What was the last planet in our solar system to be discovered during the 19th century?

Answer 1

Neptune.

Answer 2

During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846.

Answer 3

The only planet to be discovered in the 19th century was Neptune. Mercury, Venus, Earth, Mars, Jupiter, and Saturn were known to the ancients. Uranus was discovered in the 18th century, and Neptune was discovered and identified in the 19th century. (It was actually first seen by a human in the 17th century, however, but not then identified as a planet.) Pluto was discovered in the 20th century and defrocked in the 21st century.

Answer 4

Neptune: Pluto was discovered in the 20th.