The following was originally published on the Stellaris forums. I wrote it as a request from a fan.
Hi all! Today we’re going to take a request, and discuss a subject that’s very dear to many people’s hearts. I must warn you, however, that this may be controversial. If you find yourself disagreeing with me then I highly advise you to go and read Mike Brown’s excellent book on the topic.
That’s right, we’re going to talk about Pluto.
Pluto is very cold. How cold, you ask? To properly answer this question, take a deep breath. What you just inhaled is 78% nitrogen. As well as being the prime ingredient of air, nitrogen is one of the critical gases needed for plant life on Earth to flourish. When humans want to cool something to a very low temperature, we use liquid nitrogen. The stuff is fun to play with: if you live in a university town I highly recommend that you befriend a chemist or engineer and get them to get you a thermos full of it. If you cool nitrogen down even further (you’ll need a lab for this) you can get it to freeze solid. There are not, to my knowledge, many uses for nitrogen ice.
Here’s a picture of Pluto from 2015.
(Image courtesy NASA.)
The surface is mostly made up of nitrogen and methane ice. To the layman, this means that it’s cold enough that air stops being that stuff you breathe and starts being the crunchy stuff underfoot. When Pluto gets closer to the Sun then some of this turns back into a gas for a while, giving it a thin temporary atmosphere. Nitrogen ice isn’t very strong, so we suspect that Pluto’s mountains are made up of water ice over a rocky core.
In the past, we called Pluto a planet. This is because our definition of “planet” was “one of the list of objects that schoolkids memorised.” There are many objects more massive than Pluto: the Moon, for example, or Eris, or Ganymede. There was a brief period in which schoolkids were taught to call Ceres a planet, but there was never any logic or consistency to why certain bodies were planets and others weren’t.
In the mid 2000s, two things happened that changed this. The first was that we discovered Eris, a similar ball of ice on the outskirts of our Solar system. The second is that we started discovering lots of exoplanets around other stars. It quickly became apparent that unless we wanted schoolkids to learn a constantly-changing list, we would have to come up with a definition of “planet” which wasn’t based on tradition. The definition we came up with was of three parts as follows:
1. It must orbit the Sun. (Sorry, Moon and Ganymede.)
2. It must be massive enough to have become spherical via hydrostatic equilibrium. (Sorry, Vesta.)
3. It must be more massive than all the other stuff in its orbit. (Sorry, Pluto and Ceres.)
(Number 3 is the most controversial and is often written “must have cleared its orbit of other bodies”, which is a wording that often ends up being confusing to non-scientists.)
So what does that make Pluto?
At the moment we call it a “dwarf planet”, but that’s a terrible term which we’re using more as a placeholder than as anything meaningful. Pluto is a ball of ice and rock (mostly ice by volume, mostly rock by mass) which loops the Sun on an eccentric orbit. You know what else fits this description? Comets. Clearly, Pluto is not a comet. But… what is it? If you thought there was acrimony about Pluto being reclassified, you have not seen the arguments we’re having right now on the correct classification. Does it fit in the same class as Ceres? Should we classify based on orbit? Distance from the star? Chemical composition? We have not yet agreed.
This is important not just because of disagreements about our Solar system, but also about exoplanets. We need to have a proper language to describe what we find in other systems, because we’re finding more and more of them. Imagine if explorers didn’t have the proper words to differentiate between “mountain” and “bay”. How would they communicate their discoveries to others?
We can’t go back to calling everything a “planet”. Look at some of the things I’ve covered in previous episodes. Look at Fomalhaut and H98800, for example. Are all the lumps of ice and rock orbiting them planets? If so, then the word is basically meaningless.
Some people think that the word “planet” is too broad even as it is: they differentiate between “rocky planets” and “gas giants.” Some people go even further and separate gas giants into gas giants and ice giants, depending on what they’re made of. Some people prefer the term “Jovians” for them, after Jupiter. The only thing that we all agree on is that we cannot use the same word for everything.
Which word would you use?
What would it be like to live there?
Really, really cold. You couldn’t even heat up your dwellings properly either, because if your house was at room temperature then the ground it was built on would boil. That’s how cold Pluto is. Nobody could live there. Nobody could set foot on it, if only because the ground would explode with every step they took.
We could mine it for ice. A lot of science fiction stories about terraforming talk about redirecting ice comets into planets in order to provide water for oceans. Capturing comets is fuel-expensive and potentially dangerous, and crashing them into things is often bad if we want to keep those things afterwards. Mining the outer icy dwarf planets might be an alternative.
In the nineteenth century, wealthy people imported ice from icebergs for their drinks, and transported Canadian lake ice across the Atlantic to Europe. A truly decadent spacefaring society might use ice-dwarf ice as a similar status symbol.