Astronomers Just Discovered a New Hidden World Beyond Neptune

Around 4.3 billion miles from the sun, hiding beyond the orbit of Neptune, a mysterious world was spotted flitting about in the darkness by the omnipotent Vera C. Rubin Observatory. This orb is very likely to be the solar system’s newest dwarf planet—a large, round object that doesn’t quite fit the official definition of a full-blown planet.

Astronomers announced the discovery of this cosmic island—which, for now, is referred to as 2026 ED138—earlier this week made at a meeting of the American Astronomical Society’s division for planetary sciences in Spokane, Washington. If upcoming observations with other telescopes verify its size and round shape, then it would join an exclusive club of just five other confirmed dwarf planets, each formally recognized by the International Astronomical Union: Pluto, Ceres, Eris, Haumea, and Makemake.

“This is incredibly exciting,” says Paul Byrne, a planetary scientist at the Washington University in St. Louis. “It heralds a new era of solar system and planetary science.”

Very little is known about the dwarf planet so far, but already, astronomers are thrilled that Rubin, which only recently became fully operational, caught such a large object. “I love it. It makes me very happy,” says Samantha Lawler, an astronomer at the University of Regina in Saskatchewan. “It’s nice to have confirmation that the giant, expensive, complicated machine is working.”

What is known is that ED138 is located in an enigmatic, donut-shaped region of icy objects beyond Neptune called the Kuiper Belt. “This part of the solar system is largely unknown, and every detection such as this brings up a step closer to understanding how ours and planetary systems as a whole came to be,” says Byrne.

Thanks to Rubin, that’s about to change. The observatory is set to discover an estimated 32,000 objects beyond Neptune’s orbit over the next decade—many of which will be relatively small, but several of which will also be dwarf planets.

The category came out of a huge debate in the astronomy community around Pluto’s status as the solar system’s ninth planet. In 2005, the discovery of Eris, a similarly sized object nearby, had raised questions. Ultimately, the International Astronomical Union determined that, to be a planet, an object needed to orbit the Sun, be massive enough for gravity to squash it into a ball, and have an orbit clear of other, similar objects. Pluto tripped over that last hurdle and was “demoted” in 2006.

(Why the definition of a planet remains controversial today.)

Dwarf planets can be found anywhere. Icy Ceres is, oddly, found in the asteroid belt between Mars and Jupiter. Eris is located right on the edge of the solar system. But Pluto, Haumea and Makemake are found within the Kuiper Belt.

Now, ED138 looks set to be the latest addition to the dwarf planet crew.

Earlier this month, Matthew Holman, an astrophysicist at the Center for Astrophysics in Cambridge, Massachusetts, ran some custom code through some of Rubin’s autonomous observational data. This code, which is designed to examine billions of detections of moving celestial bodies, suddenly flagged something: a bright, but very distant, object.

Distant is putting it mildly: It’s 46 times further from the sun than the Earth. (For reference, Neptune is 30 times that distance.) Based on estimates for how reflective its surface could be, scientists think the object is no smaller than 370 miles across, and no more than 930 miles long.

“Given how big it is, it should be round, which is why we think it’s a dwarf planet candidate,” says Bernadinelli.

Although its status as a dwarf planet is yet to be confirmed, astronomers are now confident it’s a real object, and not an image artifact. ED138 was subsequently spotted in older images, including some taken in 2016 and perhaps others taken way back in 1954. It orbits in the Galactic Plane, a background packed with the Milky Way’s countless bright stars, and this may have made it more difficult to spot.

ED138 also has a peculiar voyage around the sun. The solar system’s planets orbit very close to the same invisible plane, making all the orbital paths look relatively flat. But the new object’s orbit is tilted by nearly 25 degrees away from it. This may be because it got caught in a gravitational tug-of-war with Neptune long ago and was thrown off its original orbit.

Almost nothing about ED138 can be said with any certainty at this point. But some reasonable speculation is possible. “It is likely similar in composition as the other large Kuiper Belt objects, being rich in ices,” says Scott Sheppard, a planetary astronomer at Carnegie Science in Washington D.C.

“It’s quite red, which points to an icy surface darkened by organic material,” says Mario Jurić, Rubin’s lead solar system scientist who was also involved in the work.

Could ED138 also be more than a giant snowball? Potentially.

When NASA’s New Horizons spacecraft flew by Pluto in 2015, it saw signs that the surface was convulsing and painting over itself, suggesting it was geologically active. Ceres, too, contains a huge reservoir of liquid water and might even harbor ice volcanoes. If ED138 is on the larger side of things, perhaps it’s still alive and kicking too.

Right now, astronomers only really know how bright the object is. To narrow down its true size, they need to get a better look at it. NASA’s James Webb Space Telescope, which has a smaller field-of-view than Rubin but an incredible zoom capability, is ideally suited to address this, and some astronomers are already taking steps to collect those observations.

(The Vera Rubin Observatory could solve myriad mysteries about our solar system.)

Meanwhile Rubin will continue its quest to illuminate the Kuiper Belt. ED183 “is certainly a harbinger of many more fascinating discoveries to come,” says Heidi Hammel, an interdisciplinary scientist at the Association of Universities for Research in Astronomy.

The Kuiper Belt is a bit like the far-flung version of the asteroid belt: Instead of mostly rocky objects, this band of material is filled with icy things, including Pluto. Despite it being one of the solar system’s largest structures, little is known about it owing to its incredible distance from Earth.

Detecting more dwarf planets and other objects in this region could address fundamental questions about our corner of the galaxy. “How many dwarf-planet-sized objects are there in our solar system?” says Pedro Bernardinelli, an astronomer at the University of Washington. “This will help us nail down the total mass we have in the outer solar system,” which matters for our understanding of all the different processes at play in the system’s history.

One of the most mysterious objects on Rubin’s hitlist is the hypothesized “Planet Nine”—a world perhaps bigger than Earth, but smaller than Neptune, that some astronomers think is hiding somewhere in the region. If “Planet Nine” is there, most expect Rubin to discover it. But even if it doesn’t, ED138 is proof that the observatory’s Kuiper belt explorations could turn up “many more things that go bump in the night,” says Sheppard.