The Violent History of the Martian Moons

Mars is a funny little place, but it seems that Mars orbit may be even funnier. As we all know, Mars has two small and lumpy moons which would be no more than a pair of mundane asteroids if they weren’t orbiting Mars. Phobos and Deimos. But this seems to be only their latest incarnation.

No one’s entirely sure about the origin of the two potatoes in orbit around Mars. Sharing their planet’s orbital space with 14 satellites we’ve sent there, many people believe that they were captured main belt asteroids. They do seem to share a lot in common with C-type and D-type asteroids, but their orbits seem a little too perfect. Their orbits are quite close to being circular, and both lie perfectly in Mars’ equatorial plane. Orbital drag and tidal forces can do this, but with Mars’ low gravity and thin atmosphere, it’s difficult to make this theory work. Additionally, infrared spectra of Phobos and Mars show very similar phyllosilicate minerals, suggesting a common origin.

A slightly more interesting possibility is that Mars may once have had rings, a little like Saturn, and Phobos and Deimos coalesced out of them. This may sound outlandish, but it’s a similar hypothesis to where we think Earth’s moon came from (with the big difference being that the Earth-Moon system is huge compared to the Mars system). And even asteroids can have rings, under the right circumstances!

There’s certainly evidence for a huge impact on Mars sometime in the past. The Martian Dichotomy is the name given to the interesting fact that Mars’ northern hemisphere is dramatically different to its southern hemisphere. The northern hemisphere is 1-3 km lower in elevation, and has a crust nearly half as thick as the crust in the south. At least one theory is that the entire northern half of Mars is, essentially, a huge impact basin.

An impact big enough to produce that kind of effect would be big enough to knock a lot of material into orbit. Some believe that this caused Mars’ orbit to have once been full of Phobos/Deimos-like objects. Others think it may have caused a ring system to form.

Similarly to Earth, a ring system like that is thought to have formed into a large moon. Unlike Earth, that wasn’t the end of the story.

In celestial mechanics, any object orbiting another has what’s known as a Roche limit. It’s described conveniently by this simple little equation;

where Rm is the radius of the secondary, Mm is the mass of the secondary, and MM is the mass of the primary. The equation gives d, the distance at which the secondary starts to be torn apart by gravity. In this case, the primary would be Mars, and the secondary would be its ill-fated moon.

So, one hypothesis goes that this former moon of Mars was ultimately shredded by the gravity of its parent planet. In fact, the process seems to still be occurring. After who-only-knows how many moons have been formed and crushed, Phobos and Deimos are all that remains, but Phobos is probably doomed to the same ultimate fate.

Deimos, on the other hand, is probably safe. It’s a lot further away. Any planetary orbit also has a radius called a synchronous orbit. Around Earth, we know this as a geostationary orbit – the point at which an orbiting satellite will always be staring at the same part of Earth’s surface. But there’s an interesting ramification of synchronous orbits. They’re the most stable kind. Inside this point, an orbit is destined to eventually decay, while objects outside will gradually orbit further away due to centrifugal force, and may ultimately be lost altogether.

Phobos sits, doomed, inside a synchronous orbit, while Deimos sits outside where it will eventually be free to leave. Similar situations in the past might explain why Phobos and Deimos are all that remains of the moon that once was.

Assuming, of course, that this hypothesis is true. It’s very difficult to say, until we find an errant asteroid which happens to match the composition of the current Martian moons. Additionally, while a ring of dust and small objects has been predicted to exist between Phobos and Deimos, no evidence has ever been found for such a ring.

Either way, the idea of a small, terrestrial planet like Mars having a ring system is rather pleasing!


Heard via Gizmodo.

Gif animation/video clip of Phobos eclipsing Deimos, as seen by NASA Curiosity.
Photomanipulations created by @InvaderXan/ using
• Comanche Outcrop observed by NASA Spirit,
• Phobos observed by NASA MRO,
Deimos observed by NASA MRO,
• Dione observed by NASA/ESA Cassini, and
• Mars observed by ISRO Mangalyaan.

