Planet-like Pluto: An Introduction


By Sabine Stanley, Ph.D., Johns Hopkins University

Pluto is no longer considered a planet, but it is remarkably planet-like. Let us take a look at the process of its discovery and at its not-so-planet-like orbit, binary system, and density.

A closeup of Pluto as it shines under the light of Sun .
Pluto is smaller than several moons in our Solar system. (Image: NASA Images/Shutterstock)

In 2015, the New Horizons space mission revolutionized our understanding of Pluto. Our best previous images of Pluto from the Hubble space telescope were blurry, but they did show some color and brightness variations on the surface. The Hubble images, however, did not prepare us for the details discovered in the high-resolution images of Pluto’s surface taken by the New Horizons mission.

Thanks to the New Horizons mission, we now know that Pluto is an active world with glacial flows and convective ice. There are dunes, there are floating mountains, there may be volcanoes spewing ice, there may even be a subsurface ocean.

Binary Planet System of Pluto

And Pluto’s not alone. Pluto is the closest thing in our solar system to a binary planet system. The companion, Charon, is about half of Pluto’s diameter, and the two bodies orbit a common center of mass in a dance that repeats every 6.4 days.

Although Charon is often referred to as a moon of Pluto, there is no other satellite in the solar system that is so similar in size to the body it’s orbiting. This similarity in size is what makes it a binary system; Pluto and Charon orbit each other. And this binary system is surrounded by four other moons that are very small: Styx, Nix, Kerberos, and Hydra. Pluto has a complex system, unlike any other, we’ve seen in the solar system.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

The Size and Density of Planet-Like Pluto

Pluto is large enough for gravity to have made it spherical. Its diameter is just under 2400 kilometers. That makes Pluto smaller than seven moons in our Solar system. It’s about 10% smaller in diameter than Neptune’s moon Triton, 30% smaller than Earth’s Moon, and 55% smaller than Ganymede, the largest moon in the solar system. In terms of horizontal extent, Pluto is comparable to about half the continental United States.

Pluto’s average density is about 1900 kilograms per cubic meter. This tells us that Pluto is a mixture of ice and rock, much less dense than our Moon, and slightly less dense than Triton or Ganymede.

Learn more about Pluto and Charon: the binary worlds.

The Discovery of Pluto

The discoverer of Pluto, American astronomer, Clyde Tombaugh.
Pluto was discovered in 1930 by Clyde Tombaugh. (Image: Unknown author/Public domain)

Pluto was discovered in 1930 by Clyde Tombaugh. He painstakingly spent many a night with the telescope, looking at photographic plates where images from telescopes were stored. The goal was to find an object that had moved between photographic plates taken on different nights. Although Pluto was considered a planet for 76 years, we realized early on that its orbit is not very planet-like.

Instead of a nice circular orbit, Pluto has an orbit that’s very elliptical. This means its distance from the Sun changes dramatically around its orbit. At Pluto’s closest approach to the Sun, it’s only 30 times the Earth’s orbital distance. And at its farthest point, it’s almost 50 times farther from the Sun than Earth.

When Pluto is near its closest point to the Sun, it’s even closer to the Sun than Neptune. But Pluto and Neptune will never collide because their orbits don’t actually cross. This is because Pluto’s orbit is also inclined to the plane of the solar system by about 17°. Contrast that with Mercury which has the largest inclination of any planet in the solar system at about 7°.

Learn more about how our Sun defines our solar system.

How Many Earth Years are in a Pluto Year?

A comparison of Neptune and Pluto's orbits.
Pluto’s extremely elliptical orbit causes it to cross inside Neptune’s orbit. (Image: Eurocommuter commonswiki/CC BY-SA 3.0/Public Domain)

Planets tend to stay in their own ‘lane’, but Pluto’s extremely elliptical orbit causes it to cross inside Neptune’s orbit for about 8% of every trip around the Sun. That’s why Pluto was closer to the Sun than Neptune during the years from 1979 to 1999.

When would be the next time that it will happen again? A year on Pluto is very long, lasting nearly 248 Earth years. This means Pluto will not cross inside Neptune’s orbit until the year 2227!

In fact, humans have been aware of Pluto for less than half a Pluto year. A year on Pluto is longer than the time since the signing of the Declaration of Independence of the United States. And two years on Pluto? Two years on Pluto just about equals the entire duration of the Roman Empire, from Augustus Caesar to Romulus Augustulus.

Pluto’s Spin Axis and its Seasons

Pluto’s spin axis echoes unusual features also found at Uranus and Venus. Pluto is inclined by about 120° from its orbital plane, so it rotates on its side similar to how Uranus does, but at a bigger angle. The large angle of Pluto’s spin axis means that Pluto experiences extreme seasons.

Near summer and in winter solstices, the planet’s surface experiences total daylight and total night. And, like Venus, the angle of Pluto’s spin axis is more than 90°. That means that Pluto technically spins opposite the direction it orbits. So the spin direction is retrograde, like Venus.

Common Questions about Planet-like Pluto: An Introduction

Q: Is Pluto considered a planet in 2020?

As per the IAU’s revised criteria for planets, Pluto is no longer considered a plane. Pluto is now considered a dwarf planet.

Q: Does Pluto have volcanoes?

As per the images captured by the New Horizons spacecraft, there is a possible ice volcano on the surface of Pluto.

Q: Who discovered Pluto?

Pluto was discovered in 1930 by an American astronomer, Clyde Tombaugh.

Q: What are the criteria by which Pluto has been demoted from being a planet?

Pluto was downgraded to the status of a dwarf planet as per the IAU’s revised criteria for planets because it does not have an orbit like a planet’s.

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