Planet formation predicted plenty of small Kuiper Belt objects — Webb found fewer

The region beyond Neptune was supposed to be full of small things. Every model of how planets assemble predicts it: a swarm of icy bodies across a broad range of sizes, from Pluto-scale giants down to objects a few kilometers across, all in cold storage since the solar system formed. For the first time, two space telescopes have counted the smallest members of that frozen archive. The tally is lower than expected.

A survey combining the James Webb Space Telescope and the Hubble Space Telescope has identified 27 previously unknown trans-neptunian objects — icy bodies orbiting beyond Neptune — including one just 5 kilometers across. At that scale, the objects are a hundred million times dimmer than any star visible to the naked eye; no ground-based observatory can find them. The detection required Webb’s infrared sensitivity and Hubble’s visible-light precision working together.

The surprise is not the discovery of small objects. It is that there are not enough of them.

How two telescopes counted the solar system’s oldest debris

The survey worked by combining two types of measurement. Hubble captured visible-wavelength images that established each object’s position and brightness in reflected sunlight. Webb added infrared data, revealing the surface color and thermal emission that encode composition and formation history. Together, the instruments could characterize objects five times smaller than anything previously surveyed in the outer solar system.

Over its first two years of science operations, Webb obtained usable photometric data on more than 75 known and newly detected trans-neptunian objects. From that broader campaign, 27 objects emerged with no prior catalog entry — new bodies in a region that ground-based surveys had considered well-mapped at the sizes they could reach.

The results appear in two papers published in The Astronomical Journal, one led by Anastasia Morgan, a PhD candidate at Northern Arizona University, and the other by Marielle Eduardo at the University of Victoria. David Trilling of Northern Arizona University was a co-investigator on both.

What the models expected — and what the count showed

Planet formation models built over the past several decades predict a specific size distribution: a power-law decline, where roughly consistent ratios hold between each size class and the next smaller one. The critical test is whether the small-body population remains proportional at sizes Webb can now reach, or whether it falls off faster.

The Webb-Hubble census found a steeper falloff than predicted. Small-body counts are lower than the models require. The instruments are sensitive enough to have detected more objects if they were present — the deficit is real, not a consequence of detection limits.

One process planetary scientists are examining is streaming instability: the direct gravitational collapse of pebble clouds into medium-to-large bodies, bypassing the gradual collisional fragmentation that would leave behind a richer inventory of small debris. If the outer solar system assembled primarily through streaming instability rather than piecemeal accretion, the small-object shortage reflects how those building blocks formed, not how they were subsequently destroyed.

What the colors remember

The second paper addresses a separate question with an equally pointed result. Trans-neptunian objects fall into two dynamical groups: “hot” populations scattered into inclined orbits by early giant-planet encounters, and “cold” ones that have remained near their birthplaces in nearly circular paths. Because these groups formed under different conditions and experienced different collision rates, their smaller members were expected to diverge in surface color.

They don’t. At every size the survey can resolve, both populations show the same color relationships. A 5-kilometer cold TNO and a 5-kilometer hot TNO are indistinguishable by color. That consistency suggests the chemical raw material of the Kuiper Belt was already well-mixed when the region formed — a uniform reservoir rather than separate zones with distinct compositions.

The implication is that small bodies preserved their birth chemistry across billions of years of collisions and cosmic-ray bombardment. The surface of a 5-kilometer object records information about the early solar nebula as legibly as any larger body in the same population.

What this study doesn’t settle

The 27 newly detected objects are statistically meaningful but cover a limited patch of a region containing hundreds of millions of bodies. The size-distribution finding carries uncertainty that larger surveys must reduce before the small-body deficit becomes a formal challenge to any specific formation model.

The survey also has a hard floor: objects smaller than 5 kilometers remain invisible to both telescopes. If the size distribution bends upward again below that threshold — with small bodies returning to expected numbers — the deficit the study identified would be a transition point rather than a persistent gap.

There is also the question of timing. The Kuiper Belt today is far quieter than during the solar system’s first billion years. Whether the current small-body count reflects primordial assembly conditions or subsequent dynamical clearing driven by migrating giant planets is something neither paper addresses.

David Trilling at Northern Arizona University described the next priority as expanding the survey area and combining color data with compositional spectroscopy — connecting what the surfaces look like to what they are actually made of.

Common questions about the Kuiper Belt and trans-neptunian objects

What is a trans-neptunian object?

Trans-neptunian objects are small, icy bodies orbiting beyond Neptune, mostly concentrated in the Kuiper Belt between roughly 30 and 50 astronomical units from the Sun. They are frozen remnants of the outer solar system’s original building material — planetesimals that never merged into a full planet because Neptune’s gravity stirred the region before accretion could complete.

Why does the small-body count matter for planet formation?

Planet formation models are tested by comparing their predictions to observed populations. The size distribution of Kuiper Belt objects encodes the efficiency and mechanism of early assembly. If small bodies are rarer than models require, either the process that made them differed from expectations, or subsequent dynamical events removed them. Either outcome requires revising our picture of how the outer solar system formed.

How did Webb detect objects just 5 km across, billions of kilometers away?

Not by resolving them — at Kuiper Belt distances, Webb sees a point of light. Detection comes from photometry: measuring the faint reflected sunlight from objects a hundred million times dimmer than the faintest naked-eye star. Webb’s infrared sensitivity, combined with Hubble’s visible-light precision, extends that reach to sizes no prior survey could achieve.

What comes next for this research?

NASA’s Nancy Grace Roman Space Telescope, with a far wider field of view than either Hubble or Webb, is designed for exactly this kind of statistical census. A planned Kuiper Belt survey is expected to yield thousands of small TNO detections — enough to confirm the deficit definitively or identify where the size distribution recovers.

The outer solar system is still being mapped. Two telescopes working together just found the territory more sparsely populated than the blueprints predicted.

Reference: Morgan et al., “A Color Survey of Trans-Neptunian Objects with the Hubble Space Telescope and James Webb Space Telescope,” The Astronomical Journal, 2026. DOI: 10.3847/1538-3881/ae907f. Eduardo et al., “Size Distribution of Small Trans-Neptunian Objects from Combined HST and JWST Photometry,” The Astronomical Journal, 2026. DOI: 10.3847/1538-3881/ae9084

Tags: astronomy, James Webb Space Telescope, hubble space telescope, planet formation, planetary science, Kuiper Belt