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Because of the ambiguity about "planet", the answer depends on when you ask and what you believe are planets. If you asked up until 1989 the answer would be "we don't have good images of Neptune and Pluto yet". If you asked after the Voyager 2 fly-by of Neptune in 1989, but before Pluto's demotion in 2008, the answer would be "we don't have good images of Pluto yet". After Pluto was demoted to a dwarf planet in 2008 the answer retroactively became "1989". Weird.

All this absurdity comes down to what is a "planet". Also what it means to "know" its color and appearance. These are arbitrary definitions. Let's get "planet" out of the way first. It's the most contentious, and also of the most historical relevance.

"Clearing the neighborhood" is what knocked the TNOs (including Pluto) and the large asteroids out. Other proposals that kept Pluto in were either arbitrary or would leave us with dozens of planets. For example, a mass cut-off that included Pluto would be arbitrary and still leave us with dozens of planets. A cut-off at a certain distance from the Sun would be arbitrary and still leave us with dozens of large asteroids. Simply defining a list of planets was arbitrary. So "clearing the neighborhood" it is.

Because of the ambiguity about "planet", the answer depends on when you ask and what you believe are planets. If you asked up until 1989 the answer would be "we don't have good images of Neptune and Pluto yet". If you asked after the Voyager 2 fly-by of Neptune in 1989, but before Pluto's demotion in 2008, the answer would be "we don't have good images of Pluto yet". After Pluto was demoted to a dwarf planet in 2008 the answer retroactively became "1989". And if we find the hypothesized Planet Nine the answer will go back to "we haven't seen all the planets yet". Weird.

All this absurdity comes down to what is a "planet" and that we keep discovering new ones. Also what it means to "know" its color and appearance. These are arbitrary definitions. Let's get "planet" out of the way first. It's the most contentious, and also of the most historical relevance.

Exoplanets

In 1989 we discovered the first exoplanet, a planet around another star. As before, it started with one or two a year. In 1996 we discovered six. 13 in 1999. 30 in 2002... With the launch of Kepler dedicated to finding exoplanets in 2009 the discoveries poured in. We're now at 3730.

Now that we're finding "planets" around other solar systems, and in bulk, it became even more important to define what a "planet" is.

"Clearing the neighborhood" is what knocked the TNOs (including Pluto) and the large asteroids out. Other proposals that kept Pluto in were either arbitrary or would leave us with (then) dozens or (now) hundreds of planets. For example, a mass cut-off that included Pluto would be arbitrary and still leave us with dozens of planets. A cut-off at a certain distance from the Sun would be arbitrary and still leave us with dozens of large asteroids. Simply defining a list of planets was arbitrary. So "clearing the neighborhood" it is.

Planet Nine

And we still expect to find more planets! The outer solar system is poorly understood and everywhere we look we find more dwarf planets.

Examining the orbits of TNOs, scientists have noticed there's an unusual tendency for planets beyond Neptune's influence to be clustered on the same side of the Solar System. This could be the result of a large rocky planet with a highly elliptical 15,000 year orbit on the opposite side of the Solar System: Planet Nine.

Orbits of TNOs proposed to be influenced by a hypothetical Planet Nine. Planet Nine is in orange.

But there's a lot of ambiguity in this question. "Color", "appearance", and especially "planet". Let's get "planet" out of the way first, because that's the most contentious, and it's also of the most historical relevance.

Because of the ambiguity about "planet", the answer depends on when you ask and what you believe are planets. If you asked up until 1989 the answer would be "we don't have good images of Neptune and Pluto yet". If you asked after the Voyager 2 fly-by of Neptune in 1989, but before Pluto's demotion in 2008, the answer would be "we don't have good images of Pluto yet". After Pluto was demoted to a dwarf planet in 2008 the answer retroactively became "1989". Weird.

All this absurdity comes down to what is a "planet". Also what it means to "know" its color and appearance. These are arbitrary definitions. Let's get "planet" out of the way first. It's the most contentious, and also of the most historical relevance.

Is it even visible light? What about ultraviolet and infra-red light far more useful to astronomy. Few astronomical observations are done in the visible light range, Hubble is one of the last great visible light observatories. It's often done in other frequencies which both reveal more useful data about the object, and which our atmosphere is opaque to.

Is it even visible light? What about ultraviolet and infra-red light far more useful to astronomy. Few astronomical observations are done in the visible light range, Hubble is one of the last great visible light observatories. It's often done in other frequencies which both reveal more useful data about the object, and which our atmosphere is opaque to. For example, Voyager 2's cameras were sensitive to UV and visible light, they used filters for wavelength ranges corresponding to "UV", "violet", "blue", "green", "orange", and for the specific wavelengths of methane.