Nearly a century after the discovery of Neptune, scientists made another groundbreaking achievement by discovering Pluto. Initially, some predicted that there might be a planet beyond Neptune, and astronomers like Lowell and Pickering published orbital elements for this hypothetical planet. After Lowell's death, the Lowell Observatory continued the search for Pluto until 22-year-old Tombaugh discovered it in February 1930. The discovery of Pluto created a sensation, especially for Americans, as it marked their first honor of discovering a new planet.
Pluto's mass is significantly smaller than expected, yet it was found near the predicted orbital path, which is puzzling. It follows an elongated orbit that takes 248 years to complete, averaging 40 astronomical units from the Sun. From Pluto, the Sun appears as a bright star. The temperatures here can drop to minus 240°C, making it one of the coldest and darkest corners of the solar system. Pluto's orbit is quite peculiar, with an eccentricity of 0.256, causing it to occasionally cross into Neptune's orbit. Its farthest distance from the Sun is 49 astronomical units, while its closest approach is 20.6 astronomical units, comparable to the distance from Uranus to the Sun.
Pluto's mass is extremely small, only 0.002 times that of Earth, and it is the smallest among all planets. Its brightness variations are also strange; since its discovery, despite moving towards perihelion, its brightness has gradually dimmed, decreasing by an average of 0.1 magnitudes every ten years.