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Space Dumpling

This is not a delicious 台湾水餃子 (Taiwanese boiled dumpling) drifting through space, no matter how much it may look like it. It is, in fact, one of Saturn’s moons! This is Pan. A tiny moon, only 35 km in diameter, and the second closest to Saturn itself.

As you might expect from such a tiny moon, no one even knew it existed until we sent a craft there. It was first photographed by Voyager 2 in 1981, but wasn’t actually discovered until 1990 when those images were being reexamined. Part of the reason for this is because it’s so small, and part of the reason is because of where it is.

Pan, you see, is a shepherd moon – one of a handful of tiny moons which help maintain the shape and structure of Saturn’s rings. Pan orbits in the Encke gap, a 325 km wide space in Saturn’s A ring which is free of the icy, dusty particles that make up the rest of the ring system. The reason the gap exists at all is because Pan sits there, sweeping up all of that material.

It’s also the reason why Pan is such an odd shape. That weird ridge around its equator is made up of accreted ring material. As Pan orbits, any and all stray bits of ring material which end up in its path ultimately find themselves sticking to Pan’s surface, and because Saturn’s rings are so perfectly planar, they form a surprisingly neat ridge around Pan’s middle.

That ridge is also what makes it look so much like an errant space foodstuff. Whether you see an empañada, a gyoza, a piergoi, or a ravioli, of course, probably depends on what you personally prefer to eat. Interestingly, it’s a common feature of  shepherd moons, with another of Saturn’s moons – Atlas – also having a similar equatorial ridge. If you’re curious to learn more, planetary scientist Carolyn Porco has two journal papers about it for you to read, available here and here.

Of course, that funny looking ring of material gives it more than just a passing resemblance to its parent planet Perhaps it just wants to be just like Saturn when it grows up. How adorable!

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The Planet Quandary II – High Definition

Take a look at these five oblate spheroidal spacey things and tell me which one is the odd one out.

Not sure? I’ll admit, it’s not too easy. They’re all unique in their own ways. One of them has a dense atmosphere. Four of them contain large amounts of subsurface water. One of them has more water than Earth does. One has lakes of liquid on its surface. Two show evidence of cryovolcanism. One has an extended atmospheric haze layer which is much larger than Earth’s.

Would it help to see them all to scale?

As it happens, the odd one out is the one to the lower left. That grey, rocky orb is Mercury, and it’s the only one which is a planet. It isn’t the largest of the group, and it’s arguably the least interesting, yet it holds a privileged place in our Solar System, being one of the Sacred 8.

In discussions about defining planets, a lot of people ask, “Why does it matter?” An important question, and one which I think is a little overlooked. Between people fighting over Pluto, and people fighting to maintain the status quo of 8 planets, the actual reason for definitions gets lost sometimes.

The thing about a good definition of something is that it lets us have some idea of what it is we’re looking at. All definitions are arbitrary and human-made. Natural phenomena don’t care what we call them. They’re supposed to be made by and for us, to help with our view of the Universe and what’s in it.

To me, this is the point of trying to properly define planets. Those five objects up there are all fascinating in their own ways. And it comes down to more than just mass and orbital characteristics. Consider Enceladus and Vesta to the right, here here. They’re both roughly the same size. One orbits a planet, and one orbits the Sun. But just by looking, you can tell that they’re very different objects.

The thing is that the Universe doesn’t play by any set of rules. When new discoveries are made, they’re frequently surprising. Things aren’t always clear cut, and there are always borderline cases. But knowing loosely where to make a distinction helps us as observers.

In a similar way, hand someone a box of crayons and they’ll probably be able to pick out a red and an orange for you. But if you ask someone to point to the exact place on a spectrum where red stops and orange begins, your mileage may vary.

