Physics Ideas

Pluto (planet): characteristics, composition, orbit, movement

Pluto is a celestial object, currently considered a dwarf planet, although for a long time it was the most distant planet in the solar system. In 2006, the International Astronomical Union decided to include a new category: that of  dwarf planets , Pluto because it lacks some of the necessary requirements to be a planet.

Note that the controversy over the nature of Pluto is not new. It all started when the young astronomer Clyde Tombaugh discovered it on February 18, 1930.

Astronomers assumed that perhaps there was a planet beyond Neptune and, to find it, they followed the same scheme as their discovery. Using the laws of celestial mechanics, they determined the orbit of Neptune (and Uranus), comparing the calculations with observations from actual orbits.

The irregularities, if any, were caused by an unknown planet beyond Neptune’s orbit. That’s exactly what Percival Lowell, founder of the Lowell Observatory in Arizona and an enthusiastic advocate of intelligent life on Mars, did. Lowell discovered these irregularities and, thanks to them, calculated the orbit of the unknown “planet X”, whose mass was estimated to be 7 times the mass of Earth.

A few years after Lowell’s death, Clyde Tombaugh found the new star using a telescope made by them, only the planet was smaller than predicted.

The new planet was named Pluto, the Roman god of the underworld. Very appropriate because the first two letters correspond to the initials of Percival Lowell, the intellectual author of the discovery.

However, the alleged irregularities found by Lowell were nothing more than the product of some random errors in his calculations.

Pluto Characteristics

Pluto is a small star, so the irregularities in Neptune’s giant orbit could not be due to that. Pluto was initially thought to be the size of Earth, but gradually observations led to its mass decreasing even further.

Recent estimates of Pluto’s mass, based on joint orbital data from Pluto and its satellite Charon, indicate that the Pluto-Charon system’s mass is 0.002 times the mass of Earth.

It’s really too small a value to disturb Neptune. Most of this mass corresponds to Pluto, which is 12 times more massive than Charon. Therefore, Pluto’s density was estimated at 2,000 kg/m 3 , being composed of 65% rock and 35% ice.

A very important feature of frozen and irregular Pluto is its highly elliptical orbit around the Sun. This occasionally brings it closer to the Sun than Neptune, as happened in the period between 1979 and 1999.

In this encounter, the stars never collided because the inclination of their respective orbits did not allow it and also because Pluto and Neptune are in orbital resonance. This means that their orbital periods are related because of their mutual gravitational influence.

Pluto has another surprise in store: it emits X-rays, a high-energy radiation in the electromagnetic spectrum. This would not be surprising, as the New Horizons spacecraft has confirmed the presence of a weak atmosphere on Pluto. And when the molecules in this thin layer of gases interact with the solar wind, they emit radiation.

But the Chandra X-ray telescope found a much larger emission than expected, which surprised experts.

Summary of the main physical characteristics of Pluto

-Mass: 1.25 x 10 22 kg

-Radio: 1,185 km (smaller than the Moon)

-Shape: rounded.

Average distance to the Sun: 5,900 million km.

– Inclination of the orbit : 17º in relation to the ecliptic.

-Temperature : -229.1 ° C average. 

-Gravity: 0.6 m / s 2

– Own magnetic field: No.

-Atmosphere: Yes, dark.

-Density: 2 g / cm 3

– Satellites: 5 known so far.

-Rings: Not at the moment.

Why is Pluto not a planet?

The reason Pluto is not a planet is that it does not meet the International Astronomical Union’s criteria for a celestial body to be considered a planet. These criteria are:

-Orbit around a star or its remnant.

– Have enough mass that its own gravity allows it to have a more or less spherical shape.

Lack of proper light.

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-They have orbital domain, that is, an exclusive orbit, which does not interfere with that of another planet and is free of smaller objects.

And although Pluto meets the first three requirements, as we saw earlier, its orbit interferes with Neptune’s. This means that Pluto has not cleared its orbit, so to speak. And since it has no orbital domain, it cannot be considered a planet.

In addition to the dwarf planet category, the International Astronomical Union created another: the smallest bodies in the solar system , in which comets, asteroids and meteoroids are found.

Requirements to be a dwarf planet

The International Astronomical Union has also carefully defined the requirements for being a dwarf planet:

-Orbits around a star.

-It has enough mass to have a spherical shape.

-Does not emit its own light.

Lack of a clear orbit.

