WRITE down everything you know about Pluto, the ninth planet in our solar system. Odds are, you won鈥檛 get very far. Even if you are S. Alan Stern, one of the world鈥檚 foremost experts on planetary science.
鈥淚 can tell you everything we know for sure about Pluto on about three 3-by-5 file cards,鈥 says Stern, who heads the Southwest Research Institute鈥檚 space studies department in Boulder, Colorado. 鈥淭hat leaves a lot of room for discovery.鈥
Small wonder then that the excitement was palpable as Stern鈥檚 team gathered at Cape Canaveral, Florida, in late October. It was the last formal meeting before next month鈥檚 long-awaited launch of NASA鈥檚 New Horizons, the first mission to explore Pluto and the outer solar system. The scientists were particularly jazzed because a separate team of astronomers led by Stern had reported just days earlier that in addition to its large moon, Charon, Pluto has two smaller moons. 鈥淲e鈥檙e going to find things are not as we expected,鈥 says New Horizons team member William McKinnon, a planetary scientist at Washington University in St Louis, Missouri. That鈥檚 clear from the fact that these moons just 鈥減opped up鈥.
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There is no doubt that more surprises are in store. 鈥淲e don鈥檛 even know what we don鈥檛 know,鈥 says Stern, New Horizons鈥 chief scientist. Indeed, because Pluto is so far away and so utterly unknown, the mission鈥檚 findings could radically change our thinking about many things. New Horizons could provide important clues for understanding everything from how the solar system formed to how Earth鈥檚 atmosphere came to be.
Unlike the rocky surface of Mars, say, Pluto is made of a mix of rock and frozen water, methane and other volatile compounds 鈥 ones that would melt, evaporate and in some cases escape into space at the higher temperatures of the inner solar system. But out at the fringes, these compounds may have remained essentially unchanged since the birth of the planets. 鈥淚t鈥檚 not just that it鈥檚 ice, it鈥檚 pristine ice that鈥檚 been in that form for the entire age of the solar system,鈥 says Richard Binzel, a professor of planetary sciences at the Massachusetts Institute of Technology who has specialised in studying the composition of asteroids. Binzel, a member of the New Horizons science team, says that this primordial material means the mission 鈥渋s not only a trip in space, it鈥檚 also a trip in time鈥.
A different world
Until US astronomer Clyde Tombaugh discovered Pluto in 1930, people knew about only two types of planet: terrestrial planets 鈥 small, rocky worlds, ranging in size from the near twins Earth and Venus down to half that for Mars and even less for Mercury 鈥 and the gas giants Jupiter, Saturn, Uranus and Neptune, made mostly of hydrogen with little or no solid surface.
Pluto is different. Really different. It鈥檚 less than half the diameter of Mercury; the sun鈥檚 rays take 4 hours to reach the little planet; and it鈥檚 1000 times darker out there than on Earth. The temperature on Pluto鈥檚 surface is in the neighbourhood of -233 掳C. And while the air on a planet like Mars is thin 鈥 less than one-hundredth the density of Earth鈥檚 鈥 Pluto鈥檚 predominantly nitrogen atmosphere is 1000 times thinner still, and sometimes freezes up altogether.
No wonder Pluto and its largest moon, Charon 鈥 discovered in 1978 鈥 are called 鈥渋ce dwarfs鈥, a name that distinguishes them from the terrestrial planets and gas giants. Still, nobody suspected that rather than being the last-found member of a limited family of planets, Pluto might become a gateway to a whole new class of bodies known as Kuiper belt objects, or KBOs. We now know that Pluto is one of perhaps hundreds of comparably sized bodies inhabiting the Kuiper belt, the solar system鈥檚 largest and outermost zone 鈥 and the source of most comets visible from Earth.
鈥淜uiper belt objects may illuminate the history of Earth and the origin of life鈥
KBOs are the leftover building blocks of the solar system, the same materials that were crashing around nearer the sun 4.5 billion years ago, some of which stuck together to form Earth and the other terrestrial planets, as well as the cores of the giant planets. Preserved in the deep freeze of space far from the sun, KBOs may be relics of what was here at the beginning of the solar system. When we start studying Pluto and its environs close-up, we鈥檒l be seeing relatively undisturbed remnants from when the planets were still under construction. They may hold a chemical and structural record whose traces have been obliterated on all of the inner solar system bodies we鈥檝e examined so far.
