Webb Reveals Secrets of Neptune’s Evolution

A twinset of icy asteroids called Mors-Somnus is giving planetary scientists some clues about the origin and evolution of objects in the Kuiper Belt. JWST studied them during its first cycle of observations and revealed details about their surfaces, which gives hints at their origins. That information may also end up explaining how Neptune got to be the way it is today.

The Mors-Somnus binary is part of a collection of objects beyond Neptune. They’re called, aptly enough, “Trans-Neptunian Objects” or TNOs, for short. About 3,000 are numbered and known, and many more aren’t yet surveyed. They all lie beyond the orbit of Neptune and are divided into various classes. There are the classical Kuiper Belt Objects (KBOs) and scattered disc objects. Within those two classes, there are resonant TNOs—which move in resonance with Neptune and extreme TNOs, which orbit far beyond Neptune (around 30 AU). Then there are objects in orbits similar to Pluto’s, called “plutinos”. Mors-Somnus is also a Plutino.

Neptune and Beyond

Why is there such a varied bunch of objects “out there”? Where did they originate and how have they changed over time? One way to answer those questions is to study the surface properties of Kuiper Belt Objects and, in particular, icy rocks like Mors-Somnus. One way to do that is to take spectra of their surfaces. The data reveals information about the surface compositions of these objects. That, in turn, tells scientists something about the environments in which they formed and those they’ve experienced over time.

Neptune itself likely formed closer to the Sun but then migrated to the outer Solar System (along with Jupiter, Saturn, and Uranus). At the same time, a huge dense disk of rocky and icy planetesimals and asteroids populated space out to about 35 AU. As the giant planets migrated to more distant orbits, they preferentially scattered those smaller bodies. These icy asteroids and cometary bodies settled into the Kuiper Belt, scattered disk, and the Oort Cloud. How that activity progressed and where those icy bodies came from in the first place are questions planetary scientists are working to answer.

More About Mors-Somnus and Neptune

This is where Mors-Somnus comes in handy. The pair is a good example of a “cold classical” TNO. It was studied by JWST as part of a program called Discovering the Surface Compositions of Trans-Neptunian Objects (DiSCO-TNOs) led by Ana Carolina de Souza Feliciano and Noemí Pinilla-Alonso at the University of Central Florida. The project identifies the unique spectral properties of these small celestial bodies beyond Neptune, something that hasn’t been done before now.

The Mors-Somnus is a member of the same dynamical group as other nearby TNOs and they share spectroscopic characteristics with other cold-classical group objects. This means they probably all formed at about the same time. They probably originated beyond 30 astronomical units from the Sun. Trans-Neptunian binaries such as Mors-Somnus provide a unique way to look at the formation and evolution of planetesimals in that region of space.

Studying the composition of small celestial bodies such as Mors-Somnus gives us precious information about where we came from, Pinilla-Alonso said. “We are studying how the actual chemistry and physics of the TNOs reflect the distribution of molecules based on carbon, oxygen, nitrogen, and hydrogen in the cloud that gave birth to the planets, their moons, and the small bodies,” she says. “These molecules were also the origin of life and water on Earth.”

The Importance of Objects Beyond Neptune

The chemical and physical properties of TNOs offer a treasure trove of information about what conditions were like in the early Solar System. They likely contain pristine materials that existed in the protoplanetary disk from which our Solar System formed, including primitive ices. Those ices don’t change due to solar heating (since the Sun is so far away), but they can be darkened by ultraviolet radiation over time, as planetary scientists have seen at Pluto and other icy worlds. And, those bodies can get transported from their birth regions to other parts of the solar system. If their surfaces don’t change much, then scientists can used spectral studies to trace where groups of objects originated.

The TNO region also contains what scientists call a “dynamical structure”. That is, its distribution of objects by various characteristics, including their orbits and motions over time. Objects and events can change the dynamical structure. For example, the dynamical structure of the trans-Neptunian region bears the traces of planetary migration that occurred in the first billion years of the Solar System’s existence. The TNOs, and in particular, binaries like Mors-Somnus were affected by such migrations.

Migration and Neptune

It’s very likely that this binary pair originally formed well beyond the orbit of Neptune. The researchers found similar spectroscopic characteristics between Mors and Somnus and the cold-classical group. It’s compositional evidence that this binary pair formed well beyond 30 astronomical units (nearly 2.7 billion miles away). Then, they moved to their present positions under the gravitational influence of other planetary migrations.

Thanks to gravitational perturbations from Neptune, Mors-Somnus and its neighbors moved closer to the planet. They now orbit in resonance with the planet. All these objects are potential tracers for Neptune’s migration path before it settled into its final orbit, the researchers say.

Binaries separated by distance, as Mors-Somnus is, rarely survive outside of areas bound by gravity, where they are sheltered by other KBOs. To survive migration, they require a slow transportation process toward their destination. The migration of Neptune to its final orbit offered such a leisurely opportunity.

Using JWST to study the surface characteristics of smaller distant worlds is a great accomplishment, according to co-author Pinilla-Alonso. The telescope has studied larger worlds out there, but this is the first time it’s focused on such tiny members of the outer Solar System. “For the first time, we can not only resolve images of systems with multiple components like the Hubble Space Telescope did, but we can also study their composition with a level of detail that only Webb can provide. We can now investigate the formation process of these binaries like never before.”

For More Information

UCF Scientists Use James Webb Space Telescope to Uncover Clues About Neptune’s Evolution
Spectroscopy of the Binary TNO Mors–Somnus with the JWST and Its Relationship to the Cold Classical and Plutino Subpopulations Observed in the DiSCo-TNO Project