This year marks the 90th anniversary of the discovery of Pluto.
  Pluto was once listed as one of the nine planets in the solar system, and was later expelled from the ranks of large planets and downgraded to a dwarf planet. However, Pluto has never faded out of people’s field of vision. In July 2015, in a close-up photo of Pluto taken by the US “New Horizons” probe, a big love pattern once again made Pluto a hot topic.
Appearance-Discovery Under the Expectations

  Before Pluto was discovered, people had found 8 planets around the sun, and the last discovered Neptune was also called “the planet on the tip of the pen.” French astronomer Le Verrier used Uranus’ orbit to be affected by the gravitational effects of other celestial bodies and calculated the location of another planet that might exist on its periphery. He informed John Galle of the Berlin Observatory of his calculations and asked him to help with the search. Sure enough, in 1846, Galle and his student Heinrich Darest discovered the trace of Neptune, and its actual position was only 1° deviation from the position estimated by Levier. As a result, Neptune became the first planet whose position was obtained through mathematical calculations and then confirmed by observation with a telescope, so it was dubbed the “planet on the pen tip”.
  After Neptune was discovered, it was discovered that Neptune’s gravitational influence could not fully explain the changes in Uranus’ orbit. Therefore, it is speculated that there should be other celestial bodies moving in the periphery, affecting the orbits of Uranus and Neptune. Many scientists have given their own results, all wanting to use their own calculations to replicate the next “planet on the pen tip” legend after Neptune. But the greater distance and smaller orbital deviation will undoubtedly make it more difficult to find new planets beyond Neptune.
  One name that has to be mentioned among many people who are struggling to search for new planets is Parcival Lowell. This wealthy Bostonian founded the Lowell Observatory in Flagstaff, Arizona in 1984. This is also the first time the observatory was built at a height away from humans in order to obtain a better observation environment. Altitude area. Lowell used the telescope on the observatory to devote himself to two obsessive astronomical tasks, one is to find traces of canals on Mars, and the other is to search for unknown planets that may exist outside Neptune, which he called Planet X .
  The First World War and the dissatisfaction of astronomical research caused a huge blow to Lowell’s body and mind. He unfortunately died of a stroke in 1916, and the Lowell Observatory’s search for Planet X was also interrupted. It was not until 1929 that Clyde Tombaugh, a 23-year-old young man recruited by the Lowell Observatory, continued this work, and the mysterious planet X was revealed. Pluto was discovered by Tombaugh at about 4 pm on February 18, 1930. Everyone will be curious, how can you see the stars when the sky is not dark at around 4 pm? In fact, Tombaugh did not observe Pluto directly with a telescope, but found Pluto through a special method.
  Tombaugh discovered that Pluto uses an astronomical device called a “flicker contrast”, which can quickly switch back and forth between two pictures of the same starry sky taken at different times, using visual residual phenomena to make the human eye perceive two The slight difference in photos is a bit similar to the two-picture finding difference game we often play now. On the afternoon of February 18th, the diligent Tom Bo, through repeated comparisons of the two photographic negatives taken on January 23 and 29, found a small bright spot that was moving in Gemini at that time. This was later named Pluto. The ninth planet.
Controversy-New celestial bodies that are messing up

  Since the discovery of Pluto, controversy about it has continued to emerge. Throughout the 20th century, astronomers have been working to determine its orbit and mass. The measured mass of Pluto is very small, too small to have too much influence on Uranus’ orbit. Other new scientific discoveries in the solar system are also shaking Pluto’s status as the ninth largest planet.
  In 1978, Pluto’s first satellite, Charon Charon, was discovered by the telescope of the U.S. Naval Observatory, also located in the small town of Flagstaff, Arizona. Charon’s discovery allowed astronomers to better determine the masses of the two stars by revolving them. As a result, the mass of Pluto was smaller than previous estimates, while Charon’s mass was ten times that of Pluto. One part, the radius reaches half of Pluto. The two orbits also formed a 1:1 tidal lock orbit, which means that Pluto and its moon Charon have only one side of each other orbiting each other, which is also unique among the planets in the solar system. It can be said that Charon’s various properties have shaken Pluto’s planetary status.
  In 1992, the field of solar system research ushered in an epoch-making discovery. Professor David Juitt of the University of Hawaii and his graduate student Lixing Liu discovered an icy celestial body outside Neptune-1992QB1. This celestial body moved in the area predicted by the famous astronomer Gerard Kuiper and became a human being. The first Kuiper belt object. Since then, astronomers have discovered more than 1,000 Kuiper Belt objects. Among them, Eris, Makemake, and Haumea are all about the size of Pluto, and satellites have been found around 10% of Kuiper belt objects.
  We must know that Pluto itself is also located in the Kuiper Belt. When more Kuiper Belt objects are discovered, Pluto does not appear to be a maverick in it. If it is a planet, should these newly discovered Kuiper Belt objects of similar size also be defined? For the planet? More new discoveries have brought not only the expansion of the understanding of the outer boundaries of the solar system, but also a real threat to Pluto’s position in the solar system.
  In addition, many properties of Pluto itself are also different from the other eight planets. For example, the mass of Pluto is very small, only 5% of the mass of Mercury, the smallest of the eight planets. Its orbit is not a near-circular orbit, but a flat ellipse, and there is a cross between the orbits of Neptune adjacent to it. In addition to its very flat orbit, Pluto’s orbit is also more inclined. Its angle with the ecliptic plane of the solar system is 17°, which is much larger than that of Mercury, which has the largest orbital inclination among other planets. Some astronomers and astronomy science workers are also constantly thinking about how to better define Pluto’s status in the solar system.

  At the 26th International Astronomical Union Congress in 2006, 424 participants voted on the spot to pass the new definition of planets, and Pluto was officially excluded from the ranks of planets.

  At the end of the 20th century, the American Museum of Natural History decided to rebuild the historic Hayden Planetarium. When the first curator of the new museum, Neil Tyson, organized the design of the venue, he racked his brains on how Pluto would arrange the location in the venue. In the solar system exhibition, Tyson was struggling to find out where to attribute Pluto. He even organized a public debate and invited five authoritative experts with different positions to explain whether Pluto should be displayed as the ninth planet. The discussion did not reach the final result. However, Hayden Planetarium, led by Tyson, decided to introduce the celestial bodies of the solar system in a different way, that is, instead of listing the planets in order according to the previous convention, but according to the nature, the solar system has been discovered at that time. Many celestial bodies are classified and displayed: the sun is displayed separately as Big Brother; the next terrestrial planets include Mercury, Venus, Earth and Mars; the Jupiter, which is the gas giant planet, includes Jupiter, Saturn, Uranus, and Neptune; Pluto is due to its weight. The natural disadvantages of, naturally cannot be classified into any of the above categories, but are placed in the Kuiper belt objects; the outer part is the Oort cloud full of active comets.