It's not that long since Pluto and some occasionally visiting comets were all we knew of the Solar System beyond Neptune. Now discoveries are coming so fast the Dark Energy Survey (DES) recently picked up 316 of them – 245 never seen before – while looking for other things.
As its name suggests, the DES is not about anything so parochial as our own Solar System, even its outer reaches. Instead it has spent six years seeking supernovae in distant galaxies in the hope of gaining some understanding of the nature of dark energy, the mysterious force accelerating the expansion of the universe. However, the survey is sensitive down to the 23rd magnitude and covers 500 square degrees of the southern sky. University of Pennsylvania graduate student Pedro Bernardinelli realized there would be hundreds of trans-Neptunian Objects (TNOs) caught in the images, and set about identifying them.
"The number of TNOs you can find depends on how much of the sky you look at and what's the faintest thing you can find," said Bernardinelli in a statement.
To find supernovae the DES compares images of the same galaxies taken at different times to see which ones have changed brightness. It then needs to rule out the possibility any light in the image is coming from something much closer.
Projects designed to search for TNOs, or smaller but closer asteroids, take images only a few hours apart, so if something is found to move, its orbit can be established. Bernardinelli and his supervisors needed to develop their own techniques to do this for images taken over longer periods, where the TNOs have moved enough it is hard to match them. They ran algorithms over 7 billion dots, which they narrowed to 400 candidates, which were then followed up for confirmation.
The reward, published in The Astrophysical Journal, was a 10 percent increase in the number of known TNOs, even using a subset of the DES's data. Objects' distances range from Neptune's 30 AUs (the distance between the Earth and the Sun) to 250 AUs, although only seven are currently beyond 150 AUs. Neptune has a fairly circular orbit, but most of Bernardinelli's finds are on the nearer part of a much more elongated path.
Some of the more well-known objects skirting the edges of our Solar System beyond Neptune include the dwarf planet no one can pronounce, Makemake (mah-kay mah-kay), and the farthest world we've ever explored, Arrokoth.
Anyone still sore about Pluto's demotion from planetary status needs to wrestle with the question of how many of these new objects should also be included if Pluto was to be restored.
More important than a mere matter of categorization, however, is that all these extra TNOs provide an enlarged database that can be used to search for patterns caused by the gravitational influence of the real planet nine, if it exists. This could then help us refine our search for an object much larger, but also more distant, than those the DES has found.