Astronomical observations over a six-year period suggest Neptune, the giant icy planet near the edge of our solar system has seasons like Earth.
It took some time spot the changes because Neptune takes about 165 years to orbit the Sun. A winter there would be unbearably long, roughly four decades.
Seasons are governed in part by a planet's tilt. Earth's rotational axis leans 23.5 degrees compared to the plane in which the planet orbits the Sun.
Summer in the northern hemisphere occurs when the north pole is oriented toward the Sun. Six months later and halfway around on its orbit, the planet's south pole is leaning toward the Sun, causing summer in Antarctica while the Arctic is plunged into 24-hour darkness.
Neptune's axis is tilted 29 degrees.
Neptune's apparent seasonal change is seen in images captured by the Hubble Space Telescope. The photographs, taken in 1996, 1998 and 2002, show cloud bands in the southern hemisphere growing.
"Neptune's cloud bands have been getting wider and brighter," said Lawrence Sromovsky of University of Wisconsin-Madison. "This change seems to be a response to seasonal variations in sunlight, like the seasonal changes we see on Earth."
In the hemisphere getting the most sunlight, heating causes gases in the atmosphere to rise, which in turn creates condensation and increased cloud cover, Sromovsky explained.
The findings, consistent with earlier hints that date back to 1980, are reported in this month's issue of the journal Icarus.
Sromovsky said the evidence is remarkable given that the Sun is 900 times dimmer at Neptune compared to what Earth experiences, so there is less solar energy to drive change. Some internally generated heat may contribute.
Bolstering the suggestion of seasonal change is the fact that weather at the equator looks to be stable. On Earth, seasonal change is most pronounced at high latitudes and almost nonexistent in equatorial regions.
Neptune is currently the eighth planet from the Sun, having routinely switched places with Pluto in 1999. It's a frigid world where the average temperature at the cloud tops is minus 360 degrees Fahrenheit (-218 Celsius). Neptune is also known for wild winds, which can gust to 900 mph (1,448 kilometers per hour).
If Neptune does in fact have seasons, Sromovsky and his colleagues say the cloud brightening should continue for another 20 years.
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