D-6: Only A While

The following was originally published on the Stellaris forums.


Drink up my darling
Be brave and smile
Death is for ever
Life’s only a while
Hello and welcome to another episode of the astroknowledge series! Today I’m going to talk about a place which I find utterly enthralling. This is not particularly interesting from a scientific point of view, but as a human I can’t get over how evocative it is.

In 1993, Artie Hatzes, an American astronomer living and teaching in the pleasant Thuringian mountains of Germany, developed a hypothesis that there was a gas giant orbiting the nearby giant star of Pollux. At the time there was very little way to tell (even Latham’s Planet was only confirmed in 1991), but Hatzes kept his idea in mind. Technology soon improved and in 2006 he was able to confirm that it existed. The planet has had various designations but in 2015 it was named Thestias.

(Artist’s impression courtesy IAU.)

Thestias is not habitable: it is a gas giant and is far too close to Pollux for its moons to be habitable either. However, it may have been within Pollux’s habitable zone back before she became a giant star. This means that if anyone was living there, they would have had to evacuate.

(For the moment, we’ll ignore the fact that Pollux is too young for Thestias to have developed intelligent life. Earth took ~600 million years to develop even single-celled life, which is more or less Pollux’s entire main sequence lifespan.)

They’d have had millions of years to evacuate, especially if their astronomers were competent enough to spot it coming, so it’s not as if this would have taken them by surprise. Entire civilisations can rise and fall in that time.

However – and here’s the interesting thing – it is likely that where there’s one planet, there’s more. This means that when Pollux became a giant and her habitable zone moved outwards, some planets might have become habitable. The new habitable zone would be somewhere around 6-10 AU, which in our Solar system places Jupiter just outside of it and Saturn just inside it.

It wouldn’t last long, of course. Pollux is 742 million years old and will not live to see her nine hundred millionth birthday. This habitable zone is only temporary But even a hundred million years is a while.

What would it be like to live there?

The evacuation of a planet sounds like a rushed affair, but remember that they have advanced warning and plenty of time. However, the expansion and first dredging of a star is sudden, and Pollux’s habitable zones would not shift gently. The two planets would not be habitable at the same time. Therefore while the evacuation of people from the moons of Thestias may have taken generations, they would have been fleeing to a planet which has no atmosphere and no liquid water. Domed cities and tunnels would be their best bet here. It would not be a pleasant life for the first few generations of colonists, and they may not be pleasant people. There’s work to do here, but most of it consists of waiting and preparing.

It’s only once Pollux swells from a white star to an orange giant that Old Thestias can truly be abandoned and the work on New Thestias can begin. Asteroids can be gathered and melted for air and water. After a few centuries, the biosphere can begin to grow. People can emerge from their tunnels and domes to walk freely under the sky of their new home.

In a few hundred thousand generations they’d have to do it again, of course, and this time they would have to take a much longer journey. All that they’ve gained is time. But time is enough, because life is good at enduring, especially when that life has radio telescopes and supercomputer simulators.

Don’t cry for the people of New Thestias, because you should be commiserating: we’re in the same situation. The Sun will give us perhaps another 5 billion years before Earth becomes uninhabitable. If we survive until then, which I think we will, we’ll have to evacuate the same way – and inconveniently our new habitable zone will fall around Jupiter, none of whose moons are massive enough to hold an atmosphere. We’ll have to take the long journey out to the stars.

We’ve got perhaps two hundred million generations left on this planet. That’s enough time to think in the long term, even if it is only a while.


D-7: The Bride

The following was originally posted on the Stellaris forums.


A big thank you to Murmeldjuret¬†or another fantastically informative weekend! Coming up this week we have all sorts of interesting stuff for you in the astroknowledge series. We’re close to the end now, very close. I’ve been holding back some of the best stuff in space until now, so as to close this series with a bang. As a result, I hope that this week will be as good for you as it is for me.

We’ll also have another guest article from the pen of the delightfully learned Admiral Howe.

Let’s talk about Pollux. This is a star that humans have known about for a very long time; together with Castor it makes up the two bright stars that the Gemini constellation is named after.

