No matter what they’re called or how they originated, the question that I’m sure comes to your mind is ‘How do these little wanderers shine?’ They don’t have parent stars whose gentle light they can reflect out to the Universe at large, like the planets of the Solar System reflect sunlight. Nor are they big enough for starlike nuclear reactions to stir them into luminosity. The thinking among the pundits is that their hydrogen atmospheres may be dense enough to insulate them from the cold of space, allowing them to glow dimly in infrared light by virtue of the same sort of energy that keeps the Earth’s core warm. That energy comes from the decay of radioisotopes like uranium, as distinct from the fusion processes that power stars and brown dwarfs – that is, big atomic nuclei falling to pieces rather than smaller ones sticking together. The glowing of these lonely objects by nothing more than geothermal energy sounds a bit unlikely, but calculations show that, yes, it could happen. What is far more certain, though, is that in coming years we will find more of them, as the astronomers’ arsenal of space-based infrared telescopes increases in size and capability. So – if you’ll excuse the pun – watch this space.
I’m sure that, by now, you’re getting the picture that things are far from straightforward in the contest of planets versus stars. And we haven’t even begun to discuss the weird and wonderful array of planets known to orbit other stars in the Sun’s neighbourhood. These extrasolar planets, or exoplanets, have been discovered in steadily increasing numbers since the first was found, in 1995. Well over 800 were known by 2012, and the number is continuing to grow. But it was because of these exoplanets in all their wondrous diversity, together with the poor homeless FFLOPs, that the issue of exactly what constitutes a planet started to emerge as a pressing matter for serious debate.
So how did the IAU, as the body in which all power of definition is vested, deal with this? It did what any organisation would do, of course. It formed a committee. At a meeting in 1999, the IAU asked some of the leading lights on exoplanets to form a working group, part of whose brief would be to write a definition of a planet. Thus, the Working Group on Extrasolar Planets was formed. Throughout its six-year lifetime, the group was chaired by Alan P Boss of the Carnegie Institution, in Washington DC, and it included several people whom I number among my friends. They did a terrific job, and I hope they will forgive me if I appear unsympathetic to the working definition of a planet that they eventually came up with – especially since they were at pains to point out that it was a compromise and ‘did not fully satisfy anyone on the WGESP’. But, to be honest, it was rubbish.
The definition was a valiant attempt to highlight the differences between brown dwarfs, planets and FFLOPs. There’s not much point in recounting all the gobbledygook here, except to quote the third and final clause, which reads: ‘Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium [13 Jupiter-masses] are not “planets”, but are “sub-brown dwarfs” (or whatever name is most appropriate)’. So there you have it. After struggling to be ultra-precise in the definition of exactly what constitutes a planet, the working group wound up by shooting itself in the foot. For the FFLOPs, at least, you can choose whatever name you think is appropriate. You might as well decide to call them ‘bananas’.
And when is a planet a trans-Neptunian object?
The difficulties encountered in working things out at the upper end of the planetary size range were eclipsed – in the public’s view, at least – by what was happening at the other end. Among the diametrically challenged members of the Sun’s increasingly unruly family, all was not well.
The rot had started in the 1950s, when two astronomers, Kenneth Essex Edgeworth and Gerard Peter Kuiper, independently suggested that, out in the frozen reaches beyond the orbit of Neptune, there must be a ring of debris left over from the formation of the Solar System. They argued that asteroid-sized objects in this region of space would have been too far apart to combine into larger objects and form planets, as had happened in the inner Solar System soon after its formation, 4.6 billion years ago. At best, there might be some half-finished planets out there that were too small to be seen with the telescopes of the time.
It was also thought – correctly, as it turned out – that these trans-Neptunian objects, as they came to be known, would have a high proportion of ice in their structure. They would have been perpetually too far from the Sun’s heat for it to melt – or, as happens in the vacuum of space, to evaporate directly into water vapour. Eventually, in August 1992, the first trans-Neptunian object was discovered, a distant speck of light that’s probably an object the size of a mountain range, taking a leisurely 289 years to orbit the Sun. It was given – and still has – the incredibly boring name of 1992 QB1, which arises from the IAU’s rather inelegant naming protocol for newly discovered Solar System objects. It was also the 15 760th object discovered, in the ongoing tally of minor Solar System bodies, since asteroid number one – called Ceres – was found by one Giuseppe Piazzi, on 1 January 1801.
