Astronomers have spotted hundreds of tiny worlds lurking in the deep, dark outer reaches of the solar system, beyond the orbit of Neptune.
These minor planets, known as trans-Neptunian objects (TNOs), “are relics of major dynamical events among and beyond the giant planets,” according to a study published this week in The Astrophysical Journal Supplement Series.
Some 139 new TNOs, out of 316 detections total, are reported in the study, which was led by Pedro Bernardinelli, a graduate student in physics and astronomy at the University of Pennsylvania. That’s a big haul considering that the current catalog of TNOs contains only about 3,000 objects, and was possible thanks to the Dark Energy Survey (DES) at the Victor M. Blanco Telescope in Chile.
“Trans-Neptunian objects (except for Pluto) have been found since the early 90s,” Bernardinelli said in an email, but “a lot of the sky we covered was either never observed or not observed to the depths we can reach with the DES.”
As the name implies, the DES is adapted to probe dark energy, a mysterious force that appears to be driving the accelerated expansion of the universe. But while the survey’s main objective is to understand our universe on the largest scales, Bernardinelli’s team showed how it can be adapted to track tiny rocks here in our own solar system.
The DES has such a wide field of view that it captures an enormous amount of activity in our solar system, even though it is focused on a much more distant cosmic horizon. During its four years of observations, the survey picked up about seven billion dots of faint foreground light, which Bernardinelli and his colleagues narrowed down to about 22 million by ruling out familiar objects such as stars and galaxies.
The team then developed numerous techniques, such as stacking images and predicting orbital trajectories, to figure out which dots were actually TNOs. This process confirmed the detection of many relatively large objects from previous surveys, such as the dwarf planets Eris and 2014 UZ224, but it also revealed new tiny worlds with diameters as short as 150 kilometers (93 miles).
The DES is particularly good at finding “high-inclination” objects, which means TNOs that orbit at a relatively sharp angle to the ecliptic plane of the solar system. The team also detected 54 “detached objects,” rarely observed TNOs that have distant orbits that mostly free them from the gravitational interference of the planets. One of the new objects was particularly deep in the outer solar system, about 250 times as far away as Earth is from the Sun.
The study also found dozens of new “resonant objects,” which are tiny worlds with orbits that are modulated by the motions of larger planets. This category includes seven “Neptune Trojans” that follow Neptune’s path around the Sun, as well as 30 “plutinos” that complete two orbits for every three traveled by Neptune.
Building an accurate catalog of TNOs and their orbits can help scientists resolve questions about the past motions of gas giants such as Neptune or Jupiter, and could even reveal whether other stars had close passes with our solar system in the past.
“Many different processes can have an impact on the structure of the Kuiper Belt and the rest of the trans-Neptunian region,” Bernardinelli explained. Scientists can use TNOs to inform questions such as “where Neptune formed and how fast it got to its current location, if the early solar system had another giant planet that got kicked out to interstellar space, how much matter was initially in the solar system, and where the solar system was born in the MIlky Way, among others,” he said.
The results can also aid searches for hidden worlds, such as the hypothetical Planet Nine (or Planet X), which exert influence over the orbits of TNOs. The team would like to see “how our objects fit with the Planet 9/X hypothesis” Bernardinelli noted.
Fortunately, discoveries of these small worlds are expected to accelerate as more future observations from DES, and other surveys, are made available to scientists. This evolving catalog will be like a Rosetta Stone that unlocks new information about past interactions within our solar system, and with the wider universe.
“We're now in progress of analyzing the full DES dataset!” Bernardinelli said. “We have six years of data, and only searched the first four.”
“Besides more data, we also changed a few things in our processing that will allow us to find fainter (so smaller or more distant) things,” he added. “We expect about 200 more objects in this processing.”