Innokenti asks

 Why are observatories dome-shaped? They house a telescope… but why the dome? The things can swivel around and stuff without being encased in a dome. So what purpose does it serve?

 Short answer: they don’t have to be. Behold the Very Large Telescope array at the Paranal observatory, which is decidedly un-domey. The dome shape is just a convenient one that happens to get used a lot.

 Long answer: The dome superstructure exists to protect the telescope from the elements. Observatory telescopes contain incredibly sensitive optics, but they also tend to be build in rather remote, inhospitable places (the middle of the desert, on top of a mountain, on top of a mountain in the middle of the desert etc.) which can suffer somewhat inclement weather from time to time. Some kind of superstructure is necessary if you actually want your telescope to survive. However, the superstructure also has to be built in such a way that it allows the telescope to rotate through a full 360 degrees and to elevate to an altitude of up to 90 degrees from the horizontal in order to observe as much of the sky as possible, while also retaining the ability to open and close and protect the telescope when it is not in use.

 Now, obviously you don’t have to use a dome to achieve all this, but unless your observatory has some sort of special purpose that requires other factors to be taken into consideration a dome-shaped superstructure is rather attractive because it’s the most lightweight one that will do the job. The heavier your dome is the more difficult it is to shift it round to look at different points in the night sky, and so a dome shape was very popular before we mechanised the whole shebang. These days it’s more a matter of tradition than anything else; there’s no real reason to use domes any more, but there’s no real reason not to either, and observatories are now inextricably linked with the dome shape in the public psyche. PR is more important to large-scale scientific endeavours than you might think, which is why having an observatory that looks just like people think it should do is quite handy. And of course for low-budget observatories where cost and weight are still an issue the dome shape is still the best available: easy to source, easy to install, easy to use.

Baroness Josh asks

http://www.bbc.co.uk/news/science-environment-18803212

How is there a moon we hadn’t noticed yet? How do we expect to find habitable extrasolar planets if we can’t even lock down our own back yard? Just how much thumb-sucking goes on in astrophysics anyway?

 This is where I start wishing I’d figured out a way to write that post on the Kuiper Belt that didn’t make me sound like a lunatic. Well, more of a lunatic, anyway.

 Right, first off, calling this thing a moon is a little bit generous and the only reason we’re doing so is because we lack the proper astronomical terminology to draw distinctions between the Hubble Space Telescope and the actual Moon. According to the technical definition they’re both moons of Earth, even though there’s a fairly considerable size difference between the two. This new moon they’ve found around Pluto is “between 10 and 25 km in diameter” which is tiny when compared to the vast majority of solar system moons and more directly comparable to Phobos and Deimos, the two asteroid-like moons which were captured by Martian gravity at some point in the distant past. Considering this thing is about 27 times further away than those are – as well as likely having roughly the same reflectivity as a piece of road asphalt – I’m amazed the Hubble picked it up at all.

 Second, if you’ll recall my post on exoplanets, the vast majority of exoplanets aren’t found by direct observation but rather through techniques involving indirect measurements of the parent star such as radial velocity and the star’s light curve. Why didn’t we use those techniques to pick this new moon up? It’s really small, meaning any gravitational force it exerts on Pluto will also be really small, and Pluto is now part of a five-body system to boot which will ensure any gravitational tugs it does manage to exert will be lost in the noise created by the other three moons, which are much larger. The two situations aren’t remotely comparable.

 Third, don’t expect things to stop there. Extensive computer modelling and laboratory experiments (if you really want references on this ask in the comments and I will, after some grumbling, go and track down the relevant papers) has shown that Pluto and the other objects within the Kuiper belt are subject to something called collisional evolution. What this means is that the Kuiper belt could be densely populated enough that collisions between Kuiper belt objects are relatively common occurances, and so depending on how fast the impacting body is going a collision will either result in the two bodies “sticking” together and agglomerating into a single new body, or else blasting both bodies apart into a collection of smaller objects. That Pluto has so many “moons” bound into what appear to be stable orbits with it indicates that it was a victim of the latter as it’s highly unlikely it would be able to capture all of these things so neatly on its own. At some point in Pluto’s past, then, it was probably struck by something that blasted off a chunk of Pluto’s mass. Some of it probably escaped. Some of it probably fell back onto Pluto. And some of it formed the moons we see today. It wouldn’t surprise me if, when the New Horizons probe gets there in 2015, it manages to find a couple more moons like this one orbiting Pluto. As far as I’m concerned this further cements Pluto’s position as something separate from a planet or a moon; it’s in a class of its own, and “dwarf planet” is a hilariously inadequate term for describing what that class is.

