Are we alone in the Universe?

At the beginning of the 90s, a young Ph.D. student (Didier Queloz) and his thesis supervisor (Michael Mayor) were working at the Geneva Observatory (Switzerland) on a subject that, at that time, was not taken very seriously: they looked for planets in other stars. In 1995, they announced the discovery of 51-Pegasi-b, a world the size of Jupiter orbiting the star 51-Pegasi, 50 light-years from Earth (for reference, the Sun is eight light-minutes away). It was the first planet discovered outside the Solar System.

Thanks to this discovery, they won the Nobel Prize in Physics in 2019, and it marked the birth of a research field that has transformed astrophysics forever: Exoplanets. With thousands of scientists worldwide working on it and with billions of euros of investment in technology and instrumental development. But, what is the reason for this deployment of means, efforts and investment?

Well, it is because we are fortunate to live in an era where we can answer one of the great questions of humanity: are we alone in the Universe? And, the point is that we do not know anything about life in the Universe. We know that life on Earth flourishes everywhere, for every crack, even in the most adverse conditions, life finds a way to breakthrough. But, as far as we know, that only happens on Earth. We do not know if our planet is a rarity, a cosmic anomaly, or if, on the contrary, life is not a coincidence but a consequence, and this, whenever it can, appears.

Who has not ever fantasized, looking at a sky full of stars, with distant worlds inhabited by other forms of life? Who has not taken their breath away as they contemplate thousands of stars shining above their eyes? That fantasy has always been in humanity, and now we are on the verge of making that discovery. The new generation of telescopes and instruments will begin in the decade 2025-2035 to explore the atmospheres of Earth-like planets and super-Earths in search of signals of biological origin, the so-called biosignals. Earth-like planets are those with a mass and size similar to our planet, between 0.9 and 1.2 times its size, while super-Earths are somewhat larger, between 1.3-2.0 Earth radius. Out of the 4,900 planets discovered at the time of writing, about 1,700 belong to this category.

In this research topic works the EXOTIC team (EXOplanets in Transit: Identification and Characterization) of the research unit in Astrobiology of the University of Liège. More specifically, we lead the SPECULOOS project (Search for habitable Planets EClipsing ULtra-cOOl Stars). This project has a network of 8 telescopes (6x1-meter and 2x60-centimeter telescopes) that observe both the northern and southern hemispheres. Every night these telescopes look for signs of planets in nearby, bright and cool stars with temperatures between 3000 and 2300 K (for reference, the Sun is about 5700 K), characteristics that favor the detection of biosignals (if they exist) in planetary atmospheres. To this end, we monitor these stars' brightness by measuring the luminous flux while waiting for a "planetary transit"; that is, for the planet to come into the line of sight between us (the observers) and the star. When the planet is Earth-like, this transit translates into a slight decrease in the star's brightness of between 0.5 and 2.0%, which is measurable with our telescopes.

This project is the continuation of the TRAPPIST project, which in 2016 found one of the most fascinating planetary systems known to date: TRAPPIST-1. This system has 7 Earth-like planets, and at least 3 of them are in the habitable zone, where the planets could maintain liquid water on their surface, a key element in the development of life as we know it. Today, it is the most promising planetary system to look for biosignals. The long-awaited James Webb Space Telescope, which has recently been launched, will spend much of its observing time exploring the atmospheres of these planets.

But how far are we from being able to say unequivocally that a planet is inhabited? I fear that this may be more complex than we think. First of all, the scientific community is still defining what a biosignature is. We must be very sure that what we think is a signal of biological origin cannot be produced by an abiotic mechanism, that is, a mechanism that does not require the presence of any metabolism. This controversy happened recently with the discovery of phosphine on Venus' atmosphere. Despite having long been considered a biosignature, after its finding on the neighboring planet, many scientists now doubt whether its origin is biotic or abiotic (leaving aside the data processing debate, which also generated heated discussions). Luckily, Venus is there next door, and we can send space probes to study the origin of that signal in more detail. In fact, work has already begun on conceptual missions for this purpose, such as The Venus Life Finder Missions. But what will we do when this happens with a planet that is hundreds of light-years away? How can we be sure of what is life and what is not? As the great Carl Sagan said, "extraordinary claims require extraordinary evidence" , and although the road will be long and complex, we are already traveling. Conservative estimates suggest that after a decade or two of learning about signals in Earth-like exoplanet atmospheres, we will be ready to confidently determine that a planet is inhabited, which will place us in the 2040-2050 decade. Students around 15 years old who are today in secondary schools will be the protagonists of the search and possible detection of life beyond the Solar System for the first time in the history of humankind. Something that will be recorded in the history books and will have enormous consequences at all scales, on a scientific, social, theological, and philosophical level ... We will understand our place in the Cosmos ... could you choose a more exciting time to live?

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This article was sent by Francisco José Pozuelos Romero. Francisco studied physics at the University of Seville. In 2010 he obtained a predoctoral fellowship that allowed him to study the master of physics and mathematics at the University of Granada and the PhD at the Instituto de Astrofísica de Andalucía (IAA-CSIC) on dynamic evolution and production of dust by minor bodies of the Solar System. in 2015 he obtained a postdoctoral fellowship at the DLR-Berlin where he studied the dynamic evolution of exoplanets that are very close to their star in the context of the mission ESA-PLATO. This mission will start in 2028. One of its main objectives is to find large planets that resemble the Earth and where life could be searched for. In 2017 he obtained a postdoctoral Marie Curie Cofund at the University of Liege, where he has been since. During this time he has participated in different projects related to the search for habitable planets from terrestrial (TRAPPIST and SPECULOOS) and spatial (NASA-TESS y ESA-CHEOPS) observatories. Recently, he has obtained a permanent researcher position at the IAA-CSIC in Granada, where he will start working on July 2022. He will continue studying habitable terrestrial planets by means of artificial inteligence techniques.