Astronomers have discovered a remarkable solar system, a system of planets orbiting a nearby star. To begin with, at least six planets have been found. On the other hand, the outer five planets orbit the star in sync, moving like dancers to the tune of gravity!
The star is called TOI-178 and is a hair more than 200 light years from Earth. TOI stands for TESS Object of Interest, a star of candidate planets detected by the Transiting Exoplanet Survey Satellite (making TOI an abbreviation with an embedded acronym; that’s not important, but for some reason they’re killing me).
TESS looks for regular, periodic dips in the starlight, indicating that we see that planet passing just in front of its star and creating a mini-eclipse, making a transit. That only happens when we see the path of the edge. But from that can be found the period (the “year” of the planet) and the size of the planet – a larger planet blocks more light.
When astronomers analyzed the TESS observations from TOI-178, they found that there are six planets orbiting the star, and that the outer five planets all have periods that are simple multiples of each other!
The planets are named TOI-178b through TOI-178g (the first planet discovered is named after the star plus a lowercase letter b). The periods of the planets, in star order and in Earth 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 those numbers: it takes planet d nearly twice as long to orbit the star as planet c, so c orbits twice in the time it takes d to orbit once. The period of planet e is three times that of c, so c goes around three times for each time that e goes around once. Planet f goes around twice for every three times planet e does, and finally planet g goes around three times for every four times planet f does.
If 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 they are in resonance. In this case it is one resonance chain, with all outer five planets moving in simple multiple time periods.
We know a few of these types of systems; TOI-178 brings the number to 5. In a way, they come about 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 get closer to the star slowly. But when that happens, they can get into a resonance pattern, and their gravitational interactions tend to amplify that pattern. If a planet moves a little too fast, the planet outside it will retreat a bit, and vice versa.
On the other hand, if you have five planets in such a chain, it can be a delicate thing; if a planet deviates even a little bit, it can throw off the whole dance, 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 gentle process that allowed them to settle into these orbits. If another major planet had tugged at them, it would have disrupted the chain. The star is about 7 billion years old, so this system has been stable for a long time.
I notice that these planets are quite close to their star, what we call a K-type star, smaller and cooler than the sun. Yet they are very close and all cooked through.
Passages also tell us the dimensions of the planets: in order of the star, the dimensions of the planets relative to the Earth are 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 Earth on the bottom and mini-Neptune on the larger side. But they are all mixed up. In our solar system, the smaller planets orbit closest and the giants farther away. That is not the case here.
Strange. But there’s more. The astronomers followed the discovery with other telescopes to measure the star’s reflex velocity, which tells us how massive the planets are (as they revolve around the star, they pull on it, making it orbit in a complex pattern; the heavier the planet, the harder it pulls).
If you calculate the density of the planets (the mass divided by the volume), it gets mixed up even more. In terms of the density of the Earth (about 5.5 grams per cubic centimeter, or 5.5 times as dense as water), the planets of TOI-178 are in order b = 0.91, c = 0.9, d = 0.15, e = 0.39, f = 0.58, g = 0.19. So the inner two are a little less dense than the Earth, but d is much less, with e being much closer than d, and f even closer, and then g being much lower. They are everywhere!
The density is important because it tells you what kind of planet it is. Gas giants have densities of up to 0.2 Earths or so, and rocky / metallic planets closer to 1. Here we see that they are mixed up in their star order, completely different from our own solar system. That’s hard to explain, and it tells us something important about how these planets were formed. We just don’t know exactly what yet.
I am delighted that we find all of these systems so different from ours. I wanted to call them “weird” at first, but I wonder. If it is only 200 light years away, it means that these types of systems are common; it seems the long odds would be so close if they were incredibly rare.
Could be we are the weird system. I think that would be lovely too. Maybe we just seem normal because we are what we are used to and that is what we base our opinion on.
If there is a moral lesson there why, then maybe we should listen more to the universe.