Graduate student and astronomy writer

The Solar System

A new look at Pluto, present and past

On April 14, 2015, NASA’s New Horizons spacecraft sent back the first color image ever taken of the dwarf planet, Pluto, and it’s primary moon, Charon.  This first image, while still grainy, is remarkable.

Pluto New Horizons

True color image of Pluto and its primary moon, Charon. Released on April 14, 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The New Horizon’s spacecraft has spent more than nine years traveling towards the elusive target in the outer solar system. It is the fastest rocket ever created and has traveled more than 3 billion miles to bring us the image you see above. Before this, the only images we have had of Pluto come from ground based or Earth-orbiting telescopes, which haven’t been able to give us sufficient detail about the surface of Pluto to really answer any questions.

While our telescopes on Earth are certainly powerful, taking images of Pluto and its moons presents similar challenges to taking images of asteroids. Pluto doesn’t give off its own light, but rather shines because of reflected sunlight, like our own Moon. However, Pluto is really far away and really small, and so reflects very little light. Small target, very far away, and very dim means that ground based and Earth-orbiting telescopes can really only get small and blurry images of the former ninth planet of our solar system.

The evolution of our understanding of Pluto has a very interesting history that anyone paying even the slightest attention will be somewhat aware of. In short, we discovered Pluto a few decades ago and called it a planet. We learned more about it and about a decade ago decided it wasn’t really a planet. Chaos ensued, the public revolted, and Pluto kept on turning. Let’s look in a little more detail:

Pluto was discovered on accident in 1930. Calculations by astronomers (that later turned out to be wrong) predicted another large planet orbiting farther than Neptune based on irregularities in the orbits of Uranus and Neptune. Astronomers at the Lowell Observatory searched for objects beyond Neptune and found Pluto, the brightest object out there. It was very quickly realized that Pluto was way too small to account for the strange orbits of Uranus and Neptune, but it was accepted as a planet anyway (to end the side story, measurements from Voyager 2 gave us a more accurate value for Neptune’s mass, which showed that Neptune and Uranus are orbiting perfectly normally after all). We have since found that Pluto has a thin nitrogen-based atmosphere with some carbon monoxide* and methane, significant color variations in the surface, and a very large moon.

Pluto HST surface

“This is the most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003… The overall color is believed to be a result of ultraviolet radiation from the distant Sun breaking up methane that is present on Pluto’s surface, leaving behind a dark, molasses-colored, carbon-rich residue… The Hubble images are a few pixels wide. But through a technique called dithering, multiple, slightly offset pictures can be combined through computer-image processing to synthesize a higher-resolution view than could be seen in a single exposure. This series of pictures took four years and 20 computers operating continuously and simultaneously to accomplish.” Credit: NASA, ESA, and M. Buie (Southwest Research Institute).

Pluto’s primary moon, Charon (officially pronounced like “Sharron”, but no one really agrees) was discovered in 1978 right before it was going to be impossible to find it. Charon is a strange moon because it is so much larger with respect to Pluto than any other planet/moon combination (the size ratio of Pluto to Charon is nearly 2:1). Charon may be so big as to actually have a thin atmosphere made of nitrogen and possibly have significant subsurface water. Additionally, Charon is the only moon in the solar system that doesn’t actually orbit around the center of its planet – the gravitational center of the Pluto/Charon system actually falls outside the surface of Pluto, so in reality, Pluto and Charon are orbiting around each other.

Images from New Horizons on its approach to Pluto. We see from this series of images that Pluto and its primary moon, Charon, are actually in orbit around each other, a unique configuration in our solar system.

Work on the New Horizons, destination Pluto, was started in 2001 and launched in 2006. In 2005, two more moons of Pluto were discovered, and since named Nix and Hydra. While New Horizons was in the middle of its voyage, two more, even smaller moons of Pluto were discovered, bringing the total number of moons up to 4 small moons plus Charon.*

Pluto HST

Visible light image of Pluto, Charon, Nix, and Hydra from HST. Image Credit: M. Mutchler (STScI), A. Stern (SwRI), and the HST Pluto Companion Search Team, ESA, NASA

In 2006, the International Astronomical Union (IAU) redefined what it means to be a planet and that new definition did not include Pluto. Even before it was kicked out of the planet club, Pluto had a number of irregularities about it that made astronomers suspicious. Let’s look at a few before we delve into the IAU decision.

  1. Pluto is so much smaller than any of the other planets. Mercury, the innermost and smallest rocky planet, is nearly 30 times more massive than Pluto. Pluto is actually smaller than seven moons of this solar system: our Moon, the four Galilean moons of Jupiter, Titan, and Triton.
  2. Pluto’s “moon” is nearly half its size and mass. The ratios are so close that some astronomers have referred to Pluto and Charon as a “double planet.”
  3. Pluto’s orbit is highly inclined compared to the rest of the planets. Pluto’s orbit is also highly eccentric compared to the other planets. Combined, we see that Pluto’s orbit actually crossed Neptune’s orbit occasionally, so sometimes it’s actually closer to the Sun than Neptune, and sometimes it orbits way farther out than Neptune.
  4. Pluto rotates in the opposite direction of its orbit. No other planet does this, and it’s highly unusual to see at all. Also, it rotates at nearly a right angle to its orbit, on its side much like Uranus.
  5. Pluto and Triton (moon of Neptune) are remarkably similar in their size, orbital characteristics, potential compositions, and other bulk properties. It was once thought that Pluto might have actually started out as a moon of Neptune and somehow escaped. More likely, Triton started out as a free object like Pluto and was captured as a moon of Neptune during one of their close encounters.
  6. There are many many many more objects out where Pluto orbits that are of similar size to Pluto, similar compositions, and similar orbits. That’s not really a mark against Pluto, but rather points to Pluto belonging more to the group of similar objects than the group of dissimilar objects.

