NASA has recently released images from the Hubble Space Telescope that catch Pluto changing with the seasons. The images, which were taken of the distant dwarf planet in 2002 and 2003, show dramatic changes in the appearance of Pluto’s polar surfaces. Compared to images taken in 1994, Pluto’s north pole is getting brighter, and its south pole is getting darker. Astronomers believe that its icy northern surface is melting, while the southern hemisphere’s pole is freezing. This is evidence that summertime is approaching for Pluto’s northern hemisphere.
Earth’s seasons result from our planet’s tilt: its rotational axis is angled about 23 degrees off its orbital axis. When this tilt orients one hemisphere toward the Sun, summer occurs in that half of the Earth. The solar radiation hits that hemisphere more directly and for a longer period of time each day. At the same time, the opposite hemisphere experiences a winter season, because it receives solar rays that are less direct.
Pluto experiences seasons largely for the same reason, but its cycles are more complicated than Earth's. The dwarf planet’s rotational axis is tilted a whopping 120 degrees, so its seasonal effects are more extreme. Unlike Earth, whose orbit follows nearly a perfect circle around the Sun over 365 days, Pluto has a slightly elliptical (oval-shaped) orbit that takes 248 years. This results in complex patterns of radiation over time, making Pluto a challenging world to understand. Continued observations from Hubble as well as the New Horizons spacecraft now traveling toward Pluto will help scientists test and refine their theories about Pluto’s seasonal surface changes.