What is a planet ?

In August 2006, the International Astronomical Union, at its General Assembly in Prague, had before it a motion to specify, a bit more clearly, what a planet is.

There was certainly a need for some such clarification; but choosing a sensible specification was bound to be the difficult part. The proposed specification was unsatisfactory (as I'll explain below) and I am pleased to hear the IAU amended it (see 2006 resolutions 5 and 6 in the IAU's full list; or the PDF) into a fit state – at the expense of demoting Pluto from planetary status; but this was clearly necessary, as I'll now explain.

I am pleased to hear that the trans-Neptunian (or: Edgeworth-Kuiper belt) object which provoked the argument, 2003 UB313, has now been given an official name (as a dwarf planet): Eris. This name is taken from that of a Greek goddess associated with discord (very apt, considering the storm we've just weathered (August 2006) about nomenclature). Given their respective sizes, we can regard Eris as Pluto's big sister. (For some time, Eris was referred to affectionately as Xena, but her discoverers chose to call her Eris when asking the IAU to sanction a name.) The one known satellite of Eris has been named Dysnomia, after a daughter of Eris, associated with lawlessness. Some prefer to think of Eris as goddess of chaos and Dysnomia as goddess of disobedience ;->

A little history

The ancient Greeks (along with other cultures, some older) understood that most of the stars in the night sky keep constant positions relative to one another: the constellations do not change shape or relative position. Five objects (sometimes) visible in the night sky, however, move around relative to the (other) stars: these are named Mercury, Venus, Mars, Jupiter and Saturn, after assorted deities of the ancient world. Collectively, they came to be known as planets, from the Greek word for wanderer.

By the late 1700s, it was widely understood that these planets orbit the Sun and that the land upon which we stand is the surface of another body which orbits the Sun with them – now known as the Earth – for a total of six planets. Herschel discovered a seventh, Uranus, in 1781. Someone (possibly Titius) noticed (and Bode publicized) a remarkable coincidence among the radii of the orbits, now known as Bode's law or the Titius-Bode law. Divide each orbital radius by that of the Earth (the Astronomical Unit, or AU): you get the sequence 0.387, 0.723, 1, 1.524, 5.202, 9.56, 19.3. The first is approximately 0.4, so let's subtract 0.4 from each and discard the first: we're left with 0.323, 0.6, 1.124, 4.802, 9.16, 18.9; divide each of these by the previous and you get 1.86, 1.873, 4.27, 1.907, 2.058; all but one of which are really quite close to 2, with the exception being really quite close to the square of two. Running this process in reverse, we take a geometric series with ratio 2 between terms and arrange for its second term to be 0.6; then we prepend a zero and add 0.4 to each entry to get 0.4, 0.7, 1, 1.6, 2.8, 5.2, 10.0 and 19.6 – which are, indeed, reasonably good approximations to the first seven planets' orbital radii, as multiples of Earth's orbital radius, as long as we skip the extraneous entry 2.8. This prompted various folk around 1800 to start looking for a planet with orbital radius roughly 2.8 AU – and, on the first day of 1801, Piazzi found a body whose orbit soon enough turned out to be about 2.77 AU.

Once he'd realized it wasn't a meteor (as he initially supposed) and determined its orbit, he named it Ceres Ferdinandea (but the surname's since been dropped). Naturally enough, it was hailed as a planet: but it soon enough emerged that it was very much smaller than the others – and that there were several other bodies in similar orbits. Eventually, it became clear that it was merely the biggest of a large population of mostly small bodies orbiting the sun roughly between the orbits of Mars and Jupiter. These came to be collectively referred to as asteroids (star-like things: they were too small for telescopes of the day to show them as anything but a point of light, where planets all showed a disc or crescent of light) and Ceres was demoted from planet to mere Queen of the asteroids.

Refined study of subtle differences between the observed and predicted orbits of Uranus led Galle to look where he (at roughly the same time as others) had calculated there must be another planet: and to find Neptune, in 1846. This turned out to be as convincingly planet-like as one could hope for, so no-one has ever argued that it should suffer the same ignominy as Ceres. This left us with four terrestrial inner planets – dense balls of rock, like Earth – and four comparatively huge outer gas giant planets, with a fuzzy zone full of asteroids between them. All of the planets (and most of the rubble) followed orbits which were pretty close to circular and pretty close to a common plane (known as the ecliptic). There was a certain stately symmetry to it all.