(Image courtesy astropixels.com)

Once upon a time, back before humanity existed, Pollux was a white A-class star on the main sequence. Sadly stars of this sort, while very beautiful, don’t live very long. Let’s look at what happened next, to understand what happens to stars when they reach the end of their lives.

Gas falls in
Light comes out
That’s what stars
Are all about

A star is a balancing act. Gravity pulls gas inwards towards the core, and light (and other radiation) that streams out of the core pushes it back. This might seem weird to you if you aren’t used to thinking of light as being something powerful enough to hold up a heavy weight, but this gives you an idea of just how bright the cores of stars are. We call this “thermal pressure.” *

When Pollux exhausted the hydrogen in her core, this light began to fail. Gravity, which had been kept at bay for hundreds of millions of years, is a patient force: the core contracted and enormous amounts of fresh hydrogen poured into it.

What happens when you pour enormous amounts of fresh fuel into a dying fire? It doesn’t just restart gradually: it explodes.

This sudden reignition of the core pushes back against gravity hard. The outer layers of the star are blasted outwards, as is a lot of the still-burning hydrogen that just began to flood into the outer core. We call this the “first dredge-up.”

At this point Pollux became vastly larger, which is where she is today: she’s only twice the Sun’s mass but is almost nine times her radius and thanks to her enormous surface area is a full forty-three times¬†as luminous.

(Image courtesy NASA. That small yellow thing next to Pollux is the Sun, included for size comparison.)

Pollux, which had been a bright white star for a few hundred million years, became a larger, cooler orange giant. Her outer layers are the only part we can see, which create her orange colour. Deep inside, her core has now started to fuse helium instead of hydrogen. This gives her only another few hundred million years of life. When this fuel source ends then it will be like before: her core will contract, bringing in more helium, which will cause a second dredge-up. She’ll get even larger as her outer layers inflate even further, and her surface will be even cooler and redder.

At this point her core will be full of oxygen and carbon, which she isn’t heavy enough to fuse. She’ll continue to glow but this will be from the outer layers rather than the core. Eventually the outer layers will fade and drift away, creating a nebula that slowly cools to black; and a core of oxygen and carbon, still glowing white from residual heat.

Don’t weep for Pollux. Space is a fantastically good insulator, and so she’ll cool extremely slowly. While her life up until this point will have been less than a billion years, her lifespan as a white dwarf is projected to be considerably longer than the current age of the universe. Everything up until this point has really just been her childhood: a brief hydrogen- and helium-burning phase she went through before she settled down to be what makes her truly happy: being small, and beautiful, and white as snow.

Tomorrow we’ll talk about her planet.


* Thermal pressure is a combination of radiation pressure (that is, light) and convection pressure (that is, hot gas rising.) For small cool stars convection pressure dominates; for larger hotter stars radiation pressure dominates.

D-9: The Iron Age

The following was originally published on the Stellaris forums by Murmeldjuret.


(Today we will only discuss the visible part of the universe or 4.6% of the total mass. So when I use %mass I mean % of the visible mass.)

Carl Sagan once said we are all made of stardust. There is truth in it. 90% of our mass has been forged in the fusion reactors that power stars. Though it is equally inspiring that the 10% that is hydrogen in us is the same hydrogen big bang created during the recombination 13.8 billion years ago. 10% of your mass is 13.8 billion years old. A lot of (note understatement) hydrogen and helium was created during this recombination, and it is still the same hydrogen that powers the stars.

For most of a star’s life it fuses hydrogen into helium, but as helium content grows and hydrogen content shrinks, it will eventually start fusing helium. This is usually the end of a star’s regular life as the core process that powers it and keeps it in balance falters. The results depend on startype. The new stellar reactor begins fusing helium into heavier elements. See chart below. The heaviest naturally occurring element is plutonium, though it is possible heavier elements have formed earlier in the universe but have decayed to plutonium or lighter by now.

