astronomy · Grey Nicholsonhttps://gkn.me.uk/entries/astronomyGrey Nicholsonhttps://gkn.me.uk/style/icon.svg2023-07-02T15:30:00+00:00Planet X-4: Pluto is a Planet; so is Eris; so is Sedna; and so are Charon, the Moon, Ceres, Titan and Quaoarhttps://gkn.me.uk/planetx42006-11-21T00:55:00+00:002008-12-01T07:48:00+00:00Hang on – I thought you said...?
<p>
This summer, <a href="http://en.wikipedia.org/wiki/2006_definition_of_planet" title="2006 definition of planet - Wikipedia">a horde of astronomers decided what the word “planet” means</a>. Their definition is a sensible one; essentially for an object to be an officially-sanctioned, bona-fide, regulation Planet™, it must:
</p>
<ol>
<li>orbit the Sun directly</li>
<li>be mostly round</li>
<li>not share its orbit with lots of other similarly-sized objects</li>
<li>pay a £20,000 registration fee to Sir Patrick Moore</li>
</ol>
<p>
...I made that last one up.
</p>
<p>
Furthermore, the same gaggle of telescope-botherers decided that a “Dwarf Planet” is anything that:
</p>
<ol>
<li>orbits the Sun directly</li>
<li>is mostly round</li>
<li>
<em>does</em> share its orbit with lots of other similarly-sized objects</li>
<li>has a long grey beard</li>
</ol>
<p>
So the only difference between a “Planet” and a “Dwarf Planet” is the requirement of having <a href="http://en.wikipedia.org/wiki/Clearing_the_neighbourhood">“cleared the neighbourhood”</a>. This means that a Planet is overwhelmingly massive and has an overwhelming gravitational influence on nearby objects. And therefore all the objects with which it shares a similar orbit will have either: been flung away; assumed an orbit that is governed by the planet (such as going into orbit around it); or just crashed straight into it and thereby ceased to exist. A Planet clearly dominates the vicinity of its orbital path.
</p>
<p>
By the way, the last Dwarf Planet criterion up there was another comedy fabrication. You guessed that, right?
</p>
<p>
This means that the difference between a Planet and a Dwarf Planet is not just its size. If you stick a fairly small chunk of rock in an orbit on its own, with only a few measly specks of dust for company, it'll easily bully them into submission. But put the same chunk of rock in the ring with something much bigger than it, and it'll become the larger object's biatch without hesitation.
</p>
<p>
So the Dwarfiness or otherwise of a Planet-like object depends on whether or not it's surrounded by similarly heavy objects, rather than any intrinsic property of the thing itself, for example its size. And so “dwarf” is a bit misleading.
</p>
<p>
Note also that a Dwarf Planet is <em>not</em> a Planet. The name makes it sound like a sub-category of “Planet”, but it's not. Similarly, a minor planet (an asteroid or similar) is not a Planet under the <abbr class="caps" title="International Astronomical Union/Union astronomique internationale">IAU/UAI</abbr>'s definition of what constitutes a Planet.
</p>
<p>
And that's very confusing.
</p>
<p>
Even before the “re”definition, people had been calling asteroids “minor planets”. That name comes loaded with the implication that the other things known as “planets” were, more specifically, “<em>major</em> planets”. Whenever anyone used the word “planet” without qualification to mean only major planets it was because: they didn't know that minor planets existed; there was sufficient context to make the adjective unnecessary (such as comparing planets to stars or “asteroids”); or they were just being lazy. (Y'know, like on <a href="/weblog012" title="Pluto Isn't A Planet">the several occasions</a> <a href="/weblog020" title="Planet X">when I've said</a> <a href="/weblog021" title="Planet X-2">that Pluto isn't</a> <a href="/planetx3" title="Planet X-3: Pluto Still Isn't a Planet, Neither Are 2003 UB313 and 2003 EL61">a planet</a>.)
</p>
<p>
So I reckon the use of the word “planet” in place of “major planet” has always just been shorthand – the unqualified term has always been used loosely. This is why I think that while the <em>definition</em> the IAU came up with was good, it should have been defining the term “major planet” (or perhaps “classical planet”) rather than just “planet”.
</p>
<p>
Let's consider the situation a bit more sensibly.
</p>
<h2 id="everythingisthesame">Everything Is The Same</h2>
<p>
Mars is a giant rock. Yes, it's got some water and various other chemical substances lurking in and around it, but that just makes it a wet rock. An asteroid is just a smaller chunk of rock with a fair amount of ice chucked in for good measure. A comet is just a particularly icy chunk of rock that exhibits a tail as this ice evaporates. Earth is just another wet rock, with some animals and plants on it. If you chuck your drink on a mossy rock and then stand on it, that is then also a wet rock with an animal and some plants on it. A pebble is a smaller rock, possibly with a few animals or plants nearby.