Therefore, the only difference between planets and dwarf planets is in the last point: dwarf planets simply do not have a “clean” or exclusive orbit. 

translation movement

Pluto’s orbit is very elliptical and, being so far from the Sun, it has a very long period: 248 years, of which 20 are closer to the Sun than Neptune itself.

Pluto’s orbit is the most inclined of all in relation to the plane of the ecliptic: 17º; therefore, when it crosses Neptune’s, the planets are far apart and there is no danger of collision between them.

The orbital resonance that exists between the two planets is the kind that guarantees the stability of their trajectories.

Pluto motion data

The following data briefly describe Pluto’s movement:

– Average radius of the orbit: 39.5 AU * or 5.9 trillion kilometers.

– Inclination of the orbit : 17º in relation to the plane of the ecliptic.

-Eccentricity:  0.244

– Average orbital speed : 4.7 km / s

– Transfer period: 248 years and 197 days

– Rotation period:  approximately 6.5 days.

* An astronomical unit (AU) is equivalent to 150 million kilometers.

How and when to observe Pluto

Pluto is too far from Earth to be seen with the naked eye, as it is just over 0.1 arcseconds away. Therefore, it is necessary to use a telescope, even amateur models work. Also, recent models incorporate programmable controls to find Pluto.

However, even with a telescope, Pluto will be seen as one tiny dot among thousands of others, so to distinguish it, you must first know where to look and then follow it for several nights like Clyde Tombaugh. Pluto will be the moving point in the background of the stars.

As Pluto’s orbit is outside Earth’s orbit, the best time to see it (but we must clarify that it is not the only one) is when it is in opposition , which means that the Earth is between the dwarf planet and the Sun .

This is also true for Mars, Jupiter, Saturn, Uranus and Neptune, the so-called higher planets . The best observations are made when they are in opposition, although, of course, they may be visible at other times.

To know the opposition of the planets, it is advisable to go to specialized websites on the Internet or download an astronomy app for smartphones. In this way, observations can be planned accordingly.

In the case of Pluto, from 2006 to 2023, it moves from the constellation Serpens Tail to Sagittarius.

rotary movement

Pluto rotates around its own axis, just like Earth and the other planets. Pluto takes six and a half days to rotate around itself because its rotation speed is slower than Earth’s.

Being so far from the Sun, even though this is the brightest object in Pluto’s sky, the king star appears to be a slightly larger point than the rest of the stars.

That’s why days on the dwarf planet pass in twilight, even the brightest, because the thin atmosphere is capable of scattering some light.

On the other hand, its axis of rotation is tilted 120° from the vertical, which means that the north pole is below the horizontal. In other words, Pluto turns sideways, as does Uranus.

This inclination is much greater than the Earth’s axis, only 23.5°; therefore, the seasons on Pluto are extreme and very long, as it takes just over 248 years to orbit the Sun.

Many scientists believe that retrograde rotations, as in the cases of Venus and Uranus, or such sharp rotation axes, again such as Uranus and Pluto, are caused by fortuitous impacts caused by other large celestial bodies.

In this case, an important question still to be resolved is why the Pluto axis stopped precisely at 120º and not at another value.

We know that Uranus did it at 98° and Venus at 177°, while Mercury, the closest planet to the Sun, has its axis completely vertical.

The figure shows the tilt of the planets rotation axis, since the axis is vertical, there are no stations on Mercury:


Pluto is made up of rocks and ice, although they look very different from those on Earth, as Pluto is cold beyond imagination. Scientists estimate that temperatures on the dwarf planet range between -228 °C and -238 °C, with the lowest temperature observed in Antarctica being -128 °C.

Obviously, chemical elements are common. On Pluto’s surface are distinguished:



-Carbon monoxide

When Pluto’s orbit brings it closer to the Sun, heat evaporates the ice from these substances, which become part of the atmosphere. And when it disappears, they freeze on the surface again.

These periodic changes cause the appearance of light and dark areas on Pluto’s surface, which alternate with time.

On Pluto, it is common to find curious particles called “tholins” (the name given to them by the renowned astronomer and popularizer Carl Sagan), created when the Sun’s ultraviolet radiation breaks down methane molecules and separates nitrogen molecules. The reaction between the resulting molecules forms more complex, though more disorderly, molecules.

Tholins are not formed on Earth, but are found in objects in the outer solar system, giving them a pinkish tinge, as on Saturn’s Titan and, of course, on Pluto.

