Almost four years since its discovery, interstellar visitor ‘Oumuamua continues to remain a puzzle. Why is that shape? How did it form? Last year, a study proposed it was the shard of a planet destroyed when it got too close to its star. A new study also suggests it’s a planetary fragment, but its source is very different. ‘Oumuamua could be the fragment of a Pluto-like planet around a distant star.

The research, published in two papers in the Journal of Geophysical Research: Planets (here and here), aims to explain the curious "cigar" shape and other peculiar properties – it looked like an asteroid but sped along like a comet – of the interstellar asteroid in one swoop. The team found that they could explain a lot of what we have observed so far if the object was made of nitrogen ice, like the surface of Pluto.

If that were the case, the scientists estimate that ‘Oumuamua (oh-MOO-a-MOO-a) first entered the Solar System back in 1995. As it approached the Sun, it began to slowly evaporate. This process created its weird flattened shape, and explains the object getting faster and faster as it passed the Sun, eventually becoming visible from Earth.

“Being made of frozen nitrogen also explains the unusual shape of ‘Oumuamua. As the outer layers of nitrogen ice evaporated, the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use,” Alan Jackson from Arizona State University said in a statement.

It entered the Solar System at a slightly slower velocity than would be expected, suggesting it has not been traveling in interstellar space for more than 1 billion years. The researchers suspect a world like Pluto far away from its star got hit by something, fragments of which flew into space, where they eventually got a boost to fly out of the system and across the stars towards us.

“This research is exciting in that we’ve probably resolved the mystery of what ‘Oumuamua is and we can reasonably identify it as a chunk of an ‘exo-Pluto,’ a Pluto-like planet in another solar system,” said Steven Desch, also from Arizona State University. “Until now, we’ve had no way to know if other solar systems have Pluto-like planets, but now we have seen a chunk of one pass by Earth.”

‘Oumuamua is too small to be studied anymore. To solve this mystery, we will need new observatories that can track all the interstellar objects that pass through the Solar System. Recently, new research estimated that there should be seven within the orbit of Earth every year, so hopefully, opportunities to study more will arise soon.