Josh, 7, from London, writes:

Dear Hentzau,

            I went to the beach yesterday. The weather was nice. I had ice cream. When I was splashing around in the sea with my rubber ring wedged firmly around my waist to stop me sinking, I thought about the planets. Planets have rings, but they do not have to be prevented from sinking. Why do planet rings always go around the middle?

(And they do always go around the middle: see Uranus, which is tilted almost ninety degrees on its side but which has rings which still orbit around the equator.)

Well, little Josh, it’s because of three factors which shall be tackled in numerical order.

1)      Planetary formation from a protoplanetary disc.

2)      The Roche limit.

3)      Planets are fat.

When a planet forms, it does so out of a big cloud of stuff. Much of this stuff is fairly hefty planetesimals (stuff one kilometre plus on a side) but there is lots of smaller stuff as well. The planetesimals along with the smaller stuff collapse towards the centre of the cloud towards the proto-planet core. As we’ve seen in the post on the Nice model, when you have a big cloud of stuff that’s shrinking it will start to rotate. Rotation produces centrifugal force, and centrifugal force will flatten the cloud out into a disc. Eventually you end up with the proto-planet at the middle surrounded by a big disc of stuff. What happens to the leftover stuff is determined by how close it is to the proto-planet.

• Stuff close to planet: falls onto planet.
• Stuff far away from planet: forms moons and stuff.
• Stuff in the middle: forms rings.

Now, clearly there are some niggling factors which mean it isn’t quite as simple as that, chief of which is that only the gas giants have rings (yeah some people think Rhea might also have rings but honestly screw those people). Earth, Mars and the other terrestrials don’t. This is because the size of the “stuff in the middle” category is dictated by something called the Roche limit.

The Roche limit is, broadly speaking, the orbital radius inside which the gravitational tidal force of the planet is greater than the gravitational force keeping whatever object has been unwise enough to venture inside it in one piece. In other words, if you go inside the Roche limit and gravity is the major force holding you together¹ – so I don’t recommend this at all if you happen to be a planet or a moon — you get pulled apart. I like the Roche limit because it’s one of the few physical terms that I actually understand the derivation of, but I won’t go into that here. It’s expressed as an approximation by:

where R is the radius of the primary (i.e. the planet), ρ_M is the density of the primary, ρ_m is the density of the thing orbiting it, and d is the Roche limit.

So the Roche limit isn’t strictly a hard physical constant; it’s going to vary from body to body depending on the ratio of the densities of the primary and the orbiting body. However, because R is also in there, if you have a really, really big planet – like a gas giant, say – the Roche limit is going to be a factor no matter how dense your orbiting body is.

Using FABULOUS EXCEL TECHNOLOGY, I have created a graph which shows how the Roche limit varies with the density of the orbiting body for Saturn and Earth.

 
This makes it very easy to see that the Roche limit is big for less dense things and small for more dense things, and also that as you increase the density more and more it becomes practically static.

Now, to get the Roche limit for your proto-planet you have to put a number on the average density of protoplanetary material. I’m going to use the mass of a comet, which is about 500 kg m^-3. This gives Roche limits of:

• Saturn: 160,000 km.
• Earth: 35,000 km.

35,000 km is sod all, in orbital terms. It’s just slightly less than the altitude at which we orbit geostationary satellites; any protoplanetary material orbiting inside 35,000 km is overwhelmingly likely to fall onto the proto-Earth anyway, so that explains why the Earth has no rings. The Roche limit of 160,000 km for Saturn on the other hand matches very nicely the observed outer edge of Saturn’s ring system.

This is where the bulk of planetary ring material comes from, then; they’re formed from the leftovers of planet formation that are far enough away to avoid falling onto the planet outright, but too close to the planet to form themselves into moons. Since this disc was rotating around the “equator” of the proto-planet anyway thanks to the cloud collapse, that explains why the rings are found there. However, this doesn’t answer an important question: what about ring material that was captured after planet formation? Say a comet ventures inside the Roche limit on a highly inclined trajectory. Why isn’t it pulled apart into a ring orbiting the planet at that inclination?

Well, this is where we come to the third thing on our list: planets are fat. Literally. Because they’re rotating objects centrifugal force distorts their shape away from the perfect sphere of hydrostatic equilibrium, making them slightly fatter around the equator than they are around the poles. For example, Earth has a radius 6,378 km around the equator but only 6,356 km around the poles; those 22 km may not seem like much but they make a big difference as far as orbiting bodies are concerned, because it means things that aren’t orbiting around the equator will nevertheless be pulled in that direction thanks to the extra mass that’s present there. Over thousands of years they’ll slowly migrate to the equator, at which point they’ll probably collide with some of the other junk that’s orbiting there which robs them of their orbital momentum and stops them where they are. Planets may be a little bit flabby around the waist, but if you like to look at ring systems a little extra weight is no bad thing.

