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Welcome to the planet that was
28 | NewScientist | 13 June 2015
COVER STORY
F
Marie Bergeron
It might now be a mere dwarf, but our first visit to Pluto could rewrite the solar system’s story, says Stephen Battersby
OUR and a half light-hours from Earth, a small planet is about to get a small visitor. On 14 July, NASA’s New Horizons craft will skim within just 13,000 kilometres of Pluto, entering a hitherto-unexplored zone of the solar system. When this probe was launched in January 2006, its destination was still the solar system’s ninth planet. Pluto was controversially demoted to a dwarf planet later that year, but its allure is undiminished. Above its surface, frosted with exotic ices, is a strange atmosphere that seeps constantly into space, and a complex system of moons, including the giant Charon. Pluto will be a dazzling destination in itself, and this first visit could also give us clues to how our own planet formed, and to the ancient upheavals that shaped the solar system. With the blobs of Pluto and Charon already swelling in New Horizons’s sights, these are exciting times at the solar system’s new frontier. Until recently, this frontier seemed a lonely one, with Pluto its sole denizen. Then, in the 1990s, astronomers began finding further icy bodies in this remote region of the sort that might make comets. More and more followed, forming a swarm of debris stretching far out beyond the orbit of Neptune – an area now known as the Kuiper belt. The Kuiper belt’s icy shrapnel is thought to be left over from the birth of the eight major planets, remnants long ago hurled out into their present dark domain. Many are dwarf planets on Pluto’s scale – a diverse bunch, with different colours and shapes and satellite systems. “The Kuiper belt is littered with small planets – and Pluto is the archetype,” says Alan Stern of the Southwest Research Institute in Boulder, Colorado, who is principal investigator on the New Horizons mission. None of this was known back in the 1980s when Stern began to push for a Pluto mission: he was mainly looking for a place to explore. “This was a chance for a new generation of scientists to mount a mission to a new place. There were no other new places to go.” Pluto’s realm – dark, cold and far from home – is tough territory. Solar panels are no use at this distance, so New Horizons runs on heat from the radioactive decay of a lump of
plutonium. Under such weak sunlight an unheated probe would cool to below -200 °C, so New Horizons is cosseted in a multilayer blanket that traps waste heat from its instruments and electrical systems, keeping its interior at about room temperature. Radio signals take 9 hours for a round trip to Earth, so the spacecraft must be highly autonomous. And with a 12-watt transmitter on board, the signal is so weak that it can carry only about a kilobit of information per second across 5 billion kilometres. Precious images and other discoveries from the fly-by will take 16 months to download. Just getting the spacecraft off the ground was a challenge. Several times a mission was proposed and studied, then ditched. “If the mission had been a cat it would have been dead long ago, because cats only get nine lives,” says Stern. “It was not just political intrigues, but the road to build and launch on time, with plutonium shortages and almost impossible schedules. There were so many problems.”
9 hours for a signal to travel from Earth to Pluto and back
Progress since blast-off has by contrast been serene. In 2007 New Horizons flew by Jupiter to gain a gravitational kick, and tested its cameras and other instruments. They revealed lightning at Jupiter’s poles, the Tvashtar volcano erupting on the moon Io and signs of a recent impact in Jupiter’s rings – as well as a surprising lack of small moons. Since then, the trip has mostly been spent in hibernation mode. “We’ve been in cruise for so long you get used to the pace: ‘What’s new this month? The spacecraft is OK’,” says Bill McKinnon, a planetary scientist at Washington University in St Louis, Missouri. “Now after all this time, something’s really happening.” That “something” might yet be calamitous. When they were building the spacecraft, scientists expected the Pluto system to be > 13 June 2015 | NewScientist | 29
Destination in sight After a journey of nearly 10 years, the New Horizons probe will fly through the Pluto system in July
NEPTUNE KUIPER BELT January 2006 Launch from Cape Canaveral, Florida SUN EARTH
SATURN URANUS
JUPITER
February 2007 Jupiter fly-by
2007 -2014 Hibernation mode
December 2014 Probe woken up PLUTO
ORBIT OF PLUTO
New Horizons is the fastest solar system probe ever launched, but the record for distance travelled still rests with the earlier Voyager probes Voyager 1
Launched 1977
Voyager 2
1977
New Horizons
2006
19.7 billion km 16.2 billion km 4.8 billion km
relatively hazard-free. Then Pluto’s smaller moons were discovered: Nix and Hydra in 2005, Kerberos in 2011 and Styx in 2012. When another object from the Kuiper belt hits one of them, it could kick up a cloud of shrapnel that would escape the moon’s weak gravity. At the speed New Horizons is travelling, even a small pebble or ice shard could be catastrophic. The latest simulations suggest that the chance of a serious collision is slim, but right now the team are analysing images to look for new moons or rings that might increase the hazard. They have contingency plans to change course and avoid any danger zones, though that would mean a slightly more restricted view of Pluto. While New Horizons analyses dust and ionised gas around Pluto, most eyes will be on
