The Hubble and James Webb space telescopes have teamed up to target some of the smallest, most distant objects in the solar system, discovering that the history of these tiny objects is more puzzling than we'd realized.

The two orbiting observatories collectively discovered 27 new Trans-Neptunian Objects, or TNOs, all less than 25 miles (40 kilometers) across, with the smallest being only 6 miles (10 kilometers) in diameter. As their name suggests, TNOs orbit the sun from far beyond Neptune. Some of them were born out there, at the dawn of the solar system, as small planetesimals unable to take the extra step to form planets.

Models of how these TNOs formed predicted that they should have been peppered with impacts that mixed up their surface material so that their composition, and therefore color, would be different than larger TNOs. Yet new observations, led by two PhD candidates, Anastasia Morgan of Northern Arizona University and Marielle Eduardo of the University of Victoria found the opposite – the little TNOs still look as pristine as the day they formed.

"You could imagine a scenario where getting knocked around and fragmented would change the surface composition, and then you would see a different surface color for tiny TNOs compared to their larger siblings," said Morgan, who led the color and composition analysis, in a statement. "So it's really fascinating to see that the smallest objects are somehow 'remembering' and preserving the history of how they were made."

TNOs native to the Kuiper Belt move in near-circular orbits around the sun and are level with the ecliptic plane, the imaginary flat 'disc' on which the planets and other objects orbit our star. They are said to be dynamically 'cold' because they haven't really budged since they formed.

Other TNOs, however, formed between the seventh and eighth planets, Uranus and Neptune, but before they could be assimilated into those worlds while those planets were growing, they were ejected by gravitational resonances into the region far beyond Neptune, collectively forming a 'Scattered Disk' of objects on highly elongated orbits significantly inclined to the plane of the solar system. Such TNOs are referred to as being dynamically 'hot'.

Yet even the 'hot' TNOs seem to have resisted any changes to their surface composition.

"These dynamically hot TNOs retain a signature of where they were born, even though they've been orbitally scrambled since then," said David Trilling of Northern Arizona University.

This leads to one of two surprising possibilities. Either there are far fewer impacts taking place far from the sun than astronomers thought, which doesn't match with what we think we know about the population density of objects out there, or the impacts and collisions do take place but for some reason do not tear up the surface of the small TNOs as much as we might expect.

Thanks to the James Webb Space Telescope's (JWST's) infrared vision, Marielle Eduardo was able to figure out the size distribution of the TNOs. When we look in visible light, how bright a TNO appears depends in part on how reflective its surface is, a property referred to as albedo. A larger body with a composition that isn't very reflective might appear fainter at the same distance as a smaller object covered in shiny ice.

However, at infrared wavelengths the brightness of an object is dependent mostly on its size, allowing accurate determinations of the diameters of the 27 TNOs. Surprisingly, there seem to be fewer of the very small TNOs than what models of their formation predict.

"It's very interesting that the process of planetesimal formation ends up producing the same distribution of sizes for both cold and hot populations, despite forming in different regions of the early solar system," said Eduardo. "The process seems to be insensitive to [planet-forming] disk conditions, producing similar planetesimal sizes whether the disk is hot or cold, and dense or fluffy."

These observations push Hubble's and JWST's abilities to the limit. The TNOs are incredibly faint, shining between magnitudes 24.1 and 29.3, described as being equivalent to seeing a swarm of fireflies on the moon from Earth. As such, it is the deepest survey yet into the relatively unknown realm beyond Neptune, just as you'd expect from these two powerful space telescopes getting together.

The research was published in The Astronomical Journal as two separate papers on Sept. 8, one on color and composition, the other on the size distribution of the TNOs.

Using the James Webb Space Telescope (JWST), astronomers have discovered that the ring system of a tiny solar system body is even more interesting than they knew.

The object in question is Chariklo, which orbits the sun between Saturn and Uranus, at around 17 times the distance between Earth and the sun. Despite only being around 155 miles (250 kilometers) wide, Chariklo, part of the Centaur family of asteroids, possesses two thick rings.

Though Saturn is the solar system body most famous for its rings, other planets also have ring systems, albeit less prominent. That includes Uranus, Neptune, and Jupiter. However, Chariklo and the even smaller body Chiron, show that even the most diminutive bodies can develop rings. Now, thanks to the incredible sensitivity of the JWST, scientists know that the rings of Chariklo are even stranger than their initial discovery in 2013 suggested.

The team, led by researchers from the Institute of Astrophysics of Andalusia (IAA-CSIC), began observing Chariklo with the JWST in Oct. 2022. They used a technique called stellar occultation, which measures the decrease in light from a star when an object passes in front of it.

"By comparing JWST observations with those obtained during other stellar occultations over the last decade, we discovered opposite changes in the two rings: while the inner ring shows significantly higher opacity, the outer ring shows lower opacity," team leader Pablo Santos-Sanz of the IAA-CSIC said in a statement.

The changing rings of the asteroid suggest that it experiences more complex physics than previously thought. This is important because scientists had previously believed that small bodies had relatively stable rings.

"Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability," Santos-Sanz said. "The ability to detect these changes opens a new window for understanding the evolution of these systems and, possibly, that of other ring systems in the solar system."

The cause of these changes remains a mystery.

Another milestone for the JWST and Gaia

The research doesn't just represent an important step in our understanding of asteroids and solar system rings, but it is also an important milestone for the JWST.

"Achieving this required knowing with extraordinary precision the orbit of Chariklo, the position of the star, thanks to the European Space Agency’s Gaia mission, and the trajectory of JWST itself around the L2 Lagrange point, a region of space located about 1 million miles (1.5 million kilometers) beyond Earth, away from the sun," team member Yücel Kilic of the IAA-CSIC said. "The JWST follows an orbit around this region that requires periodic corrections through station-keeping maneuvers."

During the occultation the team used in their research, Chariklo was traveling at around 5,600 miles per hour (2.5 kilometers per second) relative to the JWST.

This is incredibly fast by Earth standards, but relatively slow for objects racing through the solar system. This low speed allowed the rings of Chariklo to be resolved in unprecedented detail. Astronomers currently rely on occultation to study Chariklo and its rings, as even the JWST isn't powerful enough to directly image this small and distant asteroid.

The team's research was published on Tuesday (Sept. 9) in the journal Science Advances.