In conclusion:

• The majority of planetary ring material forms around the equator of the planet in a disc.
• The inner edge of the rings is created by the planet, which drags material that gets too close towards it.
• The outer edge of the rings is created by orbiting moons, which sweep up material outside of the ring area.
• The rings themselves are defined by the Roche limit, which dictates the nominal area where ring material cannot form itself into moons because the planet is gravitationally dominant.
• All material added to the rings after planet formation migrates towards the equator thanks to the planet not being a perfect sphere and having more mass – and therefore gravity – distributed around the equator.

BULLET POINTS.

1. You might think of the Earth as a solid object but it isn’t really. It’s held together by gravity; no material could possibly provide the same amount of bonding force to keep all of its mass in one place. This goes for most large bodies out there; they’re so big that the only thing that can keep them in one piece is gravity, so if they come across something with a larger gravitational force then it’s bad news for them.

The post The Rochefort Limit. appeared first on The Scientific Gamer.

Topical science time! This recent news article caught my eye the other day. Russian scientists have succeeded in drilling through the 3.7 km thick layer of ice covering Lake Vostok in Antartica and hope to be able to extract samples from the lake later this year. Lake Vostok has been completely sealed off from the surface for about twenty million years. This makes it a very interesting place to look for life forms, and what they’re doing in Lake Vostok right now is not a million miles away from the way we’ll likely find the first alien life forms.

The environment inside Lake Vostok is, to put it mildly, just a little bit exotic, and not the first place you’d expect to find bacteria. The ice sheet covering it is so thick that we didn’t even know for sure that there was water down there until aircraft- and space-based radar picked it up in the early 90s. Because it’s got a 3.7 km thick ice layer sitting on top of it, that water is under a tremendous amount of pressure – enough so that it remains liquid¹ despite having an average temperature of -3^oC. This high pressure also results in that water being supersaturated with high concentrations of oxygen and nitrogen than you’d find in an equivalent surface lake, much of which has been forced into “cages” of water molecules called clathrates. There is no light at the bottom of Lake Vostok, but that’s okay because there’s no light at the bottom of the ocean either and we find plenty of stuff living – in fact, thriving – down there, most of which tends to cluster around geothermal vents on the ocean floor – and we suspect the same might be true of Lake Vostok.

Not that you particularly need a geothermal vent to support life these days. Hopefully you’ll have heard of these guys – the extremophiles. Over the last few decades we’ve kept finding them in locations which were thought to be completely inhospitable to life – too hot, too cold, too acid, too alkali, too saline, even too radioactive. In all of these places extremophiles thrive, and some of them even need those harsh conditions in order to survive; for example the halophile loves salt and can’t grow without at least a reasonably high concentration of it present in its surrounding environment.

This has completely revolutionised our view of the conditions required for life to exist, and it’s entirely possible that at the bottom of Lake Vostok there will be a colony of extremophiles that has been evolving entirely on its own for the last 15-20 million years, and which has adapted to exist in the freezing, oxygen-rich environment down there. If they do exist, then those extremophiles will be completely unlike anything we’ve so far observed; bear in mind that twenty million years ago the ancestors of the human race looked something like this and then consider what those twenty million years of divergent evolution might do to bacteria. If they’re down there, and the Russians do find them, then it’ll be as close as we can get to finding alien life without actually going to another planet to do it.

What does this mean for the chances of alien life existing? Well, it improves them considerably, at least in terms of micro-organisms — you might not be able to have a conversation with one but you can at least be comforted that somewhere out there in the Milky Way is the alien equivalent of a tardigrade. However, it also improves the odds of us, personally (or at least a probe controlled by us) going out into space and actually digging some of these things up. You see, Lake Vostok isn’t the only place in the Solar System where there’s a large quantity of water trapped under several kilometres of ice. Several of the moons of the gas giants also qualify; notably Europa, Ganymede² and Callisto.

Europa is the really interesting one. The surface of Europa is entirely composed of ice, making it by far the smoothest body in the Solar System, but we think that ice surface is only 10-30 km thick. Beneath that is a layer of conducting fluid – overwhelmingly likely to be saltwater – that was detected by the Galileo probe via its magnetic moment. This water is kept relatively warm by tidal heating³ provided by Jupiter, and it’s possible, although nobody knows exactly how possible, that there might be microbial life down there living in environments similar to the ones we find extremophiles inhabiting on Earth – in cold, dark environments like the bottom of Lake Vostok, or next to deep sea thermal vents that spit out heat from the moon’s interior (it does have radioactive decay heating, although this is deemed insufficient to create the subsurface oceans). Missions to visit Europa to try and find some of this life have been mooted for years, although I don’t believe any of the proposals have satisfactorily tackled the question of how exactly you drill through ten kilometres of ice on another planet with a robot probe when it’s taken the team at Lake Vostok decades to bore through four kilometres with the benefit of having humans on hand to operate the equipment. Regardless, it’s likely that it’ll be tried sooner or later depending on what future technology does or does not render possible, and so if you’re unlucky enough to be alive seventy or eight years from now you might end up having the news of the first discovery of alien life piped into your brain through your neuro-cortical machine interface.

