Five years after the discovery of the first known object from outside our solar system, scientists are still figuring out what the UFO says about planetary systems.
Invasion of icy giant planets like Neptune It can eject several trillions of small objects into interstellar space, and some of them visit us Solar Systemas such أوOumuamua Especially in 2017. If true, the number of such rogue interstellar objects could be in Hundreds of trillions of trillions (This is a number followed by about 26 zeros.)
It was Oumuamua Discover On October 19, 2017, after arriving from interstellar space, it is headed again after swinging across our solar system. The presence of small objects visiting from interstellar space was no surprise. In fact, interstellar interlopers like Oumuamua and Borisovthe only two discovered so far, were predicted long beforehand.
“We know that when the solar system was forming, dozens of small, icy bodies equal to masses of Earth would be ejected into the interstellar medium,” Greg Laughlin, an astronomer at Yale University, told Space.com. “So if you take our solar system as a representative example, then you would expect to have a great deal of stuff drifting through interstellar space.”
Related: Oumuamua: The Solar System’s First Interstellar Visitor Explained in Pictures
The mechanism that ejects these myriad small objects is the result of planetary migration, in particular the rampage of giant planets. In 2005 astronomers proposed the “Nice Model,” so named because the astronomers who developed it worked at the La Côte d’Azur Observatory in Nice, France. The NES model depicts how it catalysed interactions within a rich disk of asteroids and comets SaturnAnd the Uranus And Neptune to emigrate abroad and Jupiter to migrate inland slightly over hundreds of millions of years.
Since then, the Ness model has somewhat fallen out of favor, to be replaced by similar alternatives such as the “Grand Tack” model, which describes how Jupiter initially moved inward, only relative to Saturn. gravity to stop it and pull it back. But according to Laughlin, in the context of interstellar objects, it does not matter which model is correct.
“Any model that has any kind of motion of giant planets as they form in the midst of a large sea of small planets, will produce interstellar objects,” he said.
When the planets excite the living
Agency astronomer Laughlin Konstantin Batygin coined the term “throw the line“As a description of where excretions can occur.
The ‘throw line’ is just an anomaly on the term ‘snow streak,’ Laughlin said, referring to the distance from a star where water is more stable as ice than as steam. The throw line in turn is where the giant planet is able to slingshot a small body with sufficient acceleration. To achieve a gravitational escape velocity from its star.The farther away the planet is, the easier this is because the star’s gravity decreases with the radial distance.
In our solar system, according to Loughlin, the casting line is located about 372 million miles (about 600 million kilometers) from the sunwhich is about the same distance as the snow line.
All four gas giants in our region – Jupiter, Saturn, Uranus and Neptune – are outside the throw line, and they could all eject objects into interstellar space, but the process does not necessarily need all four.
“It doesn’t take something as dramatic as Jupiter,” Laughlin said. “Neptune does the trick easily.”
Being the farthest planet orbiting in an area where the escape velocity is low and there are a lot of icy bodies to be thrown around, Neptune will act as the sentinel of the solar system when the planet migrates outward, expelling many of the smaller ones it got. on its way.
“If ‘Oumuamua is typical, it indicates that the average star has a planet similar to Neptune, just like our solar system,” Laughlin said, adding that there is observational evidence to support this, in the form of images taken by Almalarge millimeter/millimeter Atacama matrix, from Planet discs of dust around young stars. Many of these discs appear to have ring-shaped gaps in them that may have been removed by the gravity of the Neptune-like worlds.
Although this may not seem like a suggestion, it is of interest to astronomers who have been seeking to determine how our typical or non-model solar system compares to the systems around other stars.
Many giant gas outer planets discovered so far is calledHot Jupiters” And the “hot neptunewhich migrated inward and now orbits near their stars. These worlds cannot eject small objects into interstellar space because the escape velocity approaching their star is too large. Moreover, these systems with hot giant planets are very different from our solar system worlds , whose inner worlds are small, rocky, and relatively far from the sun.
However, the predicted abundance of interstellar objects suggests that the structure of our outer solar system, at least, may be somewhat regular.
A recipe for an interstellar body
This ejection mechanism explains traditional interstellar comets such as Borisov.
