The solar system follows the galactic standard, but it is a rare race

Researchers at the Niels Bohr Institute, University of Copenhagen, have studied more than 1000 planetary systems orbiting stars in our galaxy, the Milky Way, and have discovered a number of connections between planetary orbits, number of planets, occurrence and distance from their stars. It turns out that our solar system is in some ways very rare and in others very ordinary.

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

The eccentric orbits of the planets are the key to determining the number of planets

There is a very clear correlation between the eccentricity of orbits and the number of planets in a given solar system. When planets form, they begin in circular orbits in a cloud of gas and dust. But they are still relatively small in size, up to comparable in size to the moon. On a slightly longer time scale they interact through gravitation and acquire increasingly eccentric or elliptical orbits. This means that they start to collide because the elliptical orbits cross and then the planets grow in size due to the collisions. If the end result of the collisions is that all the pieces become only one or a few planets, they remain in elliptical orbits. But if they end up becoming many planets, the gravitational attraction between them causes them to lose energy and thus form increasingly circular orbits.

The researchers found a very clear correlation between the number of planets and the circularity of the orbits. ‘Actually, this is not a real surprise,’ explains Professor Uffe Gråe Jørgensen. “But our solar system is unique in the sense that no other solar system with as many planets as ours is known. So perhaps our solar system might not be expected to fit into the correlation. But it does, as a matter of fact, that’s it “.

The only solar systems that do not fall under this rule are systems with a single planet. In some cases, the reason is that in these single-planet systems, the planet orbits the star very close, but in others, the reason is that the systems may actually contain more planets than initially assumed. “In these cases, we believe that deviating from the rule can help us reveal more planets that have been hidden until now,” explains Nanna Bach-Møller, first author of the scientific paper. 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. “This would be a very important tool for detecting planetary systems such as our solar system, because many exoplanets similar to the 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 basic universal physics is at stake. Researchers can use this to say: how many systems have the same eccentricity as our solar system? – which we can then use to evaluate how many systems have the same number of planets as our solar system. The answer is that there are 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, this is, however, still no less than a billion solar systems. There are about 10 billion Earth-like planets in the habitable zone, i.e. 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, Uffe Gråe Jørgensen points out. “Something is causing there aren’t a huge amount of UFOs out there. When the conquest of planets in a solar system has begun, it happens pretty quickly. We can see it in our own civilization. We’ve been to the moon and Mars. we already have several robots. But there aren’t many UFOs from the billions of Earth-like exoplanets in the habitable zones of the stars, so life and technological civilizations in particular are probably still quite scarce. “

Earth isn’t particularly special – it’s about the number of planets in the system

What else does it take to host life than to be an Earth-sized planet in a habitable zone? What is truly special here on Earth and in our solar system? Earth isn’t special – there are many 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 all. Could it be that the existence of the great gas planets is the cause of our existence here on Earth? Part of that debate involves the question of whether the great gas planets, Saturn and Jupiter, redirected the water-carrying comets to Earth when the planet was half a billion years old, allowing life to form here.

This is the first time that a study has shown how unique it is for a solar system to host eight planets, but at the same time it shows that our solar system is not entirely unique. Our solar system follows the same physical rules for planet formation as any other solar system, we just happen to find ourselves at the unusual end of the scale. And we still have the question of why, exactly, we are here to do this.