The other possibilities are somewhat less interesting. Ganymede has an ice-silicate lithosphere over 200 km thick covering its ocean, so it’s doubtful we’ll ever be able to tunnel down there. Callisto too has a surface layer up to 150 km thick. Enceladus on the other hand is kind of promising; if you’ve watched that EXPLETIVE DELETED Brian Cox on his EXPLETIVE DELETED EXPLETIVE DELETED series Wonders of the Solar System, you’ll be aware that Enceladus has a surface which, like Europa, is composed entirely of ice, but that Enceladus is unique in that the Cassini probe observed water vapour outgassing from the surface in 2008. This points to the presence of liquid water in some form or other, the most likely source of which is yet another subsurface ocean – and since it’s outgassing directly from the surface, it points to this ocean being somewhat more accessible than that of Europa (after all, it needs some way to get there from the interior). At the very least it should be possible to collect material that has come directly from the ocean and determine its composition, and thus its likelihood of being able to support life. While Enceladus isn’t necessarily a better bet for finding life than Europa, it’s potentially far more useful for telling us what the odds of that bet actually are.

The first step, though, is to look at these subsurface water environments on Earth. The Lake Vostok team has been painstakingly drilling through the ice for years while constantly at the mercy of the Antarctic elements (Lake Vostok is the coldest place on Earth). They used a thermal sensor to detect when they were approaching a source of free water and stopped the drill, the general idea being that the decrease in pressure would allow the ice separating the borehole and the lake to melt and then refreeze; they can then retrieve this portion of ice to examine water that has come directly from the lake. Meanwhile they’ve examined the last piece of ice extracted from the borehole – which contains ice thought to have frozen onto the bottom of the ice sheet hundreds of thousands of years ago – and they’ve managed to turn up evidence of extremophile microbes, so the odds are good that there’s a colony of something down there. The team has plans to go even further by sending a robot down in late 2013 to retrieve samples from the sediment making up the lake bed. I suggest you pay attention to the results, if only to make sure that this hasn’t happened.

1. If you think back to the geophysics post you’ll recall that the normal rules of physics don’t really apply when you crush something underneath hundreds of millions of tons of pressure. Rock cannot melt despite being at a high temperature because melting would require it to have some room into which it can expand – and because it’s compressed so much by the high pressure, this is the one thing it doesn’t have. Because water is unusual in that it actually expands in volume upon freezing into ice, the same rule applies to Lake Vostok.

2. CORRECTION: I stated in the geophysics post that Earth is the only body in the Solar System known to have a liquid iron core. While this is broadly correct, it is also suspected that Ganymede, Venus and Mercury might have them as well; Ganymede is very uncertain, Venus is likely based on its similar size to Earth (with the absence of plate tectonics being explained by there being no water on the surface of Venus to soften up the crust enough to allow plate formation/subduction) and Mercury has a strong magnetic field relative to its size which also points to the existence of a partially liquid core. Tectonic behaviour is observed on many of the icy moons of the gas giants, but their tectonics all involve chunks of ice shifting around and not rock. Earth is still a unique case in terms of the strong magnetosphere and plate tectonics.

3. Ever seen a picture of Io? I don’t particularly recommend it as a holiday destination because it’s absolutely covered in volcanoes. In terms of raw geological activity it puts the Earth in the shade, but unlike the Earth this is entirely down to outside forces: tidal heating caused by a constant tug of war on Io between Jupiter and the other Galilean moons which constantly squeezes and stresses it like… well, like somebody constantly squeezing and stressing a stress ball. Io has to relive this internal pressure somehow, and it does it by forming volcanoes which constantly spew out poisonous sulphur gas, which has stained the entire planetary surface a sickly yellow colour.

The post Just Another Bug Hunt. appeared first on The Scientific Gamer.

Here’s an interesting fact: if you smooshed together everything in the Solar System that was not a star — all the planets, moons, asteroids and other assorted junk out there — you would end up with a ball of stuff with just one seven-hundredth the mass of the Sun. The Sun has 99.86% of the overall mass of the Solar System.

Here’s another interesting fact: if you took a pea and put it next to a basketball, you would achieve roughly the same visual effect as if you took the largest planet, Jupiter, and put it next to the Sun.

