The story of Oumuamua continues to turn – Sky & Telescope

Two new studies leave most options open for the origin and composition of Oumuamua, the first interstellar visitor to the solar system

It has been a year since the detection of Oumuamua (1I / 2017 U1), the first known object to penetrate the solar system from interstellar space. Following the discovery, the astronomers reviewed their observations and held a lively debate about the possible origin and nature of the visitor.

Now two articles highlight new ground in this debate. It is claimed to have restricted the star of the object, the other arguing that it is not a comet, contrary to previous reports.

First captured by the Pan-STARRS robotic telescope in Hawaii, terrestrial and space telescopes monitored Oumuamua as he rapidly left the solar system, disappearing only a few months after its discovery. At first, astronomers thought that "Oumuamua was a dry piece of rock and metal, devoid of gas or ice. They could not see the typical signs of comets as a tail or a coma, which would have revealed a release of gas triggered by the proximity of the sun.

But in June 2018, a team of astronomers led by Marco Micheli (ESA-NEO Coordination Center of ESA, Italy) announced that forces were slightly altering the trajectory of Oumuamua. The researchers argued that the most likely source of this surge was the eruption of ice or gas into an otherwise invisible space. The dust grains in the surface may have been too thick, or the composition of the gas made it invisible to telescopes. In any case, they claimed that "Oumuamua was a comet after all.

But a new study by Roman Rafikov (University of Cambridge, UK), published September 17 on the arXiv preprint of astronomy, is a simple but convincing argument against the cometary character of "Oumuamua". Rafikov argues that if Oumuamua releases enough gas to change its trajectory, it should also rotate faster and faster, which the observations have not shown. Instead, the observations show a steady rotation, although tumbling, about once every eight and a half hours. The only exception, according to Rafikov, would have been if the force propelling "Oumuamua" was directed very close to its center of mass. "For an object with a very elongated shape and dimensions of hundreds of meters, it seems almost impossible," he says.

Although Rafikov does not directly question the results obtained by Micheli and his colleagues, he suggests that another explanation of the trajectory is necessary and encourages "other analyzes of the astrometric data to better characterize its abnormal acceleration".

Micheli declined to comment on this article.

Determining the nature of "Oumuamua" and other future visitors could reveal clues about the composition of other planetary systems in the galaxy. Current theories argue that during the formation of planets, a lot of material is thrown into space. Most of these materials are scattered out of the system by giant planets like Jupiter. But how close these planets are to the sun and what kind of material is thrown away. If the giant planets are on the periphery of the systems, as in our solar system, the material would be cold and cold. If the planets are very close to the star, as in the case of hot Jupiters, the material should be hot and dry. "Oumuamua and his ilk could tell astronomers what the most common planetary arrangements are.

Follow the interstellar traveler to his origins

Just days after Rafikov published his article, a group of researchers led by Coryn Bailer-Jones (Max Planck Institute for Astronomy, Germany) reported his attempt to find the star of Oumuamua, or at least candidates likely. Their study appeared on the arXiv preprint of astronomy on September 24th.

The team used Micheli's estimated trajectory to do the research – indeed, Micheli is co-author of the paper. This trajectory assumes that "Oumuamua accelerates alone. By following the trajectory backwards and comparing it to the positions, motions and distances observed for 1.3 billion stars contained in the second version of the Gaia mission, they identified four dwarf stars as candidates for them. more likely. These stars were chosen according to the similarities between their velocities and those of Oumuamua, because astronomers believe that ejected objects do not generally leave their domestic system at high speed.

The best candidate is HIP 3757, a red dwarf star. The estimated trajectory of Oumuamua took about 2 light years from this star about 1 million years ago. The HD 292249 is another of the finalists, a star closer to the Sun, who would have had a close encounter with Oumuamua about 3.8 million years ago, but with a lower relative speed. Other candidates have combinations of relative speeds and more improbable distances.

We do not know much about any of these stars. None of them have yet been surveyed for the planets – so we do not know if they are sheltering giant worlds that could launch Oumuamua mines – but now that astronomers have a mystery to solve, they could look closer in the near future. In addition, the third version of Gaia is expected for 2021 and will include ten times more stars, offering more candidates a better speed and trajectory.

Maybe by then, we will have understood if "Oumuamua is a comet or not.