鈥淭HERE鈥橲 something funny going on out there.鈥 Marc Buie of the Lowell Observatory in Arizona is talking about a strange feature at the far edge of our Solar System beyond Pluto, among the swarm of small worlds called the Kuiper Belt.
It鈥檚 a wild, uncharted place out there, teeming with icy celestial bodies that may give us essential clues to how the planets formed. It may even be a breeding ground for life. But what鈥檚 intriguing Buie at the moment is the very edge, about 50 times further out from the Sun than the Earth鈥檚 orbit. Here, at the 鈥淜uiper Cliff鈥, the number of astronomical objects drops off precipitously. Buie won鈥檛 be drawn too far but, when pressed, he speaks of the possibility that some 鈥渕assive object鈥 has swept the zone clean of debris.
And his is not a lone voice. Alan Stern, a planetary astronomer at the Southwestern Research Institute in Boulder, Colorado, has been saying for years that there could be another large planet out there. Just how large became clear last month when Mario Melita from Queen Mary, University of London, and Adrian Brunini from the University of La Plata, Argentina, published a paper on the very same anomaly (Icarus, vol 160, p 32). They used computer models of the orbits of nearby objects to pin down the kind of celestial object that could carve out the Kuiper Cliff and concluded that a planet about the mass of Mars or Earth would provide 鈥渁 remarkable match鈥 with the observations.
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Planet X is back on the map.
The last time the idea of a tenth planet created a stir was in 1983, when IRAS, the Infrared Astronomy Satellite, was widely rumoured to have located it. Although the rumours turned out to be false, the idea has never quite gone away.
That鈥檚 because the basic idea of a swarm of small worlds beyond Pluto has been around for a long time. The notion was first touted in 1943, by Kenneth Edgeworth, an aristocratic Irish astronomer. Gerard Kuiper, a Dutch astronomer, came to the same conclusion a few years later. But they were lone voices until the 1980s, when planetary scientists began to realise that some comets were coming from a region not far beyond Neptune and Pluto.
In 1992, Dave Jewitt and Jane Luu, working at the Mauna Kea observatory in Hawaii, discovered the first of its inhabitants. It was 10,000 times fainter than Pluto, and just one-tenth its size. It has since been joined by many others: the running tally of known Kuiper Belt Objects reached 700 on 4 November this year.
This is just the tip of the iceberg, though. Jewitt calculates there are some 10 billion KBOs bigger than one kilometre. Add them together and the objects total 0.2 Earth masses, roughly a thousand times the total mass of the asteroid belt between Mars and Jupiter. 鈥淭he asteroid belt is just a small village, compared to the bright lights and big city of the Kuiper Belt,鈥 observes Stern. 鈥淚t鈥檚 a region of planetary formation, with 100,000 objects that are 鈥榤iniature planets鈥.鈥
And some of them are not so miniature. Since 2001, astronomers have discovered four KBOs bigger than 1000 kilometres across. Caltech astronomers Michael Brown and Chadwick Trujillo announced the latest one, fully half the size of Pluto, in October this year. They have provisionally called it Quaoar, after a native god of the indigenous dwellers of the Los Angeles region. Quaoar is over 1200 kilometres across and orbits the Sun every 288 years. It鈥檚 the largest object discovered in the Solar System since Pluto.
Quaoar suggests that KBOs will soon be found to rival Pluto in size. 鈥淚t would be incredible if Pluto proved to be the only 2000-kilometre object,鈥 Jewitt says. 鈥淚 think we鈥檒l have Pluto II, Pluto III鈥 within a few years.鈥 Stern agrees. 鈥淭here are more likely 900 planets in the Solar System than nine,鈥 he says, 鈥渁nd all but eight are in the Kuiper Belt.鈥
As if that wasn鈥檛 enough to make planetary astronomers sit up and take notice, there are indications that these planets may come complete with the seeds of extraterrestrial biology. That is to say the Kuiper Belt is a rich repository of the raw materials for life. With the help of infrared spectroscopy astronomers have spotted water-ice and hydrocarbons on the surface of some objects. That鈥檚 much better evidence for life than on Mars, where there is no exposed water or organics. The Kuiper Belt, on the other hand, contains most of the Solar System鈥檚 water and organic molecules. There may not be life there just yet, but it seems almost inevitable at some point in the future. 鈥淲hen the Sun goes red giant, the Kuiper Belt will be a Miami Beach and perhaps life will even evolve there,鈥 Stern says.
As well as containing the key to the origin of life, the Kuiper Belt, and Pluto in particular, may hold the key to how planets form.
