At one point, the distant world Ultima Thule was assumed to be more or less a rocky ball. As New Horizons got closer, its shape was described as a bowling pin, then a snowman – and now it's changed once again. New images snapped as the probe sped away from the object show that Ultima Thule is more like a flat "pancake" stuck to a "dented walnut," leaving astronomers puzzled as to how such a shape is even possible.

Unlike previous images that were taken on approach, these new shots were snapped about 10 minutes after New Horizons made its closest pass of the object. At the central point of the 14 images, the craft was 5,494 mi (8,862 km) from Ultima Thule.

From that perspective, New Horizons was able to make out a few new details about Ultima Thule. The "crescent" of the object – the area visible from behind, as light hit it from the other side – was remarkably thin and sharp, rather than rounded. And the science team was also able to trace the shape of the darkened area by mapping how it blocked light from stars behind it.

These insights revealed that Ultima Thule is much flatter than previously thought. The team describes the larger section of the object as being shaped like a lumpy pancake, while the smaller lobe resembles a dented walnut. New Horizons, it turns out, had been approaching at exactly the right angle to only see the flat face.

New data shows that Ultima Thule is much flatter than previously thought, and New Horizons was approaching at just the right angle to see the flat face NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

"We had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby, but seeing more data has significantly changed our view," says Alan Stern, principal investigator on New Horizons. "It would be closer to reality to say Ultima Thule's shape is flatter, like a pancake. But more importantly, the new images are creating scientific puzzles about how such an object could even be formed. We've never seen something like this orbiting the Sun."

The team compared the new shape to a model of Ultima Thule, created from pre-flyby images and ground-based observations, and found that even though the flatness hadn't been noticed, it was surprisingly consistent.

The next big question is how such an oddly-shaped object can even form in space. Things like Ultima Thule and the cigar-shaped interstellar asteroid 'Oumuamua are shattering our understanding of what's possible out there.

The best part is we haven't even seen the clearest images of Ultima Thule yet. NASA is still awaiting shots snapped during the closest pass of the object, which brought New Horizons to within 2,200 mi (3,500 km) of the surface of this strange world.

Check out Ultima Thule's odd shape in the animation below.

Source: Johns Hopkins Applied Physics Laboratory