Scientists continue to study the solar system, and it looks very interesting. For example, modern planetary orbits harbor clues that reveal the harsh conditions of the origin of the solar system - and, possibly, the existence of an interstellar giant that went astray long ago. Our solar system is like a crime scene that happened 4.6 billion years ago.
Modern orbits harbor clues that reveal the harsh conditions of the origin of the solar system - and possibly the existence of an interstellar giant that went astray long ago.
Our solar system is like a crime scene that happened 4.6 billion years ago.
Surfaces strewn with craters, displaced planetary orbits and clouds of interplanetary debris are cosmic analogs of blood spatter on the wall and skid marks of a car leaving a pursuit. These and other clues tell of the chaotic origins of our planetary family.
Lurking among these footprints are clues about a lost sibling, planet 9 (no, not Pluto), thrown away in the gravitational tug-of-war that accompanied the original formation of the solar system.
Nowadays, four huge planets dominate the periphery of the solar system: Jupiter, Saturn, Uranus and Neptune. Behind them is the Kuiper Belt - a field of ice shards, among which Pluto is found.
“Don't think that the periphery of the solar system has always been the same as it is now,” says David Nesvorny, planetary scientist at the Southwest Research Institute in Boulder, Colorado, who first spoke out in favor of the existence of a fugitive planet in 2011. year.
Nesvorni is a member of a group of scientists who are trying to figure out how the solar system evolved in the first few hundred million years of its existence. Using sophisticated computer models, the researchers compiled a chronology of collisions between newborn planets that arose relatively close to each other - they alternately glided and jumped from one orbit to another. These models have revealed many small details about how planets, asteroids and comets rotate around the sun today.
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There was only one problem. Typically, the simulated scenarios ended with either Uranus or Neptune being driven out of the solar system, as Nesvorni wrote in September in the Annual Review of Astronomy and Astrophysics.
Since in reality Uranus and Neptune remain in their places - spacecraft have visited both of them - something in these scenarios did not work out. However, as many researchers suspect, a key player in this mystery and a missing link in the history of the solar system may well be the fifth giant planet.
Lost planet
Astronomers rely on computer models to recreate these ancient scenes, creating thousands of different solar systems in thousands of different ways. They translate the laws of physics and whatever initial planetary positions they can think of into program code. The researcher sets the parameters - one planet here, a bunch of asteroids there - and then leans back in his chair and lets the simulated environment do all the work for him. After a couple of weeks in real time - millions of years in the model - the astronomer checks the results to see what happened to the solar system. The closer it is to reality, the more successful the model is.
This is what Nesvorni did in 2009. He delved into virtual solar systems in an attempt to rescue virtual Uranus and virtual Neptune from their virtual paths in deep space.
The problem was Jupiter, a giant hooligan planet whose gravity can reach far enough to be pushed around by smaller planets and various debris. In the most successful simulation to date, Jupiter and one of the two outer planets bounced off each other and eventually settled into their present orbits. But this happened only in one percent of all models. In the remaining 99% of cases, Jupiter threw Uranus or Neptune so hard that they left the solar system and never returned to it.
“This made the situation very mysterious, since we knew that Uranus and Neptune continued to exist in their present form,” says Nesvorni. So he continued to experiment. After a year of simulating countless different scenarios, he began to think about adding martyr planets - extra planets sacrificed to save the rest.
“I just simulated their existence to see what happened, and not because I was serious about the idea itself,” says Nesvorni. "But then I realized that there could be a reasonable grain in it." He ran about 10,000 scenarios, changing the number of extra planets, their original location and the mass of each of them.
The best option, which most accurately predicted the current state of our solar system, turned out to be one in which the extra planet was located between the original orbits of Saturn and Uranus. In terms of mass, the planet was approximately equal to Uranus and Neptune, and was almost 16 times larger than Earth. It is such a planet that could collide with the orbit of Jupiter and fly out of the solar system.