In the grand scheme of things — certainly from the perspective of any astronomer who doesn’t make a living studying Solar System science — planets are insignificant specks. They’re tiny. Yet we keep finding more and more of the bloody things every year. The first extrasolar planet orbiting another star (or exoplanet, as they will be referred to from here on) was discovered in 1992. When I was learning about this stuff during my undergraduate degree in 2003, we’d bumped the discovered number up to just over a hundred. Today, if Wikipedia is to be believed, the number of known exoplanets stands at 725. Seven-two-five, despite the vast difficulties inherent in finding these tiny dust motes in a galaxy of comparative colossi. When you think about it you might wonder how the hell we’d found even one exoplanet, let alone seven hundred. Good thing you have me here to tell you all about it.

 Any astronomer who wants to discover a new exoplanet and name it after his cat has several hurdles to overcome; not least of which is that the IAU are killjoys who won’t tolerate a planet called Mrs Fluffingham even if it is several dozen light years away. Other stars appear to us as tiny points of light. If you used a really powerful telescope to beef up the magnification several thousand times this tiny point of light would resolve itself into… a tiny point of light. Interstellar distances are so vast that stars will always appear as points of light to us; we’re never going to be able to resolve them as objects and observe fine detail as long as we’re stuck here on Earth. That we see them at all is merely a happy byproduct of every star being a raging nuclear inferno that throws out billions of photons every second. A very few of these photons, after spending many, many years whizzing through space, will happen to enter the objective lens of a telescope – or even your naked eye – which is enough for you to perceive a tiny point of light, but nowhere near enough to make out anything more.

So if this is the case for stars, which are a) really sodding big and b) natural light sources, how on Earth (har) do we find planets? It’s certainly not through the conventional methods used to locate the other seven in our Solar System, which helpfully reflect the light emitted by the Sun and appear to us as distinct discs. Extrasolar planets have about one-millionth the brightness of their parent star, and finding this preposterously faint wisp of light is further complicated by the fact that it’s sitting right next to a light source one million times brighter – it tends to get lost in the glare of the star. That’s not to say it can’t be done; in some cases we can “block” the light from the star (basically phase-shifting the image so that the star is subtracted from it while leaving everything else intact) allowing us to directly view the planet, but this is really goddamn hard and a major technical challenge, not to mention only being possible if you’re looking for a planet that’s really big, emitting a lot of infra-red radiation and orbiting far enough out from its parent star that it can be viewed as a separate object.

 Directly viewing an extrasolar planet is pretty much out of the question, then. Fortunately astronomers are very ingenious people who aren’t going to let a little thing like not being able to actually look at the planet get them down, and so several ingenious methods have been developed for indirectly detecting an extrasolar planet over the last twenty years. I’ll be talking about the three most successful ones.

 The first is radial velocity measurements. Stars exert gravitational force on planets which causes the planet to orbit the star, but what people often forget is that all gravitational effects are mutual and that the planet is also exerting a gravitational force on the star. This means the centre of mass of the star-planet system won’t be at the centre of the star but will instead be offset from it by a small distance, causing the star to orbit around this point and move back and forth relative to the Earth. This movement Doppler-shifts the light emitted from the star in exactly the same way as stellar redshifts; when the star is moving towards us the wavelength of the light will be shorter, and when it is moving away from us the wavelength will be longer. By looking at the wavelength of a star’s light as it changes over time we can measure the star’s radial velocity, and so determine whether or not the star possesses an orbiting planet. It’s a bit imprecise for determining what the exoplanet is like – since a smaller planet orbiting close in will produce the same radial velocity as a bigger planet orbiting further out – but it’s an excellent method of finding out whether or not it’s there in the first place, with the vast majority of exoplanet detections being attributable to the radial velocity method.

Then there’s the transit method. If a planet is orbiting so that it passes directly between the Earth and the star, the planet will block out some of the star’s light. Astronomers study what is called the star’s light curve; essentially a big graph of the brightness of the star as it varies over time. If they see a small temporary dip in the light curve, and that dip repeats itself at periodic intervals, it’s a fairly good bet that’s it’s being caused by an exoplanet. The drawback to this method is that it’s only applicable in very specific situations; you need a big planet orbiting close in to the star so that it’ll block out a lot of light, and you also need the exoplanet to have this very specific orbit which places it on a direct line between the Earth and the parent star, which is thought to be the case for only 10% of exoplanets. However, there is also an advantage in that we can use spectroscopy to examine how the starlight changes as it passes through the planet’s atmosphere – whenever you see a headline about a newly discovered exoplanet with a vaguely Earth-like atmosphere, chances are it was discovered using the transit method.

