Occasionally Pluto will cross in front of a reasonably bright star - an event called a stellar occultation. The diagram below shows how the orbital motion of the Earth changes the perspective of an observer looking at Pluto. From the perspective of an observer on Earth the star appears to move behind Pluto. From beginning to end the occultation of the star by Pluto only lasts a couple of minutes.
An important stellar occultation by Pluto occurred in June 1988 and provided the first direct evidence of Pluto's atmosphere. If Pluto had no atmosphere the starlight would blink instantly off and then back on again at the end. For a brief time at the beginning and end of the occultation, however, Pluto's atmosphere was backlit by the star and the starlight dimmed more gradually. By carefully modeling how the atmosphere "bent" the starlight - called refraction - researchers detected the planet's tenuous atmosphere.
Several research groups observed the 1988 stellar occultation by Pluto using telescopes on Earth - such as in Hobart, Tasmania and Charters Towers, South Africa - and from a plane, the Kuiper Airborne Observatory.
Pluto's is one of four known atmospheres in the solar system that is predominantly molecular nitrogen. The others belong to Saturn's largest moon, Titan; to Neptune's largest moon, Triton; and to Earth.
Like its solar system twin, Triton, Pluto's thin atmosphere has a surface pressure of only 3 to 100 microbars (3 to 100 millionths of the surface pressure of the Earth). The temperatures are extremely cold, with surface temperature in the range of minus-400 to minus-360 degrees Fahrenheit. The atmosphere is controlled by interaction with surface ices, primarily molecular nitrogen ice.
From our understanding of the properties of the ices of Pluto, we know that some of them slowly evaporate from the surface and enter the atmosphere as gases, much in the way that ice cubes evaporate in the freezer of the refrigerator. Ices of carbon monoxide and methane have also been detected on Pluto's surface using telescope measurements of reflected sunlight (at near infrared wavelengths). Scientists therefore believe that the atmosphere also contains trace amounts of carbon monoxide and methane gases supplied by sublimation of their ices.
Look carefully at the gradual dimming of starlight shown in the occultation data in The Discovery section on this page - do you see a slight bend or kink in the curve? You might also notice that starlight does not penetrate all the way to Pluto's surface, a fact that has led some scientists to suggest that obscuring clouds and/or haze mask the surface. Alternatively there may be a very steep temperature gradient in the lower atmosphere, where the temperature increases from minus-397 to minus-360 F at the surface to minus-280 F in less than 10 kilometers (about 6 miles) of altitude increase.
The large tilt in Pluto's rotational axis (120 degrees) creates very large seasonal variations. As surface ices evaporate in the migrating sunlight, their gases are transported and redeposited in dark, cold surface regions as frost. At the same time, in other locations on the planet or as Pluto's seasons change, some of the gases of the atmosphere freeze on the surface, causing a slow but continuous exchange of materials between the surface and the atmosphere. This constantly refreshes the surface, perhaps covering much of the planet's topography with freshly deposited ices on time scales of thousands of years.
Thus, even frigid, distant and tiny Pluto is a dynamic world where the processes of nature continuously change the surface and the atmosphere, creating an alien and exotic world that beckons us from Earth to visit, explore and learn.
Pluto is currently moving away from the Sun, having reached its closest approach distance in 1989. Generally, the closer an object is to the Sun, the warmer it should be and the more rapidly its surface ice should sublime into space. The sublimation of ices on the surface of Pluto is responsible for its tenuous atmosphere. As Pluto moves away from the Sun it will get colder and, eventually, its atmosphere will almost completely condense back onto the surface.
The actual situation is a bit more complicated than the simple illustration discussed above. Because of "thermal lag," the time of Pluto's closest approach to the Sun in 1989 was probably not when its surface temperature was greatest, just as the temperature on Earth is hottest at mid-afternoon rather than noon, or high summer is about a month after solstice. In the case of Pluto, the latest observations reveal that the atmosphere has thickened during the past decade. But this trend will definitely reverse as Pluto continues moving away from the Sun. Scientists don't know exactly when the condensation will start to dominate sublimation - which is why they want to get to Pluto as soon as possible!
The New Horizons mission has a plan for measuring Pluto's atmosphere. After flying by Pluto, the New Horizons spacecraft enters the planet's shadow. As the spacecraft moves into Pluto's shadow, sunlight must pass through the planet's atmosphere before reaching the spacecraft. Absorption of sunlight by Pluto's atmosphere is detected as characteristic "dips" in the ultraviolet part of the spectrum of light measured by New Horizons' Alice instrument. This technique is a very powerful method for measuring even trace amounts of atmospheric gas.
In addition, radio waves sent from Earth to New Horizons will bend as they pass through Pluto's atmosphere. The amount of bending of the radio waves is detected by the New Horizons Radio Science Experiment (called REX) and is related to both the average molecular mass and the temperature of the atmosphere.
Together, these ultraviolet and radio "occultation experiments" provide powerful probes of Pluto's tenuous atmosphere.
Since no one has observed an atmosphere on Charon, no one can say for sure if Pluto's largest moon has one. Scientists can say, however, that if Charon does have an atmosphere it must be a very thin one, at least compared with that on Pluto.
Spectroscopic studies of Charon show no clear evidence of at atmosphere, and in fact suggest that if an atmosphere is present on Charon it must be at least 10 times less dense than that on Pluto. It is possible that Charon did have a substantial atmosphere, perhaps soon after it formed approximately 4.5 billion years ago. But with Charon's small mass and correspondingly small gravitational attraction, any atmosphere would "evaporate" into space (see discussion of sublimation on this page) on a relatively short time scale.