Comet or alien spaceship? The appearance of this strange body from outside the solar system puzzled researchers in 2017. A new study explains why its trajectory and displacement did not match those of other previously observed comets
On October 19, 2017, on the island of Maui in Hawaii, astronomers with the Pan-STARRS1 telescope observed an object in space that they identified as a possible comet or asteroid. When analyzing the orbit it described and its high speed -87 kilometers per second- they realized that this object came from outside the solar system and they gave it the name 1I/Oumuamua, which in Hawaiian means “messenger from afar who arrives first”. It is the first star of interstellar origin observed inside our planetary system (in 2019 a second will be observed, 2I/Borisov).
In 2017, telescopes around the world focused on Oumuamua and astronomers were able to recompose its orbit in more detail, determining that it had already gone around the sun and was heading outside the solar system. However, the fact that it was accelerating away from the star in a way that scientists could not explain gave rise to all sorts of theories, some even suggesting that it was actually a craft made by aliens.
This week two American researchers publish in Nature an article explaining the mysterious deviations of the comet Oumuamua in its path around the Sun. allows to explain the behavior of this body with a physical mechanismwhich in fact has been observed in other icy comets: the degassing of hydrogen that occurs as the body warms up in sunlight.
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Oumuamua and Borisov: alien spaceships or big rocks?
Oumuamua and Borisov: alien spaceships or big rocks?
Most comets are objects that accumulate cosmic material and periodically approach the Sun from the confines of our solar system, fundamentally affected by the orbit of other objects. When heated by sunlight, they expel water and other molecules, producing a glowing halo – called a coma – around them. It also often produces gas and dust tails. These expelled gases act like the thrusters of a ship and slightly alter their trajectory with respect to the typical elliptical orbits of other objects, as also occurs with asteroids or planets.
“We can say that cosmic radiation cocina to those comets that travel through the interstellar medium, hydrogen as a result,” explains Jennifer Bergner, UC Berkeley associate professor of chemistry and co-author of the paper. solar system and reheat, releasing that hydrogen? And could this process produce the force needed to explain a non-gravitational acceleration?
small size
Precisely one of the differences of Oumuamua with respect to the other comets of our solar system was its relatively small size. It is estimated that it had approximate dimensions of 115x111x19 meters. To date, all comets observed in our solar system have ranged from one to several hundred kilometers in diameter. However, astronomers could not be sure of its actual size because it was too small and too far away to confirm their measurements.
According to the authors of the article Nature that makes it more plausible that its gravitational deflection around the Sun was altered by the small push created by the hydrogen emanating from the ice. The problem is that astronomers couldn’t detect any comas, outgassed molecules, or dust around Oumuamua. In addition, the calculations showed that the solar energy incident on the comet would be insufficient to sublimate the water or organic compounds on its surface and give it the observed non-gravitational momentum. Only hypervolatile gases such as diatomic hydrogen (H2), el dinitrgeno (N2) or carbon monoxide (CO) could provide sufficient acceleration to match the observations, given the incoming solar energy.
“We had never seen a comet in the solar system that didn’t have a dust coma, so non-gravitational acceleration was really weird,” summarizes Darryl Seligman, a postdoctoral researcher at Cornell University and co-author of the study. Seligman had already published early work suggesting that a solid hydrogen comet—a hydrogen iceberg—would give off enough hydrogen in the heat of the Sun to explain the strange acceleration. And, under the right conditions, a comet made of solid nitrogen or solid carbon monoxide would also give off gases with enough force to affect its orbit.
The authors found experimental research published in the 1970s, 1980s, and 1990s showing that when ice is struck by high-energy particles such as cosmic rays, abundant molecular hydrogen (H2) is produced and trapped in the ice. In fact, that cosmic radiation can penetrate tens of meters into its interior, turning a quarter or more of water into gaseous hydrogen.
“In the case of a comet several kilometers in diameter, the outgassing would occur in a very thin layer relative to most of the object, but in this case, both from a compositional and acceleration point of view, you can’t expect it to be a detectable effect,” Bergner says. This idea also explains the absence of dust coma. “Even if there is dust in the ice matrix, the ice is not sublimating, it is just rearranging itself, allowing H to be released.2 [del interior]so there won’t even be dust,” Seligman stresses.
“However, because Oumuamua was so small, we think it produced enough force to drive this acceleration,” Bergner adds. Therefore, the researcher argues that “The main conclusion is that it is a normal interstellar comet that had just undergone a very strong process.”
News about the universe
A comet is an icy block of rock, the result of the formation of the solar system 4,500 million years ago. Their study regularly provides astronomers with information about the conditions that existed when our Solar System formed. “Comets hold a snapshot of what the Solar System was like when it was in the phase of evolution protoplanetary disks are in now,” Bergner explains.
In the case of interstellar comets that are beginning to be detected, they also provide clues about the existing conditions around other stars, in other planetary systems. And some occasionally cross over to other corners of the galaxy. “Arguably, comets and asteroids in the Solar System have taught us more about planet formation than the planets themselves,” Seligman says. “And I think interstellar comets will be able to tell us more about extrasolar planets than the same exoplanets we’re trying to get measurements of today.”
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