</p>
<p>
The only differences between any of these are their size, their shape and the details of their composition. (Note that in planetology, “ice” needn't be water – it means any simple chemical compound, usually but not necessarily a solid. For example, Uranus and Neptune contain water ice, ammonia ice and methane ice in various proportions.) So what's a sensible way to categorise them?
</p>
<p>
The universe is generally homogeneous – everything is made of the same stuff. Some clever people pointed telescopes at the most distant galaxies they could find and <a href="http://en.wikipedia.org/wiki/Spectroscopy" title="Spectroscopy - Wikipedia">spectroscopically analysed</a> the hell out of them, and it turned out that they were made up of much the same stuff as our galaxy. That's how we know. There's no enlightenment to be had by categorising things based on their composition, because everything's pretty much the same anyway.
</p>
<p>
Size is also a dodgy naming criterion because sizes are usually continuously distributed. You'd have to pick a number and say that anything slightly bigger than that number is a widget, and anything slightly smaller is a fudget. Then if it turns out you measured its size wrongly, your widget becomes a fudget.
</p>
<p>
Picking an object up and dumping it somewhere else doesn't change what the object actually is. That's pretty obvious. (Well, unless you stick some bread in a toaster, but then it's the heat that's causing it to change, not the movement; anyhow, the protons, neutrons and electrons are all still the same, just rearranged.) So a body orbiting a planet wouldn't become something else if it suddenly started orbiting the Sun.
</p>
<p>
It's not even sensible to use an object's shape as a naming criterion. It's impossible to quantify by how much an irregularly shaped blob differs from some pre-defined shape. Even if it was possible, you'd still have to pick a boundary and we'd be back to the widget/fudget scenario.
</p>
<p>
<strong>So let's just call anything in space that's important enough to be identified individually a “planet”.</strong>
</p>
<p>
...Unless there's nuclear fusion going on inside it and it's therefore a star.
</p>
<p>
I don't care whether it's successfully bullied everything out of its way, or if it's been the victim of the bullying; nor whether it goes 'round the Sun in a near-circular orbit in the plane of the ecliptic, or if its orbit's the shape of Bert's head and it's flying 'round the Sun at a funky angle; I don't care if <em>it</em>'s the shape of Bert's head; I don't even care if it's going 'round another planet instead of the Sun. It still the same thing – it's still a planet.
</p>
<p>
This is actually what most people mean when they say “planet”, and that's why minor planets and Dwarf Planets have the word “planet” in their title, and why the constituent particles of Saturn's rings <em>aren't</em> called planets.
</p>
<p>
That's not to say that all planets are equally important, though, and it's wrong to suggest that a planet like Earth or even Saturn is equivalent to Jupiter in magnitude. Jupiter's <a href="http://en.wikipedia.org/wiki/Barycentre">barycentre</a> with the Sun lies above the Sun's visible-light surface; this is one of the proposed criteria for distinguishing binary planets from primary-and-satellite pairs of planets – it's that big.
</p>
<h2 id="somethingsaredifferent">Some Things Are Different</h2>
<p>
I said earlier that moving something doesn't change what it is, but that's not strictly true. If a planet shares its orbit with thousands of other planets, it's a member of a larger population. If it's the only thing in its orbit, it <em>is</em> essentially the population. This is the difference between being <em>an</em> object in that orbit and being <em>the</em> object in that orbit. The latter is a <em>major</em> planet – an IAU-sanctioned Planet™; I suppose you could also call them “individual planets”. (It's also the difference between being part of a loaf and being a sandwich. Plus or minus some filling, of course.)
</p>
<p>
Another of the IAU's criteria for Planets and Dwarf Planets is that they are generally round. Roundness is a useful criterion to describe planets, as it provides an overview of their size and structure and, crucially, the gravitational influence they exert over themselves and over nearby planets.
</p>
<p>
It's not an absolute, on/off, binary property though; there's no certain mass where you can add one more microgram of dust and the object suddenly stops being “not round” and instantly starts being “round”. It's a gradual thing, and it's subject to a lot of conditions, such as the body's composition and its temperature (which in turn is related to its distance from the Sun).
</p>
<p>
In fact, there are three general shapes corresponding to a planet's self-gravity: not round at all, round, and totally-über-super-well-round. I shall explain.