So Pluto may actually tell us about the early Earth. Because Pluto and Charon are very close in size and in space 鈥 the pair is often termed a double planet 鈥 they may exchange some of their atmospheres. Charon pulls hydrogen and other lightweight gases outward from Pluto in a spiral swirl that might mix with its own extended atmosphere. This kind of interplanetary weather exchange has never been seen before. Studying how Pluto鈥檚 atmosphere changes over time, through radio and camera measurements, may shed light on a related question from Earth鈥檚 early days: how quickly did the hydrogen originally present in Earth鈥檚 atmosphere escape into space (Science, vol 308, p 1014)? We need to know this to accurately simulate the environment in which life began nearly 4 billion years ago.
Half-built planet
Another attraction is that Pluto may be a kind of unfinished planet. 鈥淚t鈥檚 the equivalent of a palaeontologist finding a pregnant dinosaur with an embryo inside,鈥 says Stern. 鈥淚t鈥檚 fundamental to understanding the formation of the planets to see one half-built.鈥 Computer simulations show Pluto should have grown to be a very substantial object, at least the size of Earth, Stern says. If the same processes that built the other planets were at work, there should have been enough boulders and mountain-sized chunks of rock called planetesimals in Pluto鈥檚 region of space to crash into each other, sometimes sticking together, to form a big planet. 鈥淪omething came along and shut off the supply,鈥 he says, though nobody knows what.
The discoveries that New Horizons makes about the present number of KBOs and the size and density of craters on Pluto and Charon could help pin down this idea. Although simulations of the solar system can do a reasonably good job of producing families of planets similar to what we see today, the nature of the early stages is still mostly based on inference. Actually seeing some of the intermediate steps in the process, preserved in mid-formation, could provide substantial new evidence to bolster, or alter, theories of planet formation. It could also reveal some of the unknown details, such as the size and composition of the planetesimals themselves.
Michael A鈥橦earn, an astronomer at the University of Maryland, says that studying KBOs is the only way to address some 鈥渧ery important issues鈥 in understanding the solar system鈥檚 formation. Characterising these objects, he says, is fundamental to the big picture of how the solar system works.
Take the relative populations of objects of different sizes in the Kuiper belt. 鈥淲e can鈥檛 see the small Kuiper belt objects from Earth,鈥 A鈥橦earn says. 鈥淪o looking at the craters they make when they hit Pluto and Charon is an indirect way of getting at the size distribution of Kuiper belt objects.鈥 That, in turn, could help explain the nature of the comets that we see in the inner solar system. Are these really primordial bodies, leftovers of the disc that spawned the planets, or are they fragments of larger objects produced by repeated collisions in the outer solar system? Without such basic knowledge, it is difficult to interpret the findings from recent missions such as this year鈥檚 Deep Impact, which A鈥橦earn headed, that have given us detailed images of cometary nuclei that are startlingly different from one another (Science, vol 310, p 258).
What鈥檚 more, New Horizons will make it possible to learn more about the chemistry of KBOs and how much they may vary from one another. Spectroscopic work from Earth has already proved that different types exist. 鈥淭here are at least two flavours,鈥 says MIT鈥檚 Binzel. Some are bluish or grey in colour, probably composed mostly of a mixture of ices, while others are reddish, indicating a coat of more complex organic molecules that may have been produced by chemical reactions spurred by cosmic rays and impacts.
Some theories hold that impacting bodies from the Kuiper belt were important sources of Earth鈥檚 water, atmosphere and even complex hydrocarbons that helped to provide building blocks for the rapid appearance of living cells. So observing the detailed composition and distribution of KBOs may help to illuminate further the history of our own planet and the origins of life on it.
But getting out there will be a long haul indeed. After years of preparation and waiting, once New Horizons is sent on its way this January there will be a sudden burst of activity and planning. That鈥檚 because the spacecraft鈥檚 trajectory will depend on exactly when it is launched. 鈥淔or some of us, it will be like waking up from suspended animation,鈥 says McKinnon.
If things go according to plan, expect another flurry of activity 10 years hence. Once the long voyage to Pluto is complete, the most intensive data-collection phase will be over in just a few days as the spacecraft hurtles past the planet. Pluto rotates at a lazy rate of once every 6.4 Earth days, and after getting a slingshot-like boost from Jupiter鈥檚 gravity the spacecraft will be whizzing by at a clip of 43,000 kilometres per hour. So New Horizons will have time to see only one side of Pluto as it passes closest to the planet 鈥 and only one side of Charon as well, since it rotates at exactly the same rate as Pluto. To make up for this, New Horizons will use a telescope to map the other sides of the two bodies during the approach phase, three days earlier.
Though it鈥檚 a compromise, the plan should provide plenty of coverage. 鈥淲e鈥檙e going to have 1000 times the number of pixels on the 鈥榝ar side鈥 of Pluto as we presently have,鈥 Stern says. 鈥淚f you grab a garden-variety pair of binoculars and go outside and look at the full moon, that鈥檚 what we鈥檒l see on the poorly resolved side of Pluto [and Charon]. On the well-resolved side, we鈥檒l get down to 25 metres a pixel.鈥 That鈥檚 enough detail to make out building-sized boulders and craters.
