Starting a new thead, because I can't resist the pun in the title. Sean said, "Ceres was originally a planet when it was discovered in the 19th century, but was demoted when the data showed it was quite small. It was given the status of asteroid, "star like"."

Not exactly. Size had nothing to do with it. There was never any question that Ceres was relatively small. That was obvious, once its distance was known. (Any large object that close would be bright enough to be discovered far earlier. It could as dim as Ceres only if it were farther, or smaller, or by some odd coincidence much darker in color.)

Nonetheless, though everyone knew from the start that Ceres and the other early-discovered asteroids were quite small, they were considered planets for over fifty years.

When this changed, the change was no principled matter, but just a matter of linguistic convenience. To save writing astronomers had a symbol for each planets (♀ for Venus, ♂ for Mars, etc.), and invented new symbols for each asteroid when discovered. But after about 50 years, so many had been found that it became difficult to concoct new symbols, so the system was changed, and each asteroid's symbol was just an encirled number, in order of discovery.

With asteroids having different symbols than the classical planets, they soon ceased to be thought of as "planets". That is, when the definition of "planet" changed to exclude the asteroids, it changed not for any astronomical reason, but rather for an orthographic one..

This account is based on the longer explanation here. Quoting the conclusion:

It was recognized from the beginning that these bodies did not fit along with the rest of the planets, but during the years in which the number of known asteroids was small, they were included on the list of solar system bodies just as if they were regular planets. However, once their numbers grew too large to fit the existing scheme of classification, their uniqueness was quickly recognized and a new class of solar system objects was created. The event that appears to have triggered this reevaluation of the status of the asteroids was the introduction of a new set of symbols to use as a shorthand notation for the asteroids.

quote:

Not exactly. Size had nothing to do with it.

In my defense, I was referring to this quote from wikipedia.

quote:

Ceres turned out to be disappointingly small for a planet, showing no discernible disc, so Sir William Herschel coined the term "asteroid" ("star-like") to describe it.

The word 'asteroid' was coined and came into use soon after the first asteroid was discovered. But Seanhan was misled, and Wikipedia is confusing, because the word 'planet' has changed. For over 50 years, an asteroid was considered a type of 'planet' (contrary to today's usage) as in this 1847 newspaper account:

Another New Planet. – Another member belonging to the family of Asteroids, between Mars and Jupiter, was discovered …. This makes the eighth known Asteroid and the fifth new Planet that has been discovered within the last two years.
– (Canton) Ohio Repository, Dec. 1, 1847

To understand this quote, it will be helpful to know the discovery dates of the first eight asteroids plus Neptune.
1801-1807: 4 asteroids (Ceres, Pallas, Juno, Vesta)
next 37 years: none
1845-1847: 4 asteroids (Astraea, Hebe, Iris, Flora), plus Neptune