Imagine for a moment that you are an intrepid space explorer, traveling the solar system in search of adventure and, if you have time, scientific exploration. After launching from Earth in your rocket ship, you end up on the surface formerly known as the planet Pluto, looking up at the stars. The earth now lies six billion kilometers from the sun, rendered invisible by the distance, and you try in vain to even identify the sun. She is lost – one star among countless thousands, or so the old science fiction trope would have you believe.
I've heard variations of this vignette many times, but there's one small problem: it's wrong.
The Sun is indeed a star, not so different from the many billions of other stars in the Milky Way. However, for us humans, what matters most is its proximity. The Earth is in a fairly circular orbit around the Sun at a distance of 150 million kilometers (plus or minus several million at perihelion and aphelion) – so close on an astronomical scale that the Sun is the brightest object in the sky. Even a fleeting glance at its radiance causes tears and can damage the retina.
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What about the view from Pluto? This icy world many farther away, so the sun should appear dimmer. But how dim is “dim”? We can find out!
Like all stars, the Sun emits light in all directions. Therefore, anyone who is at the same distance from the Sun will see it shining with the same brightness. This fixed distance defines the surface of a sphere, for example, with a radius equal to the radius of the Earth's orbit.
Now imagine a person who is twice as far from the Sun. The light emitted by our star now covers a much larger spherical surface and has become diluted, or more accurately, dim. Think back to your elementary school memories and you might remember that the surface area of a sphere depends on the square of its radius. This means that someone twice as far from the Sun as Earth will see the light spread over four times the area, so it will appear a quarter brighter. Increase the distance by three times and the sun becomes one-ninth brighter. This is known as the “inverse square law” and it greatly simplifies the mathematics.
Pluto is on average 40 times farther from the Sun than Earth. This means that from Pluto the Sun appears 40 x 40 = 1600 times dimmer (or, to use better mathematical grammar, sixteen hundredths brighter) than from our brighter and more familiar world. This is a big drop! But how weak is it really?
In our sky, the sun is about 400,000 times brighter than the full moon. This means that the Sun will still be from Pluto 250 times brighter than the full moon visible from Earth. Keep in mind, the full moon is bright enough to be read, so even from very far away Pluto the sun would illuminate the landscape quite well: about the same level as twilight on Earth.
Obviously, it will be much, much brighter than even the brightest night star. Think of it this way: to make the sun look like any other star in the sky, it should be as far away from you as the other stars in the sky.. Alpha Centauri, for example, is a triple star system; it consists of two stars, somewhat similar to the Sun, and a third, much dimmer. The most massive of these stars dominates the brightness of this star system. The system is located 40 trillion kilometers from Earth – more than 6000 times further away than Pluto, but still looks like one of the brightest “stars” in the sky. Obviously, you'd have to go much further than Pluto to darken the sun to even that level of darkness.
Keep in mind, I also used Pluto's average distance from the Sun. Pluto's orbit is actually quite elliptical, deviating significantly from a circle. At its closest, it is about 30 times farther than the Earth from the Sun, and at its best, about 50. This means that at its farthest, the Sun is one twenty-five hundredths brighter than it is on Earth—and still bright enough to be seen. And at most it's one-nine-hundredth brighter, more than 400 times brighter than the full moon, and will shine so brightly you'll have to squint when you look at it. It's best to wear sunglasses, even this far out in the solar system.
The situation with science fiction is also getting worse. From Pluto, our Sun is not only much brighter than any other star, but it can also be seen, that is, seen as a disk. In the Earth's sky, the size of the solar disk is half a degree. A person with average vision can distinguish objects up to one-sixtieth of a degree in size. The apparent size of an object decreases linearly with distance; that is, when Pluto is 30 times farther from the Sun than the Earth, the Sun will appear one-thirtieth larger. This is still large enough to see our star as a disk. Of course, it will be a small point, but not a star-shaped point of inconspicuous size. Of course, when Pluto is farthest from the Sun, our star will appear fifty times larger than from Earth, so you would think of it as an unresolved point, but this idea depends on where Pluto is in its orbit. For most of the Plutonian year, the Sun was a blindingly bright but tiny disk.
What an alien look that would be! The Sun, so dominant in our sky, will still rule Pluto's sky, but with much less energy.
It so happens that Pluto has an atmosphere, although it is incredibly thin, its density is only one hundred thousandth the density of Earth's air at sea level. Even at noon, when the sun is high, Pluto doesn't have enough atmosphere. scatter sunlighttherefore the sky would be black. But the bright sunlight will still make it difficult to see any stars, let alone thousands of them (though if you block the sun with your hand in your space suit, you'll probably be able to see a few).
However, during Pluto's twilight, the situation will change. When the sun is low on the horizon, you will see much more of the thin atmosphere, so much so that the scattering couldmake the sky near the sun bluevery similar to heaven on Earth!
I'm not sure if the blue skies on Pluto would make this otherworldly appearance any more comforting or just highlight its alienness. But I love how science-based science fiction can give us a realistic sense of fantastical places. Of all the weird things to see on Pluto, seeing the Sun tops the list, but what might be even weirder is that such an alien place can sometimes remind you of Earth.
