After Pluto was discovered by William Thompson in 1930, it was considered as the ninth largest planet in the solar system, next to mercury. But after 76 years, they were demoted by the vote of scientists and kicked out of the ranks of the nine planets. Why?
The “eight planets” are the inner planets of the solar system. According to the distance from the sun, they are Mercury, Venus, earth, Mars, Jupiter, Saturn, Uranus and Neptune.
In 1992, many ice bodies with mass similar to Pluto were found in the Kuiper belt, challenging Pluto’s planetary status.
The mass of Eris discovered in 2005 was even 27% more than that of Pluto, so the International Astronomical Union formally defined the concept of planet the following year.
Planets are defined as:
1、 It has to be a celestial body orbiting a star.
2、 The mass is large enough to make the celestial body spherical by its own gravity.
3、 There should be no other objects near its orbit, or it can “clean up” the objects in its orbit within 3 billion years.
From this definition, Pluto doesn’t agree with the third one. Pluto’s orbit intersects with Neptune, and Pluto’s moon is too large. So remove Pluto from the planet list and reduce it to a dwarf planet.
Pluto’s surface is frozen methane, the temperature is very low, about – 200 degrees Celsius, is a real frozen world.
Compared with the earth, Pluto is very terrible. Its atmosphere is thin, mainly composed of nitrogen, methane and carbon monoxide. Therefore, its atmospheric pressure is extremely low, which is about one million to one hundred thousandth of the earth’s atmosphere. Therefore, there is great uncertainty about whether there are clouds.
Pluto has a long rotation period, about 248 years. So one year on earth, 248 years on Pluto. The polar days and nights of Antarctica will be very long. In 1987, Pluto’s South Pole ended 120 years of polar night. So if you live at the south pole of Pluto, you’ll never see the sun in your life.
Although Pluto has been kicked out of the nine planets, many people still hope and support its return today. But there is no way, Pluto does not meet the definition of a planet, so it can only be excluded from the nine planets.