First look: The official list of dwarf planets in the solar system includes five names as of 2008. But a new observatory in Chile has discovered an icy planet beyond Pluto that could become the sixth. It could also be the largest object discovered beyond Neptune in about 20 years.
Currently, the object announced by the Vera C. Rubin Observatory team earlier this week only has the provisional designation 2026 ED138. It is currently about 46 astronomical units from the Sun. In comparison, Pluto averages around 40.
So far, no one has measured his height directly. Instead, astronomers went from how bright it looks at that distance, plus an estimate of how reflective its surface is, working backwards to arrive at a diameter between 600 and 1,500 km. Even the middle of this range is close to Ceres, the smallest dwarf planet on the official list. Mario Jurić, head of the Ruby Solar System, put the distance at about 1,000 km and called it a unique find.
It remained hidden for so long mainly because of its position in the sky. From Earth, ED138 is aimed close to the plane of the Milky Way, where it is just another faint dot against millions of background stars.
But Rubin photographs the entire southern sky every few nights with the largest digital camera ever built for astronomy, so the same spot always appears in a slightly different place. On October 13, Harvard & Smithsonian astrophysicist Matthew Holman searched data from earlier in the year using a search code he wrote with colleague Kevin Napier.
ED138 caught his eye because it looked far too bright for something so far out. Holman admitted that his first instinct was to check whether he or the code had made a mistake.
Jurić’s search of Rubin’s catalog dispelled this doubt – the object appeared in 30 images over 20 nights between March and July. Older survey images also date back to 2016.
However, this does not make it clear whether it is a dwarf planet. To receive this designation, an object must be massive enough to be pulled around by its own gravity. This confirms that ED138 requires infrared observations that can determine its true size.
Even then, it might be a while before there is an official title. Quaoar and Gonggong are already considered dwarf planets in many research papers, but the International Astronomical Union has not yet added either planet to its list. In the meantime, anyone curious can follow ED138’s orbit in 3D on Orbitviewer.
