A New Eye on the Outer Solar System

What happens when one of the most powerful wide-field astronomical surveys ever built turns its gaze across our own Solar System? We will soon find out! NASA's Nancy Grace Roman Space Telescope has successfully launched aboard a SpaceX Falcon Heavy on the 30th of August, beginning its journey to a position roughly one million miles from Earth. From there, Roman will survey enormous areas of the sky, searching for answers to some of the biggest questions in astronomy: dark matter, dark energy, the formation of galaxies and the discovery of exoplanets.

And while all of this is all very exciting, there is another part of Roman's mission that is close to my heart: it will also look at our own Solar System. More precisely, the outer Solar System from Uranus and beyond.

I have written about Trans-Neptunian Objects, or TNOs, and why I think they deserve much more attention. These are the small worlds that orbit the Sun beyond Neptune, occupying the vast and largely unexplored region we call the Kuiper Belt and the scattered disc. Some are only a few kilometres across. Others, such as Pluto, Eris, and Makemake, are large enough to be classified as dwarf planets. What makes them so fascinating is that they are, in many ways, time capsules. The planetary bodies we know best have been through billions of years of geological evolution. Earth has been reshaped by plate tectonics, erosion, and Life. Mars had massive volcanoes and oceans three billion years ago. Venus has undergone enormous changes. Even the giant planets are still evolving. But far beyond Neptune, the story is different.

Many TNOs have spent most of their existence in the deep freeze of the outer Solar System. Their surfaces contain mixtures of water ice, nitrogen, methane and other volatile materials. They have been subjected to far less geological alteration than the planetary objects closer to the Sun. In other words, they preserve clues about the Solar System as it was forming. And we have barely scratched the surface.

Another fascinating aspect of TNOs is that some of the largest ones we know of have undergone remarkably complex geological transformations. When NASA's New Horizons spacecraft reached Pluto in 2015, many people expected to see a cold, inert ball of ice and rock. Instead, we saw mountains, glaciers, enormous plains of nitrogen ice, possible cryovolcanic features and evidence that Pluto has been geologically active. I even wrote about the possibility that Pluto might have a subsurface ocean. That single encounter forced many researchers to change their perception of the outer Solar System. Pluto was not a frozen relic; it was a world. And if Pluto could be so interesting, what about all the other worlds out there?

That question has become increasingly relevant as our telescopes have improved. The James Webb Space Telescope (JWST) has already demonstrated what is possible; its observations of TNOs such as Eris and Makemake have revealed surprising information about their surfaces and, potentially, their internal evolution. I wrote about these observations previously because they raised an extraordinary possibility that even worlds orbiting billions of kilometres from the Sun may have retained enough internal heat to sustain subsurface oceans.

So what can Roman add? At first sight, Roman might not seem like the obvious telescope for this job. It will primarily investigate dark energy and dark matter, conduct huge surveys of galaxies and stars, and is expected to discover around 100,000 exoplanets(!). To do this, Roman has been designed to survey the sky extraordinarily quickly. Its field of view will be at least 100 times larger than Hubble's while retaining comparable infrared resolution. That combination is important. Roman will not simply stare at individual objects for long periods, it will repeatedly survey enormous areas of sky. And that means it will inevitably encounter objects much closer to home, including TNOs.

Some of these objects are incredibly faint and their apparent motion across the sky is also very slow compared with asteroids and planets closer to the Sun. And that makes them difficult to find. Roman's combination of wide-field infrared observations, sensitivity, and repeated surveys should allow astronomers to identify and track populations of distant Solar System objects that are currently difficult to study. And this is where things become particularly interesting as the distribution of these objects is not random. It contains information.

Indeed, the orbits of TNOs can tell us whether the giant planets moved from their original positions and provide clues about gravitational encounters that occurred billions of years ago. For example, some of the most distant objects have highly elongated or unusual orbits, and these are particularly interesting as they may be telling us that something else is out there. One of these might be the possibility of Planet Nine. The proposed planet has not been observed directly, and its existence remains unconfirmed, but some astronomers have argued that the unusual clustering of the orbits of a small number of distant TNOs could be explained by the gravitational influence of a large planet orbiting far beyond Neptune.

The real value of finding these objects, however, is not simply to add more names to the catalogue. The more TNOs we discover, the clearer the overall picture of the outer Solar System becomes. Their sizes, colours, compositions and, especially, their orbits can be compared with one another. In much the same way that a palaeontologist can reconstruct an ancient ecosystem from many different fossils, planetary scientists can use these distant worlds to reconstruct the Solar System's early history.

As such, Roman will offer the possibility of discovering thousands of previously unknown TNOs and as importantly, be able to examine how their orbits are distributed. The most promising objects will then be studied by JWST or other powerful telescopes to better characterise their surfaces and geological evolution.

Our understanding of the outer Solar System is about to take another major step forward. And I suspect Roman will have a few surprises in store for us. These are exciting times.

As always, onwards and upwards. Image Copyright: NASA