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Astronomers have discovered a remarkable solar system, a system of planets orbiting a nearby star. For one thing, there are at least six planets found there. For another, the five outer planets revolve around the star in sync, moving like dancers to the sound of gravity!
The star is called TOI-178, and it is a hairline 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 acronym built in; it doesn’t matter but for some reason it cracks me up).
TESS looks for regular, periodic dips in starlight, indicating that we are seeing this planet pass directly in front of its star, making a mini-eclipse, which we call a transit. This only happens when we see the orbit on the edge. But from there, we can find the time period (the ‘year’ of the planet) and the size of the planet – a bigger planet blocks more light.
When astronomers analyzed the TESS observations from TOI-178, they discovered that there were six planets orbiting the star, and the five outer planets all have periods that are simple multiples of each other!
The planets are called TOI-178b to TOI-178g (the first planet discovered receives the name of the star plus a lower case b). The periods of the planets, in order from 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: it takes almost twice as long for planet d to orbit the star than planet c, so c circles the star twice in the time it takes to Once. The period of planet e is three times that of c, so c turns around three times for each time e circles once. Planet f spins about twice for three times planet e and, finally, planet g spins around three times for four times planet f.
When a planet has a period that is a simple multiple (a number that can be expressed as a fraction with two integers, like 2/3 or 5/4), we say they are in resonance. In this case, it is a resonance chain, with all five outer planets moving in several single periods.
We know a few systems like this; TOI-178 brings the number to 5. In a sense, they happen naturally and easily. Planets form from a disk of gas and dust around the star, and as they interact with this disk, their orbits change. They tend to slowly approach the star. But when that happens, they can enter a resonance pattern, and their gravitational interactions tend to reinforce that pattern. If a planet moves a little too fast with the planet outside, it 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 shifted even slightly, it can interrupt all the dancing and the periods of the planets will change, disrupting the resonance. This tells us something about how they formed: it must have been a relatively smooth process, allowing them to settle into those sockets. If there had been another large planet shooting at it, it would have disrupted the chain. The star is about 7 billion years old, so this system has been stable for a very long time.
I will note that these planets are quite close to their star, which we call a K-type star, smaller and cooler than the Sun. Yet they are very close and all cooked by him.
The transits also tell us the size of the planets: in the order of the star, the size of the planets in relation to the Earth is b = 1.18, c = 1.71, d = 2.64, e = 2 , 17, f = 2.38, g = 2.91. They’re all bigger than Earth, but smaller than Neptune, so we call them super-Earths on the low end and mini-Neptunes on the bigger one. But they are all mixed up. In our solar system, the smaller planets revolve around us and the giants further away. That is not the case here.
Odd. But there is more. Astronomers continued the discovery with other telescopes to measure the star’s reflex speed, which tells us how massive the planets are (when they revolve around the star they shoot at, making it spin in a complex pattern; the more massive the planet the more it pulls).
If you calculate the density of the planets (mass divided by volume), it’s even more mixed up. In terms of Earth’s density (about 5.5 grams per cubic centimeter, which is 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. So the two interiors are a little less dense than the Earth, but d is much less, with e being much denser than d, and f even denser, and then g is much lower. They are everywhere!
Density is important because it tells you what type of planet it is. Gas giants have densities up to around 0.2 Earth, and rocky / metallic planets closer to 1. Here we see that they are mixed in their order relative to the star, unlike our own. solar system. It’s hard to explain and tells us Something important about how these planets were formed. We don’t know exactly what yet.
I’m glad we find all of these systems so different from ours. I was going to call them “weird” first, but I wonder. If this is only 200 light years away, that implies that systems like this are common; it seems like long chances would be so close if they were incredibly rare.
May be were the strange system. I think it would also be delicious. Maybe we just seem normal because we are what we’re used to and that’s what we base our opinion on.
If there is a moral lesson here, why, perhaps we should listen more to the Universe.
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