5 ms·

Why would it be gaseous if Pluto is rocky? Is it the size?

• dr_zoidberg 9y ago
  I'm not an astronomer, much less planetary scientist[5], but it's got something to do with the size, and location[0]. Rocky planets are suposed to be formed near the star, and the stellar winds "blow" the gaseous elements away from them. The gas giants, like Jupiter and Saturn, form farther away and hence get to hold the more "volatile" elements. Hot jupiters, a class of giants that orbit very close to their stars, are weird and there a two competing hypothsies about their formation[1] so we could say that the precise mechanisms aren't fully understood (but we do have a pretty good idea). As for ice giants, the precise mechanisms for their formation are not completely understood[2]. But what we do know, and it gives a strong(er) foundation for Planet Nine, is that super-earths (or Uranus/Neptune sized planets) are actually the most common type of (exo)planet we've observed so far, and it's a class that's surprisingly missing from our solar system[3] and a "slot" that Planet Nine would fill[4]. [0] https://en.wikipedia.org/wiki/Nebular_hypothesis#Formation_of_planets https://en.wikipedia.org/wiki/Nebular_hypothesis#Formation_o... [1] https://en.wikipedia.org/wiki/Hot_Jupiter#Formation_and_evolution https://en.wikipedia.org/wiki/Hot_Jupiter#Formation_and_evol... [2] https://en.wikipedia.org/wiki/Ice_giant#Formation https://en.wikipedia.org/wiki/Ice_giant#Formation [3] https://en.wikipedia.org/wiki/Super-Earth https://en.wikipedia.org/wiki/Super-Earth [4] https://en.wikipedia.org/wiki/Super-Earth#Planet_Nine https://en.wikipedia.org/wiki/Super-Earth#Planet_Nine [5] I'm a sort of an astronomy geek though, and I pretty much enjoy reading this stuff up until the point I don't understand any more. Which over the years has luckily been moving forward. If you're interested in the subject, follow @plutokiller and @kbatygin on twitter, they're active and link to a lot of articles and their scientific papers.
  • sulam 9y ago
    I’m suspicious about one statement you’ve made here. Either you haven’t made it precisely, or it may not logically follow. We’ve observed many super earths because that is what we can observe. The sensitivity of our instruments make it hard to observe Earth-like planets unless they are at Mercury-type orbital distance or even closer. There are some recent counter-examples, but to say that these are the most common because that’s what we’ve been able to observe is a selection bias of the first order. Now, perhaps planetary formation models predict many closely orbiting Neptune class planets, and independently observations are demonstrating that this is true. That would be a stronger statement, although still perhaps somewhat driven by observational data, since any model which doesn’t predict this might be kicked to the curb by now.
    • dr_zoidberg 9y ago
      Yes, I see your point. Basically I mentioned the Kepler mission results, without providing the relevant link[0]. Right there you can see that super-earths are quite high in frequency. However, I remember a talk held sometime last year (or maybe this year) where the mission team talked about the Kepler results and they specifically said that super-earths were now the most common planet type found. But I can't find it, and can't back that up with a strong source. What's more, looking at the published results[1], gas giants are far more frequent (and that would line up with the selection bias you noted). The only thing I can think that would back the claim about super-earths, would be considering them on radius alone, in which case yes, the 2 < R ≤ 6 R_Earth class is the most common class[1]. Edit: I was looking mainly at the mass results (in which case a SE is any planet between 2-10x Earths mass), not the radius results. But then, the definition is still a bit shaky, with different criteria[2]. [0] https://en.wikipedia.org/wiki/Kepler_(spacecraft)#2011 https://en.wikipedia.org/wiki/Kepler_(spacecraft)#2011 [1] https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail... [2] https://en.wikipedia.org/wiki/Super-Earth#Definition https://en.wikipedia.org/wiki/Super-Earth#Definition