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Q&A

If planet 9 exists, is it correct to say that it is a "dark planet"?

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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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You need to define "dark planet". Olin Lathrop‭ 28 days ago

I mean to a planet that doesn't reflect enough light to easily be seen from telescopes ; it would just appear in any normal telescope so dark to be "swollen" in the dark void, so it just might be very hard to watch it with a telescope. JohnDoea‭ 27 days ago

The extra information you supplied about "dark planet" belongs in your question, not buried in a comment. Olin Lathrop‭ 27 days ago

1 answer

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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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