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Did you make it yourself?

We’ve known for a long time the broad details of how the Solar System formed. The Nebular Hypothesis was first proposed back in the 18^th century and has been refined over time with the aid of detailed observations of areas where other star systems are in the process of being born into its current general state. The theory goes that you start with what is called a molecular cloud – a big clump of gas that is mostly hydrogen. At first the matter in this cloud will be very diffuse and spread out, but since it isn’t exactly evenly distributed the cloud will have areas where there is a slightly higher concentration of gas molecules than the average. This is enough to get the gravitational ball rolling: those small concentrations of gas molecules are heavy enough to attract and absorb other gas molecules, which makes them heavier, which lets them attract yet more molecules etc. etc. It’s a runaway process that eventually draws most of the surrounding gas cloud in towards a common centre of mass. While this is happening the Brownian motion of the gas molecules in the cloud will average out so that the cloud starts to rotate in the direction of its net angular momentum¹, causing the outer regions of the cloud to flatten into a disc shape. The centre of the cloud collapses to the point where the pressure and temperature are enough to kickstart a nuclear fusion process, creating a star. This leaves a disc-shaped remnant of gases that is rotating around the star in the direction of the original cloud’s rotation, and this is where we eventually get the planets from.

This protoplanetary disc then goes on to form the planets in much the same way as the molecular cloud formed the star. This time the starting point is a dust grain within the disk. This dust grain collides with another dust grain in the disk, and the two stick together. Repeat this process a couple of million times and you now have a clump of matter several hundred metres on a side hurtling around the proto-star. Smash many of these clumps together and eventually a planetesimal will be created; this is a chunk of stuff about 1 km in diameter. The planetesimal stage is the point where the process becomes runaway since planetesimals are large enough to attract one another through their own self-gravity – they collide, merge and the resulting boost in mass means they attract even more planetesimals.

Eventually you end up with a load of planets that have cleared all the matter immediately surrounding their orbits (hence the inclusion of this condition as part of the IAU’s definition of a planet in 2006). Any gas remaining in the protoplanetary disc either falls onto the Sun or else is blown away by the solar wind. So far, so good. But this is where the Nebular Hypothesis runs into a few problems; namely, that the Solar System structure predicted by this model of accretion from a protoplanetary disc does not match what we see in the Solar System today. There should be a lot more planetesimals, the gas giants should be located closer to the Sun than they are today (this goes back to the “hot Jupiters” we often find orbiting other stars), and there should be a fairly dense cloud of leftover material outside the orbit of Neptune. Since none of these things are true, something obviously happened in the four billion years separating the formation of the planets and the present day to account for the difference in what we should see and what we do see.

Current best candidate for that something is the Nice Model. It explains the following:

• Where all the planetesimals went.
• Why the gas giants are where they are.
• Why the Kuiper Belt, Scattered Disc and Oort Cloud are structured the way they are.
• The Late Heavy Bombardment, an approximately 300 million year-long period just after the formation of the Solar System during which all the inner planets were subjected to a much, much higher rate of asteroid and comet impacts than they are today (as inferred from the very large number of impact craters on the surface of the Moon which date from around this time).

As the Nice Model would have it, none of these things makes sense on its own but when you put them all together you can in fact come up with a model of the evolution of the Solar System that works.

The starting point is the one suggested by the Nebular Hypothesis: all the planets clustered in a narrow 15 AU radius from the Sun, with a very dense disc of icy detritus containing many planetesimals that didn’t quite make it as planets surrounding them from 15 up to about 30 AU. One of the planetesimals gets nudged inwards and approaches the outermost gas giant. The gas giant gives it a gravitational velocity kick by exchanging angular momentum with it; conservation laws dictate that this leads to an equivalent loss of angular momentum – and thus orbital speed – in the gas giant, shifting it to an orbit further away from the Sun.

