The Ariane 5 rocket carrying the James Webb Space Telescope left its launch pad in French Guiana on 25 December 2021. Anyone who had worked on the project for two decades knew the riskiest part was still ahead: the sunshield and the mirror had to unfold, hundreds of mechanical steps, with no chance of repair. They did. By late January the telescope had settled into orbit around the second Lagrange point, about 1.5 million kilometres from Earth, and the first science images were released in July 2022. Since then the observatory has produced results at a pace that has surprised even the people who built it.

Why infrared matters more than you might think

Webb is a near- and mid-infrared telescope, sensitive to light between roughly 0.6 and 28 micrometres. That choice explains almost everything about its design. Light from the most distant galaxies has been stretched by the expansion of the universe on its way to us, so visible light emitted early in cosmic history arrives as infrared. Closer to home, newborn stars are wrapped in dust that blocks visible light but lets infrared through. Cool objects — planets, brown dwarfs, the outer reaches of planetary systems — glow brightest in the infrared too.

To see that faint heat, Webb needs to be cold. Its five-layer sunshield, roughly the size of a tennis court, keeps the mirrors and instruments shaded from the Sun, Earth and Moon, while the 6.5-metre primary mirror of eighteen gold-coated beryllium segments gathers about six times the collecting area of Hubble's. The result is a telescope that can pick out a candle-like signal from a planet trillions of kilometres away, and separate it from the glare of its star.

The early universe is busier than models predicted

The headlines that followed Webb's first deep fields were about maturity, not just distance. Astronomers found galaxies at redshifts that correspond to a few hundred million years after the Big Bang, and several of them looked brighter, more massive or more chemically evolved than simulations had suggested was likely so early. The record holder among spectroscopically confirmed objects, JADES-GS-z14-0, sits at a redshift of about 14.3, meaning its light has travelled for more than 13 billion years.

Whether this counts as a crisis in cosmology depends on who you ask. Redshift measurements are solid; the interpretation of mass and brightness depends on assumptions about stars, dust and star formation. A likelier explanation than broken physics is that galaxy formation was faster and more efficient in some environments than our models allowed for. Webb has also turned up a population of compact, red, oddly bright objects in the early universe — nicknamed little red dots — that may be growing black holes, dense star clusters, or something we have not thought of yet.

Exoplanet atmospheres: from guesswork to chemistry

Before Webb, atmospheric studies of exoplanets mostly produced suggestive hints. Now, for a handful of favourable targets, astronomers can detect specific molecules. When a planet passes in front of its star, a thin sliver of starlight filters through its atmosphere; Webb splits that light and reads the fingerprints.

Early results included a clear detection of carbon dioxide in the atmosphere of the hot gas giant WASP-39b, along with sulphur dioxide produced when starlight breaks down and recombines atmospheric chemicals — a process predicted but never observed outside the solar system. Webb has also measured thermal emission from the daysides of hot Jupiters and found methane and carbon dioxide in the atmospheres of some cooler, larger-than-Earth worlds. For small rocky planets, the news has mostly been sobering: several around nearby M-dwarf stars appear to lack the thick atmospheres some had hoped for, though a few results remain contested and are being re-observed.

A sharper look at stars being born and dying

Some of Webb's most striking images are of regions Hubble could only hint at. The Carina Nebula's "Cosmic Cliffs", the Pillars of Creation, and the Rho Ophiuchi cloud complex show infant stars, jets and cavities carved into dust in detail that makes the physics visible. In the Southern Ring and Ring nebulae, the messy outflows of dying stars are mapped in molecular hydrogen and dust. In the Crab Nebula, Webb has refined what we know about the supernova remnant's composition and structure.

One recurring theme is how quickly things move. Webb has spotted jets from very young stars still gathering mass, and discs where planets are forming, complete with gaps and spiral structure. The timeline from cold cloud to glowing star is being pinned down with far better precision than before.

Our own solar system, up close

Planetary scientists have found Webb unexpectedly useful. It has detected carbon dioxide on Europa's surface in a region where the ice is disturbed, mapped water vapour in the plume thrown out by Enceladus, watched volcanic activity on Io, and imaged Jupiter's auroras and faint rings, Saturn's rings and moons, and the polar cap and rings of Uranus. None of these replace a dedicated mission, but they help decide where to send one.

The questions still open

• How did black holes grow so quickly in the first billion years?
• What reionised the universe, and which objects did most of it?
• Do planets around small red dwarf stars usually keep their atmospheres?
• How does cosmic dust form so early and in such quantity?
• What are the Jupiter-mass binary objects seen drifting through the Orion Nebula, unattached to any star?

How to follow Webb's science yourself

You do not need a physics degree to keep up, but a little scepticism helps. Mission updates from the space agencies are reliable and readable; the fuller picture comes later, when results pass peer review, and a few early claims will inevitably be revised. When a paper is published, the underlying data usually lands in the public archive, and amateur astronomers have used it to produce their own images.

Two habits make the images easier to read. First, remember that colour is assigned: infrared wavelengths are mapped onto visible colours so our eyes can make sense of them, which means what you see is a translation, not a photograph. Second, check the scale bar and the wavelength labels in the caption — they tell you whether you are looking at something a few light-years across or a few hundred million.

Webb is designed to last well into the 2030s, and it is only just getting started. The most interesting results will probably be the ones nobody predicted.

Photo: Jason Pittman / Pexels