Spoiler: Nucleosynthesis table



Above is a chart of the different types of elements in the solar system. The same ratio is about correct for the universe in general, except hydrogen andhelium are even more common. Hydrogen is about 74% by mass and helium is about 24% of the mass in the Milky Way. That leaves 2% to be shared by all the other elements. We thus say the Milky Way has a metallicity of 2%. Chemists will probably not consider carbon a metal, but since anything heavier than helium has to have been made after big bang it is a more clear distinction from an astronomical perspective.

Interestingly enough, the third most abundant element is oxygen, with a much higher concentration than any element heavier than helium. In fact, a full 1% of the galaxy is oxygen. This is why ice is not uncommon in the universe, it being a stable bond of hydrogen and oxygen, the most and third most common elements. The fourth most common element is carbon, so score one for carbon lifeforms on watery worlds being the most abundant.

Another interesting thing to note from the curve is that even numbers are much more common. This is due to helium being the main source of heavier elements and helium is two protons. There is no way to get the odd numbered elements without something to alter the regular process. Of the ten most common elements by mass, only nitrogen and hydrogen are odd numbered.

The next thing to note is the peak of iron (Fe) and nickel. Iron is a relatively heavy element, but it is still the sixth most common element by weight. 0.1% of the visible Milky Way is iron by mass. All planetary cores are iron and/or nickel based, and iron can be found everywhere. This is strange, element nr 26 beats 21 preceding elements in abundance. The reason is actually quite straightforward. Iron is the most stable nuclei in the universe. It has the lowest binding energy of all nuclei. All the lower elements give net energy when you fuse them, and all the heavier elements give net energy when you split them. There is no way to change iron without adding energy, both splitting or fusing iron costs energy. When the core of star turns to iron the process that powers a star turns to a process that takes energy from the star.

Fissile elements usually decay to lead, see its peak (Pb). The heaviest stable element lead-208 is still radioactive but with a halflife of 10^19 years. Compare that to the 10^10 years the universe has existed. Everything heavier than iron might be radioactive but with halflives unimaginably long.

The hydrogen created at big bang is limited, and it is being burnt in every star, in every galaxy, in every cluster, every second. For each atom of iron it creates, there is less fuel left to power the universe. If nothing else ends the universe as we know it, the hydrogen and helium will run out, and all that would be left is a universe sized cloud ofiron glittering from the light of long gone stars.

D-10: The Cloud World

The following was originally published on the Stellaris forums. No pictures today, sadly.


Once again, a big thank you to Admiral Howe for terrifying us with an explanation of how safe it is to stay on Earth. He’ll be back next week to share more knowledge.

Let’s talk about a planet that we might flee to if and when the Siberian Traps recur and all multi-cellular life needs to find a new home. (Good luck, archaea! Let us know how it turns out!) We’re going to discuss Kepler 442b.

You know the drill by now: first we ask what the star is like. Kepler 442 is K-class, which means she’s orange and slightly smaller than the Sun. Through an Earthlike atmosphere her light would look yellow and there’d be about two-thirds as much of it. We’ve seen worse: Gliese 667Cc, for example, only gets reddish light at 20% of Earth levels. Kepler 442 is, however, quite metal-poor. Because planets and stars are normally formed from the same material, this means that the planets in the Kepler 442 system would be too.

Kepler 442b is a little larger and a little heavier than Earth. The extra size helps to offset the increased pull of gravity, meaning that if you stood on her surface you’d probably only feel 30% heavier.

(We cover a lot of larger-than-Earth planets on the astroknowledge series. This is because they’re easier to spot. There are probably lots of smaller-than-Earth planets too, but we haven’t found them yet.)

I’ll come out and say it directly: unless she had a thick atmosphere, Kepler 442b would be cold. Her surface temperature on the equator during the day would be 233K, which is -40C. However, if she had a thicker atmosphere then she could retain much of this heat and nurture a greenhouse effect.

Too much greenhouse effect, of course, is a bad thing. It’s all about balance.