When defining anything, you need to use its characteristics, and these can be conveniently subdivided into two types. Intrinsic characteristics and extrinsic characteristics. Intrinsic characteristics originate solely from the thing itself. An object’s colour, mass, and shape, for instance, are all intrinsic. Extrinsic characteristics, on the other hand, originate from outside the object in question. An object’s location, and the energy it receives from elsewhere are examples of extrinsic characterstics.

So how does this all apply to planets? Well, the thing is that the current definition for a planet is needlessly convoluted, using both intrinsic and extrinsic characteristics. In case you missed the background, you can catch up here. Long story short, the current definition of “planet”, according to most, meets the recently proposed geophysical definition:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameter.

As well as the astronomical definition:

  • Orbits its parent star.
  • Dominates its orbit in terms of mass and orbital distance.
  • Would clear out any debris in their orbit in well under 0.1 billion years.
  • Its orbit, barring any outside influences, will be stable as long as its star exists.

The problem in this, as some of us see it, is the use of extrinsic characteristics in the definition. There seem to be 3 spanners in the works here.

A stray planet is not a planet

The galaxy is a big, chaotic mess of dancing stars. As they orbit the galactic centre, they stray and occasionally interact with each other. Sometimes they come close enough to perturb each other gravitationally. Tidal interactions happen. Planetary orbits get tugged and tweaked. If two stars pass close enough to each other, a planet may be ejected into space.

Except it’s not called a planet anymore. Its planet card has been revoked. Rogue planets aren’t really planets at all. I want my money back!

In astronomy, there’s a weasel term sometimes used – planemo or PMO (an abbreviation of “planetary mass object”). Technically, a planet is actually a sub-class of planemo. But this feels like a needless subdivision, and it’s questionable whether this nomenclature would be necessary if we simply referred to these objects as planets.

A planet orbiting a planet is not a planet

This is something which has always bugged me. In astronomy, I started out looking at stars. Stars frequently orbit stars. In fact, there are a huge number of multiple star systems out there. At no point has anyone seen fit to question whether or not these all qualify as stars. Mostly because that would be ludicrous.

Similarly, galaxies orbit galaxies and asteroids orbit asteroids. In all of astronomy, the only class of object which isn’t allowed to orbit another of its kind is a planet. This is not logical.

Planetary requirements vary dramatically

This seems to be the central point which I’m refuting. As I pointed out in part I, this means that the definition of a spheroidal planet sized object actually changes according to where that object is. I’ve spent awhile thinking about this, and I can’t think of a single other physical phenomenon anywhere else in science whose definition is dependent upon its location♣︎. Feel free to leave a comment if you can think of one.

Additionally, subdivide any physical phenomenon into its component parts, and they too are defined by intrinsic characteristics. Clouds on Earth are made of water droplets. An iron nail is made of metal crystals, made of atoms, made of subatomic particles. All of these things are defined intrinsically. An iron atom doesn’t cease to be an iron atom because you remove it from its crystal lattice, and a neutron doesn’t cease to be a neutron if you pluck it from an iron nucleus.

Personally, I’d be much more comfortable with a definition based on intrinsic characteristics, such as the geophysical definition I included above. The criticism of using the geophysical definition alone is, apparently, that it’s too inclusive. We’ve catalogued over 4000 comets, with estimates for the total of up to 100 billion. It’s estimated there are 1.9 million asteroids over 1 km in diameter in the main belt alone. But 110 planets is too many? I find this argument… unconvincing.

However, there’s one line in the proposed geophysical definition which I’m unsure about – “a spheroidal shape adequately described by a triaxial ellipsoid” has obviously been included so that the dwarf planet Haumea would be included. This would also include the asteroid Vesta (shown in that image above). I think it’s fair to say, Vesta’s claim to planethood is a can of worms best saved to open in a later blog post.

Of course, there is one thing the current astronomical definition can tell us about an object, and that’s its formation and history – which is what I intend to discuss in part 3 of this series. Thanks for reading!

In the meantime, for no particular reason, here’s the most hilarious photograph of Alan Stern that I could find.

