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The story begins in 1781 when the "amateur" astronomer William Herschel suspected of discovering traces of a new planet in his own yard. In 1783, it was confirmed by Laplace that it was a new planet, Uranus. In 1785, the young mathematician Bouard went to Paris to make a living and finally met Laplace in 1794. At that time, Laplace was working on his masterpiece, Celestial Mechanics, which had a lot to calculate, and Bovard did this work. In the days that followed, Bovard discovered eight new comets and published catalogues of Jupiter, Saturn and Uranus.

However, Boward soon discovered that something was wrong: the orbit of Uranus calculated from observations after 1781 did not match previous observations (it had been observed before, but did not know it was Uranus). And the modified orbit quickly deviated from the orbit. Boward has created a system of 77 equations that is still unable to correct this error. Therefore, Boward believes that there must be a celestial body outside affecting the movement of Uranus. Then comes the familiar story. In 1846, three years after Bouard's death, Levier and Adams calculated that Neptune, the eighth planet in the solar system, was discovered. So Neptune is also known as the planet under the tip of the pen.

Fig. 1 Adams and Levier ✦✦✦

But soon, the astronomical community discovered a new problem: Neptune can not fully explain Uranus's orbital anomaly, so there should be another planet. Astronomers look beyond Neptune, looking for further planets!

The search for new planets is not easy. In the last 50 years of the 19th century, astronomers found all kinds of evidence of the existence of new planets, and some even predicted the orbits and cycles of new planets, but no trace of new planets was found. In 1906, Lowell, who was scheduled to observe life on Mars, began to find a new planet, which he called Planet X. In 1915, Lowell gave what he thought were the elements of the orbit of Planet X after a decade of calculation, but still did not find it in the predetermined area. In the following year, Lowell died. Lowell's death brought the search for Planet X to a standstill, and in 1925, Lowell's brother sponsored a 330mm telescope to the observatory, with which the exploration of Planet X began again.

Fig. 2 13-inch (330mm) telescope ✦✦✦ of Lowell Observatory

In 1929, the director of the observatory entrusted the task to the young Clyde Tombaugh. Tombo's job can be described as simple and boring-using a blink comparator to compare the differences between different night sky photos. By the early 1930s, he had compared pictures of nearly 2 million stars and finally found a moving object on film taken between January 29 and 23. After repeated confirmation, Tombaugh thought it was Planet X. On March 13, the Observatory announced the discovery of a new planet on the occasion of Lowell's 75th birthday, a difference of only 5.9 °from Lowell's previous prediction, meeting Lowell's prediction of the orbit of Planet X. Eventually, the planet was named Pluto. It was later discovered that Pluto had been photographed by the observatory before Lowell's death, but it was not screened out from tens of thousands of photos at the time.

Fig. 3 A photo taken by Pluto, ✦✦✦

For ordinary people, champagne can be opened here. But students who have been champagne in their previous lives all know that opening champagne rashly without being completely sure is often half-time. After all, people still don't know enough about Pluto at this time.

As soon as Pluto was discovered, people began to wonder if Pluto could take the position of Planet X. because Pluto looks very small and its brightness is six orders lower than Lowell predicted, supporters believe this may be due to low reflectivity. Opponents think this is an asteroid or even a comet.

But in the end, the trend of history has made supporters more and more disappointed. At first its mass was estimated to be similar to that of Earth; by 1976, by calculating Pluto's albedo, it was only 1% of Earth's mass; in 1978, Pluto's moon Charon was discovered, and Charon astronomers calculated that Pluto's mass was only 0.2% of Earth's. Another important problem is that Pluto's moon Charon is relatively large in size and mass, and the two can even be tidal locked to each other, which in theory affects Pluto's ascending point longitude and orbital inclination. In this way, a new problem arises, why is the previous calculation correct?

Obviously, there are new planets affecting the orbit of Uranus.

After 1978, some astronomers continued to look for Lowell's Planet X, while others believed that Planet X did not exist. In July 1992, E. Myles Standish used data from Voyager 2 to reduce the mass of Neptune by 0.5%. The 0.5% difference is not much, but the difference in Uranus's orbit has disappeared for Planet X. In addition, Voyager and other probes have not yet discovered the gravity of large celestial bodies. It seems that Lowell's Planet X really doesn't exist.

Can we open the champagne? Wait a minute!

