Last year, the interstellar space intruder Oumuamua passed through the inner solar system. Initially thought to be a comet, then an asteroid, and then an alien ship altogether - this visitor had properties unusual for typical space rocks. It moved too fast and at an odd angle to have the roots of our system; neither Jupiter, nor Neptune, nor the Oort cloud could send us an object with such properties. Having studied the asteroid in detail, scientists came to the conclusion that its icy interior is covered with something like carbon, and the asteroid itself does not leave a tail, despite the temperature of 290 degrees Celsius. What's strangest is its cigar shape with a 1: 8 width to length ratio. They offered a variety of explanations, but in the end they came to the simplest:Traveling through the Milky Way over billions of years turned the asteroid into the object of our interest.
Looking at the solar system today, you can find inner solid worlds, outer gas giants, and a handful of smaller objects grouped into four different peoples:
- asteroids, mineral-rich objects formed near Mars and Jupiter: on the border, where solar radiation will allow ice to form in the clear light of the sun;
- Kuiper Belt Objects, ice-rich objects formed outside Neptune that become comets if they enter the inner solar system;
- centaurs - hybrid objects between the orbits of Jupiter and Neptune;
- objects of the Oort cloud, which lies outside the Kuiper belt and is the remnants of the formation of the solar system.
Although Kuiper Belt and Oort Cloud objects are similar in composition and extremely numerous, there were many more in the early days of the formation of the solar system.
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Over billions of years, mutual gravitational interactions between objects and planets throw a huge number of the former into interstellar space. For every star in the galaxy, we will have thousands or millions of objects flying through the Universe, not tied to any star. And just as stars move relative to the Sun at a speed of 20 km / s, most of these interstellar intruders move on average.
From a certain point of view, it is striking that we have been looking for our first interstellar asteroid for so long. It is likely that such encounters should occur many times a year, but very rarely do such large objects appear close enough to the sun for us to capture them. And when we discovered this asteroid, we were immediately surprised by its properties: its rotational motion, its dimming curve, the composition of the surface and the interior, as well as its strange elongated shape. The rotation did not come as a surprise, since in the absence of a massive object to regulate it, an asteroid of this shape would rotate. But other properties remained a mystery.
We have never seen interstellar objects before, so astronomers and astrophysicists have seriously considered how to explain Oumuamua. Some are trying to track its movement in the past, since there is a possibility that the asteroid was ejected from the system very recently. Others are looking for an explanation as to how such an elongated object, protected by carbon, could have formed, especially against the backdrop of these shapeless objects that we see everywhere. The simplest explanation is that this icy object has flown through the galaxy for billions of years, and its interaction with the interstellar medium has turned it into what we see today.
We think that space is emptiness, but in fact it contains a lot of dust particles, neutral atoms, ions and cosmic rays even where there are no stars. As the object moves through space at hundreds of kilometers per second, it is constantly bombarded by numerous small, fast particles. Just as water and sand smooth and erode pebbles and boulders in the ocean, the space environment affects the ejected ice bodies in the same way.
Because objects are rarely spherical, they tend to elongate more in one direction than others, resulting in elongated, flattened shapes. The lightest molecules wear off faster, while the heavier ones with a stronger lattice can hold on. The presence of carbon components bombarded by the particles means that they can heat up, bond into more stable molecular configurations, and then freeze again. This is how a "cigar" could have formed over billions of years, which was mistaken for an alien ship.
If such an object does not come close enough to the star for its interior to break through the crust, we will not see the tail, coma, or behavior of the comet. In addition, over billions of years, most of the external volatiles will evaporate. It's just not typical for bodies in the solar system. Modeling, new observations and collecting statistics on this new class of objects will eventually provide us with an answer, but until then we can only guess where "it" came from.
Ilya Khel