The graph shows how the distance between the planets and the sun has changed over time. The first few million years in the computer model, the orbits changed slowly, then there was a close contact between Saturn (green) and an extra planet (purple), which led to the destabilization of the orbits. Dotted lines indicate the current sizes of the orbits. (Source: taken from materials of D. Nesvorny / section of astronomy and astrophysics of Knowable magazine, 2018.)
The chances are still slim. In subsequent models, this alignment ended in success in about five percent of the time. “The existence of the solar system in its current form is neither typical nor predictable,” Nesvorny noted in 2012 in a paper co-written with his colleague Alessandro Morbidelli of the French Riviera Observatory. Despite this, the model was a significant improvement over the 1% success rate for those models that included only the four giant planets we know and love today.
“Assuming a fifth planet makes it much easier to explain what is happening,” says Sean Raymond, planetary scientist at the University of Bordeaux in France. And although the evidence is mostly circumstantial, "it is much more logical to assume that then there was also a fifth planet."
This may seem like a very controversial assumption. How can astronomers know anything about what happened four billion years ago, even with the planets that we can observe now, let alone those about which we know nothing? However, it turns out that the planets have left a lot of battle scars of youth as evidence for detectives of the future.
Interplanetary Blood Spatter
“We are more than confident that the planets did not originate where they are today,” said Nathan Keib, a planetary scientist at the University of Oklahoma in Norman.
However, this realization happened quite recently. For most of history, astrologers have had no doubt that the planets have always been in their present orbits. But in the early 1990s, researchers realized that something was missing from such a model.
Neptune and Triton.
Just beyond the orbit of Neptune lies the Kuiper belt, a scattering of ice debris that encircles the sun. “This is our blood splatter on the wall,” says Konstantin Batygin, a planetary scientist at the California Institute of Technology.
The location of Kuiper belt objects led researchers to the inevitable conclusion: Neptune should have formed much closer to the Sun than its current location suggests. Many Kuiper Belt objects clump together in concentric orbits that vaguely resemble grooves on a musical record. These orbits are hardly random - they are directly related to Neptune.
For example, Pluto is the most famous inhabitant of the Kuiper belt. He and a couple of hundred of his fellow travelers known to us make exactly two revolutions around the Sun in the three that Neptune makes during the same period. Other streams of debris in the belt make one complete revolution for every two that Neptune completes - or rather, four for every seven.
The Kuiper Belt could not be made this way without external influence. However, assuming that Neptune arose closer to the Sun and then moved outward, its gravitational force would be strong enough to catch interplanetary debris in its nets and send it into these unusual orbits.
This model shows how the close arrangement of the outer planets (image on the left) can change over time. The orbits of Jupiter and Saturn are converging (center image), which leads to a change in all other orbits. Specifically in this model, Uranus and Neptune are swapped. After a while, (image on the right) space debris is scattered - some of it settles in the Kuiper belt, while the planets begin to move towards their present orbits. (Source: adapted from Astromark / Wikimedia Commons.)
This coincided with the predictions of some models obtained a decade earlier.
The formation of the planets left behind a mess of debris scattered throughout the solar system. Any fragments that got too close to Neptune would inevitably fall under the influence of its gravity. Since every action is followed by an equal force of opposition, every time Neptune pushed the shard, he himself moved in the opposite direction. Slowly but surely, Neptune crept away from the sun.
The migration process of Neptune also applies to other giant planets. After all, Jupiter, Saturn and Uranus made their way through the same debris field and dealt with similar gravitational interactions. And if Neptune moved to a new place, the same should have happened with all the other giant planets.
And this process was clearly not smooth.
Incessant collisions with all this debris should have turned the orbits of the giant planets into perfect, slender circles - just like clay on a potter's wheel is smoothed by the firm hand of a potter. However, the orbits turned out to be quite different. Instead, the giant planets move in slightly elongated and distorted orbits. As if someone hit a wheel, reshaping the once round pots.
