Pluto – the (Formally) 9th Planet in our Solar System (#159)

Did you know that the IAU (International Astronomical Union) have three criteria for what can be considered a full-sized planet? They are:

• It is in orbit around the Sun.
• It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
• It has “cleared the neighbourhood” around its orbit.

And it is because of the 3rd criteria in question that officially demoted my favourite planet, Pluto, from planet status back in 2006 - a demotion that attracted controversy and stirred debate in the scientific community and among the general public.

So rather than being known as the smallest and most distant planet in our solar system, Pluto is now classified the largest known dwarf planet in the solar system.

The strange world is located in the Kuiper Belt, a zone beyond the orbit of Neptune brimming with hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across as well as 1 trillion or more comets.

American astronomer Percival Lowell a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed “Planet X.” He first suggested that Pluto existed in 1905 when he observed strange deviations in the orbits of Neptune and Uranus. Lowell thought there must be another whose gravity is tugging on these ice giants, causing discrepancies in their orbits. Lowell proceeded to predict the mystery planet's location in 1915 but died 15 years before its discovery. Pluto was eventually discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers.

Pluto got its name from 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world get its name from the Roman god of the underworld. Venetia was interested in classical mythology as well as astronomy, and considered the name for the god of the underworld appropriate for such a presumably dark and cold world. Her grandfather then passed the name on to Lowell Observatory. The name also honours Percival Lowell, whose initials are the first two letters of Pluto.

Pluto officially became Pluto on March 24, 1930. The name was announced on May 1, 1930, and Venetia received £5 as a reward.

Funnily enough, the Disney cartoon character Pluto, Mickey's faithful dog, also made his debut in 1930, the same year Tombaugh discovered the dwarf planet. There is speculation that Walt Disney named the animated dog after the recently discovered planet to capitalize on its popularity,

Since Pluto is so far from Earth, little was known about the dwarf planet's size or surface conditions until 2015, when NASA's New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto is about 1,400 miles (2,380 km) wide. That's about half the width of the United States, or 2/3 the width of Earth's moon.

Due to it’s distance, a year on Pluto is 248 Earth years, and because it spins so much slower too, a day on Pluto lasts 153 hours, or about 6 Earth days.

Observations of Pluto's surface by the New Horizons spacecraft revealed a variety of surface features, including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth. While methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to support such enormous peaks, so scientists suspect that the mountains are formed on a bedrock of water ice.

Pluto's surface is also covered in an abundance of methane ice, but New Horizons scientists have observed significant differences in the way the ice reflects light across the dwarf planet's surface. The dwarf planet also possesses ice ridge terrain that appears to look like a snakeskin when captured on camera.

Another distinct feature on Pluto's surface is a large heart-shaped region known unofficially as Tombaugh Regio. The left side of the region (an area that takes on the shape of an ice cream cone) is covered in carbon monoxide ice, whilst the center-left of Tombaugh Regio is a very smooth region unofficially known by the New Horizons team as "Sputnik Planum," after Earth's first artificial satellite, Sputnik. This region of Pluto's surface lacks craters caused by meteorite impacts, suggesting that the area is, on a geologic timescale, very young — no more than 100 million years old. It's possible that this region is still being shaped and changed by geologic processes.

These icy plains also display dark streaks that are a few miles long and aligned in the same direction. It's possible the lines are created by harsh winds blowing across the dwarf planet's surface.

Speaking of the surface, Pluto's surface is one of the coldest places in the solar system. Temperatures here can drop to around minus 226 to minus 240 degrees Celsius. When compared with past images, pictures of Pluto taken by the Hubble Space Telescope revealed that the dwarf planet had apparently grown redder over time, apparently due to seasonal changes.

It is believed that at one time, Pluto may have had a subsurface ocean. If the subsurface ocean existed, it could have greatly affected Pluto's history. For example, scientists found that the zone of Sputnik Planitia redirected Pluto's orientation due to the amount of ice in the area, which was so heavy it affected Pluto overall; New Horizons estimated the ice is roughly 6 miles (10 km thick). A subsurface ocean is the best explanation for the evidence, the researchers added, although looking at less likely scenarios, a thicker ice layer or movements in the rock may be responsible for the movement. If Pluto did have a liquid ocean, and enough energy, some scientists think Pluto could harbour life.

So what is Pluto made up of?

Atmospheric composition: Methane, nitrogen. Observations by New Horizons show that Pluto's atmosphere extends as far as 1,000 miles (1,600 km) above the surface of the dwarf planet.

Magnetic field: It remains unknown whether Pluto has a magnetic field, but the dwarf planet's small size and slow rotation suggest it has little to no such field.

Chemical composition: Pluto probably consists of a mixture of 70 percent rock and 30 percent water ice.

Internal structure: The dwarf planet probably has a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane, carbon monoxide and nitrogen ice coating the surface.

Unlike other planets orbits, which tend to be circular, Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. Since the dwarf planet's orbit is so eccentric, or far from circular, Pluto's distance from the sun can vary considerably. The dwarf planet actually gets closer to the sun than Neptune is for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world.

As a result of that orbit, after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet).

When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, consisting mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds. The atmosphere also has brightness variations that could be explained by gravity waves, or air flowing over mountains.

While Pluto's atmosphere is too thin to allow liquids to flow today, they may have streamed along the surface in the ancient past. New Horizons imaged a frozen lake in Tombaugh Regio that appeared to have ancient channels nearby. At some point in the ancient past, the planet could have had an atmosphere roughly 40 times thicker than on Mars.

In 2016, scientists announced that they might have spotted clouds in Pluto's atmosphere using New Horizons data. Investigators saw seven bright features that are near the terminator (the boundary between daylight and darkness), which is commonly where clouds form. The features are all low in altitude and roughly about the same size, indicating that these are separate features. The composition of these clouds, if they are indeed clouds, would likely be acetylene, ethane and hydrogen cyanide.

And finally, to take us back to the top of the episode, we must talk about Pluto’s moons. Pluto has five moons in total: Charon, Styx, Nix, Kerberos and Hydra, with Charon being the closest to Pluto and Hydra the most distant.

In 1978, astronomers discovered that Pluto had a very large moon nearly half the dwarf planet's own size. This moon was dubbed Charon, after the mythological demon who ferried souls to the underworld in Greek mythology.

So remember when I said that the reason Pluto was downgraded was because it only met two of the three criteria, losing out on the third? In all the billions of years it has lived there, it has not managed to clear its neighborhood. You may wonder what that means, “not clearing its neighboring region of other objects?” Sounds like a minesweeper in space! This means that the planet has become gravitationally dominant — there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space.

Now this is where the moons come into play – particularly Chiron! Because Charon and Pluto are so similar in size, their orbit is unlike that of most planets and their moons. Both Pluto and Charon orbit a point in space that lies between them, similar to the orbits of binary star systems. For this reason, scientists refer to Pluto and Charon as a double dwarf planet, double planet or binary system.

So when Pluto was reclassified in 2006 from a planet to a dwarf planet, there was widespread outrage on behalf of the demoted planet. As the textbooks were updated, the internet spawned memes with Pluto going through a range of emotions, from anger to loneliness. But since the release of New Horizons images showing a very prominent heart-shaped feature on the surface, the sad Pluto meme has given way to a very content, loving Pluto that would like to once again be visited by a spacecraft.