Astronomers have found another weird exoplanet, and this one is weird even given how weirdly weird most of them are.

The host star is called TOI 1408 (TOI = Target of Interest, a catalog of stars that have planet candidates, in this case ones observed by TESS, the Transiting Exoplanet Survey Satellite). It’s slightly hotter and more massive than the Sun (an F8 star if you’re curious) about 450 light-years from us. In 2023 astronomers found a gas giant orbiting it pretty close in; it takes just 4.4 days to circle the star once. That planet, called TOI-1408b, is about 1.9 times wider than Jupiter and has a little over twice its mass.

It was found using the transit method, when the planet passes directly in front of the star and causes a mini-eclipse, dimming the star for a short time. A lot can be gleaned from that, including the planet’s size and orbit.

The new planet is called TOI-1408c. It was found using what’s called the Transit Timing Variation method. If there are two planets orbiting a star, the gravity of one can tug on the other when one laps the other in their orbits. This can speed up or slow down the planets, sometimes enough to be measured as a change in the timing of their transits. At least 30 planets have been found this way.

Usually the amount of variation is small, but in this case it’s not at all subtle. 1408c takes about 2.2 days to orbit the star, so it’s closer to the star than the gas giant. It’s a mini-Neptune, about 2.2 times Earth’s diameter and 7.6 times our mass. Still, that’s a lot less than the outer giant so it gets tugged hard when it passes. Not only that but the two planets are in a near-resonance, meaning their orbital periods (their years) are very nearly a simple fraction of each other, in this case 2:1. That means the tugging is extra hard because they line up so often.

The astronomers find the variation in the inner planet’s period can be as much as 8 hours — 15% of its orbital period! To scale, that would be like the Earth sometimes taking 11 months to orbit the Sun, and other times 13. Imagine how screwed up the calendar would be! Not to mention the weather. Yikes.

The outer gas giant exhibits these variations as well, but they’re much smaller, only about 8 minutes, because the gravity of the smaller planet is of course weaker so it doesn’t pull as hard on the much more massive planet. 

I’ll note the same thing does happen in our solar system, because we have lots of planets that pull on each other. However the changes are pretty small. You’d never notice them on your own, but astronomers are pretty picky about such things (they do affect a lot of our observations on a noticeable scale, so we have a right to be fussy). Also, over decades, it makes the exact positions of planets difficult to predict, which is why we’re constantly updating our formulas on orbital characteristics and predictions. That affects spacecraft, and engineers get testy when they don’t know where their targets are to sufficient accuracy.

So yeah, TOI-1408c is pretty peculiar. It’s also a rare beast; a smaller planet known to orbit closer in than a giant one, something like our own solar system. There must be lots more like it, but as usual I’ll point out we’re just getting started finding alien worlds at all; only ~5,750 have been discovered as I write this. But as those numbers increase we’ll see more rarities (that’s how numbers work, after all). I wonder what other surprises are waiting?

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