Significantly, Ceres itself was controversial in its day, since Piazzi and his chums at first thought it was a planet. It was found in what had been suspected to be a vacant lot between the orbits of Mars and Jupiter. But when it was discovered to be much smaller than the other planets and circulating with a clutch of even smaller objects in the same part of the Solar System, astronomers realised they had some quite un-planetary items on their hands. What could they be? It was the British astronomer William Herschel who eventually coined the term ‘asteroid’ to give them all a decent – if slightly comical – identity.
But back to the outer Solar System. Today, there are more than 1000 known trans-Neptunian objects. So many are known, in fact, that they can be categorised into different types, depending on the sizes, shapes and tilts of their orbits. Astronomers love to classify stuff, and this particular stuff is eminently classifiable. A minor problem is that specialists in the esoteric field of ‘classifying stuff at the edge of the Solar System’ don’t entirely agree on what the categories should be. Broadly, though, there’s an understanding that the trans-Neptunian objects divide into two types. Nearer to the Sun are objects belonging to the Kuiper Belt (or, more correctly, the Kuiper-Edgeworth Belt), which occupies a zone extending for about 2 billion kilometres beyond the orbit of Neptune. Since Neptune itself is, on average, 4.5 billion kilometres from the Sun (about 30 times the Sun–Earth distance), this is very remote stuff indeed. But trans-Neptunian objects in the second category are even further out. They are known, rather obscurely, as ‘scattered disc objects’, a reference to their wildly disparate orbits, and they extend out to staggering distances from the Sun – 12 billion kilometres and beyond. In comparison with the scattered disc objects, Kuiper Belt objects have better behaved orbits, although they also divide into a couple of subcategories that needn’t concern us here. Suffice it to say that Pluto sits in the middle of the Kuiper Belt, at an average distance of 6 billion kilometres from the Sun.
It was some unexplained irregularities in the orbit of Uranus that had prompted the search for a ninth planet, beyond the orbit of Neptune, during the early years of the 20th century. The irregularities also suggested where in the sky astronomers should search. When, in January 1930, Pluto had been discovered in more or less the right place, there had been jubilation. At last, the ledger of gravitational forces would be balanced. So Pluto was originally thought to be a large planet – probably larger than Earth. But, as time went by and measurements of its diameter became ever more accurate, estimates of its size decreased. In a paper published in 1980, two US scientists derived a mathematical formula for Pluto’s apparently diminishing girth, whimsically suggesting that by 1984 the planet would have disappeared altogether. Very droll. And so, as with Ceres a century or so earlier, jubilation over the planet’s discovery eventually turned into consternation. Pluto was far too small to have any appreciable effect on a gas giant planet like Uranus. Indeed, it is now known to be only two-thir
ds the size of our own Moon.
The disappointment over Pluto’s half-pint dimensions spurred a renewed search for a hypothetical planet that was the scapegoat for the outer Solar System’s lack of equilibrium. But some people doubted there was any need for such an object. At last, in the 1980s, the idea of Planet X ran out of steam completely, when the mass of Neptune was carefully measured from the trajectory of a robotic interplanetary spacecraft called Voyager 2. That re-evaluation brought everything back into balance and eliminated the need for any new planets. Surprise, surprise: the discovery of Pluto had been nothing more than a happy accident.
In the meantime, however, we had a planet of minuscule proportions on our hands that seemed entirely at odds with the lumbering giants of the outer Solar System. Moreover, Pluto was in an orbit quite different from the other planets – very elongated, with a 17-degree tilt to the rest of the Sun’s family. With the discovery of the first Kuiper Belt object, 1992 QB1, suspicions began to arise that maybe Pluto wasn’t what it had at first seemed. Perhaps it wasn’t a planet at all, but one of these pesky new icy asteroids.
