If planet 9 exists, is it correct to say that it is a "dark planet"?
If planet 9 exists, is it correct to say that it is a "dark planet"?
By "dark" here I mean to a planet that doesn't reflect enough light to easily be seen from normal telescopes (in the current common telescope technologies of 2021 which I am not familiar with) ; it would just appear in any such telescope so dark to be "swollen" in the dark void, so it just might be very hard to watch it with such a telescope
I would assume that it is indeed a "dark planet" by that definition because I don't think that what makes us humans having hard time to directly watch it short-scale telescopes is all the objects that screening it, rather, the vast proximity of it from the sun which should make it so dark so to be a too dark spot in a dark background and thus a "dark planet" in the similar way to how sunless galaxies would be considered "dark galaxies".
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Planet 9 would certainly be "dim", but whether it would be dark according you your definition is impossible to say.
Planet 9 needs to be smaller or further away than Pluto, otherwise its gravitational effect would have been noticed more clearly by now. This means it probably reflects less light than Pluto does. Pluto is too dim to see with the naked eye, but was detected by optical telescopes decades ago.
Whether a dimmer object fits your definition of "dark" can't be judged because the definition is too vague. You define dark as not visible with a "normal" telescope. The norm for telescopes, at least leading edge scientific ones, keeps progressing. There is also huge variation in the light gathering power of scientific telescopes used today. If you build a big enough mirror and keep other light away from it well enough (like putting it in space), then you can eventually see arbitrarily dim objects. How big a mirror is "normal"? 3 meters, 10 meters, a much larger space telescope that might be normal before the end of the century?
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There are perhaps two questions here. The first is whether the planet is intrinsically dark, i.e. it reflects only a small fraction of the light that reaches it from the Sun and emits only a small amount of blackbody radiation. This can be quantified by the planet's albedo, which depends on the composition of its atmosphere and, if the atmosphere is thin enough, its surface. Given its rough distance from the Sun and a reasonable guess as to its mass, it's likely to be an ice giant like Neptune, and as such would have a similar albedo (Fortney et al. 2016, Brown & Batygin 2016). If it's icy and, in particular, at all rocky, it would be unlikely to emit much radiation on its own - the Kelvin-Helmholtz mechanism isn't significant for such bodies.
To recap: It probably won't emit much light of its own, but it's unlikely to have an albedo low enough to qualify as "dark" for any reasonable definition of the word.
The second question is whether the planet has a reasonably normal albedo (likely) but appears too dim to be seen by our telescopes because of a combination of size and distance. Certainly, the further away an object is, the lower the flux of its reflected light, and the smaller an object is, the less light it will reflect. The answer to this is that we simply don't know, because it's difficult to place constraints on its orbital or physical parameters (although that might have changed - see Brown & Batygin 2021!). There's also the possibility that it could be detectable in some regions of the spectrum but not others. For instance, we don't have instruments that can search the far-infrared with large enough fields of view to make an initial detection.
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