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SYFY WIRE Bad Astronomy

Putting the spin on a rogue planet

By Phil Plait
Artist’s conception of the free-floating planet PSO J318. Credit: MPIA/V. Ch. Quetz

Sometimes, a new bit of research in astronomy comes out that is very specific, an observation of one aspect of one object that may not seem that important — it might make you say, "Huh, that's cool" under your breath — but then, when you think about what it implies, where we are with astronomy, that phrase might change to "Oh, wow."

Let me tell you about one.

I think one of the most amazing discoveries in modern astronomy is that planets orbiting other stars not only exist, but are common. When I was a kid we only knew of nine planets in the whole Universe, the ones in our solar system (and now it's eight, but don't get me started).

Without any examples of planets around other stars we didn't know if our solar system was unique, or one of many. That's important to know; besides the philosophical ramifications (which are extraordinarily profound), the scientific implications are huge. What processes make planets? How easily does it happen? Are these processes super rare, or do they happen to every star?

And now we know a lot better. The first planet orbiting a star like the Sun was definitively found in 1995 (though the timeline is a bit twisted), and now we know of thousands. By extrapolation, there must be billions in our galaxy alone, maybe hundreds of billions. They could easily outnumber the stars in the Milky Way!

But it gets cooler. Newly forming planets migrate; as they interact with the disk of material from which they form, and with each other, they can move in and out from their host star. If one of those planets is really big, and massive, and it passes another, less massive planet, the encounter can literally eject the smaller one from the system, casting it out to roam the galaxy, cold and alone.

Rogue planets (scientists generally refer to them as "free-floating planets") were the subject of a few science fiction stories I remember reading as a kid. But it turns out (like so many predictions of sci-fi) they're real. We've seen them. Quite a few!

An actual image of the rogue planet PSO J318.5338-22.8603, a faint, young, hot object just 80 light years away. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium

Some are found indirectly (if one passes between us and a much more distant star, the gravity of the rogue planet acts like a lens, making the star brighten and fade as it moves past), but some are directly seen in images, a small faint dot against the sky. It helps when they're very young, and still warm from their formation process. This makes them glow in infrared, and easier to spot.

In 2013, the rogue planet PSO J318.5338−22.8603 was discovered. Its motion across the sky revealed it to be a part of a group of nearby stars called the Beta Pictoris moving group, which is super helpful: From physical models of how stars form we know those stars are about 23 million years old or so (though, to be fair, this is uncertain, and they could be somewhat younger). Why is that important? Because once a planet is done forming it starts to cool, and as it does so it gets dimmer and the light it emits changes color (just as a glowing bar of iron that starts off white hot changes to yellow, red, and then infrared as it cools). Given its age, brightness, and color, we can infer that PSO J318 is about eight times the mass of Jupiter — a true planet.

Except one without a star. It likely got ejected during the chaos that ensues when planets form and move around. We don't know what star is its parent, and we may never know. But we do know something else about it that is also amazing: how fast it spins.

Using observations made by the Hubble and Spitzer Space Telescopes, astronomers watched PSO J318 over the course of many hours. They saw a clear change in its brightness in the Spitzer observations in infrared light: It got about 2% brighter and 2% fainter than average in an obvious sinusoidal pattern that repeats every 8.6 hours. That's a dead giveaway they were seeing it rotate: Some feature on its surface was spinning into and out of view, causing the variation in brightness.

The “light curve” of PSO J318, the change in brightness over time.

Given the planet's mass — far higher than Jupiter's — it almost certainly doesn't have an actual surface. Instead, we must be seeing some change in its cloud pattern. But what?

That's where the Hubble observations come in. They also show this same pattern, but offset in time almost exactly half a cycle — when the planet was getting brighter in the Spitzer observations, Hubble saw it getting fainter, and vice versa. The Hubble observations were in the near infrared, just outside the range the human eye can see. This strongly implies in these we're seeing different layers of clouds.

Atmospheres of planets are layered. Ours is! Sometimes you can see puffy cumulus clouds a few kilometers up, and cirrus clouds far higher above them. Not only that, but sometimes the constituents are layered too; on Jupiter ammonia forms ice crystals when it gets high and cold enough, and these clouds are very reflective in visible light. Lower down, where it's warmer and ammonia can't form ice, it's transparent.

We must be seeing something like this in the atmosphere of PSO J318. Given the conditions there and the fact that Spitzer sees through them while Hubble sees light reflected from them, the astronomers who analyzed the observations posit these might be made of materials we'd consider exotic here on Earth: Na2S (sodium sulfide), chromium, or MnS (manganese sulfide). These may exist as patchy clouds above a thicker, more homogeneous layer of clouds. We can't be sure just yet, but this is a reasonable guess.

Now allow me to take a step back here and stress something. In 1990 we didn't know if planets existed outside or solar system. And now, less than three decades later, not only do we know they exist by the billions, but there may also be billions of free-floating planets. We've seen some, and we're so good at observing them that not only can we detect their rotation, but we can also take educated guesses at what their clouds are made of!

This is astonishing. Astonishing. This planet is 800 trillion kilometers away, a distance so vast it crushes our sense of scale, yet we can watch it and see it changing because it's having a partly cloudy day.

Oh, wow.

This kind of thing makes me want to jump to my feet and shout it from the rooftops. Yes, we dumb hairless apes are tribal and petty and self-centered and sometimes easily confused by our haphazardly evolved brains misinterpreting the world. But despite that, when we look up, when we focus, when we reach out, we can grasp the stars.

And the things between them, too. This will never cease to be a source of pride for me in my species. We are capable of amazing things when we try.