Neptune and Triton. NASA; JPL/Calvin Hamilton/Michael Benson, Kinetikon Pictures

Neptune and Triton. NASA; JPL/Calvin Hamilton/Michael Benson, Kinetikon Pictures. Courtesy Flowers Gallery

Factfile: Neptune

With frigid temperatures and ferocious winds, our solar system's outermost planet may be last but it's certainly not least. Get the facts on this planet of extremes.

Orbiting the Sun at a distance of 4.5 billion kilometres - more than 30 times further out than the Earth - Neptune is a dark, icy world. Receiving just one nine-hundredth as much light from the Sun as the Earth does, it can experience temperatures as low as -218°C, making it one of the coldest places in our solar system.

The planet's sheer distance from Sun makes viewing it from Earth a challenge, and it was only discovered in 1846 after calculations of Uranus's orbit suggested there was an as-yet unknown planet affecting it gravitationally.

Since then, the planet has only been visited once by spacecraft when, in 1989, Voyager 2 completed its 'Grand Tour' of our solar system's outermost planets. It took this image of Neptune and its moon Triton, one of 77 composite photographs appearing in Otherworlds: Visions of our Solar System, before speeding towards interstellar space.

Wild weather

Despite its extremely cold atmosphere, the interior of Neptune is far hotter than that of its ice giant twin, Uranus. In fact, Neptune gives out 2.6 times more heat than it receives from the Sun.

The resulting temperature difference is thought to contribute to Neptune's formidable weather - winds on the planet can reach almost 2,200 kilometres per hour, the fastest in our solar system. In comparison, the fastest winds recorded on Earth, during tornadoes, are between 400 and 500 kilometres per hour.

Neptune has also been seen with giant storm spots, similar in appearance to the Great Red Spot on Jupiter. When Voyager 2 flew by Neptune in 1989, it saw a storm 13,000 kilometres across, which was subsequently named the Great Dark Spot. However, the storm disappeared before it could be photographed by the Hubble Space Telescope in 1994, meaning very little is known about how these storms form or dissipate.

Audio commentary extract

If Uranus and Neptune are gas giants, why do they appear blue, unlike Jupiter and Saturn? Dr Joe Michalski talks to Museum planetary scientist Dr Ashley King about this image of Neptune and Triton, and asks about the planets' unusual appearance.