Five years after spotting the first known object passing beyond our solar system, scientists are still trying to figure out what the strange object says about planetary systems.
Marauding ice giant planets like Neptune could launch several trillion small bodies into interstellar space, some of which visit our solar systemas ‘Oumuamua notably done in 2017. If this is true, then the population of such rogue objects moving between stars could be in the hundreds of trillions of trillions (it is a number followed by about 26 zeros).
‘Oumuamua was discovered on October 19, 2017, arrived from interstellar space, where it is once again heading after passing through our solar system. And the existence of small bodies from interstellar space was no surprise. In fact, interstellar intruders such as ‘Oumuamua and Borisovthe only two discovered to date had been predicted long before.
“We know that during the formation of the solar system, several dozen land masses of small icy bodies would have been 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, you would expect to have quite a bit of stuff drifting through interstellar space.”
Related: ‘Oumuamua: the first interstellar visitor to the solar system explained in photos
The mechanism that ejects these myriads of small bodies is the result of planetary migration, specifically the sacking of giant planets. In 2005, astronomers proposed the “Nice model”, so named because the astronomers who developed it worked at the Côte d’Azur Observatory in Nice, France. The Nice model describes how interactions within a rich disk of asteroids and comets caused Saturn, Uranus and Neptune to migrate outward and Jupiter migrate slightly inward for hundreds of millions of years.
The Nice model has since fallen out of favor somewhat, to be replaced by similar alternatives such as the “Grand Tack” model, which describes how Jupiter initially moved inward, only to Saturn. gravity to stop and remove it. But according to Laughlin, in the context of interstellar objects, it doesn’t matter which model is the right one.
“Any pattern that has some kind of motion of giant planets as they form in the middle of a large sea of planetesimals is going to produce interstellar objects,” he said.
When the planets agitate a neighborhood
Laughlin and Caltech astronomer Konstantin Batygin coined the term “throwing line” as a description of where such ejections can take place.
“The ‘projection line’ is just a riff on the term ‘snow line,'” Laughlin said, referring to the distance from a star where water is more stable as ice than as ice. steam. The line of projection, in turn, is located where a giant planet is able to launch a small body with sufficient acceleration to reach escape velocity from its star’s gravitational pull. The further away the planet is, the easier it becomes because the star’s gravity decreases with radial distance.
In our solar system, according to Laughlin, the projection line is about 372 million miles (about 600 million kilometers) from the sunwhich is about the same distance as the snow line.
The four gas giants in our vicinity – Jupiter, Saturn, Uranus and Neptune – are beyond the line of projection, and all of them could have ejected bodies into interstellar space, but the process does not necessarily need all four. .
“It doesn’t require something as dramatic as Jupiter,” Laughlin said. “Neptune easily does the trick.”
As the outermost planet that orbits in a region with low escape velocity and lots of icy bodies to launch, Neptune would have acted as the solar system’s bouncer as the planet migrated outward, ejecting many small bodies in its own way.
“If ‘Oumuamua is typical, that suggests the middle star has a Neptune-like planet, just like our solar system,” Laughlin said, adding that there is supporting observational evidence, in the form images taken by ALMAthe Atacama Large Millimeter/submillimeter Array, planet forming discs of dust around young stars. Many of these discs appear to have ring-shaped gaps which may have been obliterated by the gravity of Neptune-like worlds.
While this may not seem like a revelation, it is important for astronomers looking to determine how typical or atypical our own solar system is compared to systems around other stars.
Many gas giants exoplanets discovered so far are supposedly “Hot Jupiters” and “hot Neptune“, which have migrated inward and now orbit very close to their stars. These worlds cannot eject small bodies into interstellar space because the escape velocity near their star is too great. In addition, these systems with hot giant planets are very different from our own solar system, whose innermost worlds are small, rocky, and relatively far from the sun.
However, the predicted abundance of interstellar objects implies that the architecture of our outer solar system, at least, may be quite regular.
Recipe for an interstellar object
This ejection mechanism would explain conventional interstellar comets such as Borisov.
However, ‘Oumuamua was anything but conventional. Its shape was most likely that of a flattened, disc-shaped ribbon, rather than a long fragment as originally suggested. We saw a somewhat similar shaped body in the form of Arrokoththe Kuiper Belt object that NASA New Horizons spacecraft flew over on new year’s day 2019.
