Scienceline’s Guide to the Exoplanets: The Bouncy Ball

The Bouncy Ball
HR 5183 b
Discovered by: Sarah Blunt et al.
Discovered: 2019
Distance from Earth: ~100 light-years
Mass: ~3.23 Jupiter masses (~1020 Earth masses)
Radius: ~1.2 Jupiter radii (~13 Earth radii)
Surface climate: Bouncy?
Habitability from humans: No.

On its surface, were you to look at HR 5183 b at a single moment frozen in time, you’d find nothing particularly unique about it.

HR 5183 b’s sun is yellow, and not particularly different from Earth’s. HR 5183 b is a gas giant, three times the mass of Jupiter; while that is rather impressively big, among exoplanets Earthlings have discovered, that’s more or less the traditional norm. Even if HR 5183 b were solid, it would likely be too far away from its sun to host liquid water in a form we would recognize.

HR 5183 b probably doesn’t have a particularly notable appearance. What HR 5183 b does have, on the other hand, is a strangely eccentric, elongated orbit.

To understand what that means, let’s place HR 5183 b in our own solar system. At its furthest, it would drift out past the orbit of Neptune. It would start to fall inwards, slowly gathering speed, before reaching somewhere around the orbit of Jupiter and snapping all the way back. Such an orbit would last decades — and it would mean that, in those decades, the planet would bounce between some interesting extremes of heat and light.

Sarah Blunt, the astronomer who led the planet’s discovery, described the planet’s motion as “whiplash,” a term that seemed to catch on in the media. It’s the sort of orbit you might expect from a comet like Halley’s — not a gas giant that dwarfs Jupiter.

But, while HR 5183 b may be an extreme, it’s not alone. Astronomers theorize that there’s actually a whole category of such “eccentric Jupiters,” gas giants with similarly unusual elongated objects. In fact, they may be as common, if not more, than the hot Jupiters that orbit close to their suns.

So the obvious question, then, is this: how do massive planets like HR 5183 b get themselves stuck in such odd orbits in the first place?

In a common theme among exoplanets, astronomers aren’t sure. HR 5183 b in particular is a recent discovery, meaning that astronomers haven’t yet been able to study its system in-depth to learn more. More generally, the mechanisms of star system and planet formation still aren’t well-understood — although HR 5183 b and its ilk offer a chance to learn more.

And astronomers have put out a few ideas. One is that such planets form in circular orbits, before other planets in their star systems push them out into these much longer, comet-like orbits. Perhaps more intriguing, however, is the possibility that such planets might exist because of passing stars — pulling from afar.