An expelled "planet": What did Pluto offend?

Pluto, as the first Kuiper Belt object detected, is also the largest known dwarf planet in the solar system (by volume), and its mass ranks second. Its orbit is mostly outside Neptune, and its composition is mainly rock and ice. Its mass is relatively light, only one-sixth of the moon's mass and one-third of the moon's volume.

On March 19, 1915, American researchers used a survey telescope to capture a blurry image of Pluto, but it was not accurately identified at the time. It was not until 1930 that American astronomer Clyde Tombaugh successfully discovered Pluto, and it was subsequently regarded as the ninth planet in the solar system, a recognition that lasted until 2006.

The discovery of Pluto was full of twists and turns

The discovery of Pluto is a legendary story. In the late 19th century, astronomers observed Neptune and speculated that there was another planet that affected the orbit of Uranus. In 1906, American astronomer Percival Lowell, founder of the Lowell Observatory, began searching for this hypothetical ninth planet.

In 1909, Lowell and William Henry Pickering, the discoverer of Saturn's ninth moon, proposed several possible locations for the celestial body and launched a search accordingly, but no discovery was made until Lowell's death in 1916. Unfortunately, on March 19, 1915, a fuzzy image of Pluto was captured by the survey telescope, but the opportunity to discover it was missed because it was not accurately identified at the time.

After Lowell's death, the ownership of the observatory sparked a legal dispute that lasted more than a decade until it was resolved in 1929. Subsequently, the search for Pluto was restarted, led by American astronomer Clyde Tombaugh. After nearly a year of hard work, Tombaugh confirmed the existence of a moving object on February 18 after comparing photos taken in January 1930. After further verification, the discovery of the ninth planet was officially announced on March 13.

Since its discovery in 1930, Pluto has been regarded as the ninth planet in the solar system. However, with the in-depth study of Pluto and other celestial bodies in the solar system, its planet status has gradually been challenged. Initially, scientists overestimated the mass of Pluto and believed that it was larger than the Earth, so they named it a major planet. But subsequent observations showed that Pluto's diameter is only 2,300 kilometers, smaller than the moon.

In August 2006, the 26th International Astronomical Union (IAU) voted to reclassify Pluto as a dwarf planet and remove it from the list of planets.

On July 14, 2015, Pluto once again became the focus of scientific attention. After a nine-and-a-half-year, five-billion-kilometer journey, NASA's New Horizons probe successfully flew past Pluto, with a closest approach of only 13,695 kilometers and a flight speed of 13.78 kilometers per second. On the same day, New Horizons also flew past Charon, with a closest approach of about 29,473 kilometers and a slight increase in speed to 13.87 kilometers per second. As the first probe to fly past Pluto, New Horizons sent back a large number of high-definition images and valuable data to Earth, revealing in depth for the first time this mysterious and unexplored corner of the solar system.

A dark and cold alien world

In 1930, Pluto was named Pluto by astronomers. Given that it has the same name as the god of the underworld in ancient Roman mythology, people are more accustomed to calling it Pluto.

Pluto's rotation period is about 6.387 Earth days, and its axial inclination is as high as about 120 degrees, resulting in large areas of polar night on Pluto. At the peak of polar night, a quarter of Pluto's surface is dark all day long.

Due to its great distance, the sun on Pluto appears dim. At perihelion, the brightness of the sun on Pluto is only about one thousandth of that on Earth, and at aphelion, this ratio drops to about one 2,500th. Pluto is extremely cold due to the scarcity of sunlight. The average surface temperature can drop to minus 229 degrees Celsius. This extremely low temperature causes almost all substances such as nitrogen, methane and carbon monoxide to be frozen.

This dark and cold planet is undoubtedly an extremely harsh environment for humans, but fortunately, Pluto's atmosphere is extremely thin, and its surface atmospheric pressure is only one millionth of the Earth's atmospheric pressure at its lowest, and only one hundred thousandth at its highest. This thin atmosphere leads to extremely low efficiency in heat dissipation. Therefore, in theory, as long as humans are equipped with suitable space suits, they can stay safely on Pluto for a long time. In addition, Pluto's gravity is only 6.3% of that on Earth, which means that on Pluto, people will experience an unprecedented sense of lightness, as if they were "as light as a swallow."

Charon Pluto's largest satellite

Reference sources: Charming Science, The Paper, Science Popularization China