27 tiny worlds have been discovered beyond Neptune: they may hold the key to understanding the birth of the Solar System

The James Webb Space Telescope has discovered 27 new, extremely faint objects beyond Neptune’s orbit — in the most extensive search for such objects to date. Meanwhile, joint observations by Webb and Hubble have revealed an unexpected detail: the surface colours of the smallest objects studied are similar to those of their much larger ‘relatives’.

Scientists believe that this consistency may indicate that small trans-Neptunian objects have retained traces of the conditions in which they formed more than 4 billion years ago. However, this is merely an interpretation of the data: the researchers measured brightness and colours, rather than observing the ancient surface directly.

Details

Trans-Neptunian objects are small icy bodies orbiting the Sun beyond Neptune. Many of them are thought to be remnants of the early stages of planet formation, when planetesimals – the building blocks of future planets – formed from dust and rocky particles.

In the first study, a team led by Mariel Eduardo used the Webb telescope’s NIRCam camera and a method of digitally stacking consecutive frames to detect extremely faint moving points. The search covered just 0.05 square degrees of the sky, but enabled the confident detection of 27 new trans-Neptunian objects. The authors describe this work as the deepest search for small bodies in the Solar System to date.

The faintest confirmed discovery in the publication corresponds to a diameter of approximately 10 km, assuming a surface albedo of 15 per cent. This is not a direct measurement of size: the diameter is calculated from brightness and depends on how well the object reflects sunlight.

For the second study, Hubble observed the same region almost simultaneously with Webb. Due to its smaller coverage – around 65 per cent of Webb’s field of view – the Hubble data made it possible to identify 13 of the objects detected by Webb. Together, the two telescopes provided colours in the range of approximately 0.35 to 3.2 micrometres: Hubble measured visible light, whilst Webb measured near-infrared.

For the so-called ‘cold’ trans-Neptunian objects in calm, nearly circular orbits, the colours were grouped in the same way as for the larger members of this population. Among the dynamically ‘hot’ objects in more elongated and inclined orbits, a greater variety of colours was observed — and this pattern also resembles that seen for larger bodies.

Separately, the researchers studied the distribution of objects by brightness and size. There were fewer small bodies than might be expected if one simply extrapolated certain previous models down to small sizes. At the same time, the ‘cold’ and ‘hot’ populations exhibited very similar size distributions, even though, according to current models, they formed in different regions of the early Solar System.

Why this is important

If the colour patterns do indeed reflect the original surface composition, then even small bodies could have survived billions of years of collisions without their chemical ‘signature’ of their place of origin being completely erased. This makes them a kind of archive of the early Solar System.

The scarcity of the very smallest objects is also significant. The authors suggest that the mechanism for the formation of planetesimals may have been inefficient at producing bodies below a certain size: a cloud of dust and ‘pebbles’ would have needed to accumulate sufficient mass to allow it to collapse under its own gravity. For now, this remains a hypothesis rather than a proven mechanism.

There are several significant limitations. The colour study is based on just 13 objects that were simultaneously observed by both the Webb and Hubble telescopes. Colour serves as an indirect indicator of composition, rather than a full chemical analysis. The sizes of the faintest objects also depend on the assumed value of albedo — the surface’s reflectivity. Furthermore, the deep survey covered a very small patch of sky, so new observations are needed to accurately characterise the entire population.

Source

Marielle R. Eduardo et al. The Luminosity Function of Ultra-Faint Trans-Neptunian Objects Detected by JWST, The Astronomical Journal, 2026. DOI: 10.3847/1538-3881/ae907f.

Anastasia N. Morgan et al. Combined JWST and HST Deep Imaging to Characterise the Smallest Known Trans-Neptunian Objects, The Astronomical Journal, 2026. DOI: 10.3847/1538-3881/ae9084.

Official NASA material: NASA’s Hubble and Webb Discover Distant Solar System Objects That ‘Remember’ the Past.