Which was the first planet to be discovered using a telescope?

The first planet to be discovered using a telescope was Uranus. This historic discovery occurred in 1781 when the German-born British astronomer William Herschel observed an object that appeared to move differently than the fixed stars. Initially, Herschel thought it might be a comet, but further observations confirmed it was indeed a planet — the seventh from the Sun.

Before Uranus, the planets visible to the naked eye — Mercury, Venus, Mars, Jupiter, and Saturn — had been known since ancient times. Uranus’s discovery was groundbreaking because it extended humanity’s knowledge of the solar system beyond Saturn and demonstrated the power of telescopic observation for scientific discovery.

Uranus is a gas giant, composed mainly of hydrogen, helium, and methane, which gives it a pale blue color. It has a unique axis of rotation, tilted almost 90 degrees, making it appear to roll on its side as it orbits the Sun. Uranus also has a system of rings and moons, further enriching our understanding of planetary systems.

The discovery of Uranus inspired astronomers to search for other distant planets, eventually leading to the discoveries of Neptune and Pluto (although Pluto is now classified as a dwarf planet). Herschel’s meticulous observations highlighted the importance of systematic astronomy, precise measurement, and careful documentation in expanding scientific knowledge.

Therefore, the correct answer is Uranus (Option A), as it was the first planet discovered using a telescope, marking a major milestone in the history of astronomy and opening the door for modern planetary science.