Thread by @DrPhiltill

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." jstor.org/stable/pdf/392…

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]

[Back to this short thread] Earlier, in 1950, Kuiper wrote: "The largest asteroids, like Ceres, are regarded as such original condensations; they are in effect true planets."

He thought the smaller asteroids were all rubble from the original 5 to 10 "true planets" colliding.

But in 1953 he wrote that the smaller asteroids could have formed directly (without gravitational instability!) through accretion. Some were rubble from collisions, others were direct accretions. It was in this 1953 paper that he stopped calling the smaller asteroids "planets".

Before that 1953 paper, papers on planet formation were calling the asteroids "planets". After that 1953 paper, all the scientists who cited Kuiper also stopped calling them planets. Other theory papers soon stopped. Then observational astronomers also stopped during the 1960s.

So it was fascinating to read this @ScienceNews article from 1951, which was penned right between Kuiper's two seminal papers when he (and then all astronomers) decided that the smaller, non-round asteroids are not planets. jstor.org/stable/pdf/392…

It's fascinating, because the journalist's need to explain scientific terminology shows us that the public *already* didn't consider asteroids to be planets. In fact, we know the public lost interest in asteroids in the mid-1800s, because discovering so many seemed boring.

Here is an 1835 almanac with astrological interpretations of the planets. (Almanacs were the public's main source of info about planets until the late 1800s when education improved.) You can see it still teaches the old geocentric set of planets, plus Uranus ("Herschel").

Here is an 1842 almanac. The Sun is the "governing planet" for 1842😆 It's funny how it gives an astrological interpretation for "Hershel" (Uranus) even though it was never part of astrology before. How was this knowledge acquired? But note the asteroids still aren't on the list.

Finally we see asteroids on the list in this 1857 almanac. But no astrological interpretation was given for them. One historian pointed out that there were quickly too many planets to use in astrology. You couldn't make up a system that made sense out of so many planets.

But here's an 1826 almanac that includes THE EARTH as a planet. (Shocker!) And it includes the Sun (!), and it includes the Moon (which wasn't strange since scientists counted the Moon & other moons as planets). And it omits the asteroids (!). The 1820/30s was a period of chaos.

Through the 1800s into the 1900s, textbooks were including asteroids as one of the four types of planets (terrestrial, minor, giant, and secondary planets) but stopped listing them all because it was impossible to fit them on the page. The public never knew about them all.

So this is why the @ScienceNews reporter had to explain that scientists considered even the small, city-block sized asteroids to be planets. And this was exactly when Kuiper was concluding that most formed by accretion with insufficient gravity to be "true planets". Fascinating!