James Webb Space Telescope: The Most Important Discoveries of Its First Three Years
The James Webb Space Telescope launched on Christmas Day 2021 after two decades of development and $10 billion in cost. Its first science images, released in July 2022, stunned the world — thousands of galaxies in a single frame, the atmospheric composition of a distant exoplanet, the death throes of a dying star. Three years later, the telescope has exceeded even the most optimistic expectations, upending several long-held assumptions about the early universe and confirming others. It is arguably the most productive scientific instrument in history, generating over 3,000 peer-reviewed papers in its first two years of operation.
Rethinking the Early Universe
The most surprising discovery has been that the early universe matured faster than cosmologists expected. Webb identified fully formed galaxies as early as 300-400 million years after the Big Bang — galaxies that are massive, well-ordered and have relatively low rates of star formation, resembling galaxies that took billions of years to evolve in the standard model. In February 2023, a team announced the discovery of six massive galaxies at a redshift of 13 — seen when the universe was only 500-700 million years old — with stellar masses estimated at billions of times the mass of the Sun. If confirmed, these galaxies would challenge the timeline of galaxy formation. A 2024 spectroscopic follow-up study, however, found that some of these galaxies appear smaller than initially estimated, and their star formation rates and masses were lower than the photometric estimates suggested. This is science in action — a cycle of discovery, scrutiny and refinement.
Exoplanet Atmospheres
Webb’s ability to analyse the atmospheres of exoplanets — planets orbiting other stars — represents a leap forward in the search for potentially habitable worlds. The telescope has measured the atmospheric composition of dozens of exoplanets by observing how their atmospheres filter starlight during transits. In 2023, Webb detected carbon dioxide in the atmosphere of WASP-39b, a hot Jupiter, with unprecedented clarity — the first definitive detection of CO2 on any exoplanet. It has detected water vapour, methane, sulphur dioxide and clouds of various compositions. Most tantalisingly, Webb has begun studying the atmospheres of rocky planets in the habitable zones of their stars — planets where liquid water could exist on the surface. The TRAPPIST-1 system, a red dwarf star with seven Earth-sized planets about 40 light-years away, has been a particular focus, with Webb finding that some of the planets lack thick atmospheres while others may retain them. The result does not confirm life — but it is the first step on a path that could, within a decade or two, detect biosignatures in the atmosphere of a distant world.
Star Formation and Stellar Death
Webb’s infrared sensitivity allows it to see through the dust clouds that hide the process of star formation from visible-light telescopes. It has captured images of protostars still forming within their natal clouds, jets of material being ejected from young stars, and the complex chemistry of the interstellar medium. In one notable result, Webb observed the Ring Nebula — a well-studied planetary nebula — and revealed a complex structure of filamentary arches and clouds that visible-light telescopes entirely missed. In the Carina Nebula, Webb’s image revealed hundreds of previously unseen stars and dozens of jets and outflows from young stars, transforming a familiar object into a new scientific landscape. Each of these observations adds to our understanding of how stars are born, how they live and how they die — the processes that forged the elements that make up our planet and our bodies. Webb has not rewritten all of astronomy, but it has rewritten enough of it to justify its cost many times over. The telescope is funded to operate through at least 2032, with a propellant budget that could extend its life beyond 2040. The best discoveries, astronomers say, are probably still ahead.