All-seeing eye
As well as its keen-sighted camera, the golf-cart-sized New Horizons craft carries an impressive array of other instruments to give every object it passes a thorough once-over (see Diagram). The biggest constraint on the amount of data we鈥檒l get is not the sensors, though, but the strength of the radio signal that New Horizons can beam back to Earth, which NASA鈥檚 Deep Space Network of antennas in California, Spain and Australia will pick up before sending the data on to mission control at the Johns Hopkins University Applied Physics Laboratory in Maryland. That communication system will be pressed into service for research, too. Once New Horizons is on the far side of Pluto, the Deep Space Network will transmit radio signals so that the craft can measure how they are bent and absorbed by Pluto鈥檚 atmosphere. That will yield valuable information about the planet鈥檚 air temperature, density and composition.
And of course, the frenzy of data acquisition during the Pluto fly-by is hardly the end of the mission. There will be at least one more Kuiper belt object in New Horizons鈥 path, and maybe more. The actual selection of a target won鈥檛 take place until shortly before the rendezvous with Pluto in 2015 鈥 once the craft鈥檚 exact route is known it will be relatively easy to locate a good candidate using powerful telescopes such as those at the Keck Observatory in Hawaii.
Given the importance of the questions New Horizons might answer, it seems extraordinary that the project has taken so long to reach the launch pad. Stern has been trying to get a Pluto mission off the ground since 1988. During the 1990s, he led the design of a camera for a Pluto and Kuiper belt mission planned by NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. That mission was axed by NASA in 2000 after costs reached more than double the original estimates, but the basic concept refused to die. 鈥淭he idea just wouldn鈥檛 stay down. It鈥檚 too interesting, too compelling, and a number of us just wouldn鈥檛 keep quiet,鈥 says McKinnon.
鈥淭he further you go from home, the weirder it gets. It鈥檚 going to be jaw-dropping鈥
Although New Horizons is budgeted at a fraction of the price 鈥 an estimated $500 million 鈥 its science capabilities will match and in some cases exceed those of the previous proposal. Crucially, the new Pluto mission has also received overwhelming support from high up in the administration. In 2000 NASA鈥檚 solar system exploration subcommittee said flatly, 鈥淲e must go there.鈥 And the US National Research Council has listed exploration of Pluto and Charon as one of its top priorities.
There鈥檚 good reason for all this: now is the perfect time to go. Pluto鈥檚 elongated orbit carries it more than 40 times farther from the sun than Earth, so it will become both harder to reach and less interesting to study after 2020. Because its entire atmosphere may freeze and collapse to the planet鈥檚 surface for many decades, the next good viewing might not come until 2230. Stern is convinced the mission will be well worth the investment 鈥 and the wait. 鈥淭he further you go from home, the weirder it gets,鈥 he says. 鈥淲hen we get to Pluto in 2015, it鈥檚 going to be jaw-dropping. I think it鈥檚 going to be transformational.鈥
Two to tango
One of the big questions about the solar system鈥檚 formation is how Pluto and Charon came to be. The prevailing idea is that, like Earth and its moon, they were the product of a massive collision.
Robin Canup, a planetary scientist at the Southwest Research Institute who is not involved with New Horizons, has done the most extensive simulations of the formation of both the Earth-moon system and of Pluto and Charon, the results of which were published earlier this year (Science, vol 307, p 546). The computer models demonstrate that the collision scenario does work, but they leave questions unanswered: in some simulations the impact creates a large ring or disc of debris that slowly clumps together to form a moon, while in others the moon forms right away.
New Horizons might determine which scenario actually took place, Canup says. In the disc model, Charon would have a very different overall composition and density from Pluto, whereas in the intact-formation model the two would be very similar. Detailed measurements of their sizes, relative motions and surface compositions should help resolve that.
As for Pluto鈥檚 newly discovered moons, the likely scenario is that they were produced by the same collision that formed Charon, because they are relatively close to Pluto and orbit in the same plane and direction as Charon. The idea that they were 鈥渁 by-product of the impact that formed Charon seems the simplest and therefore the most attractive place to start鈥, Canup says. But it leaves open the question of whether all three moons formed slowly out of a disc of debris or were born immediately in the collision鈥檚 aftermath. It is also still possible that the new moons were separate objects captured at a different time.
