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When astronomers spotted for the first time the celestial object now known as Oumuamua who crossed the sky last October after diving around the sun, its elongated trajectory and rapid speed quickly revealed that he was coming from outside the solar system.
However, learn anything else about our first interstellar visitor, such as whether it 's an asteroid or a small comet, s & # 39; proved much more difficult, as he left our planetary neighborhood as quickly as he arrived. The one or the other clbadification would have important implications not only for Oumuamua itself, but also to understand how planetary systems are formed.
Now, a team of researchers who are monitoring the object as it returns to the stars say that they have the answer: "Oumuamua is almost certainly a comet, although perfectly alien to those we find in orbit around of the sun. Using NASA's Hubble Space Telescope and other ground-based instruments, the team observed the changing position of Oumuamua over time and traced its outgoing trajectory, noting that, remarkably, she did not follow the path she had planned. The result appears in the June 27 edition of Nature.
"Once gathered, these positions showed that Oumuamua's movement was slightly different from what we expected," said lead author of the study, Marco Micheli, of the SSA Coordinating Center. -NEO of the European Space Agency.
It turns out that the movement of Oumuamua always changed slightly over time, suggesting a force other than the decreasing gravitational pull of the sun acting on it.
The strange thrust was small, about two million times weaker than gravitational attraction on the Earth's surface and about 1000 times smaller than the effect of gravity of the sun, said Micheli.
Despite everything, over time, this little push has made big changes: At Jupiter's distance, the team's measurements show that 'Oumuamua's position has been shifted in relation to the width of the giant planet. But what was driving the mysterious visitor?
"Oumuamua was swaying around the sun and its closest planets looked like a giant boomerang from above. Image by NASA / JPL
To find out, Micheli and his colleagues first simulated his journey through the solar system, taking into account the gravitational thrusts of the eight planets, Pluto, the moon and the larger bodies of the belt. asteroid. They also investigated other possibilities such as the influence of "radiation" from the sun, adjusted rotation rates of uneven solar heating of the surface of Oumuamua or potential collisions with d & # 39; s. other objects that could have affected the trajectory of the visitor. None of them explained the observed changes.
"There are a whole lot of potential reasons that the comet could get," said cometary scientist and co-author of the study Karen Meech, a global scientist at the University of Hawaii at Mnoa. "We have systematically excluded them, the only one that remains physically plausible is degbading."
In other words, "the displacement of Oumuamua was self-induced, caused by the rocket effect of gas currents coming out of the ice warmed by sunlight at or near its surface . Such a phenomenon is regularly observed in ordinary comets that pbad near the sun, as had Oumuamua.
A tale without a tail
Understanding what all of this means requires a quick history lesson: Our solar system was a violent and chaotic place when it was young. During the first few hundred million years – a wink in the astronomical scales – it is thought that a wealth of material was expelled from the solar system as a result of gravitational interactions between giant planets and other smaller bodies.
Most of these outcasts were rich in comet-like, icy materials from the outer solar system, rather than rocky asteroids. If our solar system is typical (and there is still little reason to believe that it is not), most of the young planetary systems should suffer from a similar violence and l? Space between the stars should be littered with comets more ejected than asteroids. Comets would thus be the default emissaries of other stars.
But as telescopes around the world turned to Oumuamua, it became clear that the visitor was showing no sign of cometary activity. It was missing the tail of a comet or any sign of ice and gas emerging after sizzling so close to the sun. Some astronomers have suggested that the comet was fried by interstellar radiation, forming a crust of material that protected light ice under the evaporative heat of the sun.
So, if the data of Micheli and Meech are healthy, why did not previous observations detect gas or, incidentally, the badociated displacement of the Oumuamua motion? One of the main reasons is that the gas emission – and the resulting change in motion – was very small.
"[The push] Micheli said – in particular, according to Meech, given the brevity of Oumuamua's close encounter with the Earth and the inherent darkness of the object. In the space of one week of discovery, Oumuamua was so far away from our planet, that it was ten times weaker than when it had been spotted for the first time . after a month, it was a hundred times darker. This made some observations difficult, if not impossible, said Meech.
When astronomers study comets, they chase cyanide, which, when excited by starlight, emits a distinct, revealing blue glow, easily detectable with advanced telescopes. The compound is stirred in the comet during its formation, a fingerprint of the early planetary system. However, if Oumuamua diffused gaseous cyanide, it was below the detection limits of the current instruments.
This nil result suggests that "Oumuamua must have a cyanide / water ratio at least 15 times lower than the cyanide-depleted comets of our solar system – further evidence that the object was not really born in our solar system.
"I would not be surprised that a different solar system has a totally different environment and that we can find a cyanide depletion," said scientist scientist Matthew Knight of the University of Maryland, College Park, who was not part of the team.
A shortage of small dust particles reflecting light on its surface could also explain the masked Oumuamua cometary emission. Knight says that the apparent lack of small dust from the object could have occurred in two different ways: Either he could have pbaded several times through his star in his home system, in which case the stellar winds would eventually blow out his smaller ones dust particles; or the dust could slowly erode eons of exposure to cosmic radiation during its long stay in interstellar space.
He doubts the first explanation, simply because most objects ejected from a planetary system are so early, before they can be so intensely cooked by their stars. Although it is still possible to expel things from the solar system today, a last-minute pariah would be rare in contrast to the wealth of material ejected in the early years.
Statistically speaking, "Oumuamua should not have had enough time to lose its small particles before leaving its home system. The second option – the gradual erosion of dust from cosmic radiation – is the explanation that Meech and others find the most likely.
"Oumuamua has long since disappeared, never seen the best telescopes in the world. But astronomers are now busy preparing the next visitor. Mr Meech said that the completion in the 2020s of several next-generation observatories planned will allow more detailed reviews of future intruders of solar systems.
"Now that we know from experience how interstellar objects behave, we hope that the next time such an object is discovered, we will be able to get even more detailed observations," Micheli said. "I hope that the next can also stay observable a little longer, giving us more time to study its movement and composition in more detail."
This article is reproduced with the permission of Scientific American. It was published on June 27, 2018. Find the original story here.
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