At nearly 5900 million km (3670 million miles) from the Sun, Pluto is a cold, dark world, even in the middle of the day. The Sun appears over 1000 times fainter from the surface of Pluto that it does from Earth — little more than a bright star. Because of this, surface temperatures average around —230°C (-382 °F). In summer, Pluto has a slight atmosphere because the surface warms up enough to turn some of the ice to gas. As Pluto moves away from the Sun, the gas freezes and becomes ice again.
Pluto lies in an area classified as the Kuiper belt in the trans-Neptunian region of our solar system. The Kuiper Belt is composed of thousands of icy, solid objects extending from near the orbit of Neptune to nearly 5 billion miles from the Sun. Pluto is the largest known body in the Kuiper Belt with enough mass to exhibit a spherical shape. Even though Pluto has enough mass to give it a spherical shape, Pluto is in fact smaller in physical size than seven of the solar system’s moons, including Earth’s. Pluto rotates on its axis every 6.39 days and takes 247.8 years to revolve around the Sun. It has five known satellites. The satellites in order of distance from Pluto beginning with the closest are Charon, Styx, Nix, Kerberos and Hydra. Charon is the most interesting as it is about half the size of Pluto and spherical. Charon orbits Pluto every 6.39 days and also rotates once during this time frame, matching Pluto’s rotation. Therefore, Pluto and Charon are acting like a dumbbell in space, rotating around each other in a near perfect lock-step. This is the closest thing to a binary-planet system that it is in our solar system.
NASA’s New Horizons space probe flew by Pluto on July 14th, 2015, giving scientists a wealth of data previously not known about the icy body. Pluto’s very thin atmosphere extends 100 miles from its surface, about 5 times higher then earlier models had predicted. Although the atmosphere is thin, it’s possible that it is enough to give Pluto day to day weather variances. The atmosphere primarily exists as a gas when Pluto is closest to the sun (perihelion), but then slowly freezes onto the surface as it moves further away. New Horizons was able to observe Pluto’s atmosphere as the probe was moving away, photographing the haze of the atmosphere as Pluto was backlit by the sun (shown below). The atmosphere actually gives off a bluish haze, which is caused by sunlight being scattered by haze particles abundant in its atmosphere. Before the New Horizons flyby, scientists had expected that much of the nitrogen encompassing Pluto’s atmosphere had escaped into space. However New Horizons showed a surprisingly thick haze, revealing that a replenishment of nitrogen has to be coming from somewhere, possibly from the planet’s interior via ice volcanoes or geysers. Indeed, New Horizons showed that Pluto’s surface has plenty of diverse geography such as craters, ice dunes, mountains, plains, and erosional features such as surface troughs and ridges.
New Horizons gathered data from Pluto for over 5 months. Therefore New Horizons was able to image one side of Pluto and found an extremely interesting large, heart-shaped body (shown above), primarily composed of nitrogen ice. This feature gives rise to evidence that Pluto has a large, sub surface, salty, slushy-liquid ocean, perhaps up to 60 miles in deep. How can a body this cold, whose surface temperature ranges from about -370 to -400 degrees F, support liquid water, even under its surface? Given Pluto’s internal pressure and heat budget, it is possible. Furthermore, the heart-shaped feature, named Tombaugh Regio (after Pluto’s discoverer Clyde Tombaugh) holds a key. This region is nearly exactly opposite Pluto’s largest moon, Charon (shown below), exhibiting a constant pushing and pulling of gravity in that region. Astronomer’s focused their attention on the western side of the heart, a region named Sputnik Planum, an area thought to have formed from an impact with a meteor. An impact would basically blast material away from it, giving that area a “negative mass anomaly”. But that is not the case with this area. It has a positive mass anomaly. For this to happen, simulations show that a sub-surface ocean would have had to of spread out across the planet below the surface after the impact. Therefore it is possible that Pluto’s rocky interior is surrounded by a slushy-ice ocean, which in turn is surrounded by the icy surface.
Like Uranus, Pluto’s rotational axis is highly tilted at 122.5 degrees. This would give one side of the planet extremely long periods of darkness or light, depending on that side’s orientation to the Sun. However it is likely that the planet’s temperature is very uniform around the globe. This is due to the fact that the sun casts such feeble light at that distance.