</p>
<p>
For a planet to qualify as an IAU Planet™ or Dwarf Planet™, it must be in hydrostatic equilibrium. This means that rather than being simply a cluster of rocks held together by their mutual gravitational attraction, a (Dwarf) Planet is massive enough that <em>its own gravity distorts its shape into a sphere</em>. The planet's rotation will also distort its shape into an ellipsoid, but that's inconsequential – it's the effect of the planet's gravity that's important.
</p>
<p>
Jupiter, Saturn, Uranus and Neptune are often referred to as Gas Giants; Uranus and Neptune are sometimes also referred to as Ice Giants. These names aren't particularly helpful – the composition of each planet can be classified and sub-classified as much as you like. So let's call them all “giant planets” for now. Like all (Dwarf) Planets, they are in hydrostatic equilibrium and like most things, they're made up of a variety of different substances.
</p>
<p>
But unlike other planets, a giant planet exerts so much gravitational force that <em>there is no definite boundary between the layers of different substances</em> inside the planet; the transitions between layers are smooth. There is no surface and no atmosphere, just a mostly-symmetrical sphere of stuff that gradually attenuates to nothing as you go further out. This is the aforementioned so-round-it-hurts state, and it's the same sort of thing as goes on inside a star. (This doesn't make giant planets stars, mind – there's no nuclear fusion happening inside giant planets.)
</p>
<p>
To avoid naming them by size, you might call these self-gravitationally-internally-smooth things “fluid planets”, as the entire the planet is in a fluid state. (“Gas planet” is a little misleading, because the pressure deep inside the planet can compress gases into liquids and solids.)
</p>
<p>
So, planets can be classified broadly into three categories: “spheroidal planets” in hydrostatic equilibrium, “fluid planets” with no definite layers or surface, and “irregular planets”, which includes anything else notable enough to be identified.
</p>
<h2 id="summaryofstuffinthesolarsystem">Grey's Summary of Stuff in the Solar System Based On What's Actually There Rather Than On Dogma</h2>
<ol>
<li>
Firstly, there's a <em>star</em>, <a href="http://en.wikipedia.org/wiki/Sun">the Sun</a>. Nuclear fusion happens inside it, releasing lots and lots of shiny energy.
</li>
<li>
There are also lots of <em>planets</em>. There are <em>ten major planetary systems</em>, divided into three broad categories:
<ul>
<li>
There are <em>four</em> planetary systems with a <em>fluid planet</em> at their centre:
<ul>
<li>
The <a href="http://en.wikipedia.org/wiki/Jupiter">Jupiter</a> system, comprising the fluid planet Jupiter; four spheroidal planets (Ganymede, Callisto, Io and Europa); at least 59 irregular planets; and several coherent rings of débris
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Saturn">Saturn</a> system, comprising the fluid planet Saturn; seven spheroidal planets (Titan, Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas); at least 49 irregular planets; and several coherent rings of débris
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Neptune">Neptune</a> system, comprising the fluid planet Neptune; the spheroidal planet Triton; at least 12 irregular planets; and several coherent rings of débris
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Uranus">Uranus</a> system, comprising the fluid planet Uranus; five spheroidal planets (Titania, Oberon, Ariel, Umbriel and Miranda); at least 22 irregular planets; and several coherent rings of débris
</li>
</ul>
</li>
<li>
There are <em>four</em> planetary systems with a <em>spheroidal planet</em> other than a fluid planet at their centre:
<ul>
<li>
The <a href="http://en.wikipedia.org/wiki/Earth">Earth</a> system, comprising two spheroidal planets, Earth and Moon
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Venus">Venus</a> system, consisting solely of the spheroidal planet Venus
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Mars">Mars</a> system, comprising the spheroidal planet Mars and two irregular planets, Phobos and Deimos
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Mercury">Mercury</a> system, consisting solely of the spheroidal planet Mercury
</li>
</ul>
</li>
<li>
There are <em>two annular</em> planetary systems – <em>rings</em> populated evenly with planets:
<ul>
<li>
The <a href="http://en.wikipedia.org/wiki/Asteroid_belt">asteroid belt</a>, comprising at least two spheroidal planets (Ceres and Vesta); and hundreds of thousands of irregular planets
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Kuiper_belt">Kuiper belt</a>, comprising at least 800 (but probably hundreds of thousands of) irregular planets; and many spheroidal planets, including the binary spheroidal planets Pluto and Charon and their irregular satellite planets, Nix and Hydra; the spheroidal planet Haumea and its irregular satellite planets, Hi'iaka and Namaka; and the spheroidal planets Makemake, Quaoar, Orcus, Varuna and Ixion
</li>
</ul>
</li>
<li>
There are also lots of other minor planets strewn about all over the place, many of which orbit beyond Neptune, but not all. Some of them are arranged into distinct structures:
<ul>
<li>
The <a href="http://en.wikipedia.org/wiki/Scattered_disc">scattered disc</a>, a roughly disc-shaped collection of planets beyond Neptune, scattered there by an interaction with a major planet, that includes the spheroidal planet Eris and its irregular satellite planet Dysnomia
</li>
<li>
The <a href="http://en.wikipedia.org/wiki/Oort_cloud">Oort cloud</a>, a spherical cloud of icy minor planets about a thousand times further out than the Kuiper belt, of which the spheroidal planet Sedna may be a member; planets from the Oort Cloud that fall towards the Sun and exhibit a tail of evaporating ices are called “comets”.
