Showing how concepts change, here's a delightful Science News article from 1951: "Not just nine, but thousands of planets are known to circle our sun. Most of them are tiny bits of matter, ranging from several hundred miles across down to a city block." https://www.jstor.org/stable/pdf/3928808.pdf?ab_segments=0%2Fbasic_SYC-5187_SYC-5188%2Fcontrol&refreqid=fastly-default%3A1e75ccf801f0b38164ef1b0728bc0fce

The 1st sentence: "There are thousands of known planets circling our sun. Yet it is still quite right to say the chief planets are Mercury, Venus, our own earth [sic], Mars, Jupiter, Saturn, Uranus, and Pluto...The other planets are little bits of matter...down to a city block."

Also (written 1951), "When a new planet is discovered...it is possible to compute its position...But these tiny planets are easily pulled out of their path by large planets, particularly giant Jupiter...[So they might] get lost or be mistaken for a new planet when spotted again."

I find this fascinating because it clearly shows how the scientific view had diverged from the public's understanding of planets. The science journalist felt she needed to explain to her audience that scientists consider the asteroids planets, because the public didn't know that.

Then, she mentions the *key* scientific development that was about to change everything in our concept of planets vs. asteroids, although this article was written just two years before the actual change took place. (Fascinating!) She wrote,...

"Five to ten baby planets the size of Ceres were formed between the orbits of Mars and Jupiter about the time our earth was born, according to Dr. Gerard P. Kuiper of Yerkes Observatory of the @UChicago. Several small planets were created instead of a single big one...

..."because of the disturbing pull exerted by the near-by giant planet Jupiter, which likewise was being formed from the cosmic cloud about that time.

"Sometime within the past three billion years two of these minor planets bumped into each other, Dr. Kuiper reasons...

..."Numerous tiny planets were thus created. Collisions between these baby planets became increasingly frequent until thousands of asteroids, flying mountains known to exist in this region today, had formed."

That was Kuiper's thinking in 1951. But then, just two years later...

he wrote: "The hypothesis made before was based on the assumption that gravitational instability**, clearly operative in the formation of the 8 major planets, was also responsible, though in modified form, for the formation of the original group of minor planets." (**key point)

But now in 1953 he knew better. He understood that small bodies can accrete by slowly adding little bits of matter from the solar nebula even though they are too small for gravity to attract matter to themselves. This changed his idea of a planet [Will finish this thread ASAP]