the infrared, phototrophic life on the surface of these planets would have to develop strategies to utilize these photons while protecting themselves from the very strong extreme ultraviolet (XUV) radiation and stellar winds.   

On Earth, phototrophic organisms have developed mechanisms to capture photons in the visible, but also in the infrared, and strategies to protect themselves from UV. Phototrophy appeared more than 3.4 billion years ago, when the Earth's anoxic atmosphere was devoid of ozone, exposing the Earth's surface to powerful UV radiation. Later, around 2.4 billion years ago, oxygenic photosynthesis had a major influence on the chemical composition of the atmosphere and the oceans, and contributed to the diversification and complexification of life (eukaryote which have one or more nucleated cells). Phototrophy can therefore impact the evolution of planets and life. The JWST will allow the remote exploration of the various types of exoplanets and the composition of their atmospheres, thus offering an essential knowledge base which will then allow to better define the diversity of uninhabitable worlds, possibly habitable worlds, and to search for anomalies that may be explained by biology.