Greenhouse effects are caused by triatomic gases in the atmosphere: that is, gases which have three atoms per molecule. While there are many gases which are triatomic, the most famous and probably most important are carbon dioxide and water vapour. Climate scientists have modelled atmospheric carbon dioxide extensively, and we’re just starting to learn about what water vapour does in the atmosphere. While it’s not my field, it’s my understanding that water vapour is much better at causing a greenhouse effect than carbon dioxide is.

However, if the planet is below 0 C, there’s going to be no water vapour in the atmosphere because it would be frozen. This means that we’ll need enough carbon dioxide to get the greenhouse started and get the temperature above 0 C, at which point water vapour can start to amplify the process.

In other words, if Kepler 442b doesn’t have enough carbon dioxide then she’ll be an arctic planet. If she does have enough, however, then she could be quite warm.

How warm?

Answer: pleasantly, the answer is “not too warm.” As water vapour increases in the atmosphere it forms clouds. Clouds are white [citation needed.] White things reflect heat. Therefore, the hotter the planet gets the more clouds it has and the more of Kepler 442’s light it reflects away. We call this albedo.

James Lovelock pointed out that this creates a self-balancing system: a hotter planet has more clouds and so has a higher albedo, which means it receives less light and so cools. A cooler planet has a lower albedo, so it receives more light and so warms up. He then got very weird and mystical about it but the principle itself is sound.

If Kepler 442b has a greenhouse effect dominated by water vapour (like Earth does) then her temperature will stabilise between 0 C and 100 C, which is good news because that’s what humans find comfortable.

Considering Kepler 442b’s higher gravity, her atmosphere will be denser than Earth’s, which means that she may well be more humid. This, in turn, means that where Earth has deserts she’ll have jungles, and where Earth has plains she’ll have swamps and forests. Plant life loves humidity, and even if it isn’t hotter it feels hotter and more unpleasant.

What would it be like to live there?

The very densest forests are the sort known as cloud forests: they grow where the air is saturated with water, so that plants can feed directly from it rather than needing deep roots. In the cloud forests, trees grow to immense heights and are covered with secondary vegetation. The humid air of Kepler 442b might lead to this being common.

Innately fertile regions often have extremely fragmented government. If every glade can fend for itself without the help of the next, and if it’s possible to escape a lord’s rule simply by moving elsewhere, then the lord has less ability to enforce their will on the people. In historical Europe, lords tried many elaborate schemes to prevent people “voting with their feet” in this way, from serfdom to border controls, but none of it worked very well. Society naturally fragments into lots of little states, each with their own unique slant on government, philosophy and religion.

Humid, fertile regions are also cauldrons of disease. This would encourage primitive peoples to spread out rather than concentrating in any one region, and so would naturally limit the size of cities.

Both of these factors can be overcome with technology, of course, but their effects upon culture would linger. The people of Kepler 442b might grow up suspicious of foreigners, taking pride in their petty differences, and fearful of unified government. When they went into space this might cause a profound dislike of aliens, but also an attitude to space colonisation as being less about spreading their culture than about creating new cultures. They wouldn’t have colonies so much as alternate home planets, owing little allegiance to Kepler 442b itself.

D-12: Journey to the Core, Part Two

The following was originally published on the Stellaris forums.


Welcome back! Last time we were in the midst of a journey to the centre of the galaxy, and had just passed into the molecular gas cloud known as Sagittarius B2. This is the point, beginning around 120 parsecs from the centre, beyond which the stars are dense enough and gas is common enough that we can no longer think of space as being empty. The gas is very thin by atmospheric standards, but if we tried to travel through it at high speeds then it would create enough resistance to slow us down. It’s also bright enough that it gets harder to see beyond it.

What’s beyond it is terrifying, but the stars get even denser and I have no pictures for you. Astronomy into this mass of light and gas is like staring into fog.

