Can all the planets be seen from Earth with the naked eye?

Neptune and Pluto can be seen only with telescopes. The other planets, including a faint Uranus, can be seen with the Naked eye.

Astronomers in ancient times were hence unaware of the planes beyond Saturn since they did not have telescopes.

When the planets beyond Saturn were first discovered?

In 1781, Sir William Herschel discovered the Existence of the planet Uranus. It is twice as far out as the sixth planet Saturn.

Which planet is farthest from the Earth?

The eccentric orbit of Pluto regularly brings it closer to the Sun than Neptune. Hence Neptune and Pluto will take turns to be the farthest planet from time to time.

Do all the planets have an atmosphere similar to that of the Earth?

Mercury and Pluto have no atmosphere. The other planets have atmospheres containing different gases. Earth’s atmosphere, containing mainly nitrogen and oxygen, is unique. Venus has a carbon dioxide atmosphere. The atmosphere of Mars is mainly argon and carbon dioxide. Jupiter, Saturn, Uranus and Neptune have thin atmospheres containing hydrogen, helium and methane.

Why are Mercury, Venus, Earth and Mars Called “Terrestrial” planets?

“Terrestrial” means Earth-like. All the terrestrial planets have large, solid, metallic cores surround by a rocky outer layer. All of them have atmospheres.

Why Jupiter, Saturn, Uranus and Neptune are called “Gas Giants”?

The gas giants or Jovian planets do not have any significant solid cores and are almost entirely composed of gases or liquefied gases. They are also enormous in size compared to the other planets.