248 years for Pluto to orbit the sun 30 | NewScientist | 13 June 2015
the dwarf planet itself. Already the probe has seen signs of a polar cap and other surface features, and as it closes in two imagers will scan Pluto’s surface in more and more detail. LORRI, equipped with a small telescope, will capture black-and-white images with a resolution of about 70 metres at closest approach; another, called Ralph, will furnish the colour pictures. Ralph can also analyse infrared light, and should reveal the detailed chemical make-up of Pluto’s landscape. From the way Pluto’s brightness changes as it spins, we know that methane, nitrogen and carbon monoxide ices form changing patterns on its surface, but what that landscape looks like remains a mystery. Probably the closest model is Neptune’s giant moon Triton, thought to be an ex-Pluto long ago captured from an independent life in the Kuiper belt. “Triton has very unusual landscapes,” says McKinnon. In 1989 NASA’s outer solar system probe Voyager 2 sent back pictures of Triton’s “cantaloupe terrain”, which looks like nothing in the solar system so much as the skin of a cantaloupe melon. Along with fresh lava flows, Triton’s relatively new terrain is a sign
of geological activity driven from within. “This is a mystery,” says Stern. “We don’t know how to power small worlds like this. Pluto will be a second data point.” McKinnon will be looking for signs that may betray a buried ocean of water. “We don’t have an ocean detector device, we can’t X-ray Pluto, so we’ll be looking for clues from its shape and landscape,” he says. Those include fracture patterns that might indicate the presence of a subsurface reservoir. “The holy grail would be evidence of past eruptions, or active vents,” says McKinnon. Perhaps, like Triton, Pluto has active geysers of nitrogen gas, powered by faint sunlight. It certainly has a tenuous nitrogen atmosphere with pressures a few millionths that of Earth’s. Without a supply of fresh gas the atmosphere would not last long, because under Pluto’s weak gravity it is leaking into space, even enveloping Charon. Nitrogen on the surface must be going direct from the frozen to the gaseous state; winds are then thought to blow from the day to the night side, where the gas refreezes. New Horizons will find out more about the >
Pluto and family PLUTO
HYDRA
1930
1978
2005
2005
2011
2012
Diameter
2370 km
1207 km
57-177 km
42-148 km
13-34 km
10-25 km
Surface gravity
6.7% of Earth’s
2.8% of Earth’s
—
—
—
—
Composition
70% rock, 30% ice
55% rock, 45% ice
—
—
—
—
Orbital period
—
6.3 days
38 days
25 days
32 days
20 days
Mean distance from system centre
—
19,600 km
65,000 km
49,000 km
58,000 km
43,000 km
HYDRA
STYX
EARTH’S MOON
CHARON
STYX
NIX
KERBEROS
CHARON
Discovered
14 July 2015 Pluto closest approach
KERBEROS
PLUTO
PATH OF NEW HORIZONS
2017-2020 Flies further into the Kuiper belt
NIX
PLUTO
Winds
Nitrogen atmosphere Ice DAY SIDE
NIGHT SIDE
Ocean? Rock core
Relative sizes
Dwarf or planet? Pluto’s elongated orbit is now taking it further from the sun. But even as it heads into colder, darker parts of space, the little world may be edging back into a more cosy position – at least in some people’s eyes – as a planet. Whatever that means. In 2006, competing definitions of the term were put to a vote at a meeting of the International Astronomical Union in Prague. The initial proposal included any star-orbiting object large enough that its own gravity has pulled it into a “nearly round” shape. That would have extended our solar system’s pantheon of planets to encompass not only Pluto but also the asteroid Ceres, along with Eris and Makemake in the Kuiper belt and probably dozens more planets-in-waiting beyond Neptune. A rival proposal added the criterion that to be a planet, an object should be massive enough that its gravity has cleared out its orbital neighbourhood of most debris. Pluto fails on this count as it is just part of the swarm of bodies criss-crossing one another
in the Kuiper belt. Under the final agreed definition, therefore, it became classed as a “dwarf planet”. Before the final vote, the astronomer Jocelyn Bell-Burnell had wielded an umbrella to clarify what would, without qualification, count as an planet. The umbrella extended over rocky bodies such as Earth and giants like Jupiter, but left dwarf planets out in the rain. The new definition was and is contentious. Some find it confusing or arbitrary. Others are sentimental about poor little Pluto, or dislike the terminology. “I coined the term ‘dwarf planet’ in 1991,” says Alan Stern, head of the New Horizons mission. “Some members of the public think it’s an insult, so when I use the term ‘small planet’, it is meant to be harder to misinterpret as pejorative.” Last year a debate at Harvard confirmed that the public, at least in the US, is pro planet Pluto. The IAU has no plans to reopen the debate, but perhaps “dwarf planet” might creep back in under the planetary umbrella, in both popular and scientific parlance.
