Astronomers have found a remarkable solar system, a system of planets orbiting a nearby star. For one thing, there are at least six planets found there. On the other hand, the five outer planets orbit the star synchronously, moving like dancers to the tune of gravity.
The star is called TOI-178 and is about 200 light-years from Earth. TOI stands for TESS Object of Interest, a star with candidate planets detected by the Transiting Exoplanet Survey Satellite.
TESS looks for periodic and regular falls in the light of the stars that indicate that we see that the planet passes directly in front of its star, making a mini-eclipse, what we call traffic. This only happens when we see the orbit on the edge. But from here you can find the period (the “year” of the planet) and the size of the planet: a larger planet blocks more light.
When astronomers analyzed TOI-178’s TESS observations, they found that there were six planets orbiting the star, and the five outer planets have periods that are simple multiples of each other.
The planets are called TOI-178b to TOI-178g (the first planet discovered is named after the star plus a lowercase b). The periods of the planets, in order of departure of the star and in terrestrial days, are b = 1.91, c = 3.24, d = 6.56, e = 9.96, f = 15.23 and g = 20.71.
Take a look at these numbers: planet d takes about twice as long to orbit the star as planet c does, so c revolves around the star twice in the time it takes d to if once. The period of the planet e is three times that of c, so that c makes three turns each time e makes one turn. Planet f makes two turns for every three times the planet ei, finally, planet g makes three turns for every four times that planet f does.
When a planet has a period that is a simple multiple (a number that can be expressed as a fraction with two integers, such as 2/3 or 5/4), we say that they are resonance. In this case, it is a resonance chain, with all five outer planets moving in simple multiple periods.
We know some systems like this; TOI-178 brings the number to 5. In a sense, they occur naturally and easily. Planets form from a disk of gas and dust around the star, and as they interact with that disk, their orbits change. They tend to slowly approach the star. But as this happens, they can move to a resonance pattern and their gravitational interactions tend to reinforce that pattern. If a planet moves a little too fast, the outer planet makes it recede a little and vice versa.
On the other hand, when you have five planets in a chain like this, it can be a delicate thing; if a planet is off even a little, it can throw all the dance and the periods of the planets will change, interrupting the resonance. This tells us something about how they formed: it must have been a relatively smooth process that allowed them to settle in these orbits. If there had been another large planet pulling on them, it would have altered the chain. The star is about 7 billion years old, so this system has been stable for a long time.
I will observe that these planets are very close to their star, which is what we call a K-type star, smaller and cooler than the Sun. Still, they are very close and cooked.
The transits also tell us the sizes of the planets: in order of the star, the size of the planets with respect to the Earth is b = 1.18, c = 1.71, d = 2.64, e = 2.17, f = 2.38, g = 2.91. They are all larger than Earth, but smaller than Neptune, so we call them super-Earths at the bottom and mini-Neptunes at the top. But they are all mixed up. In our solar system, the smaller planets orbit closer and the giants farther away. This is not the case here.
Odd But there is more. Astronomers tracked the discovery with other telescopes to measure the star’s reflex speed, which tells us the mass of the planets (as they orbit the star they drag, rotating it in a complex pattern; the hardest planet he pulls).
If you calculate the density of the planets (the mass divided by volume), it will mix even more. In terms of density of the Earth (about 5.5 grams per cubic centimeter, or 5.5 times denser than water), in order, the planets of TOI-178 are b = 0.91, c = 0, 9, d = 0.15, e = 0.39, f = 0.58, g = 0.19. Therefore, the two interiors are slightly less dense than Earth, but d is much smaller, with e being much denser than d, if even denser, and then g is much lower. They are everywhere!
Density is important because it tells you what kind of planet it is. Gas giants have densities of up to about 0.2 Earths, and rocky / metallic planets closer to 1. Here we see that they mix in their order from the star, unlike our own solar system. . This is difficult to explain and it explains it to us something important about how these planets formed. We still don’t know exactly what.
I am delighted to find all these systems so different from ours. At first I was going to call them “weird,” but I wonder. If this is only 200 light-years away, it implies that systems like this are common; it looks like the odds would be very long if they were incredibly rare.
May be we are the strange system. I think it would be delicious too. It may seem normal to us, as we are used to it and we base our opinion on it.
If there is any moral lesson, perhaps we should listen more to the Universe.