The Search for Life

The question of whether life exists beyond Earth is the oldest in astronomy and, until thirty years ago, the most embarrassingly speculative. That changed in 1995, when Michel Mayor and Didier Queloz detected a planet around the Sun-like star 51 Pegasi. Today the catalog of confirmed exoplanets exceeds 5,800. A handful — fewer than thirty, depending on which definition of habitability is used — sit in the right temperature zone around their stars for liquid water to exist on their surfaces.

The most important target of all is forty light-years away. TRAPPIST-1 is a small, red, faint star with seven Earth-sized rocky planets transiting it. Three are in the habitable zone. The James Webb Space Telescope has now spent two years measuring their atmospheres in transmission spectroscopy. The results are clarifying and, in places, disappointing — the two innermost worlds appear to have lost their atmospheres entirely to the star's flare activity. The outer worlds are still being characterized. The TRAPPIST-1 program is the closest thing the field has to a clean experiment on whether terrestrial planets around red dwarfs can hold onto the chemistry life needs.

The other strand of the search is closer to home. Europa, the second of Jupiter's Galilean moons, is sheathed in ice and hides a salt-water ocean estimated to contain twice the volume of all Earth's oceans combined. Enceladus, around Saturn, sprays its ocean into space through cracks in its south polar terrain — material that the Cassini spacecraft flew through and sampled. Both moons appear to have hydrothermal vents on their seafloors, and on Earth, the equivalent vents in the Mariana Trench host the densest non-photosynthetic ecosystems on the planet. Europa Clipper is on its way. Enceladus is the leading target for the mission after.

And then there is the awkward fact that, in 13.8 billion years, no one appears to have visited. Enrico Fermi asked the question over lunch at Los Alamos in 1950. In 2018, three researchers at Oxford's Future of Humanity Institute showed that the entire premise of the paradox depends on a statistical assumption that, when corrected, dissolves it. We may be alone. We may not be. The honest answer — sharpened by the work below — is that the universe being silent means almost nothing.