In 1930, after much difficult searching, Clive Tombaugh found Pluto. Initial estimates of its size were greatly exaggerated as a result of failure to realize that what was actually seen was a mutually orbiting pair of bodies, each smaller than the Moon; the result looked bigger than the reality. It was announced with great fanfare as a planet and, unlike Ceres, adding it to the list didn't imply re-numbering any of the others. It was so distant that we took another sixty years to develop technology to see anything else so far away. In the interval, everyone got used to its status as a planet: I've even run into astrology fans who insist it should be taken into account.

However, Pluto's orbit is odd. Its plane is at a much greater angle to the ecliptic than the usual angles between the planes of other planets' orbits. The orbit is highly eccentric (i.e. obviously an ellipse rather than almost a circle) and actually crosses that of Neptune – indeed, if it were not neatly in a resonance with Neptune (Neptune completes three orbits in the time Pluto takes to complete two), it would be disturbed from its orbit by Neptune; and if it had not been in the part of its orbit closest to the Sun, Tombaugh would have had even more trouble finding it. It also became evident, as it was studied more closely, that it was not as big as it initially seemed: in fact, it's smaller than several moons of other planets, notably including our own Moon. Even Mercury (the smallest inner planet, and closest to the Sun) is bigger than all the other planets' moons (although Jupiter's Callisto gives it a close run on volume). All this made Pluto something of a second-class planet.

Eventually, we had the technology to see beyond Neptune a bit more clearly; as CCD (charge-coupled devices, the light sensors in digital cameras) became cheap and good, plenty of other objects came into view beyond Neptune. Many of these, like Pluto, are in two-to-three resonances with Neptune, in highly eccentric orbits in planes significantly tilted relative to the ecliptic. In hindsight, Pluto is merely the largest of a large population of bodies in similar orbits to its own – just as Ceres turned out to be merely the largest of the asteroids. Worse was to come: we have now found larger bodies than Pluto even further out.

It thus becomes necessary, if the designation planet is not to become absurdly arbitrary, either to demote Pluto from planet-hood (as was once done to Ceres, for similar reasons; it can still be King of the bodies in orbits like its own, as Ceres is Queen of the asteroids) or to allow many other bodies to be classified as planets. Demoting Pluto has the virtue of restoring the elegant and symmetric situation that held from 1846 to 1930, save for the addition of a second fuzzy zone full of rubble, surrounding the outermost orbit – which, if anything, merely improves the elegance of the original. It also allowed for a specification of a planet (due, I believe, to Chad Trujillo) which sums up what's satisfying about this pre-Pluto characterization: a planet has most of the mass in orbit around the sun at roughly the same radius as itself [this is roughly the definition that the IAU finally adopted].

The IAU proposal

The proposal originally before the IAU said that a body is a planet if:

it is big enough that its own gravitation forces it to be roughly spherical

formally, it must have sufficient mass for its self-gravity to overcome rigid body forces so that it is in hydrostatic equilibrium. This is a perfectly sensible criterion which many things smaller than Ceres satisfy (but Neptune's second largest moon, Proteus, doesn't); and was kept in the amended resolution.

it orbits a star and is not a star

Stars aren't planets: that sounds sensible. Having to orbit a star seems rather incidental to planet-ness, but I'm not hugely concerned about it.

it is not a satellite of another planet

and, for these purposes, one body is a satellite of another if their joint centre of gravity (formally: their barycenter) lies inside the latter. This is the constraint with which I [and, it turns out, many IAU members] took issue.

I see several problems with the last constraint.

By my calculations, the qualifying language says (assuming I've got my sums right) that Jupiter isn't a satellite of the Sun. The Sun's mass is 1047.5 times that of Jupiter, so their barycenter divides the line between them in the ratio of 1 (on the Sun's side) to 1047.5 (on Jupiter's); its distance from the Sun's centre is thus 1/1048.5 of Jupiter's orbital radius. Jupiter's orbital radius is 1119 times the surface radius of the Sun; so the barycenter lies outside the Sun.

Consider the case of two big enough bodies, of equal size, orbiting the Sun along with a third larger body. The two equal bodies have their joint barycenter half way between them, so outside their surfaces. When we pair either one of them with the larger body, this pair's barycenter shall be closer to the larger body's center than the barycenter of all three bodies. So it's possible for the group barycenter to be outside the bodies but the pair-wise barycenter of either smaller body with the larger one to lie inside the larger one. Thus each smaller body is a satellite of the larger one, but the pair of smaller bodies – when taken together – would not constitute a satellite of it.

