Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Vapor Beacon

HD 20782 b: A world caught in a violently stretched loop

October 19, 2020
A fake planet with "Scienceline's Guide to Exoplanets" labeled across its equator. Other planets zoom past behind it.
Your favorite planets, and you didn't even know they existed. [Credit: Curtis Segarra | CC BY-NC-ND 2.0]

The Vapor Beacon
HD 20782 b
Discovered by: Stephen Kane et al.
Discovered: 2016
Distance from Earth: ~117 light-years
Mass: ~1.5 Jupiter masses
Radius: ~1.2 Jupiter radii
Surface climate: A mess
Habitability for humans: Sometimes, maybe.

Last week, we talked about a so-called “whiplash planet,” HR 5183 b. That planet is a so-called “eccentric Jupiter,” circulating in a wildly eccentric orbit, swinging from far away to much closer to its sun and back again, more like what you’d expect from a comet than from a gas planet.

But HR 5183 b isn’t the most eccentric exoplanet scientists have found, not even when it was discovered. That honor goes to another Jupiter-sized gas giant, discovered three years earlier in a star system in the direction of Formax. That planet is HD 20782, and its orbit makes HR 5183 b’s surface conditions look positively stable in comparison.

Like HR 5183 b, HD 20782 b’s star is yellow and quite Sun-like. That means we can quite nicely place its orbit in our own solar system for comparison. At its furthest, HD 20782 b drifts at around 2.5 times the distance between the Sun and the Earth, which would place it beyond Mars, somewhere around our solar system’s asteroid belt. From there, it begins to descend, closer and closer, hotter and hotter. It would drift closer than Earth, closer than Venus — even closer than Mercury.

At closest, it would reach a searing 0.06 times the distance between the Earth and the Sun: something around one-fifth the distance between the Sun and Mercury. That would place HD 20782 b well in the realm of the Hot Jupiters. Then, the planet would swing back out, returning to its winter home at a range that makes it much closer to, well, the Jupiter we know.

In fact, the planet’s discovery was in large part thanks to that eccentricity; astronomers detected a flash of reflected light as it swung round close to its star.

And, because it’s in what would be our inner solar system, that orbit is relatively quick. Comets like Halley’s might take tens or hundreds of years to finish their orbits, but a year on HD 20782 b lasts only twenty Earth months or so.

For what that means on the planet’s surface, the result is colossal, cyclical instability. The repeated heating and cooling perhaps creates storms far eclipsing any of those found on Jupiter or Neptune. If HD 20782 b has moons, those would be repeatedly sterilized by immense heat, then freezing ice, then immense heat, then freezing ice, et cetera, over and over and over again, for celestial timescales.

But we don’t know much else. In a common theme, even though HD 20782 b is quite fascinating, it hasn’t been studied — yet. But astronomers might; some have said the planet’s conditions make it an opportunity to study its atmosphere, and there’s undoubtedly something bizarre waiting there.

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