When we get within one parsec of the centre, the density of stars is so great that every cubic light-year contains, on average, seven stars. (Compare that to where we are, out in the spiral arms, where the stars have empty light-years between them.) Even worse, the proportions of star types have changed: while red stars are still the most common, big blue stars are no longer fractions of one percent of the stellar population: they are now a full ten percent. These are dense enough that their radiation has ionised all the gas we find within this area. (This means that it is far beyond lethal.) Even worse, many of these are what we call Wolf-Rayet stars: they’re right on the cusp of supernovadom and are massive enough to cause a gamma ray pulse when they finally get there. This is a bad celestial neighbourhood to be in. But even that isn’t the worst thing.

The worst thing is that all these unusually-common massive stars are the same age. Not “around the same age” but “close enough that we think something caused them all to form at the same time.” I am not a superstitious man, and so I do not believe that it was witches (of the Hoag’s Object sort) who caused this to happen. Others may be more superstitious than me.

Let’s leave the puzzling stars behind us and travel even further. Deep inside this hedge of stars we find another structure, called Sagittarius A. It’s a confusing lump which is composed of two visible parts and one hidden. One of the visible parts is the remnants of a supernova; the other visible part is an accretion disk. Over a long period of time the two have partially joined together. We’ve met a lot of deadly things in the course of this series, but Sagittarius A is the worst. Each of these two monsters can devour stars without pausing for breath, and they’re so close they overlap.

They’re not friends either. We have evidence that something colossal blasted away enormous amounts of matter from the accretion disk in the past, in a way which suggests that it was the supernova that did it; and we can see the accretion disk feeding on the supernova remnant.

If you’ve ever played that childhood game of “who would win a fight between Cthulhu and Yog-Sothoth”, then what you are seeing now is evidence that the very gods themselves have played that game too.

In the middle of the accretion ring is the invisible reason why they’ve joined together to form Sagittarius A. We call this invisible thing Sagittarius A* (pronounced A-Star in a way that will make British people think about A-level marks.)

Before I tell you about Sagittarius A*, I want to tell you about a party I went to once. A new album by Muse had come out and it got played, because my university friends and I liked Muse. I vividly remember hearing the lyrics for the first time:

I’ll never let you go
If you promise not to fade away
Never fade away

I just wanted to hold
You in my arms

I asked what the song was called, and found out that it was Supermassive Black Hole. I distinctly remember thinking that this is in a way the most perfect description of a supermassive black hole that has ever been written. That’s what it does: it just tries to hold onto everything, up to and including light. General Relativity tells us that anything which passes the event horizon of a black hole will, to an outside observer, never experience time passing again. This means that light which goes past the event horizon will quite literally never fade away.

That’s what Sagittarius A* is: a supermassive black hole, lurking at the core of the galaxy.

I found out several months later that those weren’t actually the lyrics to Supermassive Black Hole: that’s the chorus from Starlight, off the same album. It’s also a good song but the two tunes are entirely different and it’s difficult to mistake the two. Weirdly, to this very day I can vividly remember hearing those words to the wrong tune. Memory is a strange thing.

Black holes are strange things too. It takes a while to appreciate how heavy they are. If you’re a scientist then 4.1×10^6 solar masses is meaningful to you. If not, then “four million suns” is just a number: it’s big, yes, but it could be three or five million suns and you wouldn’t feel any differently about it. How heavy is it? Very heavy. It is, by itself, 0.1% of the total visible mass of the galaxy. It is one-quarter of the mass of all the other black holes in the galaxy put together. That’s how heavy.

Once you’ve got a grasp on this number, the next surprise is how big it is in size terms rather than mass terms. It must be immense, right? Well, I’m sorry to disappoint you. We know that it’s less than 0.08 AU across, 17 times the size of the sun, which would still fit comfortably inside the orbit of Mercury.

Black holes are actually quite small.

What they are not, however, is quiet. Remember when I said that Sagittarius A* was invisible? That’s true only for people in the spiral arms of our galaxy who can’t see (or hear) radio waves. If you can hear radio then you’ll be able to hear it quite clearly as the loudest source in the galaxy.

Most of the radio emissions, however, are being directed directly out of the galactic plane in two tight beams. If you are in another galaxy and can pick up radio waves, then Sagittarius A* is the most easily-detectable thing in the Milky Way. (We think.) There are other galaxies like ours which are so far away that the only detectable thing is a radio source like that, which probably also comes from a supermassive black hole at the centre.