Gradually, as more trans-Neptunian objects were discovered during the 1990s, a few brave souls began openly speculating that Pluto belonged among them. Most notable was Neil deGrasse Tyson, an astrophysicist and director of the Hayden Planetarium in New York City. When Pluto failed to appear among the planetarium’s refurbished display of planets in February 2000, Tyson suddenly started to receive hate mail from US school kids. It was a sign of things to come.
Then, early in 2005, the unthinkable happened. A group of astronomers based at the Palomar Observatory in California announced that they had discovered a remote scattered disc object way beyond Pluto that was probably bigger than Pluto itself. Identified from sky images taken late in 2003, it had been given the provisional name of 2003 UB313. But the discovery team, led by Michael E Brown of the California Institute of Technology, had its own pet name, borrowed from the heroine of a TV fantasy series – Xena: Warrior Princess. (Planet X subtly cropping up again, you see.) But there was the rub. With the discovery of Xena, did the Solar System have a tenth planet? Or were both Xena and Pluto something else, in which case there would only be eight planets?
Once again, the spotlight fell on the IAU to resolve the issue. But, as we have seen, its Working Group on Extrasolar Planets hadn’t exactly covered itself in glory in arriving at a workable definition, and this was a much more acute problem. So the IAU brought together a different group of distinguished scientists, historians and science communicators to form a planet definition committee and nut out the answer. They were tasked with reporting the result of their deliberations to the IAU’s General Assembly in Prague in August 2006 and duly set about their work. When they revealed their conclusions, a week before the General Assembly’s decision, they could hardly have guessed how much the IAU’s membership would disagree with them.
Pluto’s puzzles
Ever since Pluto’s discovery, back in 1930, by a young Illinoisborn astronomer called Clyde William Tombaugh, people had wondered how this remote world was formed and what it might be like.
As Pluto traverses the frozen outer reaches of the Solar System, its surface temperature ranges between about –240° Celsius and a balmy –220° Celsius. Its orbital speed of less than 5 kilometres per second (compared with Earth’s 30 kilometres per second) places it at the lethargic end of the Sun’s family. But there are signs that past interactions between Pluto and its neighbours in the Kuiper Belt may have been more violent.
Pluto has been known since 1978 to have a large moon, Charon (usually pronounced ‘Care-on’ rather than, well, ‘Sharyn’). Its diameter of 1209 kilometres has been measured with an accuracy of a couple of kilometres by observing its passage in front of a distant star – an event called an ‘occultation’. Pluto and Charon are sometimes thought of as a binary system, because their relative sizes are fairly close – Pluto’s diameter is only twice Charon’s diameter. As a result, their combined centre of gravity, or barycentre, lies in the space between them rather than within the body of Pluto. This contrasts strongly with the situation for most planets and their moons, and may provide a clue to the origin of Charon. We know that in the Solar System’s turbulent youth, collisions between young planets and the rocky debris left over from their construction were commonplace. For example, a collision between the baby Earth and a Mars-sized object is thought to have produced our own Moon, 4.6 billion years ago. About half a billion years later there was another bad patch, incongruously known as the ‘late heavy bombardment’, during which the Moon received most of the craters we see on its surface today. Is it possible that in one of these wild and woolly periods a violent collision between icy bodies in the far reaches of the Solar System could have produced Pluto and Charon? Computer simulations have shown that this is, indeed, possible, but there is at present no way of discriminating between that scenario and those in which Charon was simply captured by Pluto’s gravity as it wandered past within the Kuiper Belt.
A further tantalising clue turned up late in 2005 in the shape of two more moons of Pluto – tiny objects no bigger than 150 kilometres across, now called Nix and Hydra. In July 2011, Pluto’s known retinue was increased again with the discovery of a fourth, even smaller moon, as yet unnamed, while a year later, a fifth moon no more than 25 kilometres across was discovered. Nix and Hydra are known to orbit Pluto in the same plane and the same direction as Charon, suggesting that they may have formed as by-products of the same collision event. A neat and tidy theory, but only a closer look by a passing spacecraft, allowing such information as crater-number counts and surface compositions to be gathered, will provide the information needed to confirm it.
