Answer:
Introduction:
The discovery of planets using telescopes has played a crucial role in expanding our knowledge of the universe. In this question, we are asked to identify the first planet discovered through the use of a telescope.

The first planet to be discovered by the telescope was Uranus. Here is a detailed explanation of the discovery:
1. Background:
- Before the invention of the telescope, the only planets known to ancient civilizations were Mercury, Venus, Mars, Jupiter, and Saturn.
- These five planets are visible to the naked eye and were known since ancient times.
2. The Discovery:
- In the 18th century, astronomers began using telescopes to explore the night sky in more detail.
- On March 13, 1781, the British astronomer Sir William Herschel observed an object in the constellation of Gemini.
- At first, Herschel believed it to be a comet, but further observations revealed that it was a planet.
- The discovery of Uranus marked the first time a planet had been identified using a telescope.
3. Confirmation and Recognition:
- Herschel initially named the newly discovered planet "Georgium Sidus" in honor of King George III of England.
- However, the name Uranus was eventually adopted, following the tradition of naming planets after ancient Greek and Roman deities.
- The discovery of Uranus was a significant achievement for Herschel and brought him international recognition as an astronomer.
Conclusion:
The first planet to be discovered using a telescope was Uranus. This discovery by Sir William Herschel in 1781 marked a significant milestone in our understanding of the solar system and expanded our knowledge of the planets beyond the five known since ancient times.