Under Pluto's Sunny Skies, You'd Have to Wear Shades

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Imagine, for a moment, that you are an intrepid space explorer, jaunting through the solar system for adventure and, if there’s time, scientific exploration. After launching from Earth in your rocket ship, you eventually find yourself on the surface of formerly-known-as-a-planet Pluto, gazing upward at the stars. Earth now lies six billion kilometers sunward, faded to invisibility by the distance, and you struggle in vain to even identify the sun. It’s lost—one star among countless thousands, or so an old sci-fi trope would have you believe.

I’ve heard versions of this vignette many times, but there’s one small problem with it: it’s wrong.

The sun is indeed a star not so very different from many billions of others in the Milky Way. To us humans, though, what matters most is its proximity. Earth resides in a fairly circular orbit around the sun at a distance of 150 million kilometers (give or take a few million at perihelion and aphelion)—so close on an astronomical scale that the sun is by far the brightest object in the sky. Even a furtive glance at its radiance makes your eyes water and can damage your retinas.

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But what about the view from Pluto? That icy world is much farther out, so the sun should appear dimmer. But how dim is “dim”? We can figure that out!

Like all stars, the sun emits light in every direction. Anyone at the same distance from the sun will therefore see it shining with exactly the same brightness. That fixed distance defines the surface of a sphere—for example, one with a radius equal to that of Earth’s orbit.

Now imagine someone who is twice as far from the sun. Our star’s emitted light now encompasses a much larger spherical surface and has been diluted or, more accurately, dimmed. Reach back into your memories of grade school and you may recall that the surface area of a sphere depends on its radius squared. That means that someone twice as far from the sun as Earth would see the light spread out over four times as much area, so it would appear one quarter as bright. Triple your distance and the sun would appear one ninth as bright. This is known as the “inverse square law,” and it makes the math a lot easier.

Pluto is, on average, 40 times farther from the sun than Earth is. That means that from Pluto the sun appears 40 x 40 = 1,600 times fainter (or, to use better math grammar, one sixteen-hundredth as bright) as from our fairer, more familiar world. That’s a big drop! But how faint is that, really?

In our sky the sun is about 400,000 times brighter than the full moon. That means that from Pluto the sun would still be 250 times brighter than the full moon as seen from Earth. Mind you, the full moon is easily bright enough to read by, so even from wee distant Pluto the sun would illuminate the landscape pretty well: at about the same level as at twilight on Earth.

Clearly it would be far, far brighter than even the brightest nighttime star. Think of it this way: for the sun to look like most any other star in the sky, it would have to be as far from you as the other stars in the sky. Alpha Centauri, for example, is a trinary star system; it’s made up of two stars that are somewhat similar to the sun and a third that is much fainter. The most massive of those stars dominates this star system’s brightness. The system is 40 trillion kilometers from Earth—more than 6,000 times farther away than Pluto—yet still appears as one of the brightest “stars” in the sky. Clearly you’d have to trek a lot farther out than Pluto to dim the sun to even that level of obscurity.

Mind you, too, I used the average distance of Pluto from the sun. Pluto’s orbit is actually fairly elliptical, substantially deviating from a circle. At its closest it’s about 30 times Earth’s distance from the sun, and at most it’s about 50. That means, at its farthest away, the sun is one twenty-five-hundredth as bright as it is from Earth—still brilliant enough to see by. And at most it’s one nine-hundredth as bright, more than 400 times brighter than the full moon, and would blaze so effulgently that you’d have to squint when you looked at it. Better wear sunglasses, even that far out in the solar system.

It gets worse for the sci-fi trope, too. From Pluto, not only is our sun far brighter than any other star, but it can also be resolved, that is, seen as a disk. In Earth’s skies, the sun’s disk is half a degree across in size. A human with average eyesight can resolve objects as small as one sixtieth of a degree. The apparent size of an object drops linearly with the distance; that is, when Pluto is 30 times farther from the sun as Earth is, the sun would appear one thirtieth as big. That’s still big enough, barely, to see our star as a disk. It would be a small one, to be sure, but not a starlike point of no discernible size. Granted, when Pluto is farthest from the sun our star would appear one fiftieth as big as it does from Earth, so you would see it as an unresolved dot, but that view depends on where Pluto is in its orbit. For a big chunk of a Plutonian year the sun would be a glaringly bright but tiny disk.

What an alien view that would be! The sun, so dominant in our own sky, would still rule that of Pluto’s but with far, far less vigor.

As it happens, Pluto has an atmosphere, albeit one incredibly tenuous, amounting to only about one one-hundred-thousandth the density of Earth’s sea-level air. Even at noon, with the sun high, there’s not enough atmosphere on Pluto to scatter sunlight, so the sky would be black. But the glare of the sun would still make it difficult to see any stars at all, let alone thousands of them (though if you blocked the sun with your spacesuit-clad hand, you could probably see a few).

During Plutonian twilight, however, the situation would change. When the sun is low to the horizon you’d be looking through far more of the rarefied atmosphere, enough that scattering could turn the sky near the sun blue, much like the sky on Earth!

I’m not sure if a blue sky on Pluto would make that otherworldly vista more comforting or just accentuate its alienness. But I love how science fiction can, when informed by science, give us a realistic sense of fantastic locales. Of all oddities to experience on Pluto, the view of the sun is near the top of the list, but an even stranger one might be that such an alien place could sometimes remind you of Earth.

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