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Comment by slg | Hacker News Reader

>There are still people alive today who were born into a world where we thought there was just one galaxy, and only nine planets anywhere.

It might be a pedantic quibble, but you have the timeline of these discoveries a little messed up. There are people alive today who were born in a world in which we thought there was only one galaxy, but they also thought there were only 8 planets at the time. Hubble proved there were other galaxies a few years before Pluto was discovered. You would have to go back to the 1850s to find anyone who thought there were more than 8 planets and only one galaxy. Back then dwarf planets and large asteroids were all categorized as planets which put us at over 20 planets before there was a reclassification.

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Fantastic quibble - I stand corrected (I had Hubble in mind but did not know the 20+ planets part.)

Perfect pedantry :-)

It is always fun to bring up the 20 planets thing when people complain about Pluto not being a planet anymore. Lots of people have never even heard of fellow dwarf planet Ceres, but it spent almost as long as a full fledged planet as Pluto. The only reason most people defend Pluto's inclusion among planets and not Ceres is because Ceres was recategorized a century before most of us were born. It shows that the primary objection from laypeople is that we don't like when something we were taught as children changes.

>>> we don't like when something we were taught as children changes.

The first rule of politics is don't take on any workers that appear in children's books (nurses, farmers, pigs that build straw houses etc). But it does remind me also of Hans Rosling explaining that the facts we learnt as kids (which countries were poor and starving) are today out dated.

I guess if we can learn to let go of Pluto we can learn to accept the world as it really is.

I don't understand what the concept of a "rogue planet" is supposed to be. How do they differ from asteroids?

Asteroids orbit the Sun (or presumably other stars), but don't clear their orbital path, generally within the inner Solar system (within Jupiter's orbit).

https://en.wikipedia.org/wiki/Asteroid

Rogue planets orbit no star. They are interstellar objects.

https://en.wikipedia.org/wiki/Rogue_planet

Dwarf planets are another classification, and can include asteroids, but also other types of bodies (Pluto is often classified as an Kuiper belt object). "A dwarf planet is a planetary-mass object that does not dominate its region of space and is not a satellite."

https://en.wikipedia.org/wiki/Dwarf_planet

The difference between asteroid and planets is just mass. There are some specific rules that dictate the classifications, but the simplest explanation is just that asteroids are hunks of space rock and planets are bigger hunks of space rock.

The "rogue" in rogue planet means isolated similar to how a "rogue wave" doesn't belong to a pattern of other large waves. A rogue planet is a planet that isn't orbiting a star.

There are theoretically also rogue asteroids, but I have never seen that phrased used in that context. That might be because "rogue asteroid" is often used colloquially to describe an difficult to predict asteroid that might hit Earth. Another reason might be is that only asteroids we have ever officially observed are orbiting our sun. Asteroids are so relatively small that it is almost impossible to find one outside our solar system with our current tech. We would basically have to hope that a rogue asteroid just happens to fly through our solar system.

ʻOumuamua[1], which was discovered a few years ago was an interstellar object that might fight the definition of rogue asteroid. However there is still a lot of debate whether it was an asteroid, a comet, some other natural space object, or even potentially some artificial object made by intelligent life. That last theory isn't particularly well supported but it is fun to think about.

[1] - https://en.wikipedia.org/wiki/%CA%BBOumuamua

I would call it planetaroid

Asteroids can already be round.

> Lots of people have never even heard of fellow dwarf planet Ceres, but it spent almost as long as a full fledged planet as Pluto.

Well, if the original IAU proposal for the definition of planet had gone through in 2006, not only would Pluto still be a planet but people would be made aware of new planets Ceres, Eris, and others. It also would have highlighted how there's still much to discover in our solar system, since we wouldn't be able to firmly say how many planets there were. The IAU ended up taking the conservative approach in choosing a definition, one that preserved an overly simplistic view of our solar system in people's minds.

The value of the word "planet" is in signaling the importance of an object (IE, no one would care if these were called "Celestial body class A"/"Celestial body class B"/etc.). Saying "rogue planets are not planets" was a way of saying "these things aren't that important, you don't have to worry about remembering them." The ignorance most people have about celestial bodies like Ceres suggests that it was effective.

Worth noting that a "rogue planet" isn't defined in the adopted 2006 IAU definition of planet (which only deals with planets in our solar system, and relies on an orbit for its criteria).

Are you sure about this? A google search indicates Pluto was discovered in 1930 and Hubble was launched in 1990.

Sorry, I could have phrased that better to avoid the confusion. I was talking about Edwin Hubble the person who the telescope is named after. He showed that other galaxies existed in 1925. [1]

[1] - https://www.discovermagazine.com/the-sciences/january-1-1925...

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