When the James Webb Space Telescope launched on Christmas Day 2021, it carried the hopes of an astronomical community that had spent more than two decades and roughly $10 billion building it. After a flawless deployment to the Sun-Earth Lagrange point L2, a million and a half kilometres from Earth, and a meticulous months-long commissioning, JWST returned its first science images in July 2022. Three years later, it has not just met expectations — it has repeatedly upended them.
A Different Kind of Telescope
To appreciate what JWST has done, it helps to understand how it differs from the Hubble Space Telescope, which launched in 1990 and remains operational. Hubble observes primarily in visible and near-ultraviolet light. JWST observes in the infrared. That distinction is fundamental.
Because the universe is expanding, light from distant objects is stretched — redshifted — toward longer wavelengths. The farther away a galaxy is, the more its light is redshifted. By the time light from the earliest galaxies reaches us, it has been stretched from visible into the infrared. Hubble, sensitive to visible light, cannot see it. JWST, with a 6.5-metre gold-coated beryllium mirror (compared to Hubble’s 2.4-metre mirror) and infrared instruments, can.
JWST also has an enormous sunshield the size of a tennis court, keeping its instruments at cryogenic temperatures so their own infrared glow does not drown out faint signals. Its position at L2 keeps it away from the heat and light of Earth and the Sun.
Rewriting the Early Universe
The most dramatic findings concern the earliest galaxies. Before JWST, astronomers expected the first galaxies to be small, faint, and few. Instead, JWST found large, bright, well-formed galaxies at redshifts previously thought impossible. In 2024, a team using the JADES survey confirmed a galaxy, JADES-GS-z14-0, at a redshift of 14.32 — meaning its light has travelled for about 13.5 billion years, from a time roughly 290 million years after the Big Bang.
These early galaxies are more massive and more luminous than most models predicted. That has forced theorists to reconsider how quickly star formation could begin and how efficiently early galaxies could build up mass. Some have suggested that early stars were more efficient or that galaxies formed faster than assumed. Crucially, the observations have not broken cosmology — but they have stressed it in productive ways.
Exoplanet Atmospheres
JWST has transformed the study of exoplanets, the planets orbiting other stars. When a planet transits in front of its star, some starlight passes through the planet’s atmosphere, where molecules absorb specific wavelengths. By analysing the resulting spectrum, JWST can identify atmospheric composition.
Among its notable results, JWST detected carbon dioxide in the atmosphere of WASP-39b, a hot gas giant, with unprecedented clarity — a first for any exoplanet. It has detected water vapour, sulfur dioxide produced by photochemistry, and signs of clouds and hazes. It has measured the atmosphere of a rocky exoplanet, TRAPPIST-1b, finding it likely lacks a thick atmosphere, a sobering result for habitability prospects around that famous system of seven Earth-sized worlds.
These observations are early chapters. JWST’s ability to characterise potentially habitable worlds — searching for biosignatures such as oxygen, methane, and their coexistence — is one of its most anticipated capabilities, and it will take years to develop fully.
Star Formation and the Interstellar Medium
JWST’s infrared vision pierces dust clouds that block visible light, revealing star-forming regions in unprecedented detail. Its images of the Carina Nebula, the Pillars of Creation, and the Rho Ophiuchi cloud complex became iconic not just for their beauty but for the new structures they revealed — jets from newborn stars, previously hidden protostars, and the complex chemistry of the interstellar medium.
The telescope has also detected complex organic molecules — including, in one notable result, a molecule called methyl cation in a protoplanetary disk — providing clues about the chemical building blocks available for forming planets and potentially life.
Surprises in the Solar System
JWST is not only a deep-space instrument. It has observed Jupiter’s auroras and atmosphere, characterised the plumes of Saturn’s moon Enceladus, mapped water on the Moon, and detected carbon dioxide on Europa, strengthening the case that its subsurface ocean could be habitable.
Hubble’s Enduring Legacy
None of this diminishes Hubble, which continues to produce science after more than three decades. Hubble’s visible-light observations complement JWST’s infrared ones, and the two telescopes are frequently used together. Hubble’s deep fields, its role in measuring the expansion of the universe, and its contributions to understanding dark energy remain foundational.
Still, Hubble has suffered gyroscope failures and has limited remaining life. JWST, with a design life of at least ten years and propellant for potentially twenty, has become the primary infrared observatory.