 Lastly there’s gravitational microlensing. This isn’t that great in comparison to transits and radial velocity measurements, but it’s notched up thirteen detections so it does deserve a mention. If light emitted by a star passes close to another star on its way to the Earth, the gravitational field of the second star will “bend” the light from the first star causing a lensing effect which effectively magnifies the light. Any exoplanet orbiting the lensing star will contribute its own gravity to the lensing effect causing it to change over time as the planet moves around the star. If astronomers are vigilant and lucky – and they have to be considering that stars and planets are constantly moving relative to each other making these events very hard to catch – they can observe this change in the lensing star’s magnification and infer the existence of the planet.

 Most the exoplanets so far discovered have been of the “hot Jupiter” type; very large gas giants orbiting their parent stars at about the same distance as Mercury orbits the Sun. This isn’t because the majority of exoplanets are genuinely of this type, but instead reflects a bias in the detection methods: all three methods described above are heavily dependent on the exoplanet in question having a large mass, and two of them are also more effective if the exoplanet is orbiting close to its parent star. While the simple existence of the hot Jupiters caused scientists to reassess the way they thought planets formed and evolved over time (as ever, I may write more on that later), a major goal of the exoplanet search is to refine the detection methods so that they’re sensitive enough to reliably detect Earth-like planets orbiting at Earth-like distances, for the obvious reason that these are the places most likely to support life of some kind. This made the detection of Kepler 22-b a big step forward – it was small enough that it was unlikely to be a gas giant, and it was orbiting in what is called the habitable zone¹ of Kepler 22 thus qualifying it as the first confirmed exoplanet discovery in this category.

 But why are we so interested in finding Earth-like planets? It’s not like we can ever actually visit these places in any meaningful way² (at least not without some physics-breaking discovery that allows FTL travel) so why the obsession with finding habitable worlds? Well, it’s mainly to put some kind of constraint on how many of those worlds are out there. There’s something called the Drake equation which describes all the factors that go into an intelligent civilization arising on a certain planet — does the star have planets, are those planets in the habitable zone, do those planets go on to develop life etc. etc. – but it hits the tiny snag that we have absolutely no idea what values we should assign to these factors because we’re currently working with exactly one example of a habitable planet that supports life: the Earth. With more examples of habitable worlds we can start to assign some vaguely meaningful values to the first few terms of the equation and start narrowing down exactly what the chances are of the Milky Way hosting extraterrestrial life. As this is probably the only way we’ll ever figure that out, it’s very much worth doing.

1. Exactly what constitutes a habitable zone is a matter of some debate. The straight definition is the zone where temperature and pressure conditions are such that liquid water can exist on a planet’s surface, and so this varies with the temperature of the star; hot stars have habitable zones which are further out, while smaller, dimmer stars would have habitable zones much closer in. However, astrobiologists call into question the prequisite that liquid water has to exist on the surface of a planet as necessary for life to exist, citing the example of Europa as a planet-sized object with a subsurface ocean of liquid water that may yet turn out to support microbial life of some kind. And of course this isn’t even getting into the “life, but not as we know it” argument.
2. By “meaningful” I mean “Seeing a probe we launched arrive at another star within our lifetimes”. If we were willing to wait a while – say, a couple of hundred years – then interstellar travel would be easily doable the old-fashioned way. This gives rise to all sorts of wacky concepts like O’Neill cylinders, generation ships and von Neumann probes, the last of which actually come into play as part of the argument for why intelligent life likely doesn’t exist anywhere else in the galaxy.

The post That’s My Favourite Kind Of Planet. appeared first on The Scientific Gamer.

The K.B. stands for Kuiper Belt. SCIENTIST HUMOUR.

I picked up a compendium of H.P. Lovecraft’s short stories a couple of weeks ago – mainly so that I could amuse myself by playing Lovecraft Bingo (counting how many times he uses words like ‘accursed’, ‘pnakotic’ and ‘squamous’ ) – and ended up being rather amused when I came across the story “The Whisperer in Darkness”. This consists of about sixty pages of the usual confused rambling while the reader waits for him to get to the damn point already, but ultimately turns out to be about advanced aliens from Pluto who have set up a mining colony in the furthest reaches of the American hills that they’re trying to keep secret. This brought a wry smile to my lips for two reasons:

1. Lovecraft’s shameless attempt to piggyback off of something that was very much part of the public zeitgeist at the time, what with him starting this story the same month that Pluto was discovered. It’s the literary equivalent of somebody sticking dark matter into their awful sci-fi novel because they think it sounds exotic and dangerous.
2.  That there was ever a time when people thought Pluto was an important place that might support life of any kind.

 The whole Pluto “controversy” a few years ago stems from a variety of roots: a bureaucracy which ran a sloppy vote, scientists with vested interests and pet theories on both sides of the argument, and –  bizarrely – a great deal of public sentiment for what is essentially a lump of ice and rock out in the middle of nowhere. If you want to trace it back to the beginning, though, you have to start with the discovery of Pluto in 1930. Are you sitting comfortably? Time to Listen with Mother.