While none of these things on their own would be enough to plant suspicion on Pluto, together they paint a picture of Pluto certainly as the odd duck out in the planetary club. However, a number of factors contributed to the IAU revising their definition of a planet. Firstly, Pluto is an odd duck, but one that astronomers were prepared to stand by before the other factors. Secondly, by 2006 there were already a number of extrasolar planets being discovered, and there was debate going on as to how to define an extrasolar planet, which brought to light our lack of formal definition for what constitutes a planet in our own solar system. Lastly, in the early 2000’s there were a slew of discoveries of objects beyond Neptune (Trans-Neptunian Objects, or TNOs) of similar size to Pluto, with moons and similar orbits. There was also the discoveries of Eris, Ceres, and Vesta, Pluto-sized objects in the asteroid belt and Kuiper Belt.

Thus began the debate of whether to include all of these objects as planets (bumping the number of planets up into the 20’s, and rising), or to somehow distinguish this group of smallish bodies from the larger planets in the solar system. The IAU wanted to avoid a classification of planet based on size, mass, orbit, or composition since the largest 8 planets are widely diverse in all of these areas. So, they came up with three criteria that distinguish a planet from a not-planet:

“A planet is a celestial body that…”

  1. “Is in orbit around the sun.”
    1. This excludes moons, which are in orbit around a planet
  2. “Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape.”
    1. This means that the object must be massive enough to pull itself into a spherical shape. This excludes most asteroids and comets, which are in orbit around the Sun, but generally have an irregular shape.
  3. “Has cleared the neighborhood around its orbit.”
    1. This means that it must have enough mass to either sweep away all debris is its orbital path or to capture those debris as moons. This excludes the rest of the asteroids, Kuiper-Belt Objects, and TNOs, which are in relatively close quarters with other similar objects that share their orbits.

The main strike against Pluto as a planet (along with Eris, Ceres, and Vesta, and other later discovered TNOs like Sedna, Makemake, and Haumea), is the third criteria “has not cleared the neighborhood around its orbit.” As we said before, Pluto has a highly elliptical orbit which takes it out into the outer solar system, in particular the Kuiper Belt. The Kuiper belt is filled with other small objects that haven’t gotten out of Pluto’s way, so none of those objects can be considered planets.

Note: we said before that Pluto and Neptune cross paths, so why doesn’t that exclude Neptune, too? Well, although Pluto’s orbit does take it inside Neptune’s orbit at some points, the way the two orbits overlap mean that the actual objects will never cross paths (or collide), so the “neighborhood of Neptune’s orbit” is considered clear, because Pluto itself never enters the “neighborhood of Neptune’s orbit.”

Yes, it’s confusing. But the IAU wrote it this way to distinguish planets from other solar system objects. Next, now that Pluto and its friends can’t be considered planets, the IAU wanted to define a category for them. They settled on “Dwarf Planet,” defined as having satisfied criteria 1 and 2, but “hasn’t cleared the neighborhood around its orbit and is not a satellite.” This still excludes moons, but now potentially includes asteroids, KBOs, and TNOs. Essentially, this means Dwarf Planets are things that are massive enough to pull itself into a sphere, but not massive enough to sweep away everything else in its path. Anything else that doesn’t qualify as a Dwarf Planet (and is not a satellite) is considered a “small solar system object.”

Note: I don’t consider the “spherical but not a cleared orbit” to be a restriction on the mass (which the IAU wanted to avoid). “Clearing your orbit” depends a lot on what is actually in your orbital path. For something like Jupiter, which is waaay larger than anything else it would come across, clearing its orbit was pretty easy. For something like Pluto, its orbit was filled with similar sized objects, which made clearing its orbit impossible. Had Pluto’s orbit been a little closer in, it might still be a planet today.

Now you have the whole story. Pluto is no longer a planet, but is the prototype for “Dwarf Planets” and “Plutoids,” which are Dwarf Planets that orbit out past Neptune. But in my estimation, and clearly the estimation of the New Horizon scientists, how we categorize Pluto doesn’t change how interesting Pluto is to study. It’s still a fascinating object in the solar system, with 3 large moons and potentially many smaller ones, a potential “double planet,” the innermost TNO, an atmosphere and strange surface. Any of these things would on their own make Pluto a prime target for any space mission, regardless of what we call it.

Pluto completed less than half of its orbit between when we called it a planet and when we didn’t. I’m sure that we’ll change our mind again in the next 100 years. Pluto’s still going strong. And as I like to say, “Pluto doesn’t care what we call it; Pluto just keeps on going.”

Fun link: Keep track of where New Horizons is and how long until it reaches Pluto here:


*Edits: Thanks to Jason Wright for passing along two small corrections. The original post stated that Pluto has carbon dioxide, and it has now been edited to correctly say carbon monoxide. Also, a note about the two recent very tiny moons of Pluto has been added. Thanks for the feedback!

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