The planetesimal is tossed from gas giant to gas giant like a gigantic mystery parcel, getting a gravity boost that shifts the orbit of each gas giant outwards. Then it encounters Jupiter, fattest of all the planets. Jupiter has no time for tiny planetesimals and gives it such a smack that the planetesimal is ejected from the Solar System altogether; this moves the orbit of Jupiter inwards rather than outwards. One single planetesimal doing this will have next to no effect on the orbits of the gas giants – they’re sodding huge, after all – but if the process is repeated several thousand times as new planetesimals are leached from the outer debris disk, it will all add up and start to produce a significant effect. Over millions of years the orbits of the outer three gas giants will slowly migrate outwards, while Jupiter migrates inwards. Eventually they get to the point where Jupiter reaches a 1:2 orbital resonance with Saturn, at which point all hell breaks loose.

The greatly-increased gravitational effect of the resonance of the two largest planets in the Solar System throws everything else into chaos. The video above illustrates quite nicely the slow migration and subsequent catastrophic interactions of the planets. Saturn gets shunted out into a wider orbit, which in turn moves Uranus and Neptune outwards³. This sends Neptune careening into that huge, densely populated disc of icy planetesimals that had previously been outside its orbit, scattering them in every direction like ninepins. Many planetesimals are scattered inwards towards the terrestrial planets, turning that region of space into a shooting gallery for the next couple of hundred million years: this is how the Nice Model explains the Late Heavy Bombardment. Others are scattered outwards at all inclinations and eccentricities, explaining the Scattered Disc and the Oort Cloud. Finally, the stuff at the innermost edge of the debris disc – the stuff closest to Neptune – is booted out of the Solar System entirely unless it is fortunate enough to fall into one of the narrow orbital bands defined by stabilising resonances with Neptune, as with Pluto and the rest of the classical Kuiper belt population.

That’s the Nice Model. I think it’s a very convincing piece of work, but it’s worth bearing in mind that the only criteria for the correctness of the Nice Model we have is that it produces something close to what we see today. It’s entirely possible that it happened entirely differently — we’ll never know for sure exactly how without a time machine — and even the Nice Model has several significant question marks hanging over it, such as the inability to explain the two distinct population types found in the Kuiper Belt and the fact that the Late Heavy Bombardment might not have even happened; the evidence for it consists of an extremely limited sampling of a few lunar impact sites and it’s possible that their common origin is just a rather large coincidence.

 As far as meshing with the currently prevailing scientific theories on the history of the Solar System goes, though, the Nice Model is the best we’ve got. Next week: Oort? Kuiper? What hell they? I attempt to explain.

1. Don’t worry if you don’t understand the specifics of this; you’re in good company. As far as I can make out exerting a unidirectional gravitational force on an object that is already undergoing Brownian motion that has a component lateral to that force will create a torque force on the molecule, which results in the gas molecules in the cloud acquiring a rotational motion about the axis of the cloud’s centre of mass. That makes sense to me, and past there I guess since random Brownian motion of all the gas molecules in the cloud won’t average out exactly to zero (it would be pretty goddamn amazing if it did) the sum of the angular momentum of the cloud will also not be zero. Hence the cloud as a whole rotates one way or the other, and this rotation will get faster as the cloud gets smaller due to conservation of angular momentum (same principle as an ice skater pulling their arms in closer to their torso to spin faster) which causes the cloud to flatten into a disc thanks to centrifugal force². This could be complete rubbish, but hey – I’m just a poor Solar System scientist. Can’t expect me to know everything.

2. One of the things they try to drill into you at Physics School is that centrifugal force isn’t a real force. It’s an approximation that only “exists” to make calculations within rotating reference frames (i.e. the surface of the Earth) easier. So whenever somebody wrote down the word “centrifugal” on their work it’d come back with red pen all over it and eventually the practice was stamped out, but I secretly keep the flame alive in my heart.

3. It’s suspected that Neptune may actually have formed as the seventh planet from the Sun, with Uranus as the outermost planet, and that when Saturn boosted Neptune and Uranus outwards they switched places. The model seems to work just as well either way.

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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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