3 ms·

Relevant XKCD: https://xkcd.com/1633/ https://xkcd.com/1633/

bkohlmann 8y ago
Great graphical depiction. one question: Why do dwarf plants on the right have an upward sloping diameter as you get farther out? Isn't a dwarf defined by an absolute diameter cutoff (i.e. above which it's a true planet, below which it's not)? or is there a formula that takes into account distance from something?
- janzer 8y ago
  The difference in definition between a planet and a dwarf planet is just that a dwarf planet has not cleared debris from it's orbit. So not directly dependent on the mass, diameter etc. I'm unsure why the further would orbits require a larger size though; my guess is something to do with longer, slower orbits needing more mass to get rid of the debris.
  - JumpCrisscross 8y ago
    > I'm unsure why the further would orbits require a larger size though Stern–Levison's Λ finds a body's ability to scatter smaller masses out of its orbital region over a period of time equal to the age of the Universe scales inversely with its semi-major axis [1]. (A circle's semi-major axis is its radius [2].) This is most simply because a wide orbit contains more volume than a small orbit. [1] http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf [2] https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes
zaroth 8y ago
Is Pluto the black circle with the question mark or is that “Planet Nine”? Not marking Pluto on this would ruin it for me, even if it’s in the “Dwarf Planet” box.
- Raphmedia 8y ago
  https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Undiscovered_Planets https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Un...
  - zaroth 8y ago
    “Pluto, no longer considered a planet (it was the ninth until 2006), is not marked on the chart, but it would be below Neptune just outside the pink region (2,300 km diameter and 30-50 AU away).“ Yeah, that sucks.
    - krylon 8y ago
      Do not take this the wrong way, but I am always a little confused about people feeling about the whole planet-dwarf_planet thing so strongly. Pluto is still out there, just as it was a hundred years ago, when nobody knew it existed. And just recently, NASA sent a probe Pluto's way, that managed to amaze me, an outspoken Pluto-hater[0]: Turns out that Pluto is much more complex than people had anticipated. Who cares what the IAU decides to call it? It is what it is. You could call a rose a turd, and it still would not smell as badly. ;-) [0] Okay, I do not hate Pluto. I just wish NASA, ESA or whoever would send a couple of probes to Uranus and Neptune. I feel very strongly about this.

Great graphical depiction. one question: Why do dwarf plants on the right have an upward sloping diameter as you get farther out? Isn't a dwarf defined by an absolute diameter cutoff (i.e. above which it's a true planet, below which it's not)? or is there a formula that takes into account distance from something?

janzer 8y ago
The difference in definition between a planet and a dwarf planet is just that a dwarf planet has not cleared debris from it's orbit. So not directly dependent on the mass, diameter etc. I'm unsure why the further would orbits require a larger size though; my guess is something to do with longer, slower orbits needing more mass to get rid of the debris.
- JumpCrisscross 8y ago
  > I'm unsure why the further would orbits require a larger size though Stern–Levison's Λ finds a body's ability to scatter smaller masses out of its orbital region over a period of time equal to the age of the Universe scales inversely with its semi-major axis [1]. (A circle's semi-major axis is its radius [2].) This is most simply because a wide orbit contains more volume than a small orbit. [1] http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf [2] https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes

zaroth 8y ago

Is Pluto the black circle with the question mark or is that “Planet Nine”? Not marking Pluto on this would ruin it for me, even if it’s in the “Dwarf Planet” box.

Raphmedia 8y ago
https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Undiscovered_Planets https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Un...
- zaroth 8y ago
  “Pluto, no longer considered a planet (it was the ninth until 2006), is not marked on the chart, but it would be below Neptune just outside the pink region (2,300 km diameter and 30-50 AU away).“ Yeah, that sucks.
  - krylon 8y ago
    Do not take this the wrong way, but I am always a little confused about people feeling about the whole planet-dwarf_planet thing so strongly. Pluto is still out there, just as it was a hundred years ago, when nobody knew it existed. And just recently, NASA sent a probe Pluto's way, that managed to amaze me, an outspoken Pluto-hater[0]: Turns out that Pluto is much more complex than people had anticipated. Who cares what the IAU decides to call it? It is what it is. You could call a rose a turd, and it still would not smell as badly. ;-) [0] Okay, I do not hate Pluto. I just wish NASA, ESA or whoever would send a couple of probes to Uranus and Neptune. I feel very strongly about this.