Ultimately all photons are very much the same. Minuscule packets of electromagnetic radiation travelling at 2.998 × 10⁸ m/s. But still, we categorise them. Infrared. X-ray. Optical. Radio. Not just for the way we experience them, the way we perceive colours, but the way they interact with the rest of the universe. Infrared photons and ultraviolet photons may be relatively similar in energy, but their effects on matter which they interact with is vastly different.

♣︎ You could, I suppose, argue cause and effect to some extent. For instance, an Auger electron is specifically an electron ejected from the inner shell of an atom due to the Auger effect. But it’s still unquestionably an electron. And it should be called the Meitner effect anyway…

I’d look for a hilarious picture of Ethan Siegel, but to be honest, it seems difficult to find one which isn’t hilarious in one way or another.

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I’m pretty sure it’s something we’ve all done. You’re so intently typing something, with your desk cluttered with various printed papers, books, and notes. It’s so easy to not notice that you hit caps lock when you started typing.

Which is exactly what happened to Julie Blommaert, aka @Julie_B92 on twitter.

Who understandably found it hilarious that she’d been effectively yelling at her computer about important science things.

And so, the #ALLCAPSPROPOSAL hashtag was born! Obviously, hilarity ensued as miscellaneous scientists from various fields started yelling about science they want to do in 140 character bites.

Hopefully when we’re all done yelling, the research funding should start pouring in. In the meantime, as you might expect, the resulting tweetfest ranged from the actually quite sensible…

…to the slightly silly…

…to the delightfully absurd.

And then there’s Chissa Rivaldi.

Yeah, we’ve all been there.

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RIP Chuck Berry


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Artificial Trees, Real Energy

Take a look at this thing. It may look like an art installation of some kind, but it’s much more than that. This is a Wind Tree, made by a French startup company called New Wind, and it may hopefully be a great way to generate energy in urban environments.

Each of those “leaves” is a small turbine, trademarked as an Aeroleaf, and is capable of generating 64 watts at peak output. This gives a typical 9.2 m tall Wind Tree with 54 Aeroleaves (Aeroleafs?) a power output of over 3.5 kilowatts. I say typical, because the modular nature of this device gives some freedom for customisation, meaning the number of turbines can vary depending on your needs.

For homes in windy areas, just one Wind Tree would provide a significant amount of energy for a home. The average home in the US uses about 10800 kilowatt hours (kWh) of electricity per year. Operating at only a third of its maximum efficiency, a single Wind Tree will account for nearly 95% of that.

Thanks to the small Aeroleaf design is that the Wind Tree can make use of lower speed winds for power generation – just 2 m/s, as opposed to the 4 m/s required for more traditional wind turbines. Another benefit of these compact turbines is that an Aeroleaf is sensitive to various types of wind. Windmills operate effectively with laminar airflow – wind which comes predominantly from one direction and maintains a constant speed. This is ideal for offshore setups like those in Scotland and the Netherlands, but not so good everywhere. Because of more complex urban landscapes, city wind can be turbulent and gusty. Thankfully, turbulent airflow works just fine to power a Wind Tree.

The guy in this video clip, by the way, is Jérôme Michaud-Larivière, CEO and mastermind of New Wind.

The intention behind the Wind Tree is to provide a safe and aesthetically pleasing way to generate wind power in places where the usual kind of windmill is inappropriate. Where regular wind turbines may be inappropriate in a city, Wind Trees may be easily included on university campuses, industrial parks, outside office buildings and on rooftops to provide a convenient way to generate power.

As it happens, a typical large office building uses around 20 kilowatt hours of energy – so on a windy day, just 6 Wind Trees would yield enough energy to actually make money by selling surplus energy back to the central power grid. Of course, this doesn’t count energy storage either. Renewable energy sources like wind operate continually even when their energy is not in use, and the Wind Tree is no exception. A storage battery saves generated power, which is then converted to AC at standard voltage when needed.

