Title: The cosmic shoreline: The evidence that escape determines which planets have atmospheres, and what this may mean for Proxima Centauri b

Authors: Kevin J. Zahnle and David C. Catling

First author’s institution: NASA Ames Research Center, USA

Status: Posted to arXiv (2017) [open access]

Escaping from prison is hard. First, you have to convince your skeptical friend to smuggle you a rock hammer and a big poster of Rita Hayworth. Then you have to spend 20 years digging a tunnel through your cell wall by night and laundering money for your despotic warden by day. You’re not even done after you crawl half a mile through a sewer pipe to break out, because you have to hike into the cornfields in a business suit to leave an inspirational note under a rock for your friend.

Escaping from Earth, on the other hand, is easy. All you have to do is go up at 25,000 miles per hour. Admittedly, that’s challenging when you’re heavy, like an Atlas V rocket, because it takes a lot of energy to make a heavy thing go fast. But when you’re light, like a hydrogen molecule, even a little jolt of energy from a ray of sunlight can free you from Earth’s gravity.

For a planet, therefore, hanging on to an atmosphere becomes a somewhat delicate proposition. Being very massive helps, because it means that the planet’s gravity is stronger, so atmosphere molecules have to reach higher speeds to escape it. Being farther away from the star helps, too, because the starlight which reaches the planet is dimmer and less able to evaporate the atmosphere.

So what separates planets that successfully hang on to atmospheres from planets that don’t? And are the planets in our own Solar System, both “atmosphered” and not, a good guide to planets beyond?

Drawing a line in the sand

The authors start by taking every big object in the solar system–planets, dwarf planets, moons, asteroids–and plotting, very colorfully, the energy it receives from the Sun (“insolation”) against the strength of its gravity. They measure gravity strength by the speed you’d have to reach to escape from the planet (“escape velocity”).

The first thing to notice is that the objects with atmospheres (the filled-in symbols: Earth, Jupiter, Neptune, Pluto, Titan, etc.) separate surprisingly neatly from those without (the hollow symbols: Mercury, the Moon, the asteroids, etc.). The authors draw a nice turquoise line, which they term the “cosmic shoreline,” to mark the boundary, and then they draw a bunch of other lines because they hate your eyes.

This tidy cosmic shoreline is just something the authors noticed, not a fit to the data, but it hints that the authors may be on to something. If a planet’s ability to hang on to an atmosphere (or not!) depended on the amount of atmosphere it was born with, or the amount of gas deposited onto it by comets crashing into it, then insolation and escape velocity wouldn’t tell us much at all. But they do, which means that the strength of a planet’s gravity and the amount of energy it receives from its star really are determining factors.

Hanging out on the beach

Next, the authors take every exoplanet in the universe of which we’ve measured the escape velocity and toss those on the plot as well. (There are relatively few of these, even though we’ve discovered thousands of exoplanets, because measuring the escape velocity is difficult–it requires a measurement of mass and an independent measurement of size.) We can’t yet observe most of these exoplanets in enough detail to know what their atmospheres are like, but we deduce that they do have atmospheres, because their masses and sizes are consistent with the gas giants of the Solar System.

To their surprise, they find that the exoplanets more or less fall on the cosmic shoreline. This is weird because they have to extrapolate the shoreline quite a ways up and to the right to get to the exoplanets. All the exoplanets fall in the upper right corner because big exoplanets (high escape velocity) orbiting close to their host stars (high insolation) are the easiest ones to discover, so they’re the ones we’ve found and studied so far.

Once these big, hot exoplanets are added, the cosmic shoreline runs from the tiny icy bodies of the Solar System to the hot Jupiters, a hundred million times more insolated. That’s a lot of real estate for one trend to cover–it’s a simplistic picture, but an intriguing one nonetheless.

If we trust the cosmic shoreline, it might have major implications for exoplanet exploration: Proxima b, our nearest exoplanet neighbor, is right on the beach with the other atmosphered planets. A cosmic Zihuatanejo?

Featured image via Wikimedia Commons.