We are very familiar with the solar system - after all, in fact, this is our home. The names of the planets included in its composition, the order of their arrangement (and maybe even the distance from the Sun) are known to many of us since school. However, as the BBC Earth correspondent found out, our house is not very similar to others.
There are four inner planets closest to the Sun, they are called terrestrial planets (or solid planets). The solid surface allows walking on them or landing spacecraft. There are four outer planets (with the exception of the relatively small rock and ice Pluto, whose planetary status was recently revised to be considered a dwarf planet), they are giant gas balls surrounded by rings. And between the inner and outer planets is the asteroid belt.
Such a slim configuration, right? Actually, for about a century we had nothing but her. But in 1995 the situation changed. 20 years ago, astronomers discovered the first exoplanet - a planet orbiting a star, but not the sun, outside the solar system. It was a gas giant, similar in mass to Jupiter, called 51 Pegasi b.
In the next two decades, thousands of other planets were discovered. According to some estimates, there are hundreds of billions of them in our Galaxy. Thus, the solar system is not unique.
And yet, despite such a large number of planetary systems, astronomers believe that in a sense, the solar system stands alone. How so?
"It is becoming increasingly clear that the solar system is atypical," says Gregory Laughlin, a planetary scientist at the University of California, Santa Cruz.
It is still not entirely clear how great this atypicality is (after all, it is one thing for a punk to wander into the evening of a meeting of veterans of the collective farm movement, and quite another thing for a leprechaun galloping down the street on a unicorn), but scientists are already trying to explain the reasons for the peculiarities of the solar system.
If it turns out to be a cosmological anomaly, then, perhaps, the Earth is also such - and with it life on our planet.
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In other words, our uniqueness in the Universe cannot be ruled out.
Unique system?
One has only to come to terms with the idea that planets in space are found no less often than stars, as a new discovery appears before us - an amazing variety of their parameters. “We've always had the hope that there are many planets in space,” Laughlin says. - And it turned out that this is really so. But the exoplanets we found are strikingly different from the planets of the solar system."
Asteroids have disappeared from the inner regions of the solar system.
With the help of the Kepler orbiting observatory, astronomers have discovered thousands of exoplanets of a wide variety of compositions and sizes. It turns out that there are very miniature planetary systems, comparable in size to Jupiter and four of its largest satellites. In other systems, the plane of rotation of the planets is at a large angle to the plane of rotation of the stars. Some planets revolve around two stars at once - like the planet Tatooine with two suns from the movie "Star Wars".
There are two types of planets in our solar system - small rocky and large gaseous ones. But astronomers have concluded that most exoplanets don't fit into any of these categories. In terms of size, they are most often something in between: smaller than Neptune, but larger than Earth.
The smallest exoplanets discovered can be rocky - they are sometimes called super-earths (not quite the correct term, since a super-earth is not necessarily similar to Earth - it is just a slightly larger planet). The larger exoplanets, known as hot neptune, are mostly made of gases.
Surprisingly, many of these planets are very close to their stars - less than the distance between Mercury and the Sun. In 2009, when astronomers first discovered orbits so close to the star, most scientists were skeptical. “It seemed completely incredible, people just couldn't believe it could happen,” Laughlin says. However, later, with the help of the Kepler observatory, launched in the same year, it was possible to confirm that such a phenomenon does not just exist, but is very common. Apparently, in our Galaxy, super-Earths rotate in orbits close to stars almost half the time.
Jupiter and one of its moons
This, says Laughlin, is one of the most important differences in the solar system: “Inside the orbit of Mercury (between Mercury and the Sun - Ed.) There is nothing at all. Even asteroids."
Another oddity in the solar system is Jupiter. Large exoplanets are not so common, and for the most part they revolve in orbits comparable to those of Earth or Venus. Only about two percent of the stars studied have Jupiter-sized planets in orbits comparable to that of Jupiter.
"The complete absence of any celestial bodies within the orbit of Mercury and massive Jupiter at a significant distance from the Sun are two factors that distinguish the solar system," Laughlin notes.
No one knows exactly why this is so, but Laughlin has one complicated theory - he believes that Jupiter once "wandered" through the solar system, destroying the nascent planets and, ultimately, creating the conditions for the formation of the Earth.
Wandering Jupiter
Planets are born after their stars. A star appears when a gas cloud collapses into a dense ball. A disk is formed from the remnants of gas and dust around it, which then turns into separate planets.
Previously, astronomers believed that the planets of the solar system formed in their current orbits. In the immediate vicinity of the hot young star, gas and ice could not be - the only possible "building materials" in this region should have been silicates and metals, which is why relatively small solid planets were formed there. Far from the Sun, the gas giants that we know today arose from gases and ices.
Hot Jupiters could migrate closer to their stars, and then move away from them again
However, in the process of searching for exoplanets, astronomers found gas giants orbiting extremely close to their stars - and this despite the fact that the temperatures in such orbits would be too high for these planets to arise. Scientists concluded that such hot Jupiters were likely gradually migrating closer to their stars. Moreover, planetary migration can be quite common - it is possible that the gas giants of the solar system have also changed their orbits in the past.
“We used to think that giant planets have been in their current orbits since their inception. That was our foundational tenet,”says Kevin Walsh, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. Now, he said, this postulate no longer exists.
Walsh is a proponent of the Grand Tack hypothesis, named after the zig-zag maneuver in sailing. According to her, Jupiter began to change its orbit in the early period of the history of the solar system, and at first the planet approached the Sun, and then began to move away from the sun - like a yacht.
According to this hypothesis, the original orbit of Jupiter was somewhat narrower than the current one - the planet formed at a distance of about three astronomical units from the Sun (one astronomical unit corresponds to the average distance between the Sun and the Earth). At the time, the solar system was only a few million years old - infancy on the scale of the universe - and it was still filled with gas.
As Jupiter orbits the Sun, gas from the outer side of its orbit pushed the planet closer to the star. When Saturn formed outside Jupiter's orbit, this led to a disturbance in the gas field, and the centripetal motion of Jupiter stopped at a distance of about one and a half astronomical units from the Sun.
Saturn's formation may have stopped Jupiter's migration process
After that, gases from the inner side of its orbit began to exert pressure on Jupiter, pushing the planet into the outer regions of the solar system. Since there was nothing to put pressure on Jupiter from the outer side of the orbit, it drifted into its current orbit at a distance of 5.2 astronomical units from the Sun.
Author: Markus Wu