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I'm currently reading Planetary Geology, in which they state

When Ceres [...] was discovered in 1899, it was considered as the lost tenth planet.

Uranus and Neptune had been discovered, but Pluto wasn't know yet at the time. What were the nine planets back then? The bodies we now call planets + the Moon?

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    $\begingroup$ Does the book really state Ceres was discovered in 1899? Ceres was discovered in 1801. If the book is off by 98 years, it could also be off on the number of planets. :-) $\endgroup$ – JohnHoltz Jun 29 at 16:08
  • $\begingroup$ It does say 1899. Maybe the authors aren't too good with numbers, then... At least it's a book about geology and not about history ¯_(ツ)_/¯ $\endgroup$ – usernumber Jun 29 at 16:28
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    $\begingroup$ The 1890s was when Ceres was reclassified as an asteroid, so maybe they got their dates mixed up. Vesta was also temporarily considered a planet by some (there was no official categorization), and though Ceres was discovered first its orbit is outside of Vesta's, so perhaps Vesta was given a lower number by this source for that reason. I have no idea why Ceres would be 10th, then, because if the numbering were based on orbit it should be 6th. $\endgroup$ – called2voyage Jun 29 at 16:59
  • $\begingroup$ @called2voyage The 1850s was when Ceres, along with Pallas, Juno, and Vesta, were unofficially declassified as planets. (There was no IAU to officially designate what was / was not planet.) Those first four asteroids were discovered in quick concession in the very early 1800s. With no notable discoveries for the next thirty-plus years, those first four asteroids were for a while deemed to be on a par with the classical planets. And then came the deluge. By 1868 there were 100 recognized asteroids. $\endgroup$ – David Hammen 2 days ago
  • $\begingroup$ I have confirmed that both parthat s of the quote, that Ceres was discovered in 1899 (sic), and that Ceres was considered as the lost tenth planet (sic), are indeed in the linked text. All I can say is "Wow" -- and it makes me sick. $\endgroup$ – David Hammen 2 days ago
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Short Answer:

I can't make any sense out of what Planetary Geology says.

Long Answer:

I can't make any sense out of what Planetary Geology says. But here is some data about the changing number of planets believed to exist in our solar system during the 19th century.

Here is a link to the Wikipedia article Planet, specifically to the section "Objects Formerly Considered to be Planets".

https://en.wikipedia.org/wiki/Planet#Objects_formerly_considered_planets 1

It has a long list of 27 objects considered to be planets at one time but later reclassified as the sun, or natural satellites (moons), or asteroids. By 1854 all of those objects - including Ceres - had been reclassified as not being planets. So after 1854 or so the number of accepted planets was 8 until 1930, 9 from 1930 to 2006, and 8 from 2006 to the present.

I note that if someone numbered the objects still considered to be planets in order of their discovery, the list of planets would included 6 planets including Earth up to 1781 when Uranus was discovered, and then 8 planets when Ceres was discovered in 1801. Then 9 planets when 2 Pallas was discovered in 1802, 10 planets when 3 Juno was discovered in 1804, and 11 planets when 4 Vesta was discovered in 1807. When Neptune was discovered in 1846 it could be classed as the 8th planet by those who discounted the asteroids or as the 12th planet by those who counted Ceres, Pallas, Juno, & Vesta.

If the planets are numbered in order of increasing size, Ceres would have been number 1 when it was discovered in 1801, become 2 in 1802, and eventually become number 9 when 9 Metis was discovered in 1848, assuming that astronomers of that era could tell that Ceres was larger than other asteroids. Ceres would become number 10 when 10 Hygiea was discovered in 1849, 11 when 11 Parthenope was discovered in 1850, and eventually number 15 when 15 Eunomia was discovered in 1851.

If the planets are numbered in order of decreasing size, Ceres would have become number 8 when it was discovered in 1801 and would have remained so until Neptune was discovered in 1846, making Ceres number 9 until being reclassified as an asteroid in the 1850s. Unless one of the other largest asteroids was ever believed to be larger than Ceres.

If planets were numbered inwards from the outermost known one, the list in 1801 would have been 1 Uranus, 2 Saturn, 3 Jupiter, 4 Ceres. in 1802 2 Pallas would become 4 and Ceres 5.

