What's Hiding On Pluto? | Unveiled

What’s Hiding On Pluto?

The sun’s gravitational pull stretches out much further than Neptune, all the out to the Kuiper Belt and beyond. To these dark and mysterious satellites, the sun is just a pinprick in the sky. And one of the most mysterious of all is Pluto.

This is Unveiled, and today we’re answering the extraordinary question: what’s hiding on Pluto?

Once classified as the ninth planet in the solar system, in 2006 Pluto was downgraded from “planet” to “dwarf planet”. To count as a planet, the International Astronomical Union decided, an object “must have cleared the neighborhood around its orbit”, which Pluto has failed to do due to its relatively small mass. Technically, Pluto is a “trans-Neptunian object” drifting through the Kuiper Belt. It was the discovery of another such object, Eris, that led to the definition of “planet” being changed. Pluto’s surface area is only 3.3% the size of Earth’s - making it about the same size as Russia. On top of being small, Pluto has incredibly low gravity, so little that you’d hardly even notice it – about 6.3% the strength of gravity on Earth.

Nonetheless, it does have its own moons: Charon, Styx, Hydra, Nix, and Kerberos. The largest, Charon, wasn’t discovered until 1978, almost five decades after Pluto itself was found. It’s large enough that Pluto and Charon were almost classified as double planets. The two are tidally locked, always presenting the same face to one another. Charon’s pull causes Pluto’s football-shaped moons Nix and Hydra, and possibly Styx and Kerberos as well, to wobble as they rotate, caught in a game of gravitational tug-of-war. What little data we have about this was gathered by the New Horizons probe in 2015, and we likely won’t know more until we can investigate further.

Considering the fact that we’ve detected planets and stars and black holes millions of lightyears away from Earth, it’s strange to think that there are so many things we don’t know about what’s hiding in our own solar system. For thousands of years, we didn’t have any way to study celestial objects apart from the seven “classical planets” - the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. Obviously, we now know that the sun and the moon aren’t planets, but this ancient definition of “planet” refers to objects visible to the naked eye. Until the discovery of Uranus in 1781, which was initially thought to be a star, we had no idea there were even more in the solar system! It was over 60 years later that Neptune was discovered in 1846, and in 1930 astronomer Clyde Tombaugh found Pluto. The existence of an additional planet had been hypothesised before this, thanks to apparent perturbations in the orbit of Uranus, that was later attributed to an overestimation of Neptune’s mass. Pluto may not be a planet any longer, but its discovery was still hugely important, since it was the first trans-Neptunian object we ever found. It wasn’t until 1992 that the second, Albion, was discovered.

Part of what makes studying Pluto and other trans-Neptunian objects so difficult is the sheer distance between us and them. The distance between Earth and Pluto ranges between 2.66 billion miles to 4.67 billion. For contrast, Mars is sometimes just 34 million miles away! The vast distance makes sending probes to Pluto and potentially going there one day incredibly difficult; if you missed the optimal launch window, you could end up almost doubling the distance you had to travel. And considering that Pluto takes 248 years to orbit the sun, you’d be waiting a long time to get a second chance. In fact, Pluto hasn’t completed even half of its journey around the sun since it was discovered.

The only manmade object to have reached Pluto so far is NASA’s New Horizons probe, which in 2015 conducted a flyby over the surface. It discovered a surprisingly youthful surface and varied geography. This included exotic ice flows, mountain ranges, and possible evidence of cryovolcanic processes. A vast haze hangs around the planet, the result of sunlight breaking up methane gas particles. Pluto’s plains are mostly nitrogen ice, but its mountains are water ice, with methane frost. A dark, reddish band named Cthulhu Macula swirls across its surface, possibly the result of organic compounds called tholins - formed when ultraviolet light strikes methane and nitrogen.

Perhaps the best adjective to describe Pluto, apart from “distant”, is “cold.” Pluto has an incredibly low surface temperature, going from -369 degrees Fahrenheit at its warmest to -387 degrees at its coldest. While this is still warmer than the average temperature of outer space itself, which is around -454 degrees, it’s still significantly colder than the coldest temperature ever recorded on Earth. That was -128.6 degrees Fahrenheit, recorded in Antarctica in 1983. So, Pluto is around three times colder than the Antarctic at its chilliest. For more context, liquid nitrogen is about -320 degrees – even at its warmest, the surface temperature of Pluto is still cold enough that if you were exposed to it you’d freeze almost instantly.

Pluto is clearly one of the most inhospitable places in the solar system. Living there would involve the same problems as living in an Antarctic outpost, or the International Space Station - only worse. Astronauts and Antarctic residents, who are mostly scientists and military personnel, are incredibly isolated and can’t remain in their respective bases for long. The ISS also has radiation to contend with, which – combined with low gravity – is one of the biggest reasons that the time astronauts spend there is restricted. The longest single stay in outer space is 437 days, a record held by Russian astronaut Valeri Polyakov. Deadlier still, Pluto’s atmosphere is made up largely of nitrogen and highly poisonous carbon monoxide.

Needless to say, our technology would have to improve drastically to get people to Pluto in the first place, let alone to set up some kind of outpost - or to have any chance of a return journey. It’s unlikely that Pluto would ever become more than a research base, even in an advanced future, much like Antarctica – though it might see the odd tourist. It would just be too difficult and risky to build any large-scale settlement with any purpose except studying the planet, or perhaps refuelling spacecraft if they journey further out.

But perhaps Pluto isn’t as bleak as it seems at first. There are many icy worlds in the solar system that we think have subsurface oceans, such as Jupiter’s moon Europa and Saturn’s moon Titan. Europa is much closer and is most likely hiding a massive, warm ocean beneath its frosty outer shell. Some scientists think that Pluto could be the same. A large basin of of frozen nitrogen and carbon monoxide ices was spotted on the surface, which shouldn’t have been able to form without an ocean underneath. This evidence confused astronomers for a long time, since they couldn’t explain how Pluto could have an ocean, but new theories suggest that Pluto might have a layer of insulating gases that keep it warm enough to harbor a vast, alien sea. If this is true, not only could there be alien life on Pluto, but we may be able to send humans there one day. Luckily, by the time we develop the technology to get there, we’ll probably already have explored Europa, so we’ll have valuable knowledge to draw from when it comes to building in this kind of environment. However, Pluto is still incredibly far away from the sun, which could be enough on its own to prevent life from evolving.

Pluto is so far away that it’s hard to know for sure whether it’s a desolate, dark wasteland or a secret haven for aquatic aliens – but until technology gives us the answer, we can at least dream of the latter.

And that’s what’s hiding on Pluto.