Pluto and Charon are locked in what is known as ‘synchronous rotation’, meaning that the two bodies always keep the same faces turned to one another as Charon trundles around Pluto in its 6.4-day orbit. The mechanism by which this has arisen is exactly what keeps the same face of the Moon turned towards the Earth – friction caused by tides raised on the two bodies by each other. No, you don’t need oceans to have tides – they can occur in solid rock, and the forces involved exert a strong braking effect on the rotation of the two objects. Someday, in a few billion years’ time, the Earth will always keep the same face turned towards the Moon – no doubt to the chagrin of the folk who live on Earth’s Moon-less side.
The presence of a moon in orbit around a planet or asteroid has an important consequence for astronomers: it allows both objects to be weighed. And, remember Xena, that distant object whose discoverers thought it was probably bigger than Pluto? In September 2005, Xena turned out to have a moon too, found using one of the two giant Keck telescopes in Hawaii. Today, Xena is no longer Xena but has been officially renamed Eris, after the Greek goddess of strife and discord – which hints at the climate in planetary science at the time. Its moon has a similarly appropriate name, Dysnomia (lawlessness) – in Greek mythology, the daughter of Eris. Observations of Eris and Dysnomia have recently confirmed that Eris is 27 per cent more massive than Pluto, though of a similar diameter. (Therefore, it must be more dense, perhaps containing a smaller proportion of ice than Pluto.)
Apart from the obvious issue concerning their planetary status, why have Pluto and Eris become such celebrities in the astronomy of the early 21st century? The answer lies in what they might tell us about the formation of the Solar System and perhaps even about the origins of life on Earth.
If the typical trans-Neptunian object is a remnant of the original disc of debris that surrounded the infant Sun, then its chemistry would be nothing less than the Rosetta Stone of our corner of the Universe, with pristine dust grains that have been forever cold and frozen organic (carbon-containing) material that might carry the progenitors of living cells.
We already know that as well as being classified by their differing orbital characteristics, trans-Neptunian objects can be sorted in a different way into at least two garden varieties, some havin
g a neutral-grey colouring and others, like a very distant one by the name of Sedna, being decidedly red. This may indicate subtly different cosmic histories throughout the age of the Solar System, with the reddish ones perhaps having a surface layer that has been modified by long-term effects such as bombardment by the subatomic particles known as cosmic rays. But whatever the reason for their different colours, any one of these objects that strayed close enough to the Sun would quickly develop features characteristic of a comet – a coma, or halo, formed by the evaporation of icy materials and the release of dust, and a prominent tail. There is a recognised class of exactly these types of objects in unstable orbits that may eventually fall into the inner Solar System as short-period comets; they are known as Centaurs: half-man, half-beast; half-Kuiper Belt object, half-comet. Who says astronomers have no soul?
The importance of this to the history of the Earth is that impacting comets are thought to have been a significant source of icy materials, such as water ice, methane and ammonia, for the planet. It is highly likely that more complex organic molecules were included in the same package, and a handful of scientists think that life itself may even have arrived in this way. Hence the extraordinary interest in investigating the various types of ice contained in comets and objects in the distant Kuiper Belt and beyond.
Larger trans-Neptunian objects, like Pluto and Eris, may have a different story to tell. With these, the process of planet formation seems to have been interrupted mid-flow, resulting in half-finished worlds that have nevertheless become big enough for their own gravity to pull denser material to the middle and, at the same time, make them spherical. This process, called ‘differentiation’, is likely to have given Pluto – and perhaps Charon too – a rocky core with an icy mantle. The process would have been greatly enhanced if a collision did, indeed, give rise to Charon, since the energy of the collision would have produced additional heat. Pluto’s surface is known to consist of frozen nitrogen, with methane, carbon dioxide and ethane also present. However, the bulk of Pluto’s icy mantle is likely to consist of water ice buried beneath the more volatile surface ices by the same process of differentiation. Its thin atmosphere, whose existence was confirmed during an occultation in 1988, is probably mostly gaseous nitrogen.
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