When you gaze at the night sky, the planets you can see with the naked eye are just the tip of the ice‑cream cone that is our solar system. Past Neptune's orbit lies a frosty realm of thousands of icy bodies—some as small as a city block, others massive enough to be called dwarf planets. In September, a joint effort by the Hubble Space Telescope and the James Webb Space Telescope (JWST) turned that frosty blur into a clearer picture, revealing 27 minuscule trans‑Neptunian objects (TNOs) and a fresh dwarf‑planet candidate. Let’s unpack what these discoveries tell us about how the solar system was built and why the “beyond‑Neptune” region matters for every planetary‑science classroom.\
⚡ Quick Answer
Key point: Hubble and JWST have identified 27 ultra‑small TNOs (as tiny as 5 km) and a new dwarf‑planet candidate, showing fewer tiny bodies than many formation models predict and hinting that collisions have a subtle but lasting effect on surface colors.\
🔭 The Hunt for Tiny TNOs
Trans‑Neptunian objects are the leftovers from the solar system’s birth‑cloud, frozen beyond the orbit of Neptune (30 AU). While the giant TNOs like Pluto and Eris dominate headlines, the majority of the population is thought to be much smaller—objects only a few kilometers across. Detecting them is a challenge: they reflect little sunlight and sit far from any bright reference point.
In a coordinated campaign, astronomers pointed Hubble’s sharp optical eye and JWST’s infrared sensitivity at the same patch of sky. By stacking dozens of exposures and using sophisticated motion‑detection algorithms, they teased out 27 faint moving points that moved consistently with distant solar‑system objects. This joint effort is the first time the two observatories have worked in tandem to hunt for such tiny bodies, and the result is a data set that bridges visible and infrared wavelengths for the same objects.
📏 Measuring Sizes: From 5 km to Hundreds
Size estimation for distant, dim objects relies on two pieces of information: how bright they appear (their apparent magnitude) and how much sunlight they reflect (their albedo). JWST’s infrared cameras can measure thermal emission, which combined with reflected light lets scientists solve for both size and albedo.
For the 27 TNOs, the smallest measured about 5 km across—roughly the length of a large city—while the largest in the sample approached 100 km. This range is critical because it fills a gap between the well‑studied large TNOs (hundreds of km) and the hypothesized “dust‑to‑rock” distribution that underpins many formation models.
📌 Key Facts:
- 🔴 Object count: 27 new TNOs identified in September 2026.
- 🌡️ Smallest size: ~5 km diameter, comparable to a small town.
- ⏱️ Observatories used: Hubble (optical) + JWST (infrared) for simultaneous measurements.
🎨 Color Clues: What Surface Hues Reveal
Color—measured as spectral slope in the visible and near‑infrared—acts like a fingerprint for the surface composition of TNOs. Larger TNOs show a distinct bifurcation: “red” objects rich in complex organics (tholins) and “neutral/blue” objects dominated by water ice and silicates.
The new tiny TNOs follow the same pattern. Despite their diminutive size, their reflected‑light spectra cluster into the same red and neutral groups seen among the big players. This suggests that the processes that set surface colors (e.g., radiation processing of ices) operated early, before the objects were broken up by collisions.
💫 Why Color Consistency Matters
The fact that sub‑10 km TNOs share the same color families as 100‑km bodies challenges a simple “collision‑shuffles‑colors” view. If frequent impacts were stripping away the reddened organic mantles, we would expect the smallest objects to appear more neutral. Instead, the color similarity hints that either collisions are less disruptive than thought, or that surface renewal (e.g., cryovolcanism) quickly restores the original hues.
For educators, this is a vivid example of how a single observable—color—can encode a history of radiation, chemistry, and impacts spanning billions of years.
🧩 Why Fewer Small Objects Matter
One of the striking outcomes of the study is the apparent deficit of objects smaller than ~10 km compared with predictions from classic collisional cascade models. Those models assume that larger bodies grind down into a steady stream of ever‑smaller fragments, producing a power‑law size distribution that rises steeply toward the tiny end.