In 1846 Neptune would be discovered and Ceres would become # 6. In 1849 10 Hygiea was discovered and Ceres would become # 7. In 1852 16 Psyche was discovered making Ceres # 8. In 1852 22 Kalliope was discovered making Ceres # 9. In 1853 24 Themis was discovered making Ceres # 10. In 1854 28 Bellona was discovered making Ceres number 11. And in 1854 31 Euphrosyne was discovered, making Ceres number 12.

But I should note that none of the asteroids after 15 Eunomia, discovered in 1851, were ever considered to be planets.

If planets were numbered outwards from the sun, as is normal, Mercury would be 1, Venus 2, Earth 3, Mars 4, and 1 Ceres would become 5 in 1801. Ceres would become 6 in 1804 when 3 Juno was discovered, 7 in 1807 when 4 Vesta was discovered, 8 in 1845 when 5 Astrea was discovered, 9 in 1847 when 6 Hebe was discovered, 10 in 1847 when 7 Iris was discovered, 11 in 1847 when 8 Flora was discovered, 12 in when 9 Metis was discovered, 13 in 1850 when 11 Parthenope was discovered, 14 in 1850 when 12 Victoria was discovered, 15 in 1850 when 13 Egeria was discovered, 16 in 1851 when 14 Irene was discovered, and 17 in 1851 when 15 Eunomia was discovered.

So if planets and "planets" were numbered in order of increasing size, Ceres would be ninth between the discovery of 9 Metis on 25 April 1848 and the discovery of 10 Hygiea on 12 April 1849, and the tenth until the discovery of 11 Parthenope on 11 May 1850.

So if planets and "planets" were numbered in order of decreasing size, Ceres would be ninth between the discovery of Neptune on 24 September 1846 and being reclassified as an asteroid in the 1850s,and would never have been considered the tenth planet in size. If Ceres was still considered to be a planet, it would have been considered the tenth largest planet after Pluto was discovered in 1830, but Ceres was no longer considered a planet by then.

So if planets are numbered inwards from the outermost known planet, Ceres would have been considered the ninth planet between the discovery of 22 Kallipe on November 16, 1852 and the discovery of 24 Themis on April 5 1853, and the tenth planet between April 5, 1853 and the discovery of 28 Bellona on March 1, 1854.

Except that as far as I know no asteroids after 15 Eunomia discovered in 1851 were ever listed as planets.

So if planets are numbered outwards from the innermost known planet, Ceres would have been considered the ninth planet between July 1, 1847 when 6 Hebe was discovered and August 13 1847 when 7 Iris was discovered, and the tenth planet between August 13 1847 and October 18 1847 when 8 flora was discovered.

After Urban Le Verrier used discrepancies in the orbit of Uranus to predict the existance of a planet beyond Uranus, and Neptune was discovered in 1846, Le Verrier studied the orbits of other planets. In 1859 Le Verrier deduced that there should be a planet orbiting inside the orbit of Mercury, which he named Vulcan.

Observations of objects believed to be Vulcan were often made but never confirmed. In 1915 Einstein's general theory of relativity provided an alternate theory of gravity which explains the motions of mercury without the planet Vulcan.

When Vulcan was was believed in it was probably considered to be larger than Ceres, which would have made Ceres the tenth largest planet, except that Ceres was no longer considered a planet.

All things considered, I believe that the only possibly ninth planet that some astronomers might have believed to exist in 1899 was Vulcan. So if Ceres was discovered in 1899 it might have been considered the tenth planet to be discovered. Except that Ceres was discovered in 1801, 98 years before 1899.

And by 1899 some astronomers had already attempted to calculate the orbit of a planet believed to be beyond Neptune, a planet X, "X" for unknown, a search which eventually led to the more or less accidental discovery of Pluto.

But as far as I know nobody announced any discovery of a new planet in 1899 which turned out to be false but would have been the tenth planet to be discovered if both it and either Vulcan or Planet X was real.

So I have no idea what Planetary Geology meant by stating:

When Ceres [...] was discovered in 1899, it was considered as the lost tenth planet.

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