For a specific example of roughly this kind: consider a moon of a planet, with a fairly large body in one of its stable Lagrange points (a sixth of a cycle ahead of or behind the moon, in the same orbit). If the moon's barycenter with its planet is only just inside the planet, and its Lagrange companion is big enough, the joint barycenter of the three-body system shall lie outside the planet.

But the real problem is that whether a body is a planet or not can depend on just how far it orbits from another body. A moon in an eccentric orbit around a planet could have ended up being counted as a planet when furthest from the planet but not when at its closest – the barycenter's path is a scaled-down version of its eccentric orbit, not a circle.

The Moon's orbit grows slowly larger with time: therefore, eventually, its barycenter with Earth shall fall outside Earth. However, to reach that point, the orbit needs to grow by a bit over 36% (which'll lengthen its orbital period to six and a quarter weeks): which, at its present rate of growth, will take about 3.7 G yr (in reality it'll take longer, as I suppose the orbit's growth shall slow as it gets bigger). The Sun is only expected to survive another 4 or 5 G yr before turning into a cool red giant, so the Moon may be inside the Sun before it gets to satisfy the original proposition's definition of a planet.

The proposal also introduced the term Pluton to describe planets with orbital periods in excess of 200 years and highly inclined orbital planes. [Apparently the geologists also had some objections to this terminology.] This struck me as a designation of little use, which would merely have served to generate gratuitous controversy (over whether given bodies qualify or not).

What we actually need is a characterization of our Solar System's parts. It is well understood that each of the eight classical planets rules its orbit – it has swept the neighbourhood of its orbit clear of other bodies. We have well-understood populations of bodies orbiting among the orbits of the planets, especially in the gap between Mars and Jupiter. We are starting to understand the classes of object which exist beyond Neptune. Singling out individual bodies from these disparate populations as plutons would have both conflated things that should be considered separately and ignored the actual, interesting structure and differences that more naturally relate the bodies in the outer solar system. There is a ring (roughly matching what Edgeworth and Kuiper anticipated) of bodies orbiting the Solar System somewhat further out than Neptune in roughly circular orbits of modest inclination: there are bodies in eccentric tilted orbits, resonant with that of Neptune, between roughly the circular ring and roughly the orbit of Neptune; there are bodies in eccentric orbits stretching outwards from the circular ring. Distinguishing which of these classes (or whatever replaces them as our understanding improves) a body belongs to is far more important than adding it to the list of planets.

Likewise, the proposal elected to lump everything smaller than planets together under one heading, small Solar System bodies – again, ignoring the meaningful and useful similarities and differences among the bodies in question. The term minor planet is undeniably a poor use of words, but it usefully distinguishes the bodies it describes from comets and rubble.

Quibble: Pluto would have been the tenth planet

The frequently-asked questions page carefully explained that the proposal would have made Ceres a planet. It then went on to ask

and, perversely, answered in the affirmative. Specifically, it said that

Historically it was indeed the ninth planet to be discovered,

which is true as long as we ignore Ceres, which we can't justify if we restore it to planet status. Ceres was the eighth planet to be discovered: at the time of its discovery it was so described and the only reason to not refer to it as such now is because we no longer consider it a planet. Restoring it to planet-hood would have made Neptune the ninth planet discovered and Pluto the tenth. The answer also says:

The classical planets can be numbered by their distance from the Sun, and there is no change in their order.

which is at least partly true, since Ceres isn't a classical planet: but the ordering of the planets would have needed to change. With the addition of Ceres as fifth from the Sun, Jupiter would have become the sixth planet (despite being, still, the fifth classical planet), Saturn the seventh and so on – with Pluto as the tenth, at least until it comes back within Neptune's radius.

Trying to order planets by when they were discovered gets highly problematic for the inner planets – we don't know the order in which the five known since antiquity were recognized as planets and we'd have to either put the Earth first (because we discovered it long before the others) or last (because we recognized it as a planet long after the first five). All the same, if we do so, and allow both Pluto and Ceres as planets, Pluto was the tenth to be discovered. Ordering them by distance from the Sun likewise puts Pluto tenth. (It would make more sense to order by semi-major axis of orbit – or, equivalently, the time taken to complete one orbit).

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