Neptune 'in middle of 40-year spring season'

Spring is blooming on the planet Neptune – to the astonishment of astronomers.

The eighth planet from the Sun was thought to be far too distant and cold to be affected by Earth-like seasons.

Yet scientists have observed an increase in brightness in Neptune’s southern hemisphere which they are attributing to the onset of spring.

Images taken over six years by astronomers in the US show a marked increase in the size and brightness of the planet’s southern cloud bands.

Dr Lawrence Stromovsky, a member of the astronomy team from the University of Wisconsin-Madison, said: “Neptune’s cloud bands have been getting wider and brighter. This change seems to be a response to seasonal variations in sunlight, like the seasonal changes we see on Earth.”

The findings, reported in the planetary science journal Icarus, add to the planet’s reputation for weird weather.

Neptune has massive storm systems and ferocious winds that sometimes gust up to 900 miles per hour.

But until the new observations scientists never dreamed that the planet also had seasons.

If they are right, Neptune would have four seasons, like the Earth. Each hemisphere would experience a warm summer and a cold winter, with transitional seasons of spring and autumn.

Unlike the Earth, however, seasons on Neptune would last for decades, not months. A single season on the planet, which takes almost 165 years to orbit the Sun, could last more than 40 years.

If what the astronomers are observing is truly a seasonal change, the planet will continue to brighten for another 20 years.

Seasons both on Neptune and the Earth are caused by the planets spinning on axes that are tilted towards the Sun.

As the Earth, tilted at an angle of 23.5 degrees, orbits the Sun over the course of a year, the seasons are marked by changing patterns of exposure to solar radiation.

Similarly, Neptune is inclined at a 29 degree angle and the northern and southern hemispheres alternate in their positions relative to the Sun.

However it is remarkable that Neptune shows any evidence of seasonal change given that the Sun seen from the planet is 900 times dimmer than it is from the Earth.

Despite its weak effect, the springtime Sun appears to deposit enough energy in Neptune’s atmosphere to force rising motions, condensation and increased cloud cover.

Evidence of a seasonal pattern is strengthened by the fact that the Hubble Space Telescope images show an apparent absence of change in the planet’s low latitudes near the equator.

“Neptune’s nearly constant brightness at low latitudes gives us confidence that what we are seeing is indeed seasonal change, as those changes would be minimal near the equator and most evident at high latitudes where the seasons tend to be more pronounced,” said Dr Stromovsky.

Scientist are still trying to solve the riddle of Neptune’s turbulent weather, which cannot simply be explained by the combined action of heat from within the planet and solar radiation.

The amount of energy available to run the dynamic machine that is Neptune’s atmosphere would normally be too small.

“It must be a very well-lubricated machine that can create a lot of weather with very little friction,” said Dr Stromovsky.