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> This object appears to be in a very eccentric orbit (0.948) from [0]: > Before its demotion from planet status in 2006, Pluto was considered to be the plane

> This object appears to be in a very eccentric orbit (0.948) from [0]: > Before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit (e = 0.248). Other Trans-Neptunian objects have significant eccentricity, notably the dwarf planet Eris (0.44). Even further out, Sedna has an extremely-high eccentricity of 0.855 due to its estimated aphelion of 937 AU and perihelion of about 76 AU > ... > Comets have very different values of eccentricities. Periodic comets have eccentricities mostly between 0.2 and 0.7, but some of them have highly eccentric elliptical orbits with eccentricities just below 1; for example, Halley's Comet has a value of 0.967 so possibly an ignorant question, as someone who's interested in astronomy but doesn't follow it very closely - when this is categorized as a dwarf planet, does that include "it might be a comet" as a possibility? or have they already ruled it out as a possible comet through other observations? 0: https://en.wikipedia.org/wiki/Orbital_eccentricity#Examples https://en.wikipedia.org/wiki/Orbital_eccentricity#Examples

mandevil 1y ago
Dwarf planet versus comet/asteroid hinges on mass, basically its "enough mass to be roughly round" (technically it's called "hydrostatic equilibrium"). Back from the 1810's to the 1870's or so, most people considered Ceres, Vesta, and things like that to be planets- they were bodies that wandered around the solar system, that meant they were planets. When the numbers started to get into the 20's, everyone decided to create a new category, "asteroid" (Greek for 'star-like') and put all of the smaller things in that. So when Pluto was discovered in 1930 it was slotted right into the planet category. Pluto was discovered mostly by accident, because Clyde Tombaugh was amazing at working the blink comparator, and finding the one dot that moved in between the two pictures of the night sky a few days apart. However, by the 1990's and 2000's you had computers and digital cameras, which are even better than Clyde at finding things that move, and quickly the number of planets started to go up- and it was clear that once we had thoroughly mapped the ~~Oort Cloud~~ (meant Kuiper Belt, see below) etc. we would have dozens of planets. And so once again astronomers decided to create a new category, just like they had with asteroids a century earlier. This time they drew the line in such a way that Ceres got moved from asteroid to dwarf planet- it has enough mass to be roughly round, so after over a century of being an asteroid it became a dwarf planet. This is how things always work in science: we discover something, then we discover more of them, and re-categorize everything based on the new discoveries. It's just more noticeable with Pluto because reciting the planets is done by every schoolkid in a way that they don't for subatomic particles or for species of voles or whatever.
- epicureanideal 1y ago
  Very informative, thank you!
- AStonesThrow 1y ago
  > thoroughly mapped the Oort Cloud So it’s interesting that the Oort Cloud is often mentioned as a real thing. Surely there are plenty of bodies discovered which are orthogonal to its existence, but Oort’s “Cloud” itself still enjoys only the status of hypothesis and not reality. https://en.wikipedia.org/wiki/Oort_cloud https://en.wikipedia.org/wiki/Oort_cloud Sadly, even Wikipedia editors seem unable to distinguish between the formal definitions of “hypothesis” vs. “theory” when delivering such a scientific article.
  - codethief 1y ago
    Oh wow, looks like I'm one of today's lucky 10,000! Thanks so much!
  - mandevil 1y ago
    You are correct, I meant to say Kuiper Belt, not Oort Cloud, pulled the wrong thing out of my memory. Unlike the Oort Cloud, we are doing a good job of mapping KBO's as we speak.
  - throwaway2k255 1y ago
    If the furthest objects of the Oort Cloud are over 3 light years away, it is relatively close to Alpha Centauri. Is there a chance that Alpha Centauri also has its own cloud that overlaps with it? Would AC influence the cloud and adjust the orbit of smaller comets?
    - TheOtherHobbes 1y ago
      There's a chance, but no one knows for sure. Oort Clouds are mostly empty space, so there wouldn't be much direct interaction. But there would certainly be gravitational effects. My guess (FWIW) is there's more out there than we suspect, likely including plenty of rogue/wandering planets between systems.
hinkley 1y ago
That wiki page needs some work. The section you linked to describes the eccentricity as a ratio, however the top of the page describes 0 as perfectly circular and 1 as an escape trajectory. If it were a ratio then 0 would be escape and 1 would be circular.
- zamadatix 1y ago
  The "Examples" section doesn't seem to talk about ratios, do you mean the end of the prior "Calculation" section? If so part is just saying you can calculate the ratio of r_a to r_p given you know e and run it through the equation, not that e itself = r_a / r_p (the formula to calculate e from r_a and r_p is higher up in the section). If not that section, apologies for missing what you're trying to point out - I'm just trying to see what needs to be cleaned up so I can make an edit if needed.

