James Webb: the greatest discoveries
The James Webb Space Telescope (JWST) is the most powerful telescope humanity has ever launched. Since its first images in July 2022, its gold-coated 6.5-metre mirror has been peering deeper into the universe — and therefore further back in time — from 1.5 million kilometres away than was ever thought possible.
The result: textbooks in need of rewriting. These are the breakthroughs that matter most so far.
1. The early universe was "too crowded, too soon"
Webb's headline mission was to find the very first galaxies. It found them — and immediately created a problem. In its first years of observations Webb discovered bright, massive, surprisingly orderly galaxies from less than 300 million years after the Big Bang, such as JADES-GS-z14-0. According to standard models, there simply had not been enough time to build that many stars.
Cosmologists have been puzzling ever since: did stars form far more efficiently in the early universe? Were the first stars extremely massive and bright? Webb's data is forcing an update to our models of cosmic structure formation — exactly what a new telescope is for.
2. Supermassive black holes that shouldn't exist yet
Webb also found supermassive black holes of millions of solar masses in the young universe — too big, too early. This strengthens the idea that black holes don't only grow from dying stars, but can also form through the direct collapse of giant gas clouds: born heavyweight rather than slowly fattened.
3. Exoplanet atmospheres, molecule by molecule
When an exoplanet passes in front of its star, its atmosphere filters a tiny fraction of the starlight. Webb can identify individual molecules in that signal. It has detected carbon dioxide, methane and water vapour in the atmospheres of distant worlds — including planets of the famous TRAPPIST-1 system and the much-debated sub-Neptune K2-18b.
4. Star birth in unprecedented detail
Because infrared light passes through dust, Webb sees what stayed hidden from Hubble: the nurseries of stars. Its images of the Pillars of Creation and the Carina Nebula reveal hundreds of newborn protostars, complete with jets and planet-forming disks. We are watching solar systems like our own being born, live.
5. Fuel for cosmology's greatest conflict
How fast is the universe expanding? Measurements of the early universe give a different value than measurements nearby — the notorious Hubble tension. Webb sharpened the local measurements (using, among others, type Ia supernovae as standard candles) and confirmed: the discrepancy is real, not a measurement error. It may point to new physics — dark energy that changes over time, or something we haven't imagined yet.
How Webb does it: the engineering in brief
- A 6.5-metre mirror of 18 gold-coated beryllium segments — six times Hubble's collecting area.
- Infrared vision: the light of the earliest galaxies has been stretched ("redshifted") into the infrared by cosmic expansion.
- Extreme cold: a tennis-court-sized sunshield keeps the instruments below −230 °C; otherwise the telescope would see its own heat.
- Location: Webb orbits Lagrange point L2, 1.5 million km from Earth — no servicing possible, and everything worked first time.
Frequently asked questions
Why does James Webb observe in infrared?
Cosmic expansion has stretched the light of the earliest galaxies into infrared wavelengths, and infrared sees straight through the dust clouds where stars form.
How far back can James Webb see?
To galaxies less than 300 million years after the Big Bang — over 13.5 billion years back in time.
Is Webb the successor to Hubble?
In practice yes, though both still operate side by side. Webb sees infrared with far greater sensitivity; Hubble remains strong in visible and ultraviolet light.