Seeding life on habitable planets

Two of the great quests of humankind are the hunt for habitable planets and the search for the signatures of life beyond planet Earth. To date, we have discovered more than 4000 planets orbiting other stars, the so-called exoplanets. Of these exoplanets, 21 have a similar size to Earth and also orbit their host star in the habitable zone, a temperate region where liquid water may be able to survive on the planet surface. However, a planet's presence within this temperate zone is only one of several criteria that determines whether or not a planet is truly habitable. So far, we know of only one place in the universe where life has begun and thrived, planet Earth. It is still not well understood why the Earth is seemingly the only planet in the Solar System where life has flourished, especially because our neighbour, Venus, also orbits within the temperate region around the Sun, yet has an atmosphere and surface that is not friendly for life. Rocky planets like the Earth and Venus are formed from cataclysmic collisions of moon-sized bodies, the energy from which would have created a molten, hot surface from which volatiles, such as water, would have boiled away to space.

So what happened to make the Earth life friendly? One theory that has stood against scrutiny is that impacts from comets, icy leftovers from the formation of the Solar System, delivered a substantial volume of water and life-friendly (carbon-rich) ingredients to the surface of the young Earth when the crust cooled and solidified. This replenished the planet with the ingredients needed for life to begin. However, this then raises questions on the role of comets in seeding *all* potentially habitable planets with life-friendly ingredients. Are cometary impacts a vital process in the formation of a habitable planet? If so, are comets formed around other stars also carriers of carbon-rich and life-friendly material?

This research will scrutinise the criteria needed for habitability by investigating the role of comets in seeding life on *all* potentially habitable planets, using state-of-the-art computational chemical and climate models, in conjunction with state-of-the-art observations of comets and exo-comet forming regions around other stars. My team and I will determine i) whether or not the comet-building material in exoplanet-forming systems are universal carriers of organic-rich material needed to seed life, and ii) whether or not cometary impacts on the atmospheres of rocky exoplanets are an observable phenomenon. The outputs from this research will provide strong constraints on the commonality of habitable planets, and provide a suite of diagnostics to search for evidence of cometary impacts using next generation telescopes that will target potentially habitable exoplanets. This research will revise the definition of "habitability" and will provide atmospheric diagnostics of habitability beyond the already proposed biosignatures.

Throughout history humanity has possessed a natural propensity for exploration. Humans have indulged this curiosity to the extent that we now populate vast areas of the land mass on Earth, and have escaped the Earth's gravity to visit other worlds in our Solar System. Through the development of increasingly sophisticated telescopes and data analysis techniques, we have now detected > 4000 worlds outside of our Solar System, the so-called exoplanets. However, to date we have not yet discovered another Earth-like world, and nor have we detected signs of life outside of Earth. This is anticipated to change in the coming decades as the new generation of state-of-the-art telescopes begin operations.

Our search for Earth-like worlds is driven by our definition of habitability which understandably is based on known limits for life and theories on the emergence of life on our planet. This research aims to refine or indeed revise the definition of habitability. This work will address big questions on the probability for organic life to develop on planetary surfaces. Are the same ingredients for life formed everywhere in space? And, are these ingredients delivered to all terrestrial planets that are potentially habitable? This research will have impact in the fields of astrochemistry (formation of organic molecules in space), astrobiology (delivery of organic molecules to planetary surfaces), and in the characterisation of habitable exoplanetary atmospheres (observable diagnostics and biomarkers). Outputs from this research will be vital in the interpretation of upcoming observations from the next generation of telescopes, in which the UK is heavily invested, and will motivate the specifications and design of future observational facilities. Outputs will also be used to inspire young people from disadvantaged backgrounds to study science at GCSE and beyond, with the aim of the development and retention of skills vital to meet the demands of the modern workplace.