The observed shallow slope implies either that the collisional grinding slowed down early—perhaps because the outer disk was dynamically “cold” and collisions were infrequent—or that many of the smallest fragments were ejected from the solar system entirely by gravitational interactions with the giant planets.
📌 Key Facts:
- 🔍 Size distribution: Fewer sub‑10 km TNOs than classic models predict.
- 🚧 Implication: Collisional grinding may have been less efficient in the distant solar system.
- 🪐 Alternative scenario: Small fragments could have been scattered outward by Neptune’s migration.
🚀 New Players: Rubin Observatory & Dwarf‑Planet Candidate 2026 ED138
While Hubble and JWST were busy characterizing the tiniest TNOs, the Vera C. Rubin Observatory—set to begin its Legacy Survey of Space and Time (LSST) in 2024—has already delivered its first high‑impact discovery: a new dwarf‑planet candidate dubbed 2026 ED138.
Using a combination of new observations and archival data, astronomers at the University of Washington refined the object's orbit, confirming it resides well beyond Neptune with a semi‑major axis of ~110 AU and an eccentricity that brings it as close as 66 AU at perihelion. Its brightness suggests a diameter of roughly 500 km, placing it squarely in the dwarf‑planet regime.
What makes 2026 ED138 especially intriguing is its perihelion distance—similar to the famous Sedna-like objects—providing a fresh data point for testing theories about how the distant “detached” population was emplaced.
💫 The Rubin Advantage
Rubin’s wide‑field camera can scan the entire visible sky every few nights, generating a massive, time‑resolved dataset. This capability not only finds new objects but also tracks their motion precisely enough to compute reliable orbits within months—a game‑changer for distant solar‑system science.
For teachers, Rubin offers a real‑world example of “big data” in astronomy: students can explore publicly released LSST alerts, practice orbit fitting, and even contribute to citizen‑science projects that hunt for more objects like 2026 ED138.
🌌 The Bigger Picture: Sedna‑like Orbits and Solar‑System Architecture
Objects with perihelia beyond 50 AU—like Sedna, 2012 VP113, and now 2026 ED138—challenge the classical view of a smoothly declining Kuiper Belt. Their detached orbits suggest an additional sculpting force, such as a past encounter with a passing star, the lingering influence of a distant “Planet 9,” or the collective gravitational push of a massive early‑solar‑system planetesimal disk.
A recent Nature Astronomy study (July 2025) highlighted a new Sedna‑like object with a perihelion of 66 AU, reinforcing the idea that these extreme orbits are not isolated oddities but part of a broader, still‑mysterious population. Each new discovery refines the statistical distribution, helping researchers test competing formation scenarios.
📌 Key Facts:
- 🪐 2026 ED138: Dwarf‑planet candidate, ~500 km diameter, perihelion ~66 AU.
- 🌌 Sedna‑like sample: Growing list of objects with perihelia >50 AU, valuable for testing outer‑solar‑system models.
- 🔭 Observational edge: Hubble + JWST + Rubin together cover visible, infrared, and time‑domain regimes, dramatically expanding discovery space.
🎯 Key Takeaways
- ✨ Tiny TNOs are real: Joint Hubble‑JWST observations uncovered 27 objects as small as 5 km, proving that the far‑out solar system contains a rich population of sub‑planetary bodies.
- ✨ Color continuity: Even the smallest TNOs share the red/neutral color dichotomy of larger bodies, suggesting early surface processing dominates over later collisional alteration.
- ✨ Fewer small fragments: The observed size distribution is flatter than classic models predict, hinting at a gentler collisional history or efficient ejection of tiny debris.
- ✨ Rubin’s breakthrough: The first dwarf‑planet candidate beyond Neptune, 2026 ED138, showcases the power of wide‑field, time‑domain surveys to map the distant solar system.
- ✨ Implications for formation theories: Each new Sedna‑like orbit provides a crucial data point for testing hypotheses about planetary migration, stellar fly‑bys, and the possible existence of Planet 9.