WHY IS THERE LIFE ON EARTH?

Earth is a dynamic planet: from the surface teeming with life to the deepest interior that maintains the protective magnetic field. For over 4.5 billion years, giant impacts, volcanism, and tectonic motions have transformed the surface and atmosphere. Despite, or perhaps because of, these extrinsic changes life has thrived. Why have we not detected life elsewhere? What type of planet is most likely to sustain life? Beyond Earth, what are the characteristics of a habitable planet? We postulate that one cannot understand the habitability of Earth or constrain the potential for habitability of other planets, particularly terrestrial exoplanets, without studying the role that the internal dynamics and bulk composition of the planet play in establishing and maintaining the surface environment. Whether a planet is hot or cold, wet or dry, active or stagnant, or alive or dead depends critically on the interior in several crucial ways:

A groundswell of interest in planetary interiors has developed over the past ~20 years with the discovery of thousands of exoplanets orbiting a variety of stars. The abundances of elements present in these stars, as inferred by their spectrum, is likely inherited by the planets in its solar system. Such spectral observations indicate that the composition of exoplanets should vary widely, in terms of volatile content, heat producing elements, and bulk mantle mineralogy. A recent survey of more than 1000 of these stars has shown that the elements present in their spectra vary by more than a factor of two. This is a huge amount! Therefore, the diversity in mass, density, composition, and orbit of exoplanets hugely expands the range of parameters that should be considered in order to understand what critical features of a planet dictate its habitability.

Composition is central to how a planet forms and evolves, and must be the starting point to understand what makes a planet habitable. The composition of a planet depends not only on the composition of the material from which it forms, which can be inferred from the composition of its host star, but where and how quickly the planet forms. These details determine the likelihood of a planet to trap the right amount of volatile elements, including bio-essential elements. The bulk composition plays a first order role in controlling the surface atmosphere and ocean state, internal structure, tectonic state, recycling efficiency, and magnetic field generation. Each of these components, in turn, have a direct impact on the habitability of the surface – providing a favorable environment for life to emerge and thrive over billions of years. At Carnegie we are perfectly set up to conduct this reåsearch as we have the astronomers, the geologists, the physicists, the chemists, the experimentalists, and the theorists, with the planetary science expertise to answer these interdisciplinary questions. We propose that our team will identify which types of exoplanets are most likely to harbor life: a crucial step to finding life beyond of Earth.