This month, new evidence was presented for the existence of a celestial body predicted in 2016 by Konstantin Batygin and Michael Brown. "Attic" briefly tells about the latest news of the race of astronomers for the right to inscribe their name in the centuries-old history of the inventory of our planetary system.
"There are eight planets in the solar system" - this statement may no longer be true in a few years. Astronomers are gaining more and more circumstantial evidence for the existence of a ninth planet far beyond the orbit of Neptune.
The hypothesis of the existence of another planet in the solar system has been repeatedly proposed since the discovery of Uranus in 1781. In 1846, Neptune was discovered, and in 1930 the presence of Pluto (in the status of a planet until 2006, now a dwarf planet) was confirmed, and both times scientists identified a celestial body by its effect on the orbits of already known planets. All subsequent time, searches for various kinds of anomalies in the motion of planets and asteroids were carried out quite actively, but by the end of the 20th century, interest in the "X-planet" subsided.
In the 1990s, the model of the solar system was supplemented by the Kuiper belt, coupled with a scattered disk beyond the orbit of Neptune. Terrestrial planets, the asteroid belt, gas giants, the Kuiper belt and, possibly, the even more extensive and tenuous Oort cloud - in this model, as many began to believe, there was no place for any other planets.
Close and invisible
In 2016, American astronomers Konstantin Batygin and Michael Brown put forward a hypothesis that there is another, ninth planet behind the Kuiper belt. Their assumption was based on the analysis of several particularly distant orbits of objects in the Kuiper belt, such as Sedna, for example, which for some reason move across the sky in the same plane and in one direction. After many months of modeling and checking the data with the actual ones, astronomers have come to a surprising conclusion even for themselves: very far beyond Neptune there is another celestial body with a mass of about ten Earths and not approaching the Sun closer than 280 astronomical units. And it is this that stretches and straightens the orbits of these "strange" Kuiper belt bodies.
Diagram showing the orbit of Planet Nine (orange) and the orbits of some of the known trans-Neptunian objects (pink). Illustration: MagentaGreen / Wikimedia.
Promotional video:
In their article, Batygin and Brown noted that finding the ninth planet would not be the easiest task. Due to the large distance to this hypothetical object, it should be so dim that it can only be seen through a telescope with a mirror diameter of several meters - this corresponds to the level of a decent observatory, which is usually loaded with other tasks. The search for a giant planet in the outskirts of the solar system turns out to be technically more difficult than detecting exoplanets many tens of light years from Earth, but besides direct observations, scientists also have indirect methods.
One of them is the search for new trans-Neptunian objects and the comparison of their orbits with the predictions of the Batygin-Brown model. Astronomers argue that the gravitational influence of the ninth planet not only sends some Kuiper belt bodies on a long voyage around the Sun, but also leads to unusually large inclinations of the orbits of a number of other objects. Sometimes so that they begin to rotate perpendicular to the ecliptic of the rest of the planets of our system.
For example, the 2015 BP519 object, aka "Cashew", described in a recently published article by an international group of astronomers, just fits into the Batygin-Brown model. It has a very high orbital inclination, which, however, does not yet allow us to confidently say that the ninth planet really exists. The authors of this discovery cautiously write about "adding indirect evidence in favor of a new planet", and Batygin and Brown shortly before that presented a number of refinements to the previously stated hypothesis: new modeling of various scenarios for the evolution of the Kuiper belt showed that the influence of the ninth planet leads to the appearance of many trans-Neptunian objects with very elongated orbits - and this is in good agreement with observations.
Another diagram of the orbits of the ninth planet (green circle labeled P9) and many of the extremely elongated orbits of trans-Neptunian objects. The elongated blue circle - Cashew orbit. Each square in the background - 100 astronomical units. Image: Tomruen / wikimedia commons.
According to Konstantin Batygin, "the newly discovered object, 2015 BP519, is exactly where the theoretical model of the ninth planet predicts it." In a commentary to Attic, he noted that “this is a fantastic confirmation of the picture that we expected to see on the basis of numerical modeling,” however, it is still too early to talk about the final discovery of a new planet. The list of evidence of its existence is growing literally before our eyes, but only a couple of photographs with a moving object marked on them will put an end to this issue. Batygin and Brown have already obtained observational time with the Subaru large ground-based telescope, which, according to Batygin, is one of the best instruments for finding the ninth planet. In addition, attempts are being made to use images from the WISE space telescope,and since 2017, the Backyard Worlds: Planet 9 project has been working, where everyone can try to find this celestial body in the pictures, so it may not be long to wait.
So what?
