Department Colloquium | Is there Another Earth?

Most stars in our galaxy have one or more planets orbiting them. Their diversity is astounding, with observed bulk densities ranging over a factor of 100. Yet there is only one planet that is confirmed to sustain life as we know it: our own. Current searches for life on planets around other stars planned with next generation ground- and space-based telescopes seek identical twins of Earth. How likely is it that we will find one and detect unambiguous signs of life around it? Perhaps the only thing harder than proving a planet hosts life is proving that it doesn’t. We are beginning to understand which aspects of the Solar System make Earth a suitable place for the biochemical origins of life, as well as assessing how common such systems are. With new instruments such as the CGI on NASA’s Roman Space Telescope (launching this year), as well as METIS on the European Southern Observatories 39-meter ELT (with first light planned for 2029), we can detect small planets in both reflected light as well as thermal emission around stars like the Sun. This enables resolution of the radius-albedo ambiguity, determination of its energy budget, and the search for an active greenhouse effect, with the possibility of identifying the responsible molecules in the spectra of its atmosphere. Doing this for even a small sample of systems will yield fundamental insights into these diverse atmospheres, confronting our theories of planet formation and evolution. These could be necessary steps to understanding potential biosignatures in these atmospheres and ultimately help answer the question “Are we alone?”.