‘Oumuamua was the first interstellar object (ISO) ever discovered crossing the Solar System back in 2017, but its origin remains mysterious. What also remains unexplained are several of its unusual characteristics, including its strange cigar-like shape. A new study has a theory for solving all these unknowns with a single explanation: ‘Oumuamua is a fragment of a planet ripped apart by a star.

Although the researchers note it doesn’t have to be a planet, exactly. The study, published in Nature Astronomy, suggests that the parent body of ‘Oumuamua could potentially be a large long-period comet, or a planetoid broken apart before it could become a fully-fledged planet. Whichever the object was, it passed too close to its host star and was likely torn apart by its tidal forces. Those rocky shards were then flung into interstellar space, eventually passing through other star systems.  

"‘Oumuamua is absolutely nothing like anything else in our Solar System," according to lead author Dr Yun Zhang from the National Astronomical Observatories of the Chinese Academy of Sciences. "It is really a mysterious object, but some signs, like its colors and the absence of radio emission, point to ‘Oumuamua being a natural object." 

Two of the main unusual properties of the ISO is its shape and its lack of hazy coma. The team think they can explain these with their sophisticated modeling. “We present a comprehensive model for the first time to address all different pieces of the puzzles associated with the first interstellar object 'Oumuamua based on well understood physical principle," Zhang told IFLScience.

The researchers used a computer simulation to show how a fragment passing within a few hundred thousand kilometers of the star could be whipped into the strange elongated cigar shape we observed when the object was discovered. The way it tumbles through space is also peculiar, and a non-gravitational acceleration was detected as it passed the Sun. This is often seen in comets as the heat of the Sun releases a lot of icy or rocky material, but astronomers were not able to discern any evaporation. ‘Oumuamua didn’t appear to be forming a coma, the fuzzy envelope of ice and dust that is seen surrounding the nucleus of a comet.

According to the researchers' model, the surface of the fragment body could become hot enough to evaporate its volatile components, making it unlikely to produce a coma. But deep within the ISO, water could still exist and once close to the Sun, that could heat up and evaporate, producing that unexpected acceleration observed.

The model not only explains ‘Oumuamua itself, but also provides a framework for interstellar asteroids as a class. Comets, such as 2I/Borisov, were thought to be the more common objects likely to become interstellar visitors as they form further from their star, but this model doesn't expect this to be the case. Planetary fragmentation is a lot rarer than a comet escaping the clutches of the star around which it formed, but comets are lost one at the time, while planet fragmentations launch over a thousand ISOs at once.

“We expect the total number of asteroidal ISOs is larger than that of cometary ISOs. But since asteroidal ISOs we predict are usually smaller than cometary ISOs and they don’t have coma, it’s much harder to detect asteroidal ISOs when they come to visit our Solar System," Zhang explained to IFLScience. "Nevertheless, we anticipate many more interstellar visitors with similar traits to ‘Oumuamua will be discovered by future observation.” 

The hunt for more ISOs, whether asteroids or comets, is on. Their discovery might confirm if this possible origin for ‘Oumuamua is indeed correct.