Rubin Observatory Finds Candidate for Solar System's Sixth Dwarf Planet

"Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation. It's the largest trans-Neptunian object found in about 20 years, and Rubin turned it up while we were still commissioning the telescope."

The telescope was not even fully operational when it made the catch.

Where does it orbit?

ED138 circles the sun at roughly 46 astronomical units (AU); one AU equals the Earth–sun distance. Pluto orbits at an average of 40 AU. The object therefore lives in the Kuiper Belt, the disk of icy bodies stretching from about 30 AU beyond Neptune out to roughly 50 AU.

That region already hosts Pluto, Haumea and many smaller, irregular bodies — including Arrokoth, the snowman-shaped object NASA's New Horizons probe flew past. Astronomers believe ED138's size makes it massive enough for its own gravity to pull it into a sphere, the key qualification for dwarf-planet status.

Why did it evade detection for so long?

A bright object this large should have surfaced earlier. The reason it didn't: ED138 sat in a patch of sky near the plane of the Milky Way, competing against a dense field of background stars.

It was, in fact, recorded before anyone noticed. After tracking it over 20 nights earlier this year with Rubin, researchers located ED138 in archival images from the Dark Energy Camera dating back to 2016. Those old data points now let both professional and amateur astronomers trace its path.

What does the orbit tell us?

Tony Dunn, using his Orbit Simulator website, showed that ED138 sits in a 7:12 resonance with Neptune: for every 12 Neptune orbits, ED138 completes seven trips around the sun. Haumea shares the same resonance; Pluto holds a 2:3 resonance. Neptune's gravity herded these worlds into such stable orbits long ago, while Kuiper Belt objects that failed to reach this gravitational harmony were likely flung out of the solar system entirely.

The resonance finding strengthens confidence in the orbit's overall shape, though dwarf-planet status still awaits confirmation through follow-up observations now being planned.

What comes next?

Holman sees ED138 as likely the first of several such finds:

"If confirmed as a dwarf planet, ED138 would join a — currently — very small family of worlds. Rubin will image the whole southern sky again and again for 10 years, so any others hiding in the places earlier surveys skipped are now within reach."

A decade of repeated southern-sky imaging means objects this size hiding in crowded star fields will no longer stay hidden.