Internal structure

So far, everything indicates that Pluto has a rocky core formed by silicates and probably covered by a layer of ice water. 

The theory of planet formation indicates that the denser particles accumulate in the center, while the lighter ones, such as ice, remain above, forming the mantle, the intermediate layer between the core and the surface.

There may be a layer of liquid water below the surface and above the frozen mantle.

The planet’s interior is very hot due to the presence of radioactive elements, whose decomposition produces radiation, part of which propagates in the form of heat. 

Radioactive elements are unstable in nature, so they tend to change into more stable elements, emitting particles and gamma radiation continuously, until stability is achieved. Depending on the isotope, a certain amount of radioactive material decays in fractions of a second or takes millions of years.


Pluto’s cold surface is almost all nitrogen frozen with traces of methane and carbon monoxide. These last two compounds are not evenly distributed on the dwarf planet’s surface.

The images show light and dark areas, as well as color variations, suggesting the existence of various formations and the predominance of some chemical compounds in certain locations. 

Although little sunlight enters, ultraviolet radiation is enough to cause chemical reactions in the thin atmosphere. The compounds that are produced in this way mix with the rain and snow that fall to the surface, giving the colors between yellow and pink that Pluto is seen with by telescopes.

Almost everything known about Pluto’s geology is due to data collected by the New Horizons spacecraft. Thanks to them, scientists now know that Pluto’s geology is surprisingly varied:

ice plains


mountains of frozen water

-Some craters

Cryo evidence of volcanism , volcanoes, releasing water, ammonia and methane, unlike the terrestrial volcanoes, releasing lava.

Pluto Satellites

Pluto has several natural satellites, of which Charon is the largest.

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For a time, astronomers believed that Pluto was much larger than it actually is, because Charon orbits so closely and almost circularly. So astronomers couldn’t tell them apart at first.

In 1978, astronomer James Christy discovered Charon through photographs. It is half the size of Pluto and its name also comes from Greek mythology: Charon was the boatman who transported souls to the underworld, the realm of Pluto or Hades.

Later, in 2005, thanks to the Hubble Space Telescope, the two small moons Hydra and Nix were found. And then, in 2011 and 2012, respectively, Cerberus and Styx appeared, all with mythological names.

These satellites also have circular orbits around Pluto and may be objects captured from the Kuiper belt.

Pluto and Charon form a very interesting system, in which the barycenter, or center of mass, is outside the larger object. Another extraordinary example is the Sol-Jupiter system.

Both are also rotating synchronously, which means they always show the same face. Therefore, Charon’s orbital period is about 6.5 days, the same as Pluto. And that’s also how long it takes Charon to travel his axis. 

Many astronomers believe these are good reasons to consider the couple as a double planet. Such dual systems are not uncommon in objects in the universe; binary systems are often found among stars.

It has even been proposed that the Earth and Moon are also considered a binary planet.

Another point of interest to Charon is that it may contain liquid water inside, which reaches the surface through fissures and forms geysers that freeze immediately.

Does Pluto have rings?

It’s a good question, since Pluto is, after all, on the edge of the solar system and was once considered a planet. And all outer planets have rings.

In principle, because Pluto has two moons small enough and with little gravity, impacts against them can lift and disperse enough material to accumulate in the dwarf planet’s orbit, forming rings.

However, data provided by NASA’s New Horizons mission shows that Pluto currently has no rings. 

But ring systems are temporary structures, at least in astronomical time. Current information about the giant planets’ ring systems reveals that their formation is relatively recent and that, as they form, they may disappear and vice versa.

Missions to Pluto

New Horizons is NASA’s mission to explore Pluto, its satellites and other objects in the Kuiper Belt, the region around the Sun within a radius of 30 to 55 astronomical units.

Pluto and Charon are among the largest objects in this region, which also contain others such as comets and asteroids, the so-called minor bodies of the solar system.

The fast New Horizons spacecraft took off from Cape Canaveral in 2006 and reached Pluto in 2015. It took numerous images showing never-before-seen features of the dwarf planet and its satellites, as well as magnetic field measurements, spectrometry and more.

New Horizons continues to send information today and is now about 46 AU from Earth, in the middle of the Kuiper Belt.

In 2019, he studied the object called Arrokoth (Ultima Thule) and is now expected to take parallax measurements soon and send images of the stars from an entirely different point of view than terrestrial, which will serve as a navigational guide.

New Horizons must also continue to submit information until at least 2030.

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