In the solar system

We know that at least two conditions were necessary for the appearance of life on Earth: stable physical conditions over biological time scale and liquid water. The question of the appearance of life from an astrophysical viewpoint is currently embodied in our knowledge of the solar system history, of its stability over time, of the origin of water present on Earth, and of its distribution over the solar system. With the Laplace hypothesis of a solar system stable for the last four billion years shattered, current investigations are actually struggling to elaborate an alternative scenario that may also explain the origin of water on Earth.
Oceans may have formed from degassing of the proto-Earth mantle, if Earth formed from
wet material. Alternatively, colliding icy bodies formed beyond the snow line and destabilized during episodes of planet migration may have brought water on Earth. Earth’s mantle and volatile reservoirs might originate at the end from a mix between a wide variety of late-arriving objects together with pre-existing accreted material. Therefore, the extensive compositional and isotopic characterization of leftover primitive material contained in asteroids, comets, planets, and planet satellites is essential. The overall distribution of water in the present Solar System provides key constraint for dynamical studies of the early Solar System aimed at understanding the water content of Earth impactors during episodes of giant planets migration. This is an area where the Observatoire de Paris has provided outstanding results thanks to its observational expertise and its involvement in a number of space-based facilities (e.g., ISO, Herschel), and space missions (e.g., Mars-Express, Rosetta, Dawn, OSIRIS-ReX, Bepi Colombo).
The question of habitability in our Solar System is not restricted to Earth. Mars, Venus and the Moon currently contain (or have contained in the past) significant amounts of water in their ancient atmospheres or on their surfaces. Jupiter’s moon Europa and Saturn’s satellites Titan and Enceladus show evidence that oceans of liquid water may lay beneath their icy surfaces. These oceans could provide a present-day setting for extra-terrestrial life. The Observatoire de Paris is involved in several instruments for the JUpiter ICy moons Explorer (JUICE/ESA) mission that will perform detailed investigations of Jupiter satellites to evaluate their potential to support life.
This broad line of enquiries will be pursued during the OCAV project with the involvement of the Observatoire de Paris in the current Rosetta mission, in the future JUICE mission that will explore Jupiter icy moons that may harbour favourable conditions for the emergence of life, and the future NASA Mars2020 mission, to name a few. These three missions are key programs dedicated to the search for signs of past or present biological activities in the solar system with the Mars2020 aiming to identify potential bio-signatures at the surface of Mars.
Venus and Mars have long been presented as sister planets of the Earth but they clearly followed very different evolutionary tracks. This illustrates that, with the quest for chemical complexity within the solar system, studying long-time stability of planets and planetary systems is necessary to identify favourable conditions for the emergence of life. This is an area where the institution has produced breakthrough results over the years. Indeed, to better understand the climatic evolution on the surface of Mars and Venus through their history, one needs to model the long-term evolution of their atmosphere and associated climatic evolution. It is also important to investigate their past orbital and rotational evolution as they can then be used as test beds for the evolution of exoplanets and help identify conditions that may be favourable to the emergence of life.

Beyond the solar system:

With the discovery of thousands of exoplanets orbiting nearby stars in the past twenty years, identifying conditions that may lead to the emergence of lifeis taking a
new turn. This field is just in its infancy, but we have already identified that there exists a broad diversity of planetary objects and planetary systems. This diversity is well beyond what is found in the solar system and amplifies long-standing questions related to the uniqueness of the solar system, the Earth, as well as their evolution. Due to observational bias, the planets identified so far are primarily giant planets like Jupiters, with some far from their host stars, the very
few Earth-like planets detected so far being on very close-in orbits.
Several space missions as well as ground-based instruments are planned in the coming years to close this gap and potentially identify a significant number of Earth-like candidates with periods similar to that of the Earth. While we wait for potentially better statistics for these objects and the direct observation of a whole class of Earth-like objects, the observations obtained so far already bring many questions. The scientific community is currently engaged along several lines of inquiries. How did these planetary systems form? What are their evolution and history? How do they relate to the solar system current state and history? What are the physical conditions on these objects? Are these environments more diverse than in the solar system? Could they host chemical complexity or even lead to the emergence of life? How can we identify it if it was the case? Within the project, we will address these broad scientific questions by focusing on the characterization of planetary systems, of their constituting planets, as well as their time-evolution starting from protoplanetary disks and the primitive nebula.
As of today, the large majority of these objects are detected indirectly. As such, physical information on these planets and whether conditions are relevant to the emergence of life or chemical complexity are inferred from modelling. This involves models of the planetary system, planetary interiors and their atmospheres. The Observatoire de Paris has a unique expertise in these three areas. The project will promote further developments that will bring us closer to the identification of planetary systems and planets hosting favourable conditions over biological time scale. With the unique expertise gathered within the project, we will not only address the traditional question of habitability as surface conditions where liquid water may exist but also explore a broader viewpoint where conditions for evolutionary chemistry and its detectable signature is what to look for. With already more than 10 Earth-like candidates identified, this issue is of primary interest to select targets of forthcoming obse
rvational programs.
The Observatoire de Paris has some ties with most ground based and space observational programs. Following up on its deep implication in the CoRoT and Kepler space missions, it is directly engaged in the two upcoming European space missions using transit, CHEOPS and PLATO as well as in the upcoming NASA JWST mission that will use both transit and direct imaging techniques. These missions focus on super-neptunes and Earth-like planets with long period and will most certainly bring us its set of unexpected results. They will at least bring us a broader view of the physical conditions that may be encountered outside the solar system as well as possible candidate planets that may be targets of further characterization. The institution is also involved in the MIRI instrument aimed at performing spectral measurements for high profile candidates in the JWST mission. The scientific exploitation of these missions is one of the main trust of the OCAV project but from an R&D standpoint, the project will promote developments around high contrast imaging technics where we have very recently developed innovative approaches.
This emphasis is motivated by the fact that analysing a planet atmosphere and, ultimately, searching for signs of life on telluric planets will require, at some point, to perform spectroscopy of these very faint objects. With this longer-term goal in mind, this approach is already bringing key results. The first specialized instruments using high contrast imaging to extract planet spectral information have started operations: SPHERE at the Very Large Telescope and GPI at Gemini telescope. The ongoing exploitation where the Observatoire de Paris is taking part is focusing on large Jupiter spectral and dynamically-induced structures in protoplanetary/circumstellar disks characterization. These two areas are providing much needed data on planet formation and planetary system histories. The project will address these key questions in the broader context of the chemical makeup of planetary systems throughout their history.
This line of enquiries goes in hand with investigations on the chemical nature of the interstellar medium and how it affects the composition of the primitive nebula. The influence of the host star on the properties of the resulting planetary system and the distribution of complex or organic molecules in the solar system will also be a line of research project. Besides being potentially prebiotically important, the inventory and the understanding of formation processes of complex and organic molecules are important to assess how planetary systems form and evolve. How this molecular complexity develops in molecular clouds, how far it progresses before the molecules are incorporated as ices into planetesimals or protoplanetary disks are key questions of the active, interdisciplinary field of astrochemistry. The Observatoire de Paris combines expertise in astronomical observations and chemical modelling, and is hosting state-of-the art laboratory astrophysics platforms for understanding the chemistry on grain surfaces and interactions with the gas.

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