The best picture of Pluto currently available, which combines HST imaging data with a surface map provided by studying the light changes caused by Charon as it passed in front of Pluto.

Once upon a time there was a man called Alexis Bouvard.  Bouvard was engaged in the tedious task of compiling astronomical tables predicting and describing the orbits of the outer gas giants according to Newtonian mechanics, but he hit a snag when he came to Uranus: it wasn’t orbiting the way Newtonian mechanics said it should. There were only two possible explanations for this: either Newtonian mechanics were wrong (unthinkable at the time, although this eventually turned out to be the case) or else there was some unknown eighth planet orbiting beyond Uranus exerting a gravitational force that was interfering with its orbit. The matter then passed to Urbain le Verrier, a specialist in celestial mechanics, who cranked through some feverish calulations based on nothing more than the minor perturbations in Uranus’s orbit and came up with a predicted position for the hypothetical eighth planet. He mailed his prediction to the Berlin observatory, who duly pointed their telescopes at the appropriate piece of sky and wound up discovering Neptune in 1846.

 This was an astonishing achievement for le Verrier, but all was not hunky dory in astronomy land. After some examination of Neptune it was determined that the new planet was, on its own, not enough to account for the irregularities in the orbit of Uranus. This led astronomers to posit the existence of a ninth planet, which they eventually dubbed Planet X.

Fifty years went by. Percival Lowell spent a decade looking for Planet X, a search which ended with his death in 1916. The search stalled for another thirteen years while a legal battle over Lowell’s bequest to his observatory was wrung out, before being handed off to the 23-year old Clyde Tombaugh in 1929. Tombaugh spent the next year examining pairs of photographic plates; each pair of plates was a picture of the same portion of the night sky taken several days apart, and Tombaugh’s job was to pretty much to see if he could spot the difference. If he saw something on the second plate which had moved from its position on the first plate, he had a candidate for Planet X. This was tedious work involving the painstaking examination of hundreds and hundreds of plates, but it eventually paid off and he discovered his candidate body in January 1930.

This is probably pretty hard to spot when you've been looking at similar plates for sixteen hours straight.

Everything up till now has been Good Science, but this is where the Bad Science starts. Telescope technology in 1930 wasn’t that great. The new “planet” — named Pluto by an English schoolgirl – appeared as a point of light to astronomers rather than a distinct disc like the other planets. This could mean any or all of a number of things:

a)      It wasn’t very big.

b)      It was a very very long way away.

c)      It wasn’t reflecting much sunlight (it had a low surface albedo).

However, because astronomers thought Pluto must be responsible for the additional perturbations in the orbit of Uranus it followed that it had to have enough mass to exert the level of gravitational force required to do so. This led them to discount point a) and estimate the size of Pluto as being about 8,000 km in diameter – or just a bit larger than Mars. In order for Pluto to be this big it would need a very, very low albedo to account for such a large object reflecting such a tiny amount of sunlight, and so astronomers decided it was about as reflective as a lump of road asphalt. Despite a vocal minority proclaiming that these astronomers were full of shit and that Pluto’s orbital eccentricity and brightness meant it had far more in common with comets than it did with planets, the size estimate and the planet title stuck.

Time continued to pass. Pluto’s size was revised further and further downwards as telescope technology improved and more accurate observations were made. Eventually Pluto’s “moon”, Charon, was discovered in 1978. This allowed astronomers to measure the total mass of the Pluto-Charon system based on their mutual gravitational effect, which turned out to be about five hundred times smaller than that of the Earth. After Charon serendipitously made a series of planar transits across Pluto from 1985-1990 their respective sizes were worked out as well*. Pluto has a volume less than one two-hundredth that of the Earth. It is tiny.

So Pluto was now known to be unlike any other planet in the Solar System. Fortunately (for Pluto) nothing else had yet been found that remotely resembled Pluto, so Pluto remained a planet simply through dint of having been called one so long that nobody could think of a good reason not to. Nothing lasts forever, though, and Pluto should have started looking over its shoulder in 1992 when the catchily-named (15760) 1992 QB1 was discovered. This was the first confirmed occupant of the long-posited region of space called the Kuiper Belt, a population of icy bodies outside the orbit of Neptune which are left over from the formation of the Solar System. I’ll be posting more on that later, but after the Kuiper Belt was discovered and more people started combing it for interesting objects it was only a matter of time before somebody found something in it that was decidedly Pluto-like. Quaoar, Haumea, Ixion and Varuna were all discovered in the period between 2000 and 2005, and all of them had sizes comparable to – but not exceeding – that of Pluto (between 500 km and 1,200 km diameter; Pluto has a diameter of 2,000 km). The straw that broke the camel’s back was the discovery of Eris in 2005; this had a diameter of 2,400 km, making it some 25% larger than Pluto.

A mock-up of large TNOs shamelessly thieved from Wikipedia.