However, most comets do not have the shape of ‘Oumuamua or Arrokoth. Moreover, ‘Oumuamua did not have the characteristic coma of a comet, the “atmosphere” around the cometthe main body. Moreover, its acceleration changed as if it were pushed by a typical outgassing of a comet, even if astronomers could not detect any outgassing.
Unconventional explanations aside, one hypothesis Laughlin likes is the idea that ‘Oumuamua was a piece of solid hydrogen ice. The only place such an object could form would be in the cold core of a dense molecular cloud of gas. Such clouds, once destabilized gravitationally, become the cradles of starsbut are they cold enough to form a piece of solid hydrogen like ‘Oumuamua?
“If the hydrogen ice theory were true, then all of ‘Oumuamua’s properties would be explained directly,” Laughlin said. The theory suggests that ‘Oumuamua formed inside a molecular cloud as a much larger object that shrank over time. Laughlin likes to draw the analogy of a bar of soap, which begins life as a thick block, but after many washes it shrinks to a thin, flattened ribbon – the same shape as ‘Oumuamua.
“The problem with this theory is that it’s very difficult to cool the environment enough for the molecular hydrogen to freeze fast enough,” Laughlin said. Molecular hydrogen freezes at about 14 kelvins, or 14 degrees above absolute zero, or minus 434 degrees Fahrenheit (minus 259 degrees Celsius). Molecular cloud nuclei can reach similar temperatures, but conditions should be ideal for hydrogen to quickly condense into a solid, and how often these conditions occur is unclear. However, if they are common, then “Oumuamua would have been something that was put together before stars and planets formed in its cloud,” Laughlin said.
Supporting evidence for this lies in the path ‘Oumuamua traveled through space before it arrived in our solar system. Astronomers have traced it back and found that 45 million years ago, ‘Oumuamua would have been in the same place where a giant molecular cloud would have been about to form the stars of the mobile group Carina.
A shortage of interstellar objects
If indeed ‘Oumuamua was a hydrogen iceberg, or even if it was just a freak of nature ejected from a planetary system like Borisov was, then surely space should be filled with more of these visitors. distant stars. Do astronomers find it surprising that apart from ‘Oumuamua and Borisov, we haven’t yet discovered any other interstellar intruders?
When ‘Oumuamua was discovered in 2017, University of California Los Angeles astronomer Dave Jewitt, who co-discovered the first Kuiper Belt Object in 1992 alongside Jane Luu, predicted it there were about 10,000 interstellar intruders in our solar system. at any time, depending on the probability of discovering ‘Oumuamua when we did.
That estimate still stands, he told Space.com. However, Jewitt admits he was surprised that Borisov arrived so quickly after ‘Oumuamua, and that he is “disappointed that we haven’t had another since”.
Laughlin always clings to the most optimistic scenario regarding the number of interstellar intruders, but fair. The current shortage of interstellar objects “isn’t quite surprising yet, but it’s starting to become surprising,” he said. Based on the current discovery rate of just two in five years, he said current estimates of the abundance of these objects should be cut in half.
Jewitt, however, points out that it is difficult to find interstellar intruders, even though they visit our solar system in large swarms.
“These 10,000 objects are spread all over the volume inside Neptune’s orbit, and none of them will be detectable unless they pass near Earthjust like ‘Oumuamua was only noticed for these reasons,” he said.
However, the Vera C. Rubin Observatory in Chile will begin to observe by the middle of this decade. With its 8.4-meter wide-field telescope, it will embark on the Legacy Survey of Space and Time (LSST) and, if predictions hold, it should discover at least one interstellar intruder every year.
(Scientists are already in a better position to understand these objects than they were five years ago. With the James Webb Space Telescope now operational, astronomers have a powerful tool to study these objects that was unavailable when ‘Oumuamua cut its way through the solar system.)
“If objects like ‘Oumuamua are discovered in a short time by Rubin-LSST, that indicates a large population of Neptune-like planets,” Laughlin said. “But if he doesn’t find such objects, then the degree to which ‘Oumuamua was unusual will become more and more pronounced.”
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