Artist’s rendition of HD 80606b compared with Jupiter. Credit: Aldaron (Wikipedia).

With all the talk of comets lately, let’s take a look at what happens when planets start acting like them. But surely, no planet could have a crazy, elongated orbit like a comet, right? They’re too big to get kicked around in those weird paths. Well, that’s probably how most astronomers would have reacted up until the mid 1990s. If you’ve been reading this blog for a while, now, you probably know what happened next.

Enter HD 80606b, the planet with the most eccentric orbit known. Eccentricity is a measure of how elongated and oval-shaped a planet’s orbit is. Zero is a perfect circle. One means it’s not coming back. Halley’s Comet, on its long orbit, has an eccentricity of 0.967. HD 80606b can’t quite match Halley’s Comet, but it does have an eccentricity of 0.9336. That means its orbit looks like this:

Schematic of HD 80606b’s orbit. The longest distance is still smaller than Earth’s orbit.

HD 80606b has an orbit that takes it almost as far as Earth is from the Sun, then plunges in to a twentieth of that distance. That means that for just a few hours of its four-month orbit, it turns into a hot jupiter. It could even have a tail, like a comet. The rest of the time, it’s just a warm jupiter.

But how did it get into such a strange orbit? It couldn’t have formed that way. Many exoplanets have somewhat eccentric orbits, but what we know about how planets form says that they have to be a lot closer to circles than this. Small planets can get kicked around by bigger ones, but HD 80606b is four times as massive as Jupiter, so there’s probably not a bigger planet around.

One theory of planetary migration is that planets start in circular orbits, then fall into elongated orbits because of interactions with debris around their stars. These planets later get pulled back into much smaller circular orbits as hot jupiters because of tidal forces caused by interactions with the star itself.

This would work as an explanation for weird orbits for some young planets, but the star HD 80606 is 3 billion years older than our Sun, so it doesn’t work here. Instead, we think the answer is something called the Kozai mechanism. It happens because HD 80606 isn’t alone. There’s another star called HD 80607 nearby. It’s much farther than Neptune is from the Sun, but the two stars still orbit each other. The planet’s orbit is tilted relative to the two stars, and over time, the complex interaction of the three bodies causes the planet’s orbit to take on this comet-like structure.