4d ago

Why are dwarf planets not considered planets but dwarf stars are considered stars?

It was a burning question of mine for a while now:

I understand that dwarf planets like Pluto and Ceres aren't considered planets of the solar system, but why are they called 'dwarf PLANETS' if they aren't planets.

And no one really says, "the sun isn't a star, it's a Dwarf Star". Nor is it declassified as one because of it.

So, why are dwarf planets not planets, but dwarf stars are stars?

31 comments

Its semantics, and a subject of ongoing debate.
Per wikipedia, I really like this proposal:
Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.[210] The formula produces a value called π that is greater than 1 for planets.[c] The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement[211] – though this may not be the case around very low-mass stars.
It basically means a planet should be big enough to consolidate all the stuff in its orbital area, not be part of an asteroid field. That makes sense to me.
https://en.wikipedia.org/wiki/Dwarf_star
"Dwarf" stars are even more confusing, as it basically a synonym for "normal," as opposed to "giant" stars (which are relatively puffy and big for their mass/temperature), or more exotic stars. But the term is also used for special cases, like the relatively exotic white dwarfs (remnants of exploded stars with very strange properties, extreme density, and not "burning" like a star traditionally does), or "barely a star" brown dwarfs.
TL:DR: If an astronomer asks you to name something, you should say 'absolutely not.'
- I don't like the idea of actually requiring a clearing out of the orbit. Is this not including Trojan areas, because there will always be stuff there for any planet.
  The phrase "clearing the neignborhood" doesn't mean the orbit is clear, it means the planet in question has gravitational dominance over anything in its orbit, so larger bodies are either captures as moons or removed via gravitational slingshot. This allows wanderers and other captured bodies.
  The same astronomer (Margot) has remarked that gravitational dominance was clearer language, and it's interesting that I've found in many reputable sources like NASA where they've dumbed down this third rule to just clearing the orbit, which is NOT correct as I mentioned above.
  As for anyone who ever pulls the "I think Pluto is a planet"...it is a planet, as a subclass with restrictions.
  
  True, I suppose I though clearing the orbit sounded more colloquial, whereas gravitational dominance may not be as clear to some?
- For instance, just wait until you get a load of what astronomers consider to be metals.
- why would he use the term pi? That seems nuts.
- I'm absolutely naming something if I'm given the opportunity,
  Planet Bob is not great but still way better than K-57743267.7654
- Why would I turn down such an opportunity? I'm great at naming things.

DEI celestial bodies in this solar system have been demoted.
- when you're a star they let you do it
- They never paid their tribute to the $Trump.

It is due to the inconsistency of language, rather than properties of the objects.

For the same reason that a little fire and big fire are both fires but a small rock is a pebble while a big rock is a boulder. Fire is more about what something is doing while rocks are more about what they are.
- I kind of agree - though I'd say they are both about what they're "doing", as the small rock can't do what the big rock can (clear its celestial neighborhood).

is it maybe because a dwarf star is a stage of a star.. a dwarf planet is just the size of a rock?

They had to define planet and that left a hole of large thing smaller than a planet but larger than what they wanted to call an asteroid.
they had other options when they defined planet. I preferred the on that turn our moon into a planet - but they didn't listen to me. (They should not listen to me - I'm not an astronomer)

Not only isn't the Sun really a dwarf compared to many smaller stars, it's also not yellow. These are just holdovers from early astronomy where things were classified before a lot of knowledge wasn't known. Which is also the case with Pluto.
- Well then Superman is going to have a bad time here
  
  Turns out Kal-El was raised by the Kents; he just never developed powers because ours isn't a true yellow sun.

Mostly because Pluto was called a planet for a while, and reclassifying it upset so many people that they couldn't just go "it's just an asteroid sorry".

In addition to the other people you're hearing, "dwarf planet" also has specific criteria associated with how the body interreacts with its solar system. A dwarf planet has to orbit its star directly and be big enough for its gravity to have pulled it into a roughly spherical shape, but small enough that it hasn't cleared its orbit.
A dwarf star is just a star that's not particularly big and bright for whatever reason. While the terminology is similar, the usage is very different.

I think it's a grammar thing, not a science thing.
For example, we consider everything from Buckingham Palace to a one bedroom house to be a "building."
Similarly, everything from a broom closet to ballroom is a 'room.'

can ...... can we say that any more?
- Little people planet doesn't sound like an improvement..
  
  Midget planet?

dwarf planets was created as a term because of pluto. It had to much significance at this point to just call it an asteroid but if it was recognized as not a planet from the get go then we likely would not have the designation. They redefined the definition of a planet in general when it was realized there were to many large objects in the oort cloud so it had to be tightened up since otherwise we would be left with two types of planets and one would be in the asteroid belt and 3 more in the oort cloud with likely many more to come up and these objects most people would not consider them a real planet.

It's basically it's large enough to be round like a planet but can't achieve orbital dominance like the big 8. So it looks like a planet but acts like an astroid. I think dwarf planet is a good term for them. Becase if they were a little bigger they would have all the same properties of a planet and are a lot closer to becoming a planet that than an asteroid.
Hell I think Jupiter shouldn't be a planet. Since it's really close to being a star. I think it should be classified as a protostar.
- How much bigger would Jupiter have to be before fusion started?
  
  Just the extra mass of 13 times I know doesn't sounds like a lot but going from Earth to Jupiter's is about 318 times. Maybe instead of protostar we should use failed star since it has all the hydrogen and seller dust of that a star would used forces fuel.

I suspect that we might not use the term "dwarf planet", were it not that the objects we initially created the category to describe were originally classed as planets. The category labelling is a bit arbitrary, we just discovered that what we now call dwarf planets are quite abundant and that there was a clear line that could be drawn to distinguish them from the rest of what we called planets, and so decided to draw that line between them.

Well, for one, a dwarf star probably was a star to begin with, but ran out of fuel or something.
A dwarf planet, on the other hand, isn't the same once you strip away the rocks that make it qualify.
I am almost certainly missing something given that this is based on stuff I read almost a decade or more ago.

31 comments