Are early universe galaxies really impossible?

Not literally impossible — but they challenge the standard model's predictions for how quickly large structures can assemble. The standard Lambda-CDM model predicts massive settled galaxies should form more slowly than JWST observations suggest. Each additional mass upward revision sharpens that tension.

JWST found ancient galaxies hiding four times their measured mass in small stars

Q: Could the hidden stars in early galaxies host planets?

Low-mass stars are the most common planet hosts in the present-day universe. If the earliest galaxies harbored far more of them than assumed, the early universe may have contained more planetary systems than standard models predict. The study makes no habitability claims; the implication is statistical.

James Webb’s spectrograph reads individual stellar populations the way a prism reads a flame. Point it at a distant galaxy, collect enough photons, and the resulting spectrum encodes not just which types of stars are present but in what proportions. That measurement was always theoretically possible; at the distances involved — light that left when the universe was less than 1.5 billion years old — no telescope had ever done it reliably. The James Webb Space Telescope changed that.

The result, published in Nature Astronomy, is that nine massive galaxies in the early universe are hiding a large population of small, dim, low-mass stars behind the blinding light of their giant counterparts. Those hidden stars add mass. For at least one galaxy, formed less than 1.5 billion years after the Big Bang, the real mass may be four times higher than any prior estimate.

That number matters because modern galaxy-formation models were already struggling to explain these objects. Now they have to explain them at four times the mass.

How the spectrograph reads what prior telescopes could not

Stars come in a wide range of masses, from bright and short-lived giants down to small, faint, long-lived dwarfs. In any galaxy, the ratio of big stars to small ones — called the initial mass function — governs how much mass is locked in dim stars too faint to detect individually at cosmological distances.

Until now, astronomers working on early galaxies had to assume that the initial mass function looked roughly like the one observed in today’s Milky Way: a well-calibrated mix. The assumption was not unreasonable. It was simply untestable.

JWST’s NIRSpec instrument — its near-infrared spectrograph — captures the combined light of an entire galaxy and splits it with precision that no prior observatory could match at these distances. The technique involves identifying subtle differences in spectral absorption features characteristic of low-mass stars, features that ordinarily drown in the glare of their more luminous companions. The team combined the JWST spectra with additional data from the Very Large Telescope to constrain their models.

“Measurements like these were simply not possible until recently,” said Martje Slob, a co-author on the study. “We needed not only a telescope capable of collecting enough light, but also exceptionally high-quality spectra and new analysis techniques. Only then could we reliably distinguish the subtle features of small stars from the light of much brighter stars.”

The study examined nine quiescent massive galaxies — ones that had already completed their main star-forming phase — using this approach. In every case, the initial mass function was steeper than the Milky Way’s standard. More small stars. Much more mass.

The impossible galaxy problem just got harder

The tension between what galaxy-formation models predict and what telescopes have observed in the early universe is one of the most discussed puzzles in modern cosmology. Under the standard model of cosmic structure formation, the first billion years should not have produced galaxies as large and gravitationally settled as the ones JWST has been finding since 2022.

Those galaxies were already difficult to explain. This study makes them harder.

Lead author Chloe Cheng of Leiden University offered an analogy: “If a galaxy were a city, the brightest stars would be the skyscrapers you can immediately see from far away. Our models show that behind the brightest stars there is a much larger population of small stars, like houses among the skyscrapers. This means that the galaxy as a whole is much more massive than previous estimates suggested.”

Previous mass estimates for early massive galaxies relied on photometry — total brightness — calibrated against assumptions about stellar populations. Spectroscopy is a more direct read, though it requires far more telescope time and sensitivity. The new estimates suggest the photometric approach consistently undercounted, in some cases by a factor of four.

What the hidden stars could change beyond cosmology

Mariska Kriek, the study’s senior author and a professor at Leiden University, pointed to an implication that extends beyond galaxy masses: “Small stars often have planets orbiting them, so if there were many more small stars in the early Universe than we thought, there may also have been more planets.”

Low-mass stars — red dwarfs and K-type stars — are the most common hosts of rocky planets in the present-day universe. If the earliest massive galaxies contained a far higher proportion of these stars than previously assumed, the early universe may have been a more planet-populated place than the standard model has allowed for. The extent of that implication would depend on how widespread this stellar bias turns out to be across the early-universe galaxy population.

What the study does not settle

The sample is nine galaxies. That number is a function of observing time and current telescope capability, not a full census. The study shows the approach works and demonstrates that the effect is real — the initial mass function in these early systems is bottom-heavy. Whether that holds across hundreds or thousands of early massive galaxies, and by how much mass, remains to be measured.

The study also does not address the mechanism. Why the earliest massive galaxies should have formed proportionally more low-mass stars than present-day galaxies is an open question. It may relate to different gas metallicities, temperatures, or turbulence in the early universe — each of those possibilities would have different implications for galaxy-formation models. The spectral measurement constrains the what; the why still requires more data and theory.

Common questions about early galaxy mass

How do astronomers weigh a galaxy they cannot see inside? By reading its light. A galaxy’s spectrum encodes which types of stars it contains and in what proportions. Bright stars dominate the visual appearance but contribute less total mass than the population of faint, low-mass stars. Spectroscopy lets astronomers measure the mass fraction locked in dim stars, which changes the total mass estimate.

What is the initial mass function? It describes the distribution of stellar masses when a population of stars forms. A top-heavy function produces more massive, luminous stars; a bottom-heavy function produces proportionally more small, faint ones. The function is not fixed — it may vary by galaxy type, age, and environment. This study found early massive galaxies have a more bottom-heavy function than the Milky Way.

Are these really “impossible” galaxies? Not literally impossible — but they challenge the standard model’s predictions for how quickly large structures can assemble in the early universe. The standard Lambda-CDM model allows for galaxy formation, but the rate at which it predicts massive settled galaxies to form is slower than what observations have found. Each additional mass upward revision sharpens that tension.

Could the hidden stars host planets? Low-mass stars are the most common planet hosts. If the earliest galaxies harbored far more of them than assumed, the early universe may have contained more planetary systems than standard models predict. The study makes no habitability claims; the implication is statistical and depends on how widespread the bottom-heavy mass function turns out to be.

The Leiden team is extending their spectral analysis to a larger sample of early-universe galaxies using JWST observing programs already scheduled. When that expanded catalog arrives, the revised mass estimates will allow a more definitive test of whether the bottom-heavy stellar distribution is typical of early massive galaxies or confined to a particular class of them. The cosmological models that struggle to explain these galaxies at their current estimated masses will then face a much larger dataset.

Reference: Cheng et al., “Hidden mass in early galaxies revealed by bottom-heavy initial mass functions,” Nature Astronomy, 2026. DOI: 10.1038/s41550-026-02932-4

Tags: astronomy, James Webb Space Telescope, JWST, early universe, galaxy formation, initial mass function