However, Oumuamua was not traditional. Its shape was most likely a flat shape, Disc-like fragment, rather than a long sliver as was initially suggested. We have seen a somewhat similar object in shape ArrokothThe Kuiper belt NASA intercept new Horizons A spacecraft flew on New Year’s Day 2019.
However, most comets are not shaped like Oumuamua or Arrokoth. In addition, ‘Oumuamua’ did not have a distinctive comet coma, ‘atmosphere’ around cometmain body. Moreover, its acceleration changed as if it was being pushed by the outgassing that was typical of a comet, although Astronomers have not been able to detect any outgassing.
Unorthodox explanations aside, one hypothesis that Laughlin likes is the idea that Oumuamua was part of the solid hydrogen ice. The only location where such an object could form would be in the cold core of a dense molecular cloud of gas. These clouds, once destabilized by gravity, become Star birth placesbut is it cold enough to form a mass of solid hydrogen like Oumuamua?
“If the hydrogen ice theory is correct, then all the properties of Oumuamua will be explained directly,” Laughlin said. The theory suggests that “Oumuamua may have formed within a molecular cloud as a much larger object that diminishes over time. Laughlin likes to draw the analogy of a bar of soap, which begins its life as a thick lump, but after several washes turns into a thin, flattened piece—the same shape as Oumuamua.”
“The problem with this theory is that it’s very difficult to make the environment cold enough that molecular hydrogen freezes fast enough,” Laughlin said. Molecular hydrogen freezes at about 14 K — 14 degrees above absolute zero, or minus 434 degrees Fahrenheit (minus 259 degrees Celsius). Molecular clouds nuclei can reach similar temperatures, but the conditions must be just right for hydrogen to rapidly condense into a solid, and it is not clear how frequently these conditions occur. However, if it is common, then “Oumuamua was something that was put together before star and planet formation occurred in a cloud,” Laughlin said.
One piece of evidence supporting this lies in ‘Oumuamua’s path through space before it reached our solar system. Astronomers tracked it down and found that 45 million years ago, ‘Oumuamua was at the same place where the giant molecular cloud was about to form the moving stars of the Carina group.
The rarity of interstellar objects
If ‘Oumuamua’ was indeed a hydrogen iceberg, or even if it was just a whim of nature ejected from a planetary system like Borisov, then surely space should be filled with more of these visitors from distant stars. Do astronomers find it surprising that besides Oumuamua and Borisov, we have yet to discover any other interstellar interlopers?
When Oumuamua was found in 2017, astronomer Dave Jewett of the University of California, Los Angeles, who co-discovered the first Kuiper Belt object in 1992 along with Jane Lu, predicted that there are in the range of 10,000 interstellar interstellar in our system. The solar system at any time, based on the probability of detecting ‘Oumuamua when we discovered it.
He told Space.com that that estimate still stands. However, Jewett admits he was surprised that Borisov came so quickly after Oumuamua, and is “frustrated that we haven’t had another one since then”.
Laughlin is still holding on to the most optimistic scenario Regarding the numbers of interstellar interlopers, but only only. The current rarity of interstellar objects, he said, “isn’t entirely surprising just yet, but it’s starting to get surprising.” Based on the current discovery rate of just two in five years, he said, current estimates of the abundance of these objects should be halved.
However, Jewett points out that finding interstellar interlopers is difficult, even if they visit our solar system in huge swarms.
“These 10,000 objects are spread out over the entire size within Neptune’s orbit, and none of them will be detectable unless they pass close to Earthas noted by ‘Oumuamua for these reasons only.”
However, the Vera C Robin Observatory In Chile, surveillance will begin by the middle of this decade. With its 8.4-meter Wide Field Survey Telescope, it will embark on the Legacy of Space and Time (LSST) Survey and, if predictions come true, expect to discover at least one interstellar interstellar each year.
(Scientists are already more prepared to understand these things than they were five years ago James Webb Space Telescope Currently, astronomers have a powerful tool for studying these objects that wasn’t available when Oumuamua was cutting its path through the solar system.)
“If objects such as Oumuamua are detected in a short time by Robin-LST, this indicates a large number of Neptune-like planets,” Laughlin said. “But if such things were not found, the degree to which Oumuamua was unusual would become more and more obvious.”
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