What the Tension Means
The recurring theme of JWST’s first three years is that the universe is stranger and more varied than models anticipated. Galaxies formed earlier and more vigorously than expected. Some early galaxies appear to contain unexpectedly mature populations of stars. The “Hubble tension” — a discrepancy between different measurements of the universe’s expansion rate — remains unresolved, and JWST has been used to rule out some measurement errors as its cause.
These puzzles are not failures of the telescope. They are the telescope doing exactly what it was built to do: forcing theory to confront evidence.
The Instrument Itself
JWST’s power comes from a set of engineering decisions that made it possible to observe the universe in infrared with extraordinary sensitivity. Its segmented primary mirror, 6.5 metres across, is made of 18 hexagonal beryllium segments coated in gold, each adjustable by tiny motors to align the array to a fraction of a wavelength of light. The sunshield, five layers of a heat-resistant film called Kapton, blocks the Sun’s heat so the telescope’s instruments stay near minus 233 degrees Celsius. The telescope operates a million and a half kilometres away, too far to be serviced by astronauts, which is why its flawless deployment was so tense.
Exoplanets and the Search for Life
The question that most animates JWST’s exoplanet program is deceptively simple: do any planets beyond our solar system have atmospheres — and do those atmospheres contain signs of life? The telescope’s transit spectroscopy has already characterised several worlds, detecting carbon dioxide, water vapour, and other molecules. The holy grail is a rocky planet in the habitable zone of its star whose atmosphere shows biosignature gases in an impossible-to-explain combination — oxygen alongside methane, for instance. No such detection has been confirmed, and the observations required are at the limit of the telescope’s capabilities on the faintest targets. But JWST has shown that the method works, and the next generation of instruments will push it further.
Black Holes and Galactic Cores
JWST has contributed to the study of supermassive black holes, observing the environments of active galactic nuclei in the early universe and probing how black holes and their host galaxies grew together. Its infrared sensitivity allows it to see through the dust that obscures these regions at visible wavelengths. The observation that some early black holes appear surprisingly massive has added to a lively debate about how they formed so quickly after the Big Bang — a puzzle that bears on the fundamental physics of the early cosmos.
The Cost and the Value
JWST cost roughly $10 billion and took decades, with several near-cancellation moments in the 2000s and 2010s when budgets ballooned. That history is a cautionary tale about managing large science projects, but it also illustrates the value of patience. The telescope is now delivering science far beyond what any single-parameter evaluation could capture, reshuffling textbooks and generating thousands of research papers. It is a reminder that the returns on frontier science are long-term, uncertain, and often astonishing — categories that traditional cost-benefit analysis handles poorly.
What Hubble Still Does Better
Hubble remains uniquely capable in some respects. Its visible-light acuity is unmatched for certain targets, and it has a long, continuous baseline of observations that makes it invaluable for studying changes over time. Hubble’s ultraviolet sensitivity is something JWST lacks, and it remains the primary tool for studying the ultraviolet universe. The two telescopes are complementary, and their combined data — infrared from JWST, optical and ultraviolet from Hubble — often produce science neither could achieve alone. As Hubble’s remaining gyroscopes fail and its orbit decays, its eventual loss will leave a genuine gap even as JWST thrives.
The Uncertainty Principle of Early Cosmology
One conceptual caution is worth stating: observations of the early universe carry intrinsic uncertainties because they rely on inferences from light that has travelled for over 13 billion years and on models of how the universe expanded and evolved. A galaxy’s distance and age are derived, not directly observed, and small changes in cosmological parameters can shift estimates meaningfully. JWST’s findings are robust enough to have prompted serious theoretical work, but they should be read as evidence pushing at the edges of established models rather than as refutations of them. Science advances by these tensions — the model that survives is the one that fits the most evidence — and JWST is supplying evidence in unprecedented quantity.
Conclusion
In three years, JWST has reshaped our picture of cosmic dawn, opened a window into the atmospheres of distant worlds, and revealed the hidden processes of star birth. It has confirmed some expectations, demolished others, and generated hundreds of new questions. For astronomers, that is what success looks like. The telescope’s greatest discoveries may still lie ahead — and the combination of JWST, Hubble, and the next generation of ground-based observatories promises a decade of extraordinary science.
