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The five planets visible with the unaided eye are Mercury, Venus, Mars, Jupiter and Saturn. If you know where to look, you can’t miss them, even in light polluted skies. But there are two planets, Uranus and Neptune, which are a little harder to find. You’ll need a telescope to even see them.
Both Uranus and Neptune were discovered after the invention of the telescope. Galileo first pointed his crude telescope at the heavens, but other astronomers actually discovered Uranus and Neptune. Uranus was found in 1781 by William Herschel, and Neptune was found in 1846 based on calculations by John Couch Adams in England and Urbain Le Leverrier in France.
Although many consider Earth and Venus to be twin planets, we’re actually pretty different when it comes to temperature, atmosphere and surface features. Uranus and Neptune; however, are extremely similar. Both planets have interiors made up of hydrogen, helium, and liquid water with ammonia mixed in. And both Neptune and Uranus are thought to have Earth-sized solid cores made of molten rock and metal. Both planets actually have very similar temperatures at their cloud tops; and this is a bit of a puzzle considering the fact that Neptune receives about 40% less sunlight than Uranus.
The atmosphere of Uranus is almost featureless, while the surface of Neptune has fast moving clouds and short-lived storms. In fact, winds on Neptune move the fastest in the Solar System: more than 2,100 km/hour. It’s possible that Uranus and Neptune have such different weather systems because Uranus is tilted over onto its side. For half of its orbit, one pole it pointed away from the Sun, and one pole constantly faces the Sun, and then the situation is reversed for the other half of its orbit. Neptune has a much less inclined axis of rotation; it’s more similar to Earth.
Both Uranus and Neptune have rings. The rings around Uranus are quite large and wide, while the rings of Neptune are hard to see without a very powerful telescope.
And finally, Uranus and Neptune have only been visited once by any spacecraft launched from Earth. NASA’s Voyager 2 spacecraft flew past Uranus in 1986 taking the first ever close-up pictures of Uranus, and then it made a flyby of Neptune, passing just a few thousand kilometers above Neptune’s cloud tops. Unfortunately, there are no plans to visit either Uranus or Neptune again in the future with a spacecraft.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.