Pluto is fuzed to all the asteroids, comets, other minor planets, etc. Would it be big enough to hold an atmosphere without forming a tail?

All minor planets, asteroids, comets, etc that aren't orbiting bigger planets are included.

Anything bigger than a minor planet, or any object orbiting something that exceeds minor planet size, isn't included in the fusion.

So our moon, for example, because it's orbiting Earth and Earth is too big to be included, would be exempt from the fusion and wouldn't have its mass and matter added to Pluto's.

Pluto keeps its orbit, after the fusion it just gets bigger with the comets asteroids and other minor planets made one with it. Anything that would stay frozen on Pluto's surface before will still be frozen, anything would melt or boil on Pluto's surface would still melt or boil, etc. There's just more stuff there. Nu Pluto is still just as cold post-fuze as it was before. it's just a much much bigger lump of still-cold rock.

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    $\begingroup$ What exactly are you asking here? It seems like your question is basically "How big does a planet have to be to hold an atmosphere". $\endgroup$
    – Phiteros
    Jul 15 '17 at 19:06
  • $\begingroup$ @Phitheros give me a moment, I read somewhere Pluto gets a tail when it's nearer the sun like a comet. It has at least a tiny atmosphere, but it also gets a tail. I can post a source. $\endgroup$
    – Latifah
    Jul 15 '17 at 19:11
  • $\begingroup$ @Phiteros aaah nevermind. nasa.gov/nh/pluto-wags-its-tail $\endgroup$
    – Latifah
    Jul 15 '17 at 19:20
  • $\begingroup$ @Phiteros Apparently I was incorrect about the 'like a comet' part, does that render the entire question invalid? $\endgroup$
    – Latifah
    Jul 15 '17 at 20:04
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    $\begingroup$ fuzed to? Do you mean classified? $\endgroup$
    – user1569
    Jul 16 '17 at 8:23

It depends what kind of atmosphere. Pluto is cold enough (even at it's hottest) that CO2, NH3 and CH4 are mostly solids. (look up boiling temperature of each and compare to Pluto's temperature if you like). There's some photo-evaporation, and perhaps some equilibrium for the heavier gases, but Pluto's primary atmosphere is Nitrogen, which has a lower boiling/freezing temperature. (the atmosphere is too thin for it to exist as a liquid).

Based on this chart and Jeans escape, Pluto would need an escape velocity of about 2.5 km/s (roughly the mass of Io, or about 7 times it's current mass) to retain it's lighter gases like CH4 and NH3 as well as it's nitrogen. (Nitrogen would require a slightly lower escape velocity). Very roughly speaking, escape velocity can be tied to mass. If it was closer to the sun and hotter, it would need to be more massive.

This is, of-course, a ballpark answer.

With no magnetic field, pluto would still lose some atmosphere to the solar wind, but that far from the sun, that loss would be slow. It wouldn't lose much atmosphere to heat-escape (or Jeans-escape), like it does now. That's not to say Pluto would have a significant atmosphere if it had 7 or 8 times it's current mass but it would likely have a fair bit more. Pluto's problem is that it's cold enough that much of it's atmosphere would freeze. If it could get a thick enough atmosphere to maintain a permanent greenhouse gas effect, then it might maintain a real atmosphere. Again the minimum mass required for that would be in the 7-8 times it's current mass range, but the atmosphere would also need to be sustained and not freeze on it's surface. Significantly more mass and a higher escape velocity would make retaining an atmosphere much more likely.

The colder the outer edge of a planet's atmosphere is, the lighter the planet (or moon or dwarf-planet) can be and still retain an atmosphere but there's also the problem of too cold and the atmosphere will freeze into ice on the surface. Titan is a good example of this. It's far enough away from the sun that it can maintain an atmosphere, mostly formed by out-gassing from it's icy surface. A few billion years ago Jupiter's 4 Galilean moons may have had atmospheres similar to Titan, lost over time due to heat from the sun and perhaps also from Jupiter's very active radiation belts.


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