So far, the simulations that Canup has published on the Pluto-Charon impact scenario are not detailed enough to reveal whether additional small moons would also form. However, in simulations of the formation of Earth鈥檚 moon involving a similar large impact that first produced a disc of debris, there are situations in which there are multiple moons and scattered debris. And in some of the Pluto-Charon models that produce an intact Charon, there are also hints of leftover material 鈥渢ossed away鈥 that, in a more detailed simulation, might show the formation of extra moons.
Pluto by numbers
1930 鈥 Year of discovery by US astronomer Clyde Tombaugh
2015 鈥 Year that New Horizons will visit (planned)
2230 鈥 Year when next good viewing begins (approximate)
5.9 鈥 Average distance from the sun in billions of kilometres
-233 鈥 Surface temperature in degrees Celsius
0.2 鈥 Mass as percentage of the Earth鈥檚
0.1 鈥 Light intensity from the sun as percentage of Earth鈥檚
6.4 鈥 Duration of Pluto鈥檚 day in Earth days
248 鈥 Duration of Pluto鈥檚 year in Earth years
What is a planet, anyway?
The biggest, most passionate debate raging about Pluto is one that has little to do with science or any of the properties of what has been, for 75 years, the solar system鈥檚 ninth planet. The burning question is whether Pluto should be considered a planet at all 鈥 and if so, what else might also be one.
A 19-member committee of the International Astronomical Union (IAU) is going to deliver a verdict 鈥 maybe, some day 鈥 on what a planet is. How that decision will be received is anyone鈥檚 guess, because lots of people 鈥 astronomers, schoolchildren, museum directors and editorial writers 鈥 have already chosen sides and dug in their heels. But it鈥檚 not likely to get resolved anytime soon. Brian Marsden, a member of the IAU committee, says, 鈥淎 lot of members haven鈥檛 shown any sign of compromise.鈥 No date has been set for a decision.
There seem to be five main ideas in circulation, each of which has its passionate advocates and its fierce detractors:
1 Planets are round things
This sets a clear limit, based on physical principles. Any object above a certain size, no matter what it鈥檚 made of, will be pulled by gravity into a spherical shape. That cut-off is a diameter somewhere around 800 kilometres. Alan Stern is a leading proselytiser for this definition. This would include Pluto and at least four other known Kuiper belt objects 鈥 Quaoar, 2003 UB313, 2003 EL61 and Sedna 鈥 and also one asteroid, Ceres. It鈥檚 a simple definition and it fits with people鈥檚 intuitive ideas about planets: 鈥淪ay 鈥榙raw a picture of a planet,鈥 and all kids will start by drawing a circle,鈥 Stern says. Downside: Inclusion of Ceres and multiple KBOs is a big change from the current nine planets.
2 Planets march alone
This is the leading argument against Pluto and its Kuiper belt neighbours. The idea is that planets are things that dominate their region of the solar system, orbiting alone in their zones. Thus the swarm of asteroids orbiting mostly between Mars and Jupiter, and the much bigger swarm of KBOs, including Pluto and 2003 UB313 鈥 discovered this year, and bigger than Pluto 鈥 do not qualify as planets, but merely as members of a different class of objects. By this criterion, there are only eight planets in the solar system, and that almost certainly will remain so. This is the view supported by Neil Tyson, director of New York鈥檚 Hayden Planetarium. Downside: Pluto is excluded, changing what people have been taught for generations.
3 Size matters
Anything over, say, a 1000-kilometre radius (or 1150 kilometres, the radius of Pluto) is automatically a planet. This has the advantage of setting a clear physical limit that also fits with everyone鈥檚 existing ideas about what is and is not a planet. By this rule, Ceres is too small, as are Sedna and 2003 EL61, but Pluto and 2003 UB313 are planets. So we now have 10 planets, with the exciting prospect of discovering a few more in coming years. Mike Brown, the astronomer who discovered 2003 UB313, favours this plan. Downside: The size limit is arbitrary, plus it鈥檚 hard to accurately estimate the size of faraway bodies.
4 History matters
And Pluto is special. This is supported by a subgroup of the eight-planet camp and holds that while the orbits would normally rule out Pluto, it should be 鈥済randfathered in鈥 out of deference to its 75 years in the planetary family. That way, a strict definition can exclude future KBOs, but would still allow schoolchildren everywhere to stick with their favourite variation of 鈥淢y Very Energetic Mother Just Served Us Nine Pizzas鈥 as a way to remember what and where the planets are. Downside: Too arbitrary.
5 There are no planets
In an attempt at compromise, some have suggested getting rid of the unadorned word 鈥減lanet鈥 altogether, instead saddling it with a series of adjectives. Thus, the solar system consists of four types: four terrestrial (rocky) planets, a bunch of minor planets (asteroids), four gas-giant planets, and a whole lot of trans-Neptunian (ice-dwarf) planets. Downside: Too complicated, and too much of a change from tradition.