Small, distributed forms of energy generation like this may ultimately be a much more effective way to generate power, particularly with the increasing energy demands of a large city. For example, based on information from a few years ago, London has approximately 680 million m² of commercial property, with around 110 million m² of that being office buildings. Taking a typical office building to occupy around 1400 m², this gives over 78500 office blocks in London. If only half of these offices installed and maintained just one Wind Tree, it would provide up to 137.4 megawatts of power. In the same region as the output of a typical UK power station. 1.1 million Wind Trees would equal the output of the ominously named Drax Power Station, the UK’s largest.

Currently, a lot of  details appear to still be concepts and plans. Future concepts include turbines shaped like flowers, apparently with solar panels in the petals, or with different Aeroleaf colours. I quite like the idea of pink ones to look like sakura petals, myself.

Previous reports have suggested that New Wind intend to start production of these lovely devices sometime this year, with the cost of a Wind Tree expected to be in the same price range as a family car – an estimated €29,500. Unfortunately, their website doesn’t currently offer details. They gave the option near the end of last year, of preordering Wind Trees for the cost of around $1000 US each, though unfortunately their store website appears to be down as I write this.

All the same, I hope they’re successful. I’d love to see these around big cities. As well as giving some visual interest to city streets, it would be deeply comforting to know that cities were generating power this way.

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I just wanted to share this gorgeous photograph – Jupiter and all four of its Galilean moons clearly visible, behind our own Moon with the sunlight illuminating the bumps and ridges on its surface. I love photographs like this. Sometimes it’s a pleasant reminder of how small our world is, to look up at night and see the other planets in the night sky. And to remember that we now know what they look like up close.

Unfortunately, I have no idea who the photographer is, and neither Google Images nor Tineye are being much help. The best I can do is to track it back to this tumblr page, which lists no other source. If you know where this photograph came from originally, please let me know so I can credit the photographer properly!


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The Planet Quandary I – The Sacred 8

So this is the first part of my series of analyses and perspectives on the ongoing planet debate. Honestly, I can’t believe it’s been like, a decade already. Which is probably the main reason I’m going into some detail on this. Admittedly, the scientific community at large is somewhat infamous for spending a long time ruminating over things. Not least because, whether or not they admit it, a lot of scientists don’t really like change. Ironic but true.

Anyway, this first in this blog series is largely in reaction to a post by Ethan Siegel on his blog, Starts With a Bang. If you have a few minutes, you may want to go and read it – I’ve been reading Ethan’s writings for several years now. He’s great at explaining things from the ground up, and his articles are well structured without skimping on the detail. However, in this case, I think he’s wrong.

This concerns the definition of planets, and what does and does not constitute a planet, scientifically speaking. I’m also going to reiterate what I said in my introductory post that Pluto is not the point here, even if I may use it as a convenient example.

The full definition given by the IAU of what a planet actually is is… complicated. I’ll go into more detail on that in my next article. For now, I’m just going to concentrate on one thing. A principle outlined in a paper by Margot (2015) which gives a line in the planetary sand, demarcating what is and is not a planet.

My own redrawn version of this figure is here:

(There’s a larger version with all my annotations here.)

Seems fairly clear, doesn’t it? Anything above the line is a planet. Simples! This is essentially a function of whether or not a planet has “cleared its orbit” of other material – things which haven’t aren’t allowed to be planets.

Trouble is, this is a paper written in 2015 to justify a decision by the IAU in 2006. To my knowledge, situations such as this are rather prone to confirmation bias. Particularly when devising a rule so carefully tailored to our own Solar System. In fact, it’s interesting how firmly people want to hold the belief that there are 8 planets in the Solar System and absolutely no more. The sacred 8. A highly exclusive club, from which all vagrant small round objects are banned.

My biggest problem with this definition and its mathematically defined line in the space sand, is that it isn’t constant. The definition of an object changes, dependent on where it is exactly. And call me old fashioned, but I like my definitions to be… well… definite.

To reiterate: The definition of what an object is changes according to its location.

