Springtime on Neptune

The planet Neptune is in the cold dark, deepest depths of our Solar System. It is the ninth planet from the Sun, but due to Pluto's odd orbit, there are times when Neptune is the most distant planet. Because of Neptune's distance, not much was known about it until 1989 when Voyager 2 spacecraft flew by the Uranus and Neptune. Since then, there have been no spacecraft out that far, but thanks to upgrades to Hubble Space Telescope we can once again pay a virtual visit to Neptune.

Recent images of Neptune by NASA's Hubble space telescope are showing there is an increase in Neptune's brightness in the southern hemisphere which is considered a change of seasons. For the past six years, astronomers at the University of Wisconsin-Madison and NASA JPL have been studying this distant world. They have watched seasonal changes in Neptune marked by increases in the amount and brightness of the banded clouds features located mostly in the planet's southern hemisphere.

The changes are not all that different from what we see on Earth. Our planet goes through cloud pattern changes which are in response to seasonal changes. What triggers them on both Earth and Neptune is a response to seasonal variations in sunlight.

Neptune's weather patterns have been studied before. The planet has a history of weird and violent weather. Massive storm systems have been observed with ferocious winds that can gust up to 900 miles per hour. The observations from Hubble though are the first to suggest seasonal changes.

To understand what seasons are like on Neptune, it helps to understand what causes seasons on Earth. The tilt of the Earth, at a 23.5-degree inclination, is the phenomenon responsible for the change of seasons. As the Earth orbits the Sun over the course of a year, the planet is exposed to varying patterns of sunlight which drives the seasons.

Like Earth, Neptune spins on an axis that is tilted at an angle toward the Sun. Neptune is inclined at a 29-degree angle and the northern and southern hemispheres alternate in their positions relative to the Sun. But it takes Neptune 165 years to make one trip around the Sun, so seasons on Neptune might last 40 years! Oddly enough, a "day" on Neptune is only 16 hours and 3 minutes! Neptune is a strange world.

If Neptune is undergoing a seasonal change, it would be expected that the planet would show continuing brightness over a long period of time. And that is exactly what the scientists are observing from Hubble. And as added evidence, it turns out that astronomer from Lowell Observatory have been observing minor brightening in Neptune since 1980.

As expected, there is little change around Neptune's equatorial regions, just as around Earth's equator it stays pretty much one season all year long. But it is amazing that Neptune has seasons at all. The Sun, from Neptune, would be 900 times more dim than the Sun from Earth. It seems like such a trivial amount of sunlight that seasons are hard to imagine!

Constellations

The Solar System