National Weather Service United States Department of Commerce

Pluto, photographed by NASA's New Horizons on July 14th, 2015


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.   

Photographs below taken by NASA's New Horizons probe, July, 2015.

Pluto's atmosphere as shown by New Horizons
Pluto and its largest moon, Charon

Ice Dunes


Data is from NASA Goddard)

Average distance from Sun 3.67 billion miles
Perihelion 2.75 billion miles 
Aphelion 4.53 billion miles
Sidereal Rotation 6.39 Earth days
Length of Day 6.39 Earth days
Sidereal Revolution 247.8 Earth years
Diameter at Equator 1,475 miles 
Tilt of axis 122.5 degrees
Moons 5 known
Atmosphere Mostly nitrogen, and small amounts of methane and carbon monoxide
Discoverer Clyde Tombaugh
Discovery Date February 18, 1930



Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  Earth's sidereal rotation is 23 hours, 57 minutes.
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  Earth's length of day = 24 hours
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a planet or moon's orbital plane is perfectly horizontal, or along the equator, the axis tilt is the amount of tilt from the body's equator relative to the body's orbital plane.  For example, Earth is tilted an average of 23.44 degrees on its axis.  On Earth's latitudes, 23.44 degrees marks the Tropic of Cancer (northern hemisphere) and the Tropic of Capricorn (southern hemisphere).  On the summer solstice, the sun is directly over the Tropic of Cancer.  During the winter solstice, the sun is directly over the Tropic of Capricorn.  If a planet or moon had no axis tilt, then its axis tilt would be zero degrees, thus there would be no change in seasons.