If you move a planet to a different orbit, it will transform into a different object. Yes, this is a mathematical consequence of this definition. And it has a number of ramifications…

Changing Identities

If you were to move Earth to an orbit greater than ~140 AU from the Sun, it will cease to be a planet. While this admittedly is unlikely to happen, barring a cataclysmic event, it has implications. Earth-sized planets are typically considered unambiguously to be planets, seeing as we live on one. But by this definition, if we find any Earth-sized objects out beyond the Kuiper Belt, they may not be planets.

Meanwhile, if you were to move Pluto inwards to an orbit somewhere between Mercury and Venus, it would be a planet. Which seems to contradict the “Pluto is too small” argument that so many people always seem to make.

You’re Simply Not Close Enough

The not-a-planet line actually crosses the x-axis in this graph around 0.04 AU, which means that literally anything orbiting inside that line may be a planet – even tiny objects like Ceres, Enceladus, or a comet which is at hydrostatic equilibrium. For the record, according to NASA’s small body database, there are over 1420 objects which pass closer to the Sun than 0.04 AU. Of course, these don’t dominate their orbit in terms of mass. But their presence does call into question whether or not Earth’s orbit should be considered “cleared”.

Planets or Moons?

It’s worth also considering the moons of the Solar System. If Earth’s moon was orbiting the Sun at the same distance Earth is now, it would be considered a planet. Meanwhile, if Titan were orbiting the Sun instead of Saturn, it wouldn’t be a planet, despite being larger than the Moon. If we were to replace Jupiter with its moon Ganymede (more massive than Titan), it would be just barely too small. It misses the planet line by between 0.008 and 0.011 Earth masses, depending on whether it’s at perihelion or aphelion.

Part-Time Planets

In fact, given that planetary orbits are elliptical by definition, this means that anything orbiting too close to that magic line may be a part-time planet, achieving planethood once every orbit when it’s fortunate enough to be sufficiently close to its parent star.

This has some interesting implications as well, for people hunting for the elusive “Planet 9“. Formerly known as Planet X, but now it wouldn’t be number 10. Interestingly, it may not always be number 9 either. Planet 9, if it exists, is predicted to have an elliptical orbit with a perihelion of 200 AU and an Aphelion of 1200 AU (which is why it’s shown in two places on that plot up there). Checking this against Margot’s magic planet line, it turns out that this is just enough to be a “planet” for its entire orbit.

It suddenly feels a little suspicious that Mike Brown, who goes by the rather unsubtle twitter name of @PlutoKiller, predicted this as Planet Nine’s most distant point. I have to wonder what will happen to this definition if it turns out that Planet Nine is slightly further away. Will people start to rethink this definition if the Solar System contains an object which ceases to be a planet once per orbit?

On a final note, Margot’s line in the sand is touted as being scalable and applicable to any star/planet system. So I did what anyone else would to investigate this. I downloaded the entire NASA Exoplanet Archive to take a look.

Out of a total of 3485 listed records, I found 1070 objects with known mass, orbital semimajor axis, and host star mass – mass of the host star was important to scale things so I could plot them on the same graph. Turns out, the overwhelming majority of these are indeed “planets” by the IAU definition. Though as anyone who works in exoplanet detection will tell you, our methods are biased overwhelmingly to detect things which are either very large or very close to their parent star. Below a certain size and above a certain orbital semimajor axis, we’re largely blind.

All the same, there are a few outliers on that plot. The low mass ones are pulsar planets, which everyone always forgets about anyway. But there are undeniably two super-Earth sized planets in the data which aren’t allowed to be real planets under Margot’s definition. In addition, two Jupiter-sized planets are borderline cases which may be only part-time planets depending on how eccentric their orbits are.

As I always say, a good hypothesis will become stronger the more you try to break it. Unfortunately, the way I see it, the IAU’s definition of a planet has cracks in it. It seems strong at first, but it has weaknesses.

