Astronomers have discovered a planetary system around a nearby star, and it’s pretty danged cool: Six (SIX!) super-Earths/sub-Neptunes all crammed in close to the star, the farthest only about 40 million kilometers out! Even cooler: they all orbit in resonances. [link to research paper]

The star is HD 110067, which is a K0-type (orange) star a little smaller and cooler than the Sun, and just over 100 light-years away from us (interestingly, it’s actually a trinary star; HD 110067 is orbited by a binary star about 2 trillion km away, a fifth of a light-year!). The planets were initially discovered in observations from the Transiting Exoplanet Survey Satellite (TESS) via the transit method; we see the orbits edge-on, and when they pass in front of the star its brightness dips due to the mini-eclipse.

Two planets — HD 110067b and c —were found this way. There were several other dips seen, but they didn’t appear to be cyclic, as you’d expect from planets with fixed orbital periods (their “years”). Observations from CHaracterising ExOPlanets Satellite (CHEOPS) were used to find a third planet (d).

Here’s where it gets fun. The three planets have periods of 9.114, 13.673, and 20.519. Look at the ratios: the second to the first is almost exactly 1.5, and the third to the second is also 1.5. That means the inner planet orbits three times for every two times the second one does, and the third one twice for every three times the second one does.

That’s called a resonance, and it’s not uncommon. As planets interact gravitationally, they pull and tug each other, changing their orbits. If they fall into simple integer ratios, like 2 to 1, or 3 to 2, then an amazing thing happens: If one of the planets moves out of that resonance, the other planet will prevent it! If the first one starts to move inward toward the star and speed up a bit, the second planet will pull it forwards as it passes and backwards once it’s past, which works to keep it at the resonance (and the same for the first planet acting on the second).

Two planets can be in resonance, but if there are more than it’s called a resonance chain. Three of Jupiter’s big moons orbit this way, for example, and so do the three inner HD 110067 planets. 

Thing is, if three are in a chain, maybe there are more…? So the astronomers searched the data and found three more planets, and all of them are in resonance too! The second planet is 3:2 with the first, the third 3:2 with the second, the fourth 3:2 with the third, the fifth 4:3 with the fourth, and the sixth 4:3 with the fifth. That’s a long chain!

If you read my book Under Alien Skies, you may remember the seven planets orbiting TRAPPIST-1 are all in such a chain, too. It’s likely very common for planets that orbit their host star closely; they affect each other strongly since they’re close together, and once you lock a resonance in it tends to remain.

It’s useful, too. Planets likely fall into resonance early on after they form, when migration is common. That means the system has been this way for a long time; the star is around 8 billion years old (though with a large 4-billion-year uncertainty; for comparison the Sun is 4.6 billion), which also implies that nothing has come along to ruin this situation for a long time too (like a passing star messing with the orbits). Studying this system will yield insight in how these chains form and maintain themselves.

This video shows the planets’ motions to scale:

The planets are cool, too. They’re all more massive than Earth, ranging from ~4-8 times our planets heft. They’re bigger, too, all between 2-3 times Earth’s diameter. Three of the planets that have well-determined characteristics have low density, implying they’re gas giants, or at least have atmospheres dominated by hydrogen (which implies a thick atmosphere, since hydrogen is a light element and escapes easily, so the planets likely retain lots of other gases as well). They’re also hot; even the one farthest out has a likely temperature around 170°C, well above the boiling point of water.

I find all this enthralling. We’re finding a lot of systems that are way different than ours, with entire fleets of planets orbiting closer to their star than Mercury orbits the Sun — like this one is. What happened in the solar system all those eons ago that prevented us from going the same way? The current thinking is the complicated gravitational interactions of Jupiter and Saturn prevented all the planets from dropping down toward the Sun. But (I’m speculating) if all the planets are the same mass this may not happen. The TRAPPIST-1 planets are all around Earth’s mass, too. Hmmm.

Anyway, finding systems so different than ours really helps us put our own in perspective, and can help us understand how planets form in the first place, and how the they evolve over time. Mind you, we’re still new at this. HD 110067 is the brightest star known to have more than four planets, and it’s only 100 light-years away! Imagine how many more systems like this there must be in the galaxy. Billions. Ours may be the exception. We just don’t know yet 

But we will. We keep finding them, and the more we find the weirder ones we’ll find as well (pretty much by definition; a bigger pool means the more likely you’ll find the outliers). Is the solar system weird, or is our family more the rule?

Stay tuned. 

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!