Astronomers have found a remarkable solar system, a system of planets orbiting a nearby star. First, there are at least six planets found there. On the other hand, the five outer planets orbit the star in sync, moving like dancers in the tone of gravity!
The star is called TOI-178 and is a hair over 200 light-years from Earth. TOI stands for TESS Object of Interest, a star with candidate planets detected by the Transiting Exoplanet Survey Satellite (making TOI an abbreviation with an embedded acronym; this is not important, but for some reason they break me).
TESS looks for regular and regular dives in the light of the stars, indicating that we see that the planet passes directly in front of its star, making a mini-eclipse, which we call a transit. This only happens when we see the edge orbit. But from this one can find the period (“year” of the planet) and the size of the planet – a larger planet blocks more light.
When astronomers analyzed the TESS observations of TOI-178, they discovered that there are six planets orbiting the star, and the five outer planets all have periods that are simple multiples of each other!
The planets are named TOI-178b to TOI-178g (the first planet discovered is named after the star, plus a lower case letter b). The periods of the planets, in order from the star and in the days of the Earth, 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 needs to be almost twice as long to orbit the star as planet c, so c surrounds the star twice as long as d is needed to orbit it once. The period of the planet e is three times longer than that of c, so that it spins three times for each time e spins once. Planet f rotates twice every three times that planet e does, and finally, planet g spins 3 times 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 in resonance. In this case, it is a resonant chain, with all five outer planets moving in simple multiple periods.
We know of several such systems; TOI-178 brings the number to 5. In a sense, they appear naturally and easily. The planets form from a disk of gas and dust around the star, and as they interact with that disk their orbit changes. They tend to slowly approach the star. But as this happens, they can move into a resonance pattern, and their gravitational interactions tend to strengthen that pattern. If a planet moves a little too fast, the outside planet pulls it back a little and vice versa.
On the other hand, when you have five planets in a chain like this, it can be a tricky thing; if a planet is stopped even a little, it can throw the whole dance, and the periods of the planets will change, disturbing the resonance. This tells us something about how they formed: it must have been a relatively gentle process, allowing them to sit in these orbits. If there was another big planet throwing at them, it would disrupt the chain. The star is about 7 billion years old, so this system has been stable for a very long time.
I will notice that these planets are quite close to their star, which we call a K-type star, smaller and colder than the Sun. However, they are very close and all cooked for it.
Transit also tells us the dimensions of the planets: in the order of the star, the size of the planets relative to 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 lower end and mini-Neptunes at the higher end. But they are all mixed up. In our solar system, the smaller planets orbit closer and the giants farther. This is not the case here.
Strange. But there are many more. Astronomers have tracked the discovery with other telescopes to measure the star’s reflex speed, which tells us how massive the planets are (as they orbit the star they are shooting, making it orbit in a complex pattern; stronger).
If you calculate the density of the planets (mass divided by volume), it is even more mixed. Regarding the density of the Earth (approximately 5.5 grams per cubic centimeter, or 5.5 times denser than water), in order, the planets TOI-178 are b = 0.91, c = 0.9, d = 0.15, e = 0.39, f = 0.58, g = 0.19. So the two interiors are a little less dense than Earth, but d is much less, with e being much denser than d and f even denser, and then g is much lower. I’m everywhere!
Density is important because it tells you what kind of planet it is. The gas giants have densities of up to 0.2 Earth or so, and the rocky / metallic planets are closer to 1. Here we see that they are mixed in their star order, completely different from our solar system. It’s hard to explain and it tells us that something important about how these planets formed. We just don’t know exactly what.
I’m glad we find all these systems so different from ours. I was going to call them “weird” first, but I wonder. If it is only 200 light-years away, it means that such systems are common; it seems that the long chances would be so close if they were incredibly rare.
May be WE strange system. I think that would be lovely too. Maybe we seem normal, because we are used to it and we base our opinion on it.
If there is a moral lesson there, why, maybe we should listen more to the Universe.