The solar system follows the galactic standard – but it’s a rare breed

Researchers at the Niels Bohr Institute at the University of Copenhagen studied more than 1,000 planetary systems orbiting stars in our own galaxy, the Milky Way, and discovered a series of connections between planetary orbits, the number of planets, occurrence and distance to their stars. It turns out that our own solar system in some ways is very rare and in others very ordinary.

It’s rare to have eight planets, but study shows that the solar system follows exactly the same, very basic rules for forming planets around a star. The question of what makes it so special that it houses life is always a good question. The study is now published in MNRAS

Eccentric planetary orbits are the key to determining the number of planets

There is a very clear correlation between the eccentricity of the orbits and the number of planets in a given solar system. When planets form, they start in circular orbits in a cloud of gas and dust. But they are still relatively small in size, down to sizes comparable to the moon. On a slightly longer timescale, they interact by gravity and acquire increasingly eccentric or elliptical orbits. This means that they start to collide because the elliptical orbits intersect – and so the planets get bigger due to the collisions. If the end result of collisions is that all the parts become one or a few planets, then they remain in elliptical orbits. But if they end up becoming many planets, the gravitational pull between them causes them to lose energy – and so they form more and more circular orbits.

The researchers found a very clear correlation between the number of planets and the degree of circular orbits. “Actually, it’s not much of a surprise,” explains Prof. Uffe Gråe Jørgensen. “But our solar system is unique in that no other solar system with as many planets as ours is known. So one might expect that our solar system would not fit into the correlation. in fact, it is fair. “

The only solar systems that do not fit into this rule are systems with a single planet. In some cases the reason is that in these single planet systems the planet revolves around the very near nearby star, but in others the reason is that the systems may actually contain more planets than which was initially assumed. “In these cases, we believe that deviating from the rule can help us reveal more planets that were hidden until now,” says Nanna Bach-Møller, first author of the scientific article. If we can see the extent of the eccentricity of the planet’s orbit, then we know how many other planets must be in the system – and vice versa, if we have the number of planets, we now know their orbits. . “It would be a very important tool for detecting planetary systems like our own solar system, as many exoplanets similar to planets in our solar system would be difficult to detect directly, if we don’t know where to look for them.”

Earth is among the lucky 1%

Regardless of the method used in the search for exoplanets, the same result is achieved. So, there is some basic universal physics at play. Researchers can use this to say: How many systems have the same eccentricity as our solar system? – which we can then use to estimate how many systems have the same number of planets as our solar system. The answer is that there is only 1% of all solar systems with the same number of planets as our solar system or more. If there are about 100 billion stars in the Milky Way, it is not less than a billion solar systems. There are approximately 10 billion Earth-like planets in the habitable zone, that is, at a distance from their star that allows liquid water to exist. But there is a huge difference between being in the habitable zone and being habitable or having developed a technological civilization, emphasizes Uffe Gråe Jørgensen. “Something is the cause of the fact that there is not a huge amount of UFOs out there. When the conquest of planets in a solar system has started, it goes pretty fast. We can see it in our own civilization. We have been on the moon and on Mars, we already have several robots. But there are not many UFOs billions of Earth-like exo-planets in the habitable zones of stars, therefore life and technological civilizations in particular are probably still quite rare. “

Earth is not particularly special – the number of planets in the system is what it is

What more does it take to house life than to be an Earth-sized planet in the habitable zone? What is really special here on Earth and in our solar system? Earth is not special – there are a lot of Earth-like planets out there. But maybe it could be the number of planets and their nature. There are many large gas planets in our solar system, half of them. Could it be that the existence of the large gaseous planets is the cause of our existence here on Earth? Part of this debate revolves around whether the large gaseous planets, Saturn and Jupiter, redirected water-containing comets back to Earth when the planet was half a billion years old, thus allowing formation of life here.

This is the first time that a study has shown how unique it is that a solar system is home to eight planets, but at the same time, shows that our solar system is not entirely unique. Our solar system follows the same physical rules for forming planets as any other solar system, we find ourselves just at the unusual end of the scale. And we always end up with the question of why, exactly, we are here to be able to wonder about this.