(Properly, the name Sagittarius A* refers to the radio source itself, not the black hole. The only people who really care about this are the same sort of people who will correct you when you say things like “I weigh 70kg” instead of “I mass 70kg”.)

I wish I could show you a picture of the beast lurking at the centre of the galaxy, but I can’t. I can’t even show you a picture of the vast duelling pair of titans that make up Sagittarius A. The core is too dense and too bright for us to see into it, let alone travel there. This is no place for mortals.

Here be dragons. Stay away.

D-13: Journey to the Core, Part One

The following was first published on the Stellaris forums. Since I missed a day posting over the weekend, I’ll post two episodes today to make up for it.


Welcome once again to another episode of what’s been dubbed the Astroknowledge series.

Today we’re going to be talking about the galactic core. What we know about galactic cores comes from two sources: things we can see about our own core despite it being too close and other things being in the way, and what we know about other galactic cores despite being too far away from them.

If we started travelling towards the core in the direction of Sagittarius, what would we encounter? The answer is “nothing, probably.” There are millions of stars out here on the spiral arms but even so they’re spread so thinly that the chances of us encountering any are small. It’s eight thousand parsecs to the core but the chances are that we’ll pass most of it safely in the black.

(Image courtesy NASA.)

The first sign of change will be when we get within three thousand parsecs of the core. Here we’ll start to see two things: firstly the sky stops being dark, and secondly the stars are dimmer to human vision. We’re now within the galactic bulge, and unlike the flat disc shape of the outer spiral arms this is much thicker. In some galaxies the bulge is almost spherical, and in others it has its own sophisticated spiral form. Since the galaxy is “thicker” here, every direction we look will have more stars. If you’ve ever looked up at night and seen the vivid bright slash of stars that’s the plane of the Milky Way galaxy, imagine that but all over the sky.

A star made of fresh hydrogen is very bright but contains no metals. When it dies, its contents (still mostly hydrogen) will spread out and will in time form new stars. Those stars will contain a lower percentage of hydrogen, and as a result more of their light will be put out as infrared rather than as visible light. (If you remember our discussion on Gliese 667Cc, some stars are very noticeable in this regard.) We call this “metallicity.” The stars within the bulge are more metallic: they’re made of the corpses of their ancestors.

Travelling closer to the core, we notice that the stars get steadily more metallic. They also get steadily larger, younger, hotter and bluer. The number of bright O- and B-class stars in the bulge is higher than out in the spiral arms. These stars live extremely short lives, less than 10 million years, too short to form planets. When they reach the end of their life they swell up into giants; some become supernovae and then neutron stars or black holes, and others dwindle into tiny white dwarfs. Either way, when they die they blast huge amounts of gas out into space which then forms the substance for new stars.

Space is now less empty: the stars are denser and they’re filling the gaps between themselves with gas. We are now in the realm of the giants, living their hot brief lives in a frenzy of activity and blasting terrifying radiation at one another when they die.

Most of the stars within the realm of the giants are main sequence M-class red stars: although they are born less frequently here they live thousands of times longer, and so end up being the majority of the population. Still, there are enough O- and B-class stars around to terrify anyone.

Within 120 parsecs of the core, the gas gets so dense that our normal rules about space being empty don’t apply. There are now an average of several thousand molecules per cubic centimeter. This is still a tiny amount when compared to our atmosphere (which has an average of 10^19 molecules per cubic centimeter) but it’s enough that space can no longer be thought of as empty. This is the point past which no spacecraft could possibly go, not least because this gas is hot, agitated and baked by radiation from the stars.

(Image courtesy Hubble.)

We are now in the great molecular gas cloud, also called Sagittarius B2. This is the combined nursery and graveyard of a vast number of stars. This is also the point at which we can’t help noticing something which has been here all along but has been too subtle to detect: we’re orbitting the centre.

Suppose we had a ship which could penetrate Sagittarius B2. What would we find?

D-14: It Had It Coming

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.