At the edge of the Solar System, 27 tiny objects have just been detected.
These objects orbit the Sun beyond Neptune's orbit. Astronomers call them trans-Neptunian objects. They are considered remnants of the material that surrounded the young Sun more than 4 billion years ago. Studying them therefore makes it possible to find traces of the conditions under which the planets formed.
This illustration represents a trans-Neptunian object, a small, icy, dim body orbiting the Sun beyond Neptune's orbit. These objects are so small that even with NASA's Hubble and Webb space telescopes, they appear only as tiny, barely visible points.
Illustration: NASA, ESA, Leah Hustak (STScI)
The James Webb Space Telescope detected 27 of them thanks to its high sensitivity in infrared light. The smallest is about 5 km in diameter. That is roughly five times smaller than the smallest objects of this type accessible to the most sensitive ground-based telescopes.
Detecting such faint bodies requires a special method. Astronomers combined several images while accounting for the expected movement of these objects across the sky. Their faint light then adds up in the right place, making sources invisible in the individual images stand out. A machine-learning system then helped eliminate false detections.
Hubble provided a second piece of information. Webb observes mainly in infrared, while Hubble captures their visible light. By combining the two, researchers were able to determine their colors. These small bodies have colors similar to those of much larger trans-Neptunian objects.
This detail reveals part of their history. Their colors depend in particular on the materials present on their surfaces. Their appearance still preserves the signature of the regions where they formed in the young Solar System.
Astronomers also compared the number of objects according to their brightness, which provides information about their size. They find fewer very small objects than some models predicted. However, the populations in so-called “cold” and “hot” orbits show a fairly similar distribution of sizes.
Objects in the cold population have remained in a relatively stable region. Part of the hot population may have formed closer to the Sun before being pushed outward when the giant planets changed orbit. Finding common characteristics in these two groups therefore provides new constraints on the birth of the first small solid bodies.
The 27 discoveries represent only a sample of a very small portion of the sky. Broader observations will determine whether the apparent shortage of tiny objects is confirmed. They will also help clarify down to what sizes these bodies retain the chemical traces of their place of formation.