In August 1992, 15760 Albion was discovered, the first Neptune object after Pluto. 15760 Albion is the prelude to the discovery of Neptune objects. So far, thousands of Neptune objects have been discovered. Most of these objects are located in a zonal region 30-50 astronomical units from the sun, which astronomers call the Kuiper Belt. In 2005, astronomer Michael E. Brown announced the discovery of a Neptune object larger than Pluto, which was later named Pluto. The discovery of Eris sparked discussion among astronomers about the definition of planets, and finally in 2006, the International Astronomical Union defined planets, the most important and controversial of which was that asteroids in the vicinity of their orbits must be cleared. There can be no larger object in orbit. According to this definition, both Pluto and Eris are defined as dwarf planets. Brown called himself the "Pluto killer" and published his memoir "How I Killed Pluto and Why It Had It Coming" in 2010. Interestingly, in 2010, through occultation observations, Eris was smaller than Pluto, so Brown, who once claimed that Eris must be bigger than Pluto, was a bit embarrassed, and he made his own comments (two websites in the references).

Fig. 4 Hercules, is so vague and inconspicuous ✦✦✦

Although Brown "killed" Pluto, he opened the new magic box of Planet X. In 2003, Brown and Rabinovitz (David Rabinowitz) and Trujillo (Chadwick A. Trujillo) discovered an object named 2003VB12, which was later named Sedna. Sedna's orbit is very exaggerated, perihelion is 76 astronomical units, aphelion is 937 astronomical units, compared with the average planet, Sedna's orbit is very abnormal, which aroused the interest of astronomers. Brown and colleagues believe that the reason for the formation of Sedna's orbit is the invisible planets outside the Kuiper belt or other stars in the early stages of the formation of the solar system. Perhaps the previous search for Planet X has become the belief of astronomers, Brown paid more attention to the planetary hypothesis and put forward the "ninth planet" hypothesis.

Fig. 5 Sedna's orbit (red) and Pluto's orbit (purple) in 2012, when astronomers modeled 92 Kuiper Belt objects, they found anomalies in the orbits of six of them (Sedna, 2012 VP113, 2004 VN112, 2010 GB174, 2007 TG422 and 2013 RF98). The perihelion of these planets are very close in longitude and all have an orbit tilted relative to the ecliptic and nearly coplanar. And because it was discovered by six different surveys by six telescopes, astronomers ruled out the influence of observation bias. By calculation, Brown believes that there is only a 0.007% chance that this situation is accidental, in other words, it is almost certainly affected by external forces.

Figure 6 the orbits of six Neptune extraterrestrial bodies and the predicted orbits of the Ninth Planet Brown and colleague Konstantin Batygin estimate the orbit of the Ninth Planet: the eccentricity is 0.2-0.5, the semi-major axis is 400-800 astronomical units, and the mass may be 10 times that of Earth. However, they still have not found the "ninth planet" in the calculated position. In Brown's view, we need more telescopes to survey the sky and public participation to finally find the lonely planet. History is so magical that the man who buried Lowell's Planet X began the search for a new Planet X. let's hope he doesn't become another Lowell.

Although there are still many astronomers who question the existence of the ninth planet, according to Battikin, who knows something about history, we must not think that we know everything about the solar system and that there is nothing we do not know. After all, the last person who said something similar (Kelvin) is still nailed to the "pillar of shame" in the history of physics.

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reference

[1] Collander-Brown S, Maran M, Williams I P. The effect on the Edgeworth-Kuiper Belt of a large distant tenth planet [J]. Monthly Notices of the Royal Astronomical Society, 2000 (1): 101108.

[2] https://mikebrownsplanets.com/2010/11/shadowy-hand-of-eris.html

[3] https://mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html

[4] Horner J, Evans N W. Biases in cometary catalogues and Planet X [J]. Monthly Notices of the Royal Astronomical Society, 2002, 335 (3): 641,654.

[5] Brown M E, Trujillo C, Rabinowitz D. Discovery of a candidate inner Oort cloud planetoid [J]. The Astrophysical Journal, 2004 (1): 645.

[6] Batygin K, Brown M E. Evidence for a distant giant planet in the solar system [J]. The Astronomical Journal, 2016 (2): 22.

[7] Batygin K, Adams F C, Brown M E, et al. The planet nine hypothesis [J]. Physics Reports, 2019: 1-53.

[8] Witze A. Evidence grows for giant planet on fringes of Solar System [J]. Nature, 2016, 529 (7586): 266-267.2007

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This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Xing Si

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