Scientists have identified 27 mysterious new objects hovering at the edge of our Solar System.
Astronomers used the combined power of NASA’s Hubble and James Webb Space Telescopes for the first time to study the trans-Neptunian objects (TNOs).
TNOs are small, faint and icy bodies orbiting the Sun beyond Neptune.
Some are among the smallest and faintest objects ever directly observed.
In two complementary papers published in The Astronomical Journal, researchers analysed the colour, composition and size distribution of 27 newly discovered tiny TNOs.
They unexpectedly found fewer small TNOs than predicted by some models of planet formation. They also discovered that the colours of the smallest objects closely matched those of their much larger counterparts. Most TNOs are more than 100 million times fainter than objects visible to the unaided eye.
Scientists believe these small bodies offer a glimpse into an early stage of planet formation. At that time, a disc of dust and pebbles orbiting the young Sun began to clump together into city-sized ‘planetesimals’ – the solid building blocks from which planets eventually formed.
Beyond Neptune, the process stopped before these bodies could merge into full-sized worlds, leaving behind a frozen population of planetesimals.
In what researchers describe as the deepest TNO survey so far, teams led by PhD candidates from the University of Victoria in Canada, working under the guidance of the National Research Council of Canada, and Northern Arizona University in Flagstaff studied the same region of sky using both telescopes.
Hubble observed visible light from the objects, while Webb detected their infrared light. By combining the observations, researchers were able to measure the TNOs’ colours, which provide clues about the composition of their surfaces. They also determined their sizes and traced their orbits.
The teams examined two populations of TNOs. The first, known as dynamically ‘cold’ TNOs, remain in relatively circular orbits close to the original plane of the Solar System.
The second, dynamically ‘hot’ TNOs, are thought to have formed between the present-day orbits of Uranus and Neptune before being pushed outwards as the giant planets migrated early in the Solar System’s history. These ‘hot’ objects now follow highly elliptical orbits and move in and out of the plane of the Solar System.
Astronomers had expected small TNOs in both populations to have experienced numerous collisions. Such impacts should have fragmented the objects and altered their surfaces compared with those of larger TNOs. But the observations told a different story. The smallest bodies appeared to resemble their larger relatives, suggesting collisions have not significantly altered their surfaces.
One possibility is that collisions occur less often than expected. Another is that the TNOs somehow preserve their original, pre-collision compositions. Researchers say the reason remains unclear.
‘You could imagine a scenario where getting knocked around and fragmented would change the surface composition, and then you would see a different surface colour for tiny TNOs compared to their larger siblings,’ said Northern Arizona University PhD candidate Anastasia Morgan, who led the study of colour and composition.
‘So it’s really fascinating to see that the smallest objects are somehow ‘remembering’ and preserving the history of how they were made.’
‘These dynamically ‘hot’ TNOs retain a signature of where they were born, even though they’ve been orbitally scrambled since then,’ said co-author David Trilling of Northern Arizona University.
Both the ‘hot’ and ‘cold’ populations appear to have retained the colours they had when they formed, with little change since the birth of the Solar System.
The researchers also found fewer very small TNOs than expected from some planet formation models. Webb identified 27 new, exceptionally faint TNOs. One was so faint that its brightness was comparable to standing on Earth and seeing a small swarm of fireflies on the Moon.
The smallest object observed was about 3 miles (5km) across. That’s roughly five times smaller than the size detectable by even the most sensitive ground-based telescopes.
The findings provide a rare look at the remnants of the Solar System’s earliest planet-building process – and suggest that some of these tiny frozen worlds have remained remarkably unchanged for billions of years.