</li>
<li>
<a href="http://en.wikipedia.org/wiki/Trojan_%28astronomy%29">Trojans</a>, groups of small planets following the same orbit as another much larger planet, but 60° ahead of or behind it
</li>
</ul>
</li>
</ul>
Arranged in order of distance from the Sun, the planetary systems are: Mercury, Venus, Earth, Mars, the asteroid belt, Jupiter, Saturn, Uranus, Neptune and the Kuiper belt. The scattered disc extends from the Kuiper belt out to at least twenty times its outer radius; the Oort cloud begins at about fifty times the Kuiper belt's outer radius.
</li>
<li>
Finally, there are lots of miscellaneous small blobs of rock and ice, and plenty of dust and débris floating about all over the place.
</li>
<li>
(Actually, there are also lots of photons, neutrinos and other elementary particles hanging around that have been emitted from the Sun or other stars, but they're mostly irrelevant.)
</li>
<li>
<ins>(And there's <a href="http://en.wikipedia.org/wiki/Dark_matter">dark matter</a> – we can only account for one-tenth of the mass that the universe must have; the missing 90% is called dark matter. Maybe if we used the other 90% of our brains, we'd find it.)</ins>
</li>
</ol>
<h2 id="summary">So, to Summarise,</h2>
<p>
Everything is a planet. Even wee, small, diddy, tiny things are planets if you care enough about them to identify them.
</p>Planet X-3: Pluto Still Isn't a Planet, Neither Are 2003 UB313 and 2003 EL61https://gkn.me.uk/planetx32005-07-30T13:59:00+00:002005-07-30T13:59:00+00:00
<p>
They finally found it - again - <a href="http://www.gps.caltech.edu/~mbrown/planetlila/" title="from Mike Brown's pages at Caltech">another tenth planet, this time unnamed but temporarily designated 2003 UB313</a>. It was also announced yesterday that <a href="http://www.gps.caltech.edu/~mbrown/2003EL61/" title="again from Mike Brown's pages at Caltech">2003 EL61</a> may be roughly 70% Pluto's size.
</p>
<p>
UB313 however, is definitely larger than Pluto, although perhaps only slightly. <a href="http://www.washingtonpost.com/wp-dyn/content/article/2005/07/30/AR2005073000403.html">According to the Washington Post, Mike Brown has labelled the object the tenth planet</a>; they acknowledge that there are astronomers who disagree with this label, but don't mention that Mike is actually one of them. <a href="/weblog021" title="Planet X-2">As I wrote about last year</a>, he doesn't consider Pluto to be a planet, and argues that if Pluto is considered a planet, many other objects (<a href="http://www.gps.caltech.edu/~mbrown/sedna/#planets" title="Mike Brown's Sedna page">including Sedna</a>) must logically also be considered planets.
</p>
<p>
So UB313 must logically be classified as a planet as long as Pluto is. I'd be willing to bet that by this time next year there'll be another object found to be larger than Pluto, if not several; then, the International Astronomical Union will have to reconsider their definition of what constitutes a planet (or come up with one). For now, if you're asked how many planets there are in the solar system, it's <em>definitely</em> wrong to say “nine”.
</p>
<p>
<a href="http://www.discover.com/">Discover Magazine</a> <a href="http://www.discover.com/issues/nov-04/cover" title="Beyond Pluto">discusses the outer solar system in more depth</a> in <a href="http://www.discover.com/issues/nov-04/">their November 2004 issue</a>; you'll have to log in to read past the first page of the article, which usually entails using <a href="http://www.bugmenot.com/">BugMeNot</a>.
