When it comes to understanding planetary formation and system evolution, until recently, we had a sample set of one. Our own solar system was the only one we could look at and analyze. Now we have thousands to look at, in all stages of formation and evolution, from the protoplanetary disk we talked about earlier, to thousands upon thousands of exoplanets found with Kepler and TESS. And still, as our ground-based observatories continue to improve and as we continue to use the Hubble Space Telescope for more local observations, we learn something new about our own system nearly every week it seems.

I know it’s still a bit of a controversy, but these improved observatories are what led to the demotion of Pluto to dwarf planet status. Pluto turned out to not be the only object out beyond Neptune. Soon there were Eris and Makemake and Haumea and a whole host of other bodies. Their orbits are just as eccentric as Pluto’s, and they changed our view of the Kuiper Belt.  We generally group them all together as Kuiper Belt Objects or as Trans-Neptunian Objects, these distant, icy worlds that erratically orbit past our ice giants. And they are always spoken of as one group.

Now, it turns out that may not be the case. In new research published in The Astronomical Journal and led by Mohamad Ali-Dib from NYU Abu Dhabi, scientists have divided these Trans-Neptunian Objects into two distinct groups. And they did it with the data we already had.

Using a 2019 data set, the team analyzed the chemical composition of the Trans-Neptunian Objects to understand how the Kuiper Belt has changed over time. What they found showed that the two distinct colors of Trans-Neptunian Objects — Very Red and Less Red, which I have also seen referred to as Gray or Blue — actually link back to separate orbital patterns. Add in some math, and these orbital patterns lead back to different locations, so our two groups of Trans-Neptunian Objects formed in different places, possibly from two separate, larger bodies.

This is the first study to combine the factors of both color and orbit of the Trans-Neptunian Objects. Since these tiny, icy worlds are considered fossils from the early days of the solar system, understanding these two new groupings gives us another piece of information about our system’s evolution.

More Information

NYU Abu Dhabi researchers discover orbital patterns of trans-Neptunian objects (EurekAlert)

“The Rarity of Very Red Trans-Neptunian Objects in the Scattered Disk,” Mohamad Ali-Dib, Michaël Marsset, Wing-Cheung Wong, and Rola Dbouk, 2021 June 16, The Astronomical Journal