Eris really brought things to a head. After the size downgrades certain segments of the astronomical community had been complaining for years that Pluto wasn’t a planet at all, but Eris presented the IAU with an outright logical conundrum. Scientists love consistency. To call Pluto a planet would also mean calling Eris a planet. But if they called Eris a planet there’d be no good reason not to call the other four planets as well – they were all orbiting in the same area of the solar system and they were all of comparable size, so why not? And – even worse – it was almost certain that astronomers would go on finding these big Pluto-like objects as more people looked with better instruments. Do they call all of them planets? If they did, it’d be entirely possible that in a hundred years time we’d end up with a list of planets that was 100-200 items long.

 And so the IAU conference in 2006 to decide what exactly constituted a planet and what didn’t. Exactly what happened at that conference I do not know and there are some scientists (notably Alan Stern, the principal investigator behind the New Horizons probe that’ll be arriving at Pluto in 2015) who are still very bitter over its result, which was that the largest of the Pluto-like bodies (the ones with the most accurate size measurements) were to be placed in a new category of objects called “dwarf planets”. Further, Pluto itself was to be demoted from planet status and put in the category of dwarf planet.

 Now, you can argue all you want about how good the new definitions for dwarf planets and planets are is. There’s definitely an argument to be had since the two defining criteria of a planet are now that it is “is in hydrostatic equilibrium” and “has cleared the immediate area around its orbit”, but nobody really knows what the hell the second one means given that a strict reading of it leads to us discounting Earth as a planet given the existence of 3753 Cruithne and other similar bodies, not to mention the Jupiter Trojans. However, what is clear to me after studying the Kuiper belt for most of the last five years is that Pluto is rather obviously a member of this discrete population of objects and not a planet in its own right. It formed with the belt, it evolved with the belt and it’ll die with the belt. Pluto might or might not be a dwarf planet, but what it definitely isn’t is a planet.

These people are being ironic. At least I hope they're being ironic.

 Which is why the outrage over Pluto’s demotion eluded — and continues to elude – me. Even I get a little bit unsettled when I run into schoolkids who have been taught from day one that Pluto is a dwarf planet and nothing else, but the scientific argument for it no longer being a planet is irrefutable. When respected scientists like Alan Stern come out with something like

“Firstly, it is impossible and contrived to put a dividing line between dwarf planets and planets. It’s as if we declared people not people for some arbitrary reason, like ‘they tend to live in groups’.”

My response is an entirely predictable collection of four-letter words. It is not in the least bit contrived to draw a distinction between planets and dwarf planets, just as it is not contrived to draw a distinction between adults and children, or between gas giants and terrestrial planets, We called Ceres an asteroid for two hundred years simply because we had nothing else to call it, even though it was demonstrably different from everything else in the asteroid belt, and we didn’t start calling it a planet when we figured out it was round. Pluto is demonstrably different from a comet, or a small Kuiper Belt Object (they’re pretty much the same thing), but that doesn’t mean it automatically qualifies as a planet since it’s demonstrably different from those as well. So if you ask me, the classification of dwarf planet works quite well even if the definition does not.

But then the media and the public came up with far more spurious reasons as to why Pluto shouldn’t be demoted and should instead be “grandfathered” in as a planet, as if science worked that way. It’s been a planet for seventy-five years! It’ll upset schoolkids who will have to find a new mnemonic to remember the planet names! We’ll have to rewrite all the textbooks! Cry me a river, guys; nobody discounted tectonic plate theory just because it would have been a pain in the ass to change our scientific paradigm of the interior of the earth. There’s always cultural resistance to a new idea but it still surprised me in this case because they were kicking up a fuss over basically nothing. Pluto is the same object discovered by Clyde Tombaugh eighty-two years ago. We just call it something different now, that’s all.

Oh, and I nearly forgot to mention: those peturbations in the orbit of Uranus that started this whole mess? When the Voyager 2 probe flew by Neptune in 1989 astronomers were able to use its orbital trajectory to get a more accurate measurement of Neptune’s mass, which turned out to be 0.5% less than previously thought. This meant that Neptune on its own was now enough to account for the variations in the orbit of Uranus, and that ultimately the entire Pluto = planet debacle has been the result of science running down this massive blind alley.

 (Of course nobody ever thinks about the real victims of the IAU’s decision: the astrologers.)

 P.S – I’ll post later on in the month in more detail about some of the science behind why Pluto is so different and the general mechanics & formation of the Kuiper belt/scattered disc. I’m by no means done on this subject.

 *In detail: sometimes a planet or a satellite will move in front of some light-emitting or –reflecting object, blocking some of the light we see from that object. If the planet/satellite is close enough to us to block out all the light from the object, this is called occultation**. Solar eclipses are the most obvious example of occultation, but stars are small enough to be occluded all the time by various Solar System objects. Charon occluded a star in 1980, allowing an estimate of its diameter to be made – astronomers knew how fast Charon was moving and they knew how long it took to move from one side of the star to the other, which gave them the size.

