Using combined observations from the Hubble Space Telescope and the James Webb Space Telescope, astronomers found 27 new trans-Neptunian objects that have been orbiting past Neptune in the Kuiper Belt.
Their optical and infrared colors show that small TNOs preserve the same broad reflectance and color sequences seen in larger members of their dynamical populations.
This suggests their surface properties may be primordial and less altered by size-dependent collisional processing than some scenarios predicted. Rather, they are well-preserved building blocks of planets 4.5 billion years old.
But what they lacked was a lot more small icy objects compared to what was expected by conventional planet formation theories.
Combining space telescopes to peer beyond Neptune
Both the Hubble and the James Webb Space Telescopes were aimed at dark areas beyond the orbit of Neptune.
The absence of atmospheric interference helped both telescopes to find small bodies orbiting over three billion miles away from the sun.
While Hubble was instrumental in providing optical images, Webb complemented Hubble with infrared observations of the outer solar system.
Observations of these space telescopes revealed 27 new trans-Neptunian bodies, which could not be found using any ground-based telescope before.
The smallest body discovered through these observations is about 3 miles wide.
The use of space telescopes together proved to be very effective for exploring faint objects in space.
Finding primordial color fingerprints on tiny worlds
Scientists used multi-band optical and near-infrared photometric colors to measure the broad reflectance properties and classify the new objects into known surface color groups.
The data revealed that the colors of small trans-Neptunian objects are almost the same as those of big icy planets.
The consistency in color signatures indicates that these small bodies fall within the same reflectance sequences and color classes as larger planetesimals.
The continuation of these color patterns down to 3-mile-wide bodies favors a primordial origin for their surface traits, suggesting key spectral properties have been largely preserved since the early solar system.”
Color signatures preserved by small icy worlds date back to 4.5 billion years ago, when the planetary system was formed.
Challenging traditional solar system collision models
For a long time, planetary scientists thought that billions of years of collisions in space would change the surface of small objects.
According to models, the violence of the impacts would break the surface crusts, leaving behind fresh layers of ice and a different coloration of the surface.
The similarity in multi-band reflectance behavior indicates that small objects belong to the same dynamical and spectral color populations as larger bodies.
These small worlds do not look like they were secondarily formed after impacts but rather like the original components of planets.
The color consistency across size scales challenges models that predict strong, size-dependent collisional resurfacing, indicating that impact-driven alteration may play a less dominant role in surface coloration than previously modeled.
The observational data falsify the assumption about the presence of changed impact surfaces on small bodies.
Counting an unexpected deficit of small icy bodies
According to the models of astronomical census, there would be millions of tiny shattered pieces created by violent collisions.
But the result of deep survey images showed a significantly smaller number of 3-mile-wide-objects compared to what was expected from the planet-forming simulations.
It clearly suggests that there were no small objects present at first because of planet formation creating large pieces instead of small pieces.
Reduced collision rates would maintain the original size distribution of planetesimals formed in the cold outer region.
This clearly indicates that the process of planet formation must have been different from the traditional model of fragmentation of planetesimals.
Outlining limits of deep space Kuiper Belt surveys
The identification of 27 more TNOs proves the significance of combined Hubble and Webb space telescope observations in deep field astronomy.
A limited number of objects from a targeted sky region cannot completely capture the whole vast Kuiper Belt area, according to NASA Hubble & Webb.
Very faint objects at great distances fall within the absolute limits of detection of existing space telescopes.
Further wide-sky surveys are needed to find out whether the deficiency of objects noted is found throughout all the outer solar system areas.
Target surveys provide important breakthroughs, but the census of Kuiper Belt objects requires wider sky coverage.
Multiple observatories’ research will enhance early solar system formation models even more.
All the details of the study can be read here: Morgan, A. N., Eduardo, M. R., Trilling, D. E., Fraser, W. C., Stansberry, J. A., Bernstein, G. M., … & Fuentes, C. (2026). Combined JWST and HST Deep Imaging to Characterize the Smallest Known Trans-Neptunian Objects. The Astronomical Journal, 172(4), 188.
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