Planetary science being rewritten thanks to solar system’s weirdest object

19 Mar 2019

Image: © JohanSwanepoel/

Ultima Thule is the object that keeps giving for planetary scientists, as new findings show that it just keeps getting weirder.

Last month the world was treated to a GIF of what NASA’s New Horizons spacecraft captured as it flew past the object known as Ultima Thule, a chunk of space debris found in the Edgeworth-Kuiper Belt unlike anything we’ve ever seen before.

Officially dubbed 2014 MU69, the 35km-long object appeared to look like a snowman, with two objects of different size fused together somehow. Now, NASA has analysed the object in further detail following its close flyby at a distance of just 3,500km.

“We’ve never seen anything like this anywhere in the solar system,” said New Horizons principal investigator Alan Stern. “It is sending the planetary science community back to the drawing board to understand how planetesimals – the building blocks of the planets – form.”

Thanks to it being the best-preserved object of its kind discovered so far, the latest analysis has been a treasure trove of information, offering the clearest look back to the era of planetesimal accretion and the earliest stages of planetary formation. This includes the discovery that the larger half (Ultima) and smaller half (Thule) once orbited each other until some mysterious force saw them fuse together.

GIF of New Horizons flying past Ultima Thule at great speed.

The ‘departure’ movie comprising 14 shots taken by New Horizons during its flyby of Ultima Thule. Image: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/National Optical Astronomy Observatory

Not finished yet

William McKinnon, a New Horizons co-investigator, said this falls within our understanding of the formation of our solar system.

“Much of the orbital momentum of the Ultima Thule binary must have been drained away for them to come together like this,” he said. “But we don’t know yet what processes were most important in making that happen.”

Further mysteries abound regarding the many different – and weird – surface features of Ultima Thule, including craters that at first glance appear to have been created by impacts, but could have very different origins. Some of these, NASA researchers believe, could be pit craters where material drains into underground cracks, or a result of sublimation, where ice goes directly from solid to gas, potentially leaving pits in its place.

The largest of these depressions is 8km in width, but right now it’s hard to pinpoint its origin, with researcher Kirby Runyon admitting that he and his fellow researchers have their work cut out to understand Ultima Thule’s geology.

New Horizons also helped the team sniff out evidence for methanol, water ice and organic molecules on the object’s surface, showing it to be similar to some of the most extreme objects seen in the outer solar system.

There is still a while yet before the researchers have all of the data on Ultima Thule to hand, with the final transmission set to arrive in late summer 2020.

Colm Gorey was a senior journalist with Silicon Republic