 Now, if the blocking object appears smaller to us than the light-emitting object, all it will do is block a portion of the object’s light and so reduce the intensity of the light we receive from that object. This is called a transit, and this is what Charon did to Pluto a number of times between 1985 and 1990 – it passed between Pluto and the Earth, which was remarkably lucky given the precession of Charon’s orbit means that happens only once every hundred years or so. Astronomers measured the decrease in light intensity as Charon moved across Pluto, and since they already knew how big Charon was from the 1980 occultation they were able to use the transit time to work out the size of Pluto.

 **Not to be confused with occlusion, which is a word dentists use to describe how the top and bottom teeth fit together when your mouth is shut.

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A load of old balls.

Kicking off the science portion of this blog, I’m going to start with an easy question I used to get asked a lot when I did Outreach for the university: why are the various planets, moons etc. round? It’s a fairly simple answer with some wide-reaching ramifications.

Baldly (or possibly badly) put, it’s because gravity. Gravity is constantly pulling every piece of matter that makes up a planet like the Earth towards its centre of mass – in this case the Earth’s core. The Earth is probably a bad example to use here because of its complicated internal structure, but it doesn’t matter that planets start out being made of what we might think are strong, non-malleable materials like rock; thanks to internal heating provided both by the compressive effect of the gravity itself and the slow decay of radioactive elements inside the planet in question, the material inside it is made just “soft” enough that it will deform under pressure and slowly flow inwards over millions of years.

But if gravity is constantly pulling every bit of a planet towards its centre of mass, what’s stopping the Earth from shrinking and shrinking until it collapses into a black hole? The answer is — again–  pressure. If you have a chunk of stuff, and you exert some external force to reduce the volume of that stuff, you consequently increase the pressure inside the stuff that pushes outwards and resists your external force. Think about compressing air in a bicycle pump: at first it is easy since the internal pressure inside the pump is low, but as the air is compressed into a smaller and smaller space working the pump becomes harder and harder until you eventually reach the point where the pump pedal won’t work even if you put all your weight on it. The pressure of the air inside the pump is now equal to the force exerted by your body weight – in other words, it has reached a state of equilibrium with your body.

The same principle applies to planets. As the planet is compressed under the force of its own self-gravity, the internal pressure pushing outwards and resisting this compression will start to rise. The smaller the planet gets, the greater the internal pressure. Eventually the outwards force provided by the internal pressure will equal the inwards force exerted by the planet’s gravity, and the planet will stop shrinking. The planet has reached what is called hydrostatic equilibrium, and since the two forces balance each other out at a fixed distance from the planet’s centre of mass the effect is kind of like taking a pair of compasses, setting them to a fixed distance, and drawing a big circle. Except in 3D, obviously.

Now, a few notes and corollaries. When the IAU introduced the new classification of dwarf planets a few years ago, one of the criteria a body needed to have in order to qualify was that it had to be in hydrostatic equilibrium – i.e. round. This was a handy way of delineating the boundary between planetoids (planet-like bodies such as Pluto) and regular bog-standard asteroids and comets, since the state of hydrostatic equilibrium requires a certain level of gravity in order to start the compression process, which in turn requires a minimum amount of mass. You won’t find any bodies below about 400 km diameter which have compressed themselves into this round shape; they’re not big enough. This is why even the larger asteroids retain their rocky, irregular shape, and it’s also why the one asteroid that was large enough to achieve hydrostatic equilibrium (Ceres) got reclassified as a dwarf planet.  However, even with a few of the smaller planetoids we find that they’re not perfectly round. Other forces besides their own self-gravity act on them and distort their shape. Tidally locked satellites like the Moon will always be facing the same way relative to their parent body and thus the parent body will always exert gravitational tidal forces on the same “side” of the satellite; this can warp the shape of the satellite if it is particularly small and the parent body is particularly large. Rotation also has an effect – even the Earth is 20 km fatter at the equator than it is at the poles thanks to its rotation, and in the further reaches of the Solar System you can find weird things like Haumea, which has a rotational period we scientists refer to as batshit insane and which has severely deformed it into an ellipsoid.

Hydrostatic equilibrium applies to stars too, except in their case they’re so big that the internal pressure due to compression is nowhere near enough to stop a star’s mass from collapsing in on itself. Instead it’s the fusion fire burning at the heart of every star which provides the outwards pressure necessary counterbalance the star’s immense gravity. But what happens when a star runs out of fuel and that fire winks out? That’s a process that can be described by several phrases. Interesting. Lethal to anyone standing within about a hundred light years. And very definitely a post for another day.

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