NASA’s New Horizons spacecraft continues to take pictures of Pluto to better refine the locations of dwarf planet and its largest moon Charon in preparation for the spacecraft’s close encounter with the small planet on July 14.
You’ve probably heard Pluto’s a small object but until you see it next to the say the moon, it’s hard to appreciate just how small.
Its diameter is 1,471 miles (2,368 km) or 68% percent as large as the moon. Unlike the moon, which is as dark as asphalt, Pluto reflects far more light because its surface appears to be covered in methane ice. The dwarf planet’s albedo – the percentage of light it reflects back from the Sun – ranges between 49-66% – making it nearly twice that of Earth. Of course we have to remember that despite its shininess, the little world is more than 4 billion miles from the Sun. Sunlight falling there resembles a dark overcast day here on Earth.
In the animation we see an entire rotation of both Pluto and Charon; a “day” for each lasts 6.4 Earth days. Charon neither rises nor sets, but hovers over the same spot on Pluto’s surface, and the same side of Charon always faces Pluto. Astronomers call this arrangement tidal locking. If you went around the side of Pluto not facing Charon, you’d never see the moon.
You’ll also notice that Charon is not exactly rotating around Pluto. Instead the two bodies are circling about the center of mass in the Pluto-Charon system. Called the barycenter, this is the point between two celestial objects where their gravities balance each other.
Let’s look at the Earth-moon system. The moon doesn’t orbit the exact center of the Earth, but a point on a line between the center of the Earth and the moon, approximately 1,062 miles (1,710
km) below the surface of the Earth, where their masses balance.
Charon is 728 miles across – nearly half Pluto’s diameter. Compared to the dwarf planet it’s massive enough that the two bodies orbit about a barycenter well outside of Pluto. Look at the animation again and you’ll see how they orbit a mutual point in space where their gravities cancel out.
The Earth and Sun also revolve about their center of mass, but because the Sun is incredibly more massive than our planet that point is very close to the center of the Sun. Not so with Jupiter which is 318 times as massive as Earth. The Sun and Jupiter orbit about a point just outside our star’s blazing surface.
In the animation, you’ll notice that Pluto appears to wobble in a cyclical way every 6.4 days during its mutual revolution with Charon. Not only does the dwarf planet describe a small circle about the barycenter, it’s also moves toward and away from us during the cycle. When moving toward us, Pluto’s speed ticks up slightly; when away it’s slightly less.
Astronomers can detect these tiny wobbles or changes in speed in stars as well. We may not be able to see what’s tugging on the star, but by measuring the changing velocity of the star as it orbits about the planet-star barycenter, we can determine the mass(es) of any orbiting planets. It’s called the radial velocity method. With this tool, we’ve discovered hundreds of extrasolar planets.