The Best Australian Science Writing 2013 (18 page)

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 having 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.

Why do we think Pluto and Eris may be half-finished planets? The evidence comes mainly from computer simulations of planet formation carried out at such institutions as the Southwest Research Institute, in Boulder, Colorado. They demonstrate that Earth-sized objects could, indeed, have formed in the outer regions of the Solar System. Why the process stopped is a mystery. But if Pluto really is a half-built planet, a close look at it would give us a unique opportunity to see planet formation in freeze-frame, providing real insights into the process.

What was obviously needed was a robotic space mission to Pluto. But there's a catch – and it's not just the extreme distance involved. Pluto's elongated orbit means the energy it receives from the Sun falls by a factor of three as it moves from perihelion (its closest point to the Sun) to aphelion (its furthest point) in its 248-year orbit. Perihelion occurred in September 1989, so by early in the 21st century the planet was already well on its way towards the zone in which its thin atmosphere will simply freeze onto its surface. And there was already evidence of seasonal changes in Hubble Space Telescope observations of Pluto's surface markings. The sooner we could get to Pluto, the more informative and interesting it would be.

Towards a new horizon

On 19 January 2006, a long-cherished dream came true. A Plutobound robotic spacecraft called
New Horizons
was successfully fired from Cape Canaveral in Florida, using the Lamborghini of launch vehicles – an Atlas V rocket with some serious go-faster accessories. If we were going to start travelling to Pluto in 2006 on a timescale that would give us the best chance of investigating its atmosphere, we needed to get there as quickly as possible – and
New Horizons
broke all the records, leaving Earth at the highest launch speed ever achieved. It crossed the Moon's orbit in only nine hours and whizzed by Jupiter after little more than
a year, the close encounter with the giant planet increasing its speed to a remarkable 23 kilometres per second. After years of planning –and a few false starts – humankind was at last on its way to Pluto.

The reliability of orbital mechanics means we can predict with pinpoint accuracy when
New Horizons
will reach Pluto. It will fly by the frozen world at 11.47 Greenwich Mean Time on 14 July 2015, passing Charon 14 minutes later. Because of its speed (the close approach will take place at nearly 14 kilometres per second) there is no possibility of
New Horizons
being diverted into orbit around Pluto, so the 0.5-tonne spacecraft bristles with sensors to take full advantage of its brief encounter. They include spectrometers to analyse the barcode of information locked up in Pluto's rainbow spectrum, subatomic particle detectors, a long-range camera and an instrument named the Venetia Burney Student Dust Counter, which will provide valuable information on the levels of interplanetary dust in the outer Solar System. The fly-by should allow detailed mapping of Pluto and Charon, as well as collection of telltale data on their surface and atmospheric composition. Alongside all the high-tech robotic sensing equipment,
New Horizons
also carries a poignant reminder of its place in human history. On board is a container carrying 28 grams of the ashes of Pluto's discoverer, Clyde Tombaugh, who died in 1997.

It's hard to overstate the importance of
New Horizons
, since our first-hand knowledge of Pluto and its environment is so sparse. The results could be the most surprising of any deepspace mission yet, notwithstanding the extraordinary discoveries about Saturn and its moons that have been made by the highly successful
Cassini
mission since it reached the planet in July 2004. But with
New Horizons
going on to target selected Kuiper Belt objects after its Pluto fly-by – and then escaping from the Solar System altogether – the excitement of new discoveries may continue well into the century.

Controversially,
New Horizons
carries 10.9 kilograms of radioactive plutonium dioxide to provide thermoelectric power for its onboard instruments. There is little alternative to this, given that the intensity of sunlight at Pluto's distance is only one-1000th of that which we receive on Earth, rendering solar panels useless. But you won't be surprised to hear that this was not the only controversy surrounding the Pluto mission.

Dwarfed by controversy

Just seven months after
New Horizons
was launched, the IAU held its much-vaunted General Assembly in Prague. Its latest planet definition committee had agreed on a draft specification of what constitutes a planet, and this was receiving substantial press coverage. To be a planet, the committee suggested, a celestial object needed to be in orbit around a star and large enough for its own gravity to pull it into a spherical shape (a condition technically known as ‘hydrostatic equilibrium'). This definition significantly extended the inventory of planets in the Solar System, since both Eris and the largest asteroid, Ceres, qualified. Moreover, there would be a significant likelihood of more to come as the exploration of the Solar System's twilight zone revealed further Eris-like objects.

Shut away from the glare of the waiting media, the membership of the IAU met on the General Assembly's final day to vote on the recommendation. But an intense debate yielded a revised definition subtly different from the committee's in that it included an additional criterion. To be a planet, went the revised version, a celestial object also had to be the dominant object in its neighbourhood, having cleared away smaller debris either by absorbing it (as the Earth does with thousands of tonnes of meteoritic dust per year) or by ejecting it through gravitational forces. Any object that hadn't done this, despite meeting the spherical shape criterion, would be termed a ‘dwarf planet' rather than a
‘planet'. What that meant in practice was that an object in the main Asteroid Belt, between Mars and Jupiter, or in the Kuiper Belt could not be a planet.

When the vote on this revised definition took place it was passed with an overwhelming majority. It was adopted by the IAU as Resolution 5A of the Prague General Assembly. Thus, on 24 August 2006, the world was given, for the first time, a formal definition of a planet – and it did not include Pluto. The former ninth planet, along with Ceres, Eris and two other Kuiper Belt objects, Haumea and Makemake, had become a dwarf planet.

If there had been any doubt about the public's interest in what constitutes a planet it was quickly dispelled by the outcry that followed. The headline I liked best appeared in a Newcastle (New South Wales) article: ‘Pluto dumped by the übernerds of Prague'. Not just the nerds, mark you, but the ‘übernerds'. Similar sentiments echoed around the world, especially in the USA, from where the former ninth planet had been discovered. There, protest marches were held in some cities.

It was a long time before the outrage subsided, and even then it didn't do so before three US states had attempted to introduce legislation to reinstate Pluto as a planet. Only one of them, Illinois, succeeded, demonstrating that if you don't like a scientific outcome you can always legislate to overturn it. Significantly, Illinois was the state in which Clyde Tombaugh was born. And on the lunatic fringe there are still conspiracy theory websites that point to the fact that the IAU's decision was taken on the final day of its General Assembly, when only one-sixth of the attending membership was still present (producing 424 votes). They cry foul, citing rogue scientists and vested interests. They also conveniently ignore the fact that at the IAU's next General Assembly, held in Rio de Janeiro in 2009, there was no change of heart on the issue.