mandevil 1y ago

Dwarf planet versus comet/asteroid hinges on mass, basically its "enough mass to be roughly round" (technically it's called "hydrostatic equilibrium"). Back from the 1810's to the 1870's or so, most people considered Ceres, Vesta, and things like that to be planets- they were bodies that wandered around the solar system, that meant they were planets. When the numbers started to get into the 20's, everyone decided to create a new category, "asteroid" (Greek for 'star-like') and put all of the smaller things in that. So when Pluto was discovered in 1930 it was slotted right into the planet category. Pluto was discovered mostly by accident, because Clyde Tombaugh was amazing at working the blink comparator, and finding the one dot that moved in between the two pictures of the night sky a few days apart. However, by the 1990's and 2000's you had computers and digital cameras, which are even better than Clyde at finding things that move, and quickly the number of planets started to go up- and it was clear that once we had thoroughly mapped the ~~Oort Cloud~~ (meant Kuiper Belt, see below) etc. we would have dozens of planets. And so once again astronomers decided to create a new category, just like they had with asteroids a century earlier. This time they drew the line in such a way that Ceres got moved from asteroid to dwarf planet- it has enough mass to be roughly round, so after over a century of being an asteroid it became a dwarf planet. This is how things always work in science: we discover something, then we discover more of them, and re-categorize everything based on the new discoveries. It's just more noticeable with Pluto because reciting the planets is done by every schoolkid in a way that they don't for subatomic particles or for species of voles or whatever.

epicureanideal 1y ago
Very informative, thank you!
AStonesThrow 1y ago
> thoroughly mapped the Oort Cloud So it’s interesting that the Oort Cloud is often mentioned as a real thing. Surely there are plenty of bodies discovered which are orthogonal to its existence, but Oort’s “Cloud” itself still enjoys only the status of hypothesis and not reality. https://en.wikipedia.org/wiki/Oort_cloud https://en.wikipedia.org/wiki/Oort_cloud Sadly, even Wikipedia editors seem unable to distinguish between the formal definitions of “hypothesis” vs. “theory” when delivering such a scientific article.
- codethief 1y ago
  Oh wow, looks like I'm one of today's lucky 10,000! Thanks so much!
- mandevil 1y ago
  You are correct, I meant to say Kuiper Belt, not Oort Cloud, pulled the wrong thing out of my memory. Unlike the Oort Cloud, we are doing a good job of mapping KBO's as we speak.
- throwaway2k255 1y ago
  If the furthest objects of the Oort Cloud are over 3 light years away, it is relatively close to Alpha Centauri. Is there a chance that Alpha Centauri also has its own cloud that overlaps with it? Would AC influence the cloud and adjust the orbit of smaller comets?
  - TheOtherHobbes 1y ago
    There's a chance, but no one knows for sure. Oort Clouds are mostly empty space, so there wouldn't be much direct interaction. But there would certainly be gravitational effects. My guess (FWIW) is there's more out there than we suspect, likely including plenty of rogue/wandering planets between systems.

hinkley 1y ago

That wiki page needs some work. The section you linked to describes the eccentricity as a ratio, however the top of the page describes 0 as perfectly circular and 1 as an escape trajectory. If it were a ratio then 0 would be escape and 1 would be circular.

zamadatix 1y ago
The "Examples" section doesn't seem to talk about ratios, do you mean the end of the prior "Calculation" section? If so part is just saying you can calculate the ratio of r_a to r_p given you know e and run it through the equation, not that e itself = r_a / r_p (the formula to calculate e from r_a and r_p is higher up in the section). If not that section, apologies for missing what you're trying to point out - I'm just trying to see what needs to be cleaned up so I can make an edit if needed.