JWST's biggest discoveries so far

The earliest galaxies

Webb's infrared design was built for this. Light from the earliest galaxies is stretched into the infrared by cosmological redshift, placing it beyond what optical telescopes can reach.

MoM-z14 is currently the most distant confirmed galaxy, at a redshift of about 14.4 — seen as it existed roughly 280 million years after the Big Bang. It is small, around fifty times less massive than the Milky Way, and shows emission lines indicating nitrogen and carbon and a rapidly rising star formation rate.

It displaced JADES-GS-z14-0, announced in 2024 at around 300 million years. The record has moved several times, which is itself the result: galaxies exist earlier and are easier to find than pre-launch expectations suggested.

Little red dots

A genuinely unexpected discovery: a population of compact, very red objects appearing in early-universe images in numbers nobody predicted.

Their properties are difficult to reconcile. If they are galaxies, they are implausibly compact and massive. If the light comes from accreting black holes, the black holes are implausibly large for the epoch and lack expected X-ray signatures.

One current proposal is a distinct class of object sometimes described as a black hole star — a black hole embedded in and enshrouded by dense gas, producing a spectrum resembling a star rather than a typical quasar.

This is unresolved, and worth watching precisely because it is unresolved. A new instrument finding a category of object nobody anticipated is the strongest argument for building new instruments.

Exoplanet atmospheres

Webb can measure the composition of exoplanet atmospheres by transmission spectroscopy — analysing starlight filtered through a planet's atmosphere as it transits.

It has produced detections of carbon dioxide, water, methane and sulphur dioxide in various exoplanet atmospheres, at a precision that was not previously achievable. Sulphur dioxide is notable as evidence of photochemistry — sunlight driving chemical reactions in another planet's air.

The high-profile case is K2-18b, a sub-Neptune where observations were reported as possibly indicating dimethyl sulphide, a molecule produced largely by life on Earth.

That claim is contested. The statistical significance has been questioned, alternative molecules can produce similar features, and independent reanalyses have disagreed. Whether the planet is even a 'hycean' ocean world is disputed.

The appropriate position is interest and patience. A biosignature claim requires far more than a marginal spectral feature, and the history of this field is full of announcements that did not survive — which is why the search for life sets its bar so high.

Why infrared, and why L2

Webb observes in the infrared for two reasons: the earliest galaxies are redshifted into it, and infrared penetrates the dust that hides star-forming regions from optical telescopes.

That imposes a hard constraint. An infrared telescope must be extremely cold, or its own thermal emission swamps the signal. Webb therefore sits near the Sun–Earth L2 point, about 1.5 million kilometres out, with a tennis-court-sized sunshield keeping the instruments at around 40 kelvin.

L2 means it is far too distant to service, unlike Hubble in low Earth orbit. The deployment sequence — unfolding the sunshield and mirror segments after launch — had hundreds of single-point failure modes and had to work first time. It did.