Ask Ethan: Can two planets exist in the same orbit?
Despite the dangers that may be a random asteroid or a comet, our solar system is a surprisingly stable place, where all eight planets remain in their orbits throughout the life of the sun. But are all solar systems like ours? After viewing the questions for the weekly column, I chose this wonderful question from Dee Hurley:
This is a very good question, and in our solar system there are several keys to answering it.
According to the International Union of Astronomers (IAU), a body in orbit requires three things to get planet status:
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1. To be in hydrostatic equilibrium, that is, to have gravity, sufficient to achieve a spherical shape (given the rotation that distorts this form).
2. To orbit around the sun, and not around another body.
3. To clear its orbit from planetesimals, from competitors.
Strictly speaking, the third point excludes the possibility of two planets in orbit.
But let's not bother about clear definitions. It is better to estimate whether two earth-like planets can be in the same orbit around their star. The main problem will be gravity, which can destroy a double orbit in one of two ways: either gravitational interaction “hits” very strongly on one of the planets, which as a result will fall into the sun or fly out of the system, or mutual attraction will cause the planets to unite in an impressive collision.
The last version just happened to the Earth, when the solar system was only a few tens of millions of years. Because of the collision, the Moon appeared, and most likely, the Earth's surface has changed.
It is not very convenient for two planets to be in the same orbit, since in such cases true stability does not happen. One can hope for a quasi-stable orbit that can exist for billions of years before one of the two incidents occurs. To describe this, I want to introduce the concept of Lagrange points.
If we consider only two masses - the Sun and one planet - there are five points known as Lagrange points, around which the gravitational effects of the Sun and the planets are mutually destroyed, and the small mass body in them can always move with the first two in orbit. Unfortunately, only points L4 and L5 are stable; everything that starts from L1, L2 or L3 points, as a result, either collides with the planet or is thrown out of the system.
But asteroids accumulate around points L4 and L5. The gas giants have thousands of them, but the Earth has one: the asteroid 3753 Cruithne , which is now in a quasi-stable orbit with our world!
And although this asteroid is unstable on a time scale of billions of years, this is how two planets can exist in the same orbit. It is also possible the existence of a binary planet, something like the Earth / Moon system (or Pluto / Charon), in which there is no clear separation between the planet and the moon. If two planets in the system have comparable masses and sizes and are separated by a small distance, this system can be called binary, or double planetary. Recent studies suggest the possibility of the existence of such systems .
But there is another way that may seem unstable to you: two planets in different orbits, one inside the other, periodically changing places. You may decide that this is insane, but in the Solar System there is an example of such a case: the two moons of Saturn, Epimetheus and Janus .
Every four years, the moon, which is closer to Saturn, defeats the outer, and their gravitational pull causes the inner moon to go to the outer orbit, and the outer to the inner one.
Over the past 25 years, we have seen their dance many times, and, as far as we can tell, this configuration is stable during the existence of the solar system. That is, somewhere in our galaxy there may well exist a planetary system in which two planets, and not moons, behave that way!
The bad news is that of the thousands of planets discovered by us, we still do not know a single binary system. News about the opening of such a system, flying several years ago, was withdrawn . Of course, our technology is still not good enough to discover the moons around exoplanets, and we expect them to be there.
In fact, such cases of sharing orbits should be rare, but not so rare that we never come across them. Give us a telescope to search for better planets, a million stars and ten years, and I can argue that we will find examples of all three cases of the existence of two planets in one orbit. The laws of gravity and our simulations state that such cases must be realized. It remains only to find them.
Can there be a solar system in which two planets are located on the same orbit?
This is a very good question, and in our solar system there are several keys to answering it.
According to the International Union of Astronomers (IAU), a body in orbit requires three things to get planet status:
')
1. To be in hydrostatic equilibrium, that is, to have gravity, sufficient to achieve a spherical shape (given the rotation that distorts this form).
2. To orbit around the sun, and not around another body.
3. To clear its orbit from planetesimals, from competitors.
Strictly speaking, the third point excludes the possibility of two planets in orbit.
But let's not bother about clear definitions. It is better to estimate whether two earth-like planets can be in the same orbit around their star. The main problem will be gravity, which can destroy a double orbit in one of two ways: either gravitational interaction “hits” very strongly on one of the planets, which as a result will fall into the sun or fly out of the system, or mutual attraction will cause the planets to unite in an impressive collision.
The last version just happened to the Earth, when the solar system was only a few tens of millions of years. Because of the collision, the Moon appeared, and most likely, the Earth's surface has changed.
It is not very convenient for two planets to be in the same orbit, since in such cases true stability does not happen. One can hope for a quasi-stable orbit that can exist for billions of years before one of the two incidents occurs. To describe this, I want to introduce the concept of Lagrange points.
If we consider only two masses - the Sun and one planet - there are five points known as Lagrange points, around which the gravitational effects of the Sun and the planets are mutually destroyed, and the small mass body in them can always move with the first two in orbit. Unfortunately, only points L4 and L5 are stable; everything that starts from L1, L2 or L3 points, as a result, either collides with the planet or is thrown out of the system.
But asteroids accumulate around points L4 and L5. The gas giants have thousands of them, but the Earth has one: the asteroid 3753 Cruithne , which is now in a quasi-stable orbit with our world!
And although this asteroid is unstable on a time scale of billions of years, this is how two planets can exist in the same orbit. It is also possible the existence of a binary planet, something like the Earth / Moon system (or Pluto / Charon), in which there is no clear separation between the planet and the moon. If two planets in the system have comparable masses and sizes and are separated by a small distance, this system can be called binary, or double planetary. Recent studies suggest the possibility of the existence of such systems .
But there is another way that may seem unstable to you: two planets in different orbits, one inside the other, periodically changing places. You may decide that this is insane, but in the Solar System there is an example of such a case: the two moons of Saturn, Epimetheus and Janus .
Every four years, the moon, which is closer to Saturn, defeats the outer, and their gravitational pull causes the inner moon to go to the outer orbit, and the outer to the inner one.
Over the past 25 years, we have seen their dance many times, and, as far as we can tell, this configuration is stable during the existence of the solar system. That is, somewhere in our galaxy there may well exist a planetary system in which two planets, and not moons, behave that way!
The bad news is that of the thousands of planets discovered by us, we still do not know a single binary system. News about the opening of such a system, flying several years ago, was withdrawn . Of course, our technology is still not good enough to discover the moons around exoplanets, and we expect them to be there.
In fact, such cases of sharing orbits should be rare, but not so rare that we never come across them. Give us a telescope to search for better planets, a million stars and ten years, and I can argue that we will find examples of all three cases of the existence of two planets in one orbit. The laws of gravity and our simulations state that such cases must be realized. It remains only to find them.
Source: https://habr.com/ru/post/400411/
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