Pluto: The World With a Heart
he most famous photograph of Pluto shows a heart. A bright, smooth plain on a darker, cratered face, roughly the size of Texas. Made of frozen nitrogen, it slowly turns over like a lava lamp on a million-year cycle. Nobody had ever seen it until 2015.
The most famous photograph of Pluto shows a heart. A bright, smooth, white plain on a darker, cratered face, roughly the size of Texas. It is made of frozen nitrogen ice, and it slowly turns over like a lava lamp on a million-year cycle. Nobody on Earth had ever seen it until 2015.
That is the year a small spacecraft called New Horizons swept past Pluto at fifty thousand kilometres an hour, took photographs and readings for a few hours, and was thrown back out the other side of the Solar System. It is the only time we have ever been there. Everything we know about Pluto's surface, its weather, its moons, the makeup of its faint atmosphere comes from that single flyby. Nine years and counting of data. From one close look.
A small dot, found by a young man
Pluto was discovered in 1930 by Clyde Tombaugh, a 24-year-old astronomer who spent a year painstakingly comparing photographic plates of the same patch of sky and watching for anything that moved. Most of those plates showed nothing. Eventually one dot moved. That dot was Pluto.
For seventy-six years, it was the ninth planet. Then in 2006, the International Astronomical Union changed the rules. To count as a planet, an object had to have "cleared its orbit" of other debris. Pluto had not. There are too many other ice-rocky bodies sharing its part of the Solar System. Pluto was reclassified as a dwarf planet.
That decision still annoys some people. I think it is fine. The interesting thing about Pluto was never whether it counts as a planet. It is what we found when we finally went to look.
What the flyby found
Before New Horizons, the best images of Pluto were from the Hubble Space Telescope, and they showed essentially a blurry beige ball. The expectation was that Pluto would be a dead, frozen, heavily cratered rock. Cold worlds don't move. They are supposed to look like our Moon, with a surface stamped by every impact since the Solar System was young.
That is not what we found.
Small worlds in the Solar System have a habit of surprising us. Mercury, the closest planet to the Sun, hides ice in its deepest shadowed craters; Pluto, almost the furthest, hides a surface that is geologically alive. There are mountains made of water ice, hard as granite at minus 230 degrees, rising up to three kilometres. There are glaciers, except they are made of nitrogen ice rather than water, and they flow. The heart-shaped plain (officially Sputnik Planitia, part of a larger region named Tombaugh Regio after Pluto's discoverer) is so smooth and so free of craters that it must be young. Probably less than ten million years old. On a body that hasn't been visited by a single rock since the days of dinosaurs, that is the geological equivalent of yesterday.
Nobody is fully sure why a world this cold and this small still has internal heat enough to push glaciers around. The leading theory is that there is a layer of liquid water deep under the ice shell, kept warm by leftover radioactive heat in Pluto's core. If that turns out to be right, Pluto is the smallest, coldest place we know of where you might find liquid water. Which is a good place to keep looking, given how often water turns out to matter.
Pluto and Charon
Pluto has five known moons. The largest, Charon, is unusual: it is more than half the size of Pluto. Earth's Moon is about a quarter the size of Earth, and we already think of that pairing as an oddly large moon for a planet. Charon goes further. It is so close in size to Pluto that the two don't really orbit each other in the usual way. They orbit a point in space between them, like two ice skaters spinning hand-in-hand.
They are also tidally locked to each other. Each one always shows the same face to the other. Earth's Moon does the same thing toward Earth, but in only one direction; Pluto and Charon do it both ways. From any spot on Pluto, Charon would hang in the same patch of sky and never move. From the back side of Pluto, you would never see Charon at all.
Charon has its own surprise: a deep red cap at the north pole, made of organic compounds that drift up off Pluto's atmosphere and freeze onto Charon's surface. One world is colouring the other.
A year you'll never see end
Pluto takes 248 Earth years to make one trip around the Sun. We discovered it in 1930. By the time it has finished one full orbit since the day Tombaugh found it on a photographic plate, it will be 2178. Nobody alive today will see Pluto complete one orbit around our Sun. It joins a longer list of astronomical events nobody alive will witness twice, some of them on much shorter clocks: comets that loop back every few generations, eclipses that happen by accident.
But we have seen Pluto once. Briefly, sharply, in detail. A small grey spacecraft, a few hours of close-up cameras, and the heart turned out to be there all along.
If you've ever wondered how light from anything that far away even reaches us, the answer for Pluto is roughly five hours. From New Horizons' position at the flyby, every photograph took that long to crawl back to Earth. The team waited the five hours, opened the file, and saw the heart for the first time.