This 47 cents postage stamp was issued by America on 31 May 2016 (= Scott Catalogue # 5077). It has an enhanced-color photo of Planet Pluto that was taken by the New Horizons spacecraft, the only one to ever visit Pluto.
The planets of the Solar System are categorized into four types: terrestrial planets, gas giant planets, ice giant planets, and dwarf planets. The terrestrial planets are Mercury, Venus, Earth, and Mars. The gas giant planets are Jupiter and Saturn. The ice giant planets are Uranus and Neptune. Dwarf planets include named and unnamed objects - for example, Ceres, Pluto, Quaoar, Sedna, Eris, Varuna, Makemake, and many others. Only two dwarf planets have been visited and imaged (Ceres and Pluto).
Pluto orbits the Sun at a distance that ranges from about 30 to 49 astronomical units (A.U.). One A.U. is the distance from the center of the Sun to the center of Earth. So, Pluto is about 30 to almost 50 times farther out from the Sun than Earth.
Pluto goes around the Sun in a prograde orbit, which means counter-clockwise, when viewed above the North Pole of the Sun. Pluto's rotation is retrograde (clockwise spin), because it is a bit "upside-down". Venus and Uranus also have retrograde rotation. Most planets have prograde rotation. The spin axis of Pluto is about 120 degrees from the vertical - it is close to spinning on its side. The large axial tilt results in strong seasonality.
One year on Pluto is about 248 Earth years - it takes about 2.5 centuries to go around the Sun. One day on Pluto is about 6.4 Earth days.
Pluto is about 18% the size of Earth - its radius is ~1188 kilometers (= from the center of the core to the surface). Pluto is smaller than Earth's Moon but is about 2.5 times larger than Ceres, a dwarf planet in the Asteroid Belt between Mars and Jupiter. Its density is about 1.85 grams per cubic centimeter, which is lower than Ceres.
Five moons are known to orbit Pluto: Charon, Hydra, Kerberos, Nix, and Styx. Only Charon has been imaged "up close".
Pluto has a solid surface, like the terrestrial planets and other dwarf planets. It has no magnetic field and has variable albedo (= reflectivity), with light and dark areas.
Surface temperatures are very cold - reported values are -238 to -213 degrees Celsius (= -397 to -352 degrees Fahrenheit).
Available information indicates that Pluto has a rocky core (probably with no metal), a liquid water layer around the core, and a crust / shell of water ice (H2O) about 100 to 200 kilometers thick (or more).
Surprisingly, Pluto has an atmosphere, but the air is very thin. Atmospheric pressure varies over time, ranging from about 4 to 12 microbars (one bar is Earth's atmospheric pressure at sea level). Backlit views of Pluto show a stratified atmosphere - about 20 layers of haze are present. What little air Pluto has is about 99% molecular nitrogen gas (N2). The rest is methane (CH4) and carbon monoxide (CO).
Pluto's surface is mostly brownish-colored. The coloration agent is tholins, a group of organic chemicals that do not exist on Earth. Tholins form by irradiation of carbon-bearing gases. The equatorial areas of Pluto are quite dark-colored from tarry tholins that condensed from the air.
The left lobe of the "heart" on Pluto is a large, low-elevation, ice-filled region called Sputnik Plain. It's about 3 kilometers below Pluto's average surface elevation.
Pluto's bedrock is mostly water ice (H2O). Mountains and landslide deposits on Pluto are made of this type of ice. Other types of ice occur as surficial deposits - these include nitrogen ice (N2), methane ice (CH4), and carbon monoxide ice (CO).
Pluto has a moderately cratered surface. The oldest surfaces are intensely cratered and about 4 billion years old. Most landscapes on Pluto are less cratered and are about 1 billion years old. Sputnik Plain has no impact craters at all, indicating that it is geologically young - less than 10 million years old.
The Sputnik Plain depression itself may be a large impact basin, now filled with a vast glacier. It is about 1000 kilometers in diameter. Glaciers on Earth are made of water Ice. The large glacier on Pluto is mostly nitrogen ice, mixed with minor amounts of carbon monoxide ice and methane ice. The ice of Sputnik Plain has convection cells (they're visible on photos taken by New Horizons) - the N2 ice slowly rises and falls. Convection cells at the center of Sputnik Plain are larger than the those near the margins of the ice sheet. Sublimation pits are common on the glacial surface - N2 ice converts directly to N2 gas, leaving holes. Some valley glaciers (= alpine glaciers) flow downward into the Sputnik Plain ice sheet.
Unexpectedly, sand dunes were seen atop the ice of Sputnik Plain. Not many worlds have dunes. Venus has them, Earth has them, Mars has them, Io has them, Titan has them, and Pluto has them. Dunes on Earth are often composed of quartz sand. Pluto's dunes seem to be composed of frozen methane sand grains. The presence of dunes indicates that Pluto has wind, despite the very low atmospheric pressure.
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