The IRAS satellite has detected the smoking gun of planetary formation in very similar regions of other solar systems. It saw glowing discs of dust around several young stars; these discs are produced by collisions between icy objects similar to the KBOs. If there鈥檚 anything our Solar System can teach us about planet formation, we鈥檒l learn it in the Kuiper Belt.
鈥淧luto is the place which offers the most leverage scientifically in studying the origin of the Solar System,鈥 says Stern. 鈥淚t鈥檚 a place where planetary formation has progressed so far, but not to completion: here we have the chance to study planetary embryos.鈥 Studying the craters on both Pluto and its moon Charon, for example, will reveal how KBOs have collided over billions of years 鈥 and provide clues to the way all the planets formed from smaller objects.
This, of course, depends on how you choose to define a planet; Pluto鈥檚 status is still being debated (see 鈥淲hat鈥檚 in a name?鈥). But, whatever the details, it is going to be difficult to spot anything significant from Earth. It makes sense to get out there and have a proper look at the region, Stern says. He has been pushing NASA and the US administration for a Pluto mission for 15 years now.
鈥淚t takes longer to get out of the Washington Beltway than to get out of the Solar System,鈥 he quips. In 2000, NASA scrapped its own Pluto-Kuiper Express mission on the grounds of expense. Under intense public pressure, it held a competition for universities and industry to design a cheaper, better mission. From this was born the New Horizons space probe, due for launch in January 2006. Stern, the mission鈥檚 lead scientist, calculates that New Horizons will return 10 times more data than the cancelled Pluto-Kuiper Express, and at little more than half the cost.
NASA has little to do with the new mission, apart from overseeing its launch and beaming signals to and from the space probe. Stern is heading up a team of mainly non-NASA scientists. The engineering expertise comes from the Johns Hopkins University in Baltimore, which built the highly successful NEAR-Shoemaker probe that landed on asteroid Eros in 2001.
Just over a year after the New Horizons鈥 launch, it will swing past Jupiter and pick up enough velocity to reach Pluto, possibly as early as July 2015. The mission will bring astronomers a rare treat, Stern believes. 鈥淭he further you go in the Solar System, the wilder it gets,鈥 he says. 鈥淓verything will be completely new. This region is a scientific wonderland 鈥 we鈥檒l be like kids in a candy shop.鈥
As it speeds past Pluto, New Horizons will examine the planet鈥檚 atmosphere to find out what it鈥檚 made of, how warm it is and exactly what鈥檚 happening to it: there are hints that the atmosphere is changing in unexpected ways (see 鈥淗otting up鈥). Also, infrared instruments will reveal the composition of the surface: so far, astronomers have only indirect clues. Rick Binzel of MIT first mapped Pluto in the 1980s, and saw bright and dark regions. He believes the bright areas must be fresh ice, probably nitrogen and methane. 鈥淪pace is a dirty place 鈥 it鈥檚 hard to keep things clean for long, and the only way to have bright areas is for them to be young and fresh,鈥 he says. The dark areas are likely to be the oldest exposed regions on the surface. New Horizons鈥 suite of cameras will also unveil objects as small as 1 kilometre over the whole sunlit side of both Pluto and Charon, and the resolution will go down to 25 metres in selected regions.
To add to this scientific bonanza, New Horizons will visit up to three other KBOs after Pluto. The targets haven鈥檛 been chosen yet; they will depend on how much fuel the spacecraft has left. And the space probe鈥檚 observations of craters on these worlds should prove whether they have been pelted by small bodies perturbed by a massive, distant world 鈥 the elusive Planet X.
Indeed, by the time New Horizons reaches the Kuiper Belt, we may have confirmed that Planet X exists. At the moment, we have only hints. In their new research, Melita and Brunini have explored three possible reasons for the existence of the Kuiper Cliff. One is simply that the Kuiper Belt was formed with an abrupt edge. Alternatively, a passing star may have swept away any more distant KBOs. The third possibility is that the region beyond was brushed clear by the gravity of Planet X.
The orbits of the KBOs would be shaken up in different ways, depending on which idea is right. And Brunini and Melita have found that the KBO orbits they have investigated so far fit in best with the influence of a Planet X (see Graphic). Melita is not sticking his neck out too far, though: 鈥淚鈥檓 not claiming that there is a planet out there, but if there is, it will explain the edge and perhaps some other features.鈥
Because of its vast distance from Earth, the only way we鈥檒l find out for sure is to visit this new frontier of the Solar System and get a closer look. But we could be doomed to disappointment, because it seems that New Horizons may never get off the ground. Despite all its scientific appeal, despite all the tantalising prospects of
discovery, the mission has yet to get the final green light.
