Webb and Hubble Find 27 Tiny Kuiper Belt Worlds That Preserve Ancient Origins

Twenty-seven newly detected worlds beyond Neptune are offering an unusually direct look at the Solar System’s smallest surviving building blocks. A coordinated James Webb Space Telescope and Hubble Space Telescope program found that these ultra-faint trans-Neptunian objects (TNOs) are less numerous than expected at small sizes—while still retaining color patterns linked to the distinct populations in which they formed. 1

That combination matters. A process powerful enough to remove many small bodies might also be expected to refresh or mix their surfaces. Instead, the new observations suggest that very small TNOs can still preserve clues to their early environments.

What Webb and Hubble found

The survey identified 27 previously unknown TNOs in the Kuiper Belt, the broad population of icy bodies beyond Neptune. Some are among the smallest and faintest TNOs directly detected so far. The work was reported in two complementary papers in The Astronomical Journal. 15

Webb supplied deep infrared observations, including detections with its Near-Infrared Camera, while Hubble provided visible-light observations that helped characterize the objects’ colors and orbits. 13

The telescopes did not resolve these distant objects as visible disks. Their sizes are inferred from their measured brightness, so estimated diameters depend on assumptions about reflectivity, or albedo. The study therefore provides a powerful population-level result, rather than definitive physical measurements for every individual object. 1

A surprising shortage of the smallest objects

The survey reaches farther down the TNO brightness distribution than previous direct searches. Its luminosity-function result indicates that the smallest bodies are less common than straightforward extrapolations from larger TNOs would predict. 13

That finding challenges models that describe how planetesimals—the early solid building blocks of planets—formed and evolved in the outer Solar System. It does not, by itself, identify the cause of the shortfall.

Two explanations remain plausible:

1. Primordial scarcity: The original planetesimal-formation process may have produced fewer very small objects than many models predict.
2. Later depletion: Small bodies may once have been more abundant but were preferentially removed or broken apart through collisions and later dynamical evolution. 1

Why the objects’ colors are the key clue

Colors measured from visible and infrared light act as a broad fingerprint of TNO surface properties. The new objects continue the color relationship already seen among larger TNOs: bodies in dynamically cold populations show a different color pattern from those in dynamically hot populations. 1

In this context, dynamically cold does not mean low temperature. It refers to comparatively less-excited orbits, such as lower-inclination orbits. Dynamically hot objects have more dynamically excited paths and are generally associated with populations that were transported or disturbed during the Solar System’s early evolution.

The important result is that this population-linked color pattern persists even among the newly observed, extremely small bodies. Their surfaces appear not to have been completely overwritten by later impacts, erosion, or other processes over the Solar System’s history. 12

The puzzle for Kuiper Belt evolution models

The size distribution and color evidence must be explained together.

If the scarcity of small TNOs is primordial, formation models need to account for a relatively low production of small planetesimals while preserving separate surface-composition histories for cold and dynamically hot populations.

If collisions later removed many small bodies, those collisions and related processes must have reduced their numbers without erasing the color differences that point to distinct birth environments. This tension makes the result especially useful: it constrains both the starting size distribution of the Kuiper Belt and how violently it may have evolved afterward. 1

The researchers and the survey’s limits

The complementary analyses were led by PhD researchers Anastasia N. Morgan and Marielle R. Eduardo, with one line of work focused on the ultra-faint TNO luminosity function and the other on color, composition, and dynamical-population interpretation. 357

Researchers describe the program as the deepest TNO study to date. It extends direct observation into a range that had previously been inferred mainly from brighter objects. 1

But 27 discoveries in a limited observed area are not a complete census of the Kuiper Belt. The survey cannot yet settle exact diameters, albedos, detailed compositions, or the ultimate cause of the deficit of small objects. Its conclusions are strongest as evidence for population trends—and as a sharper test for the next generation of planetesimal-formation and collisional-evolution models. 114

The central takeaway is not merely that astronomers found 27 faint objects. It is that these tiny worlds appear both depleted in number and rich in memory: their abundance challenges expectations, while their colors still preserve evidence of where they came from.