ARC researchers explore the Solar System’s past from beyond Neptune

Past the orbit of Neptune, faint icy bodies known as Trans-Neptunian Objects (TNOs) are helping us better understand the formation of the Solar System. Two complementary papers recently published in The Astronomical Journal detail the findings of the deepest survey of TNOs to date, where researchers investigated their size, colour, and orbits. The study was conducted by Marielle “Mars” Eduardo of the University of Victoria and Wesley Fraser of the National Research Council of Canada, both members of the Astronomy Research Centre (ARC), in collaboration with researchers at Northern Arizona University, Space Telescope Science Institute, Harvard Smithsonian Centre for Astrophysics, the University of Pennsylvania, and the  University of Waterloo. Below, Mars explains the findings of both papers.

Q: What are Trans-Neptunian Objects (TNOs) and why are they important to study? What do they tell us about our solar system?

A: TNOs are small icy bodies found beyond the orbit of Neptune, at distances from the Sun roughly 30-60 AU. They are considered planetesimals—small bodies of the Solar System that serve as one of the basic building blocks of planets. TNOs are some of the best-preserved objects in the Solar System, having experienced little alteration since their formation. As such, they provide an extremely important window into the conditions experienced at the time the planets were growing.

TNOs generally fall into two categories: dynamically “hot” and “cold” objects. This does not refer to their temperatures, but to the state of their orbits.  “Cold” TNOs formed where we find them in the Kuiper Belt and orbit on undisturbed circular orbits.  "Hot" TNOs were formed closer to the Sun and were transported outwards during the migration of giant planets, causing their orbits to be more inclined and eccentric. 

Q: How do you determine and analyze the size of TNOs? What can TNO size reveal about planet formation?

A: Directly measuring size is impossible for most TNOs as we cannot differentiate between a small body with shiny surface and large body with dull surface. As such, we rely on the brightness of a source to provide a proxy of its size. Fortunately, TNOs seem to exhibit only a small range of surface reflectivity, or albedo, and so brightness provides us with a reasonable measure of size.

One of the key findings from the study I led is the surprising result that different TNO populations share remarkably similar size distributions—the relative number of objects as a function of size—despite different formation locations and histories. This similarity indicates that the conditions during formation do not determine the products of the planetesimal formation process.

Q: What can the colour of a TNO tell you? What colour are they actually?

A: TNO’s colours are primarily determined by their material compositions, where objects rich in organic-like materials appear redder and objects with more water and ice appear bluer. This allows us to piece together a broad picture of the compositional makeup of the surface of a TNO based on the wavelengths observed. TNOs are mostly red in colour, from slightly redder than leaves in the fall, to the reddest materials in the Solar System.

The companion study found that TNO colours are consistent between large TNOs, measuring hundreds of km across, and the much smaller objects we discovered, measuring 10-50 km across. This demonstrates that TNO surface compositions do not appreciably change with size. This is particularly interesting because smaller TNOs almost certainly experienced more surface-altering collisions than large bodies, and yet somehow these objects seem to have preserved their original compositions.

Q: What was the most surprising result of your TNO investigation?

A: To me, the most surprising result was the realization that formation conditions do not seem to determine the products of the planetesimal formation process. That is, the same planetesimal sizes seem to result when the disk is hot or cold, dense or fluffy. It will take a lot of simulation to understand how this can be.

Q: What role did the Hubble Telescope and James Webb Space Telescope (JWST) play in your research? How important was it to use these two observatories together?

A: Hubble and JWST together provided critical compositional information that either observatory alone could not provide. The key to this project was the ability to observe TNOs as small as 10 km—smaller than ever before — providing a view of those bodies and their formation. Before JWST and its near-infrared camera (NIRCam), observing such small TNOs was nearly impossible. A search in JWST’s NIRCam imagery allowed us to pinpoint each object’s location, thereby enabling us to identify each object in the Hubble’s visible light imagery.

Q: What is the most difficult part of studying TNOs? How did you overcome these challenges?

A: The biggest challenge we face is with the extreme faintness of TNOs. Even with the most powerful telescope people have ever created, these objects still appear as only faint blips in an otherwise noisy image. It takes some very specific specialist efforts to tweeze these objects out of the data. Fortunately, our team consists of some of the best people in the world at performing this work.

Q: What is the future of TNO research? What are you hoping to learn next?

A: Building on everything we have learned from these projects and their results, we have a much larger and extended JWST program on deck, with the primary goal of searching for TNOs as small as 1 km. This will help us to determine the size range where collisions are dominant and TNOs are probably no longer primordial. This in turn will inform us of properties about planet formation that we have been unable to answer thus far, such as how efficient was the planetesimal formation process? Other groups are doing some amazing research with JWST piecing together specific compositional details of TNOs and icy other bodies, which tells us what these objects formed from. JWST really opened the doors for our community, and we are just starting to put together a picture of what these observations are telling us.

Learn more about this exciting research in NASA’s press release!