The James Webb Space Telescope: What It Has Discovered and Why It Matters
Since its first images in July 2022, the James Webb Space Telescope has rewritten our understanding of the early universe, exoplanet atmospheres, and star formation. Here is a systematic account of its most significant findings.
Admin
Author
What is the James Webb Space Telescope?
The James Webb Space Telescope (JWST) is a 6.5-metre infrared space observatory launched on 25 December 2021 and positioned at the second Lagrange point (L2), approximately 1.5 million kilometres from Earth. It is the most powerful space telescope ever deployed.
Its primary design goal was to observe the first galaxies that formed after the Big Bang — objects so distant and receding so fast that their light has been stretched into the infrared spectrum by the expansion of the universe. JWST was designed to see light that the Hubble Space Telescope was physically incapable of detecting.
What it has actually found, since releasing its first science images in July 2022, has surprised even its designers.
What has JWST discovered about the early universe?
The first major surprise from JWST was immediate and systematic: early universe galaxies are much larger, more structured, and more evolved than our cosmological models predicted.
Standard cosmology (the Lambda-CDM model) describes how matter clumped after the Big Bang and formed galaxies gradually over hundreds of millions of years. The earliest galaxies, in this model, should be small, irregular, and structurally immature.
JWST observed galaxies at redshifts above z = 10 — corresponding to a time less than 500 million years after the Big Bang — that are massive, structured, and show signs of having already formed most of their stars. Some are as massive as the Milky Way at an age the Milky Way had not yet formed in.
The cosmological tension
Researchers began publishing estimates of stellar masses in these early galaxies that exceeded what Lambda-CDM models permitted. A 2023 paper in Nature Astronomy described observed galaxies so massive that, if accurate, "the universe would have needed to turn 100% of its available baryonic matter into stars with perfect efficiency" — physically impossible in standard models.
Possibilities under investigation:
- The mass estimates contain systematic errors (early observations often do)
- Our understanding of star formation efficiency in the early universe is incorrect
- Lambda-CDM requires modifications — possibly regarding the nature of dark matter or the initial conditions of structure formation
This is genuinely unresolved. The community is working through whether the observations represent measurement challenges or fundamental challenges to the standard model.
What has JWST discovered about exoplanet atmospheres?
JWST's infrared sensitivity has produced the most detailed atmospheric characterisations of exoplanets ever achieved.
WASP-39b: the atmospheric chemistry benchmark
In November 2022, JWST published transmission spectroscopy of WASP-39b (a hot Jupiter 700 light-years away) that detected:
- Carbon dioxide (CO₂) — the first direct detection of CO₂ in an exoplanet atmosphere
- Water vapour, carbon monoxide, sulphur dioxide, sodium, and potassium
- Photochemical production of sulphur dioxide — direct evidence that the host star's UV radiation is chemically altering the atmosphere
This is not just a list of molecules. It is a demonstration that JWST can characterise atmospheric chemistry in sufficient detail to constrain planetary formation models, atmospheric dynamics, and (eventually) biosignature detection.
K2-18b and the tentative carbon chemistry signature
In September 2023, the JWST team reported atmospheric observations of K2-18b, a 2.6 Earth-radius sub-Neptune at 120 light-years, orbiting in its star's habitable zone.
The spectrum showed carbon dioxide and methane — consistent with a hydrogen-rich atmosphere — and a possible, tentative detection of dimethyl sulphide (DMS). On Earth, DMS is produced almost exclusively by biological processes (primarily marine phytoplankton).
The JWST team was notably cautious: the DMS signal was at the 1–2 sigma significance level (well below the 5-sigma standard for scientific claims). The follow-up observations needed for confirmation are scheduled.
If confirmed, this would not be proof of life — it would be a biosignature candidate requiring extensive further characterisation. But the fact that JWST has the sensitivity to detect candidate biosignatures in a planet's atmosphere at 120 light-years is itself a scientific milestone.
What has JWST shown us about star formation?
The Carina Nebula and Eagle Nebula (Pillars of Creation) images released in 2022 were not merely visually spectacular — they resolved individual young stellar objects and protoplanetary discs in star-forming regions at unprecedented resolution.
Key findings:
- Hundreds of previously unknown young stars in the Carina Nebula, their infrared light visible through the dust that obscured them from Hubble
- Protoplanetary discs — the planet-forming material around young stars — resolved in detail, allowing measurement of mass, composition, and lifetime
- Stellar jets from protostars, revealing the physics of the earliest stages of star formation
What is the Hubble tension, and has JWST resolved it?
The Hubble tension is a persistent discrepancy between two methods of measuring the universe's expansion rate (the Hubble constant, H₀):
- Early universe method (from CMB data, Planck satellite): H₀ ≈ 67.4 km/s/Mpc
- Late universe method (from distance ladder — Cepheid variables + Type Ia supernovae): H₀ ≈ 73 km/s/Mpc
The tension is approximately 5 sigma — far above the threshold for statistical significance. Either the methods have systematic errors, or our standard cosmological model is missing something.
JWST was expected to help resolve this by providing more precise Cepheid measurements with less contamination from nearby stars. The 2023 JWST results found that Cepheid measurements were consistent with Hubble's values after correcting for crowding — the tension did not go away. The evidence increasingly suggests a genuine cosmological discrepancy rather than a measurement error.
What JWST means for the next decade
JWST was designed for a 10-year mission with consumables for potentially 20 years (the launch was so precise that minimal thruster fuel was used). It is currently operating with fuel reserves for potentially 25+ years.
The science programme for the coming decade includes:
- Systematic atmospheric survey of rocky planets in habitable zones (TRAPPIST-1 system is a primary target)
- Deep field observations pushing galaxy detection further back in cosmic time
- Direct imaging of debris discs and giant planets around nearby stars
JWST has not resolved the early universe tension; it has sharpened it. It has not confirmed biosignatures; it has demonstrated the sensitivity to detect them if they exist. This is what good telescopes do — they turn vague questions into precise ones.