In autumn 2017, a cigar-shaped object flew through our solar system at high speed – to the surprise of astronomers. Because the object christened Oumumua was not only of interstellar origin, its hard crust and extremely elongated shape also contradicted all expectations of such foreign visitors. Now researchers may have found out why Oumumua has such an unusual shape and how it was created. According to their simulations, a close approach to an alien star and its tidal forces played a crucial role.
On October 19, 2017, the Pan-STARRS telescope discovered an unusual object in Hawaii: a cigar-shaped, several hundred meter long chunk raced through the solar system on a parabolic curve at extremely high speed. From the trajectory and the high speed of the object christened Oumuamua, the astronomers concluded that it had to be a “visitor” from interstellar space. Oumuamua probably originated in the environment of a strange star and was then thrown out into space by gravity turbulence. However, the elongated shape and nature of the object caused a riddle. Because typically it is primarily comets – icy, roundish chunks from the outside areas of a planetary system – that are catapulted out of their systems. Oumuamua, however, had a firm crust and no visible tail. At first glance, it looked more like an asteroid than a comet.
Shaped in a star passage?
However, what exactly Oumuamua was and where this object came from remained unclear – also because when it was discovered it was already on its way out of the solar system. The astronomers were therefore only able to observe Oumuamua for a few months. But now a study by Yun Zhang from the Chinese Academy of Sciences and his colleague Douglas Lin from the University of California at Santa Cruz could provide some answers on the nature and origin of this interstellar object. “It’s a really mysterious object, but some features, such as its color and lack of radio emissions, suggest that Oumuamua is of natural origin,” Zhang explains. “Our goal was to develop a scenario that can explain all of these fascinating peculiarities.”
For their study, Zhang and Lin used computer simulations to investigate whether the shape and nature of Oumuamua could possibly have resulted from a close passage at a star. Because when objects fly past a much more massive one, they are exposed to strong tidal forces. The star’s gravitational pull pulls the object so strongly that it can deform and even tear. This effect was observed in 1994, for example, when the comet Shoemaker-Levy 9 came too close to the planet Jupiter and was torn apart by it. The fragments then plunged into the gas giant’s atmosphere. In their model, Zhang and Lin simulated how icy or stony chunks of different sizes change when they fly close to a sun-like star.
The effect of the tidal forces could also explain the crust
The simulations showed that the tidal forces at such a close passage at a star can not only completely tear an object apart, they can also create fragments that are as elongated and cigar-shaped as Oumuamua. This extreme form comes about because smaller fragments are heated and merge into new, elongated structures under the influence of the tidal forces. “The ratio of the long to the short axis can even be greater than ten to one for these objects,” says Zhang. At the same time, these fragments are accelerated to such an extent that they escape the attraction of the star and can be thrown out of their home system. “We have shown that Oumuamua-like interstellar objects can be generated by tidal force-related fragmentation during close encounters with their central stars,” Lin says. Oumuamua could therefore be the ejected fragment of an originally larger chunk.
At the same time, this scenario could explain another characteristic of Oumuamua: its firm, rock-like crust and the lack of a tail. Because as the simulations showed, the heat generated by the strong tidal forces partially melts the fragments and then condenses again. In the process, more volatile substances such as water outgas and a solid crust is formed from more volatile, rock-like substances. “This process explains not only the surface colors of Oumuamua and the absence of a visible coma, but also the dryness of such interstellar objects,” Zhang says. Because the hard crust makes Oumuamua look like an asteroid, but inside it, as is usual with comets, water ice and other volatile components can be present. This assumption is supported by astronomical observations, according to which the trajectory of Oumuamua has apparently changed slightly due to invisible outgassing.
“Our scenario not only provides a way of how Oumuamua might have originated, but also explains the potentially large population of asteroid-like interstellar objects,” said Zhang. According to this, ‘Oumuamua could only be the first of a large number of interstellar visitors who have emerged from the comets around strange stars, but which look more like asteroids. “Oumuamua is just the tip of the iceberg,” says Lin.
Source: Yun Zhang (Chinese Academy of Sciences, Beijing) and Douglas Lin (University of California, Santa Cruz), Nature Astronomy, doi: 10.1038 / s41550-020-1065-8