It is highlighted in a forthcoming legislative bill that will decide NASA鈥檚 budget for next year. Within the next few weeks, the US Senate and the House of Representatives must vote for New Horizons; and then it must be signed into law by the President.
If the mission falls at these political hurdles, the impact on planetary science could be disastrous. Our chance to get a good look at Pluto seems to be running out fast. For a start, the launch window for New Horizons will close in 2008 鈥 after that, it can鈥檛 use a Jupiter slingshot. And by the time another mission could launch, Pluto will have drifted so far from the Sun that its air may have frozen to the ground, robbing astronomers of the chance to explore its unique planetary atmosphere. Binzel is convinced the mission must go ahead because the region is likely to be even wilder than the astronomers鈥 wildest dreams. 鈥淣ature is always more bizarre and more complex than we can imagine,鈥 he says. 鈥淚鈥檓 looking forward to being amazed by the unexpected.鈥
Hotting up
Pluto is experiencing global warming. We know this because astronomers had a rare chance to probe its atmosphere in mid-2002, when the planet happened to pass in front of two distant stars. By measuring how the starlight fades when occulted by Pluto, astronomers can determine how the density of Pluto鈥檚 atmosphere changes with distance from the surface. Knowing the atmosphere is made of nitrogen, it鈥檚 then a matter of simple physics to work out the surface temperature.
Seven teams spread around the globe watched the event, with astonishing results. Astronomers expected the atmosphere to be cooling, since Pluto has been pulling away from the Sun since 1989. But the planet鈥檚 atmospheric pressure has increased three-fold since the measurement of a similar occultation 14 years ago. This implies that Pluto鈥檚 surface has warmed by about 2 掳C.
No one is sure why. It could be volcanic activity on the planet, as there is on Neptune鈥檚 large moon Triton. Or it could just be a result of Pluto鈥檚 exposure to the Sun. 鈥淒ue to Pluto鈥檚 geometry, the bright polar cap is going into the arctic winter shadow, leaving the darker area exposed to the Sun,鈥 says MIT鈥檚 Rick Binzel. 鈥淛ust as a black pavement is hotter than a white sidewalk on a sunny afternoon, the dark surface of Pluto is a better absorber of sunlight.鈥
Alan Stern, who鈥檚 in charge of the New Horizons mission, sits on the fence on this one. 鈥淚t鈥檚 akin to asking a Plutonian observing the Earth from afar about the ozone hole or global warming: we haven鈥檛 yet gotten the answers right to these from on Earth!鈥
Until a space probe reaches Pluto, we probably won鈥檛 even get near the right answers for this frozen world. 鈥淲e don鈥檛 know what鈥檚 going to happen with Pluto鈥檚 atmosphere,鈥 admits Jim Elliot of MIT, who led the investigations. 鈥淲e鈥檝e seen the pressure has increased about a factor of three in 14 years, so it could continue to increase or collapse on a similar timescale. We should definitely press on with the mission.鈥
What鈥檚 in a name?
The discovery of a gargantuan object like Quaoar, and the likelihood that more 鈥淧lutos鈥 exist, has revived the debate on the status of Pluto: is it a genuine planet, or just a large icy rock?
After all, it is only 2320 kilometres across, one-fifth the size of Earth. And the 1978 discovery that Pluto is circled by a moon, Charon, whose diameter is 1270 kilometres, makes it even more distinct from the other planets we know about. Pluto and Charon make up a 鈥渢win planet鈥 鈥 the only example in the Solar System. This distinction has led many to question its status.
鈥淵ou basically have a choice,鈥 says astronomer Dave Jewitt of the Institute for Astronomy in Honolulu. 鈥淧luto is either the smallest, freakiest planet with the most inclined, eccentric and far-out orbit, or it鈥檚 the biggest known but unremarkable Kuiper Belt Object. I choose the latter view.鈥
Some astronomers have suggested that, since Pluto shares a home with comets, it could be defined as one. But, unlike the other comets, Pluto doesn鈥檛 stray within the Earth鈥檚 orbit. The 鈥渙fficial鈥 status of Pluto rests with the International Astronomical Union, and they insist it is going to remain as the ninth planet.
Alan Fitzsimmons, a planetary scientist at Queen鈥檚 University, Belfast, prefers to call Pluto a 鈥淭rans-Neptunian Object鈥, but says he can live with the official title. 鈥淚鈥檓 personally happy with it continuing being called a planet,鈥 he says. 鈥淎fter all, it has an atmosphere and a moon, and it would be a real pain re-writing all those old astronomy books鈥.