My prediction is that as our telescopes improve and the planet hunters keep doing their thing, we’ll start to find more and more objects which are inexplicably not allowed to be planets.

Perhaps then, everyone might consider a better definition. In the meantime, I’ll let Mars here describe my feelings on the matter.

It’s fair to say that every change, major or minor, that’s ever happened in science hasn’t been without its opposition. In my opinion, this is not necessarily a bad thing. I’ve been, at various points, on both sides of the debates. Sometimes, it can stimulate much needed discussion and conversation, but other times it can stand in the way of progress. It’s a double edged sword.
Consider also that very hot things tend to melt, and that liquids reach hydrostatic equilibrium very easily. Water droplets, for instance, are at hydrostatic equilibrium. Taking this to its most ludicrous extreme, a small globule of molten glass in a circular orbit around the Sun at 0.02 AU would be classified as a planet if there was nothing else orbiting there.
By the way, it wasn’t included in the archive, but the mass of PSR B1257+12 is believed to be about 1.4 solar masses.

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The Moon and The Meteor

Did you know it’s actually possible to see a meteor crash land on the moon? You need to be very lucky to do so, but it’s not uncommon. After all, both Earth and the Moon are being continually pelted with meteoritic material. Most of it is barely larger than dust, but occasionally a few larger objects leave their mark.

I’ve actually been lucky enough to see this once myself. When I was just a kid living out in the countryside under dark skies, I used to have a telescope. The Moon was always an easy target to look at, so I used to point my telescope there quite often. One night just as I was about to finish up for the night, I actually saw a bright spot appear on the Moon’s surface ever so briefly.

At first, I wasn’t entirely sure what it was I’d seen. After doing a little reading, I realised it must’ve been a meteor impact. It wasn’t quite as bright as the one highlighted in this video, but it was definitely the same thing. If it had been on the Moon’s sunlit side, I wouldn’t have seen it at all. Strangely enough, this video is one of the few I’ve ever seen which show the same thing I saw that night.

I still consider myself rather lucky to have actually seen an event like that, small though it may have been. It’s little things like that which have always kept me fascinated with astronomy.

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The Planet Quandary

As some of my readers will probably know quite well, every astronomer’s favourite argument of what exactly a planet is has been drifting in and out of discussions for years now. This delightful nest of conversational wasps was recently shaken up again by none other than Alan Stern, the scientist behind NASA’s New Horizons mission to Pluto. And I’m very much inclined to take his side in the discussion.

You wouldn’t think it would be that difficult to define something like a planet, seeing as we live on one. However, you’d be mistaken. In science, we like to have good definitions of things. They’re sometimes surprising and may take things into account that you wouldn’t normally consider. This is the way of science, after all. Unfortunately… Well. To say that scientists don’t always agree on things would be an overwhelming understatement.

Stern’s argument is that planets should be defined geophysically, as he outlines in a recent paper. To me, this makes a lot of sense – even if it would suddenly mean that the Solar System is full of 110 planets. If this sounds like a lot to teach kids in school, remember that there were 151 Pokémon in the original game. This would include, among others, such places as Titan, Ganymede, Ceres, Enceladus, and Earth’s moon.

Everyone seems to focus on Pluto as the centre of the argument, but to me it’s incidental. I don’t care so much about Pluto’s planetary status. I do care about the fact that the IAU’s definition of a planet is needlessly convoluted and all kinds of problematic. Nonetheless, this is the decision which the IAU council came to.

In the words of Nick Fury…

This is a big topic, and I have a lot to discuss on it. This includes summarising conversations I’ve had with various people. So I’m not going to try and tackle it all at once. Instead, I’m going to write a series of blog posts (probably 4 or 5) on various aspects of the whole planet not-a-planet debacle.

I would, of course, appreciate any discussion and feedback anyone else wants to give on the matter as and when I post these. Even if you disagree with what I’m saying. Perhaps especially if you disagree with what I’m saying.

Let’s break this all down and see what we can build from the pieces.

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