Neptune
Planet · Ice giant
Neptune is the most distant planet: a deep-blue ice giant with the fastest winds in the Solar System. It was the first planet found by mathematics before anyone saw it.
- 49,244 kmAbout 3.9 Earths would fit side by side across it.
- 165 yearsYear
- up to ~2,000 km/hWinds
Take a closer look
Warm from inside
Neptune is about 30 times further from the Sun than Earth, yet it gives off more than twice as much heat as it receives from sunlight. That leftover heat from deep inside the planet helps stir up its clouds and storms. Uranus, its near-twin, gives off much less, and scientists are still puzzling over why.
Rings with lumps
Neptune has thin, dark rings made of dust and ice. Its outermost ring is strange: instead of being smooth, it has bright clumpy arcs that stay bunched together. Astronomers named the arcs Liberté, Égalité and Fraternité, and the James Webb Space Telescope photographed the rings clearly in 2022.
A faraway twilight
Sunlight on Neptune is about 900 times dimmer than on Earth, so noon there would feel like a gloomy evening – though still brighter than a moonlit night here. Radio signals take about four hours to travel there, so talking to a spacecraft would mean waiting more than eight hours for every reply.
Size it up
About 3.9 Earths would fit side by side across it. Both are drawn to the same scale.
How we found out
- 1846
Urbain Le Verrier (and John Couch Adams) calculate that an unseen planet must be tugging on Uranus. Johann Galle finds Neptune within 1° of the prediction on his first night of searching.
- 1846
Just 17 days later, William Lassell discovers its biggest moon, Triton.
- 1989
Voyager 2 flies past and photographs the “Great Dark Spot” storm and geysers on Triton.
- 2011
Neptune completes its first full orbit since it was discovered.
How can you discover a planet without seeing it?
By noticing that something else is being pulled off course. Gravity leaves clues – scientists use the same trick today to find planets around other stars.
See it yourself
Needs a telescope – it looks like a tiny blue dot.
From the encyclopedia
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