There may be a new dwarf planet just beyond Pluto

The small club of dwarf planets in our solar system may have just grown by one. Astronomers using the new Vera C. Rubin Observatory in Chile have spotted an object beyond Pluto that seems to be big enough to qualify as a dwarf planet. In fact, it is one of the largest dwarf planet candidates we’ve spotted so far from the sun.

The object, designated ED138, is between 600 and 1500 kilometres across. To qualify as a dwarf planet rather than just a large asteroid, a space rock must be massive enough for its own gravity to squeeze it into a roughly spherical shape; measuring the masses or shapes of these objects directly is extraordinarily difficult, but it is generally accepted that asteroids with a diameter greater than about 600 kilometres should be massive enough.

Right now, the International Astronomical Union recognises five official dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. A handful of others, including Sedna and Quaoar, are considered unofficial members of the group as well, and given its size, ED138 seems poised to enter that larger unofficial group.

While it is relatively large and bright, ED138 was nevertheless difficult to spot because of its location in the sky, close to the plane of the Milky Way. With so many bright objects behind it, previous observatories weren’t able to pick it out.

“Rubin is giving us an unprecedented capacity to explore the sky through a combination of speed, depth, and precision,” said Mario Jurić at the University in Washington in a statement. “And the discovery is just the beginning: once we identify a new object, we can search back months, and sometimes even decades, finding it in images where nobody knew to look.”

After his colleague Matthew Holman at the Harvard-Smithsonian Center for Astrophysics in Massachusetts first spotted ED138 in October, Jurić scoured Rubin’s past catalogue and found 30 more observations of it since March. Further hunts through the archives of other telescopes found archival observations going back as far as 2017. Holman and Jurić presented their work at the Annual Division for Planetary Sciences meeting in Washington on 27 October.

The observations allowed the researchers to reconstruct ED138’s orbit, finding that it takes about 281 years to circle the sun on a path that is tilted about 25 degrees with respect to the plane of the solar system. This is exactly what we’d expect for an object that has had its orbit shifted through gravitational interactions with Neptune.

More observations are needed to confirm ED138’s size and potential status as a dwarf planet. If it is confirmed, it might soon be followed by the discovery of even more dwarf planets. “Rubin will image the whole southern sky again and again for ten years, so any others hiding in the places earlier surveys skipped are now within reach,” said Holman in a statement.