Jupiter Leaping
By 2005, researchers had identified the culprit. The new models suggested that at some point the giant planets went through what scientists call "dynamic instability." In other words, for about a million years, everything turned into a crazy whirlwind. The most likely reason for this seemed to be a series of collisions between Saturn and Uranus, or Neptune - that is, one of the ice giants - that sent one of them directly towards Jupiter. As soon as the lost planet approached, its gravity pulled Jupiter, slowing it down and pushing it into a narrower orbit. However, Jupiter pulled the invading planet in no less force. The ice giant, being much lighter, accelerated much more than Jupiter slowed down and headed away from the sun.
Such an incident would be a gravitational pogrom for the solar system. Jupiter jumped deeper inward, while the rest of the outer planets jumped outward. Such a push would bend the orbits of the giant planets into their current state. In addition, it would save the inner solar system - Mercury, Venus, Earth, Mars, and the asteroid belt - from gravity from both Jupiter and Saturn, which was another problem in the earliest models.
Which brings us to the removal of Uranus or Neptune from the system. It is at this stage of the simulation that Jupiter most often throws one of the ice giants away.
This is the very problem that Nesvorny tried to solve without breaking everything else in the simulations that worked. The extra ice giant takes the brunt of the blow from Jupiter, giving the rest of the scenario the ability to unfold unhindered.
“This is quite plausible,” says Batygin. "It's not at all a fact that there have always been exactly two ice giants instead of three." On the contrary, he says, some calculations allow for the original existence of up to five Neptune-like planets.
Batygin and his colleagues investigated this issue in parallel with Nesvorni, although for different reasons. “I wanted to demonstrate that there could be no extra giant planet,” says Nesvorni.
Jupiter's Great Red Spot. Photo taken by Voyager 1.
He reasoned that on its way out of the solar system, this putative planet must have left a trail here and there in the Kuiper belt, in an area known as the "cold classical belt." If the Kuiper belt were a donut, Batygin continues, the cold classic belt would become its chocolate filling - a cluster of objects whose orbits are located practically in the same plane within the Kuiper belt. A planet passing by should have disrupted these orbits - at least, so Batygin and his colleagues believed.
Their computer models showed that nothing like this had happened. To their surprise, the exiled planet would not have destroyed the cold classic belt on its way out. This does not prove the existence of the planet - the result obtained only indicates that the solar system could exist in its current form, both with it and without it. Could this planet have left a clearer footprint? Or, returning to the crime scene analogy, are there any traces of skidding? Nesvorni thinks that such traces could well remain.
Core of truth
There is another part of the Kuiper belt - a narrow stream of icy debris called a core, whose orbits do not correspond to the current position of Neptune. Its origin is a mystery. In 2015, Nesvorni argued that, perhaps, the reason for everything could be the movement of Neptune from the Sun, provoked by a bygone planet.
As Neptune moved into its final orbit and swept the debris into orbits consistent with its own, at some point it could be exposed that released enough of this debris to form its own stream.
Models have shown that the same gravitational impact that could cause Jupiter to jump from orbit to orbit and push the extra planet out of the solar system could have happened at the right time to push Neptune as well.
“The result is something like a kernel,” says Nesvorni. "This is circumstantial evidence … it is not conclusive."
In truth, we will never know for sure what happened in the solar system during its formation. “We cannot write the Bible of the solar system,” says Batygin. "We can only talk about these events in very general terms."
If one of the inhabitants of the solar system is indeed expelled from its borders, he is in good company. In recent years, astronomers have found several rogue planets drifting between the stars, which, most likely, were also thrown out of their homes. Projecting the results of this discovery onto the rest of the galaxy, “there are far more free-flying planets the size of Jupiter than stars,” Nesvorni says.
This may be an exaggeration - according to recent estimates, there is only one Jupiter-like planet for every four stars - but it's still billions of roaming worlds. And these are only those that are comparable in size to Jupiter. Our outcast was probably smaller - about the size of Neptune; and we have no idea how many such bodies are roaming the galaxy. But we know that the Universe tends to favor small bodies than large ones.
“I bet there are a lot of them,” says Nesvorni. Among other things, astronomers have discovered thousands of star systems in the Milky Way, and many of them show signs of collisions on a much larger scale than the one discussed above. "It's amazing," Nesvorni says, "how orderly the solar system has remained."
Christopher Crockett