Why is planet X always depicted as being blue?

[Tris Fray Potter]

I know that it is probably a gas giant, but why blue? Saturn and Jupiter, and even Uranus aren't blue, so why would Planet X be blue?

 

[Bandersnatch]

Presumably, the idea is that a planet that is far away, will look somewhat similar to the farthest planet we know, or that blue is evocative of cold temperatures.

In the end, though, I think this is for the same reason why unicorns are pink.

 

[russ_watters]

Yeah, so just to be clear; you worded the question as if you think there is such a thing as Planet X. There probably isn't. The "Planet X" you typically hear about is a fiction generated by crackpots, not a scientific prediction (or at best an abandoned one).

 

[lpetrich]

Could the OP have in mind this proposed planet?

Caltech Researchers Find Evidence of a Real Ninth Planet | Caltech
A Ninth Planet in Our Solar System?
'Planet Nine' May Exist: New Evidence for Another World in Our Solar System
Journal article: EVIDENCE FOR A DISTANT GIANT PLANET IN THE SOLAR SYSTEM - IOPscience

Its existence is inferred from the orbits of some Kuiper Belt Objects. Six of them have very eccentric orbits with similar orientations. Astronomers Konstantin Batygin and Mike Brown propose the existence of an additional planet to account for this oddity.

The AASNova page cites this set of representative solution parameters:

• planet mass of 10 Earth-masses
• semi-major axis of a = 700 AU
• eccentricity of e = 0.6

This means going between 280 and 1120 AU with a period of nearly 20,000 years.

That page and the Caltech page also have an artist's conception showing the planet as blue.

This is likely an extrapolation from Neptune, which is very blue because of its atmosphere's methane content.

This planet may likely be kept warm by its internal heat. Uranus (mass 14.536 Earth masses) radiates 1.1 times as much light energy as it gets from the Sun, and Neptune (mass 17.147 Me) 2.61 times (numbers from Wikipedia).

Assuming the same radiation rate per unit volume and scaling to 700 AU, this planet will radiate 1300 (scaled from Uranus) or 1200 (scaled from Neptune) times as much light energy as it receives from the Sun. Thus, a Neptune-like appearance is very plausible.