</p>Planet X-2https://gkn.me.uk/weblog0212004-03-15T19:52:00+00:002004-04-27T17:40:00+00:00
<p>
The press release is now online, as is Mike Brown's Sedna page, at <a href="http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/">http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/</a> and <a href="http://www.gps.caltech.edu/~mbrown/sedna/">http://www.gps.caltech.edu/~mbrown/sedna/</a> respectively. Right then - Sedna isn't a planet - it's not even a Kuiper Belt Object. According to Mike Brown, the Kuiper Belt has a fairly sharp edge at 50AU (1 Astronomical Unit is the distance from Earth to the Sun, 150 Gm); Sedna comes no closer than 70AU. The appropriate term for Sedna is “Inner Oort Cloud Object” (and I'd like to take this opportunity to lay claim to the acronym “IOCO”).
</p>
<p>
The Oort Cloud was theorised by a bloke named Oort, hence the name. It's a cloud (duh) of small icy bodies which inhabits the outer reaches of the solar system; Oort inferred the cloud's existence to explain the origins of dirty snowballs (comets). The Oort cloud is thought to lie much further out than this; it seems there is also an Inner Oort Cloud, apparently created when a passing star... passed.
</p>
<p>
Mike Brown also gives a very interesting suggestion of how to define a planet. He says a planet is any body whose mass is greater than the total of all other masses in similar orbits. By this definition, Pluto isn't a planet... but we knew that already.
</p>Planet Xhttps://gkn.me.uk/weblog0202004-03-15T16:20:00+00:002004-04-27T17:40:00+00:00
<p>
Well, they finally found it, “Planet X”, a.k.a. Sedna. Of course, if it is decided that it is actually a planet, the name will have to be changed to a Roman god. Sedna is the Inuit goddess of the ocean, and the Roman god of the ocean - Neptune - is already taken.
</p>
<p>
Interestingly, the Discovery Channel website reports (<a href="http://dsc.discovery.com/news/briefs/20040315/planet.html">http://dsc.discovery.com/news/briefs/20040315/planet.html</a>) that the body appears to have a companion (read: “moon”), which is the second reddest object in the solar system, after Mars. Wow, these NASA fellas can even measure redness accurately! Since the rules of moon-naming are less strict than those for planet-naming (or rather there's a justifiably greater set of allowed names), perhaps this moon will end up being called “Sedna”?
</p>
<p>
Of course, the big question is “is it a planet?” Well, if size is your criterion, most probably yes, given that Pluto is officially a planet and Sedna is (expected to be) not much smaller, and may even be bigger than Pluto. But size isn't the best criterion - Pluto is actually smaller than the Moon, and the Moon's not a planet (although astronomers sometimes describe Earth-Moon as a double-planet because of their relative sizes).
</p>
<p>
There's some <a href="/weblog012" title="Pluto Isn't a Planet">debate over whether Pluto should qualify as a planet</a> - it's usually described as a Kuiper Belt Object (KBO). (The Kuiper Belt is a field of small, rocky bodies orbitting beyond Neptune - essentially a second Asteroid Belt.) Pluto, Sedna, Quaoar et al. are rightly not classified as asteroids - they're too big - but they're basically the same thing... just bigger. The truth is there's really little intrinsic difference between the smallest of space rocks (meteorites and the like) and much larger objects like Mercury and the Moon.
</p>
<p>
Sedna orbits the Sun at three times the distance of Pluto (taking a whopping 10,500 years to do so). If Sedna is actually a KBO, that means the Kuiper Belt must be absolutely colossal. If it's a KBO, it's as much a planet as Pluto. If it's not part of a belt, then I think it must qualify as a planet.
</p>
<p>
NASA will give a press conference at 18:00 UTC.
</p>Pluto Isn't a Planethttps://gkn.me.uk/weblog0122004-02-21T02:45:00+00:002004-04-27T17:40:00+00:00
<p>
Officially, Pluto is a planet. In reality, it's a member of numerous Kuiper Belt objects which orbit the Sun beyond Neptune. When it was found, 74 years ago, Clyde Tombaugh was searching for a planet and so assumed what he had found was one.
</p>
<p>
Another large Kuiper Belt object was found a few days ago, as has been reported by the BBC (<a href="http://news.bbc.co.uk/1/low/sci/tech/3506329.stm">http://news.bbc.co.uk/1/low/sci/tech/3506329.stm</a>) - <a href="http://en.wikipedia.org/wiki/2004_DW" title="(Wikipedia)">2004 DW</a>. Its diameter is estimated as between 1500km and 2600km; Quaoar, another Kuiper Belt object found in 2002, is about 1200km across; Pluto is 2320km across. The evidence is mounting that Pluto is simply a large example of an ordinary Kuiper Belt object. If 2004 DW turns out to be as large as, or larger than Pluto, will it be called a planet?
</p>