The relative absence of constant collisions of the Earth with asteroids over the past billions of years can be attributed to the gas giants. They, going into their present orbits, "cleaned" our sector of the planetary system from various small (according to astronomical criteria) debris. But if Jupiter or even Neptune really influenced the Earth at least by getting rid of regular planetary catastrophes, then what about a body ten times more distant?
Russian astronomer Vladimir Surdin in his commentary to Attic noted that the discovery of each new planet affects our understanding of the fate of the solar system, which remains vague to this day. "In fact, research is just beginning," the scientist said and added that "on the periphery of the solar system, in the darkness, God knows what." Those bodies that replenish the catalogs of astronomers by hundreds are found at a relatively small distance from the Sun, but even a giant planet behind the Kuiper belt has every chance of hiding from observers for a very long time and giving itself out only by indirect gravitational effects.
Scheme: Anatoly Lapushko / Chrdk.
Outwardly, the ninth planet, if it exists, should be similar to the two gas giants farthest from the Sun. “A planet with a super-Earth mass would be similar to Uranus and Neptune, but even colder,” Surdin says. These two celestial bodies are sometimes called "ice giants" because of the supposed presence of a rocky-ice core without the layer of metallic hydrogen expected from Jupiter and Saturn. However, only one spacecraft, Voyager 2, has visited Uranus and Neptune in the entire history of mankind, so scientists have less observational data than they would like.
The ninth planet, even at perihelion, will be practically inaccessible for research probes with rocket engines. Voyagers moved away from the Sun at 117 and 140 AU. - despite the fact that they were launched in 1977. Flying even to a point of 200 AU. it will take at least half a century from our star, and reducing this period to some reasonable limits will obviously require fundamentally new technologies like a solar sail. Even combining a nuclear reactor with ion engines in a configuration roughly reminiscent of Russia's megawatt-class nuclear facility will not make it possible to reach the target in less than a decade. And when the planet is in aphelion, this time increases significantly.
Uranus and Neptune, NASA images. As you can see, the ice giants are quite different in appearance: for example, a small admixture of methane (about 1%) makes Neptune much more blue.
Direct detection of the ninth planet will confirm the correctness of Batygin and Brown, will make it possible to clarify the history of the solar system, but this celestial body itself, even with the introduction of a new generation of telescopes, will hardly remain more than a dot in the photographs. The ninth planet "in the backyard" of the solar system is paradoxically more difficult to study than any hot Jupiters near other stars, but it will provide a better understanding of the behavior of those objects that have long been known.
From paper to computers
Neptune was the first planet discovered "at the tip of a feather" - based on calculations and analysis of the motion of Uranus, which moved at a variable speed due to external attraction. However, the greater the distance between celestial bodies and the greater the number of these bodies, the more difficult it is to calculate their trajectory. Physicists and mathematicians know that the problem of rotating two bodies around a common center of mass is relatively easy to solve and has an answer in the form of an equation with an exact description of the orbit, but a combination of three bodies is much more difficult to calculate. In particular, a system of three or more bodies does not have an analytical solution, that is, it is impossible to obtain a formula describing their motion over an arbitrarily long time.
The evolution of the solar system according to the Nice model. Blue shows the orbit of Uranus, blue - Saturn, while orange and green correspond to Saturn with Jupiter. According to this model, Uranus and Neptune changed places and along the way all the giant planets "cleared" the planetary system of small objects. The model has a number of modifications - for example, suggesting the presence of another gas giant, which was completely thrown into interstellar space. Figure: AstroMark / Wikimedia.
Modeling of the solar system is carried out only by approximate methods. With a sufficiently large expenditure of computational resources, it is possible to calculate the movement of the elements of the system with an arbitrarily necessary accuracy, but sometimes negligible deviations from the initial conditions lead to a completely different behavior of the model after a while. This effect is known to the general public as the "butterfly effect". The movement of planets and asteroids, as well as the behavior of air masses, are subject to this effect, so the reconstruction of the history of the solar system is in no way inferior in complexity to a weather forecast for a long time. And attempts to calculate a hypothetical planet are comparable to the task of predicting all the consequences of a hurricane - here you have to face both a lack of accurate information and a lack of computing power.
Before the advent of modern computers, calculating the motion of many thousands of bodies at the same time remained an almost insoluble problem. The appearance of the Nice model, which describes the behavior of gas giants after their formation from a disk of gas and dust, was made possible by computers. The arguments for a ninth planet are also based on calculations that cannot be done with paper and pen. The discovery of the ninth planet, if it takes place, will be not just a repetition of the history of Neptune or Pluto, but a new history that would have been impossible a hundred years ago.
Alexey Tymoshenko