Then dwarfs would be just one subspecies of planet, along with terrestrials, gas giants, ice giants, super-Earths and probably some other exotics we haven’t seen yet. New Horizons may well help to revive Pluto’s planetude. “When people see imagery of the Pluto system, I don’t know what else they will call it,” says Stern. “If you’re watching Star Trek, the moment you see the destination you know instantly if it’s a star or a planet or an asteroid or a comet or an alien spacecraft; you don’t have to do anything sophisticated to make a decision.” On the other hand, over the coming years more and more dwarf/small planets will be identified in the Kuiper belt. It might be natural enough for people to talk about the planet Pluto, the planet Eris and the planet Quaoar; a sterner test of terminology will be whether we blithely talk about the 73 known planets of the solar system (or is it 74 this week?). Usage will show whether people find this planetary proliferation tiresome or exciting. 13 June 2015 | NewScientist | 31
4.8 billion km Current distance of New Horizons from Earth composition of Pluto’s atmosphere using an instrument called Alice to analyse the spectrum of starlight filtering through it. The spacecraft will also send radio signals through the atmosphere and back to Earth, where mission scientists can work out temperature and pressure profiles from the signal distortion.
Wispy though it is, this atmosphere allows some outlandish possibilities. Liquid neon might be stable, suggests hydrologist Jeff Kargel of the University of Arizona in Tucson. Then neon could play the part of water on Earth and methane on Saturn’s moon Titan, and run in rivers across Pluto’s surface. McKinnon is doubtful. “I’m not expecting rivers of liquid neon, but you never know,” he says. “We are going to see something we haven’t seen before, and that will delight us.” His own suggestion is that liquid nitrogen could flow in places under Pluto’s ice, and that a large impact could inject enough heat to melt lakes of nitrogen on the surface, or even cause nitrogen rainfall. So there seems little chance that Pluto will turn out to be a dull, dead world, a disappointing space rock marked only by impact craters. “But I hope to see some craters,” says McKinnon. For one thing, Pluto’s craters could be a key to understanding how planets came into being. Our traditional picture of planet formation starts with small grains that collide and build into boulders, then larger and larger objects.
The New Horizons probe almost didn’t get off the ground
If this is right, the Kuiper belt should hold leftover planetary building blocks of all sizes. But that idea is undermined by Jupiter’s moon Europa. Europa’s surface is only a few tens of millions of years old, having been renewed perhaps by a process akin to plate tectonics. Being so new, it only shows recent comet impacts. There are relatively few small craters, meaning a lack of small comets hitting Europa. “We would expect many times more kilometre-sized things,” says Hal Levison, a colleague of Stern’s at the Southwest Research Institute who specialises in the dynamics of planetary systems. “Maybe those things go
The hunt for Planet X All the clues pointed to something big out there. In the late 19th century, astronomers suspected that anomalies in the orbits of Uranus and Neptune were caused by the gravity of a large, unseen planet. They spent decades, on and off, searching for “Planet X”, to no avail. Then, in 1930, Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona, spotted something. He was checking photographic plates of an area in the constellation Gemini, using a device called a blink comparator to flick between plates taken on separate nights. On 18 February he found a small dot that had moved between two dates in January (see pictures, right). Being so far away, stars are effectively 32 | NewScientist | 13 June 2015
NASA/JHUAPL/SwRI
Rivers of neon
stationary as seen from Earth, so it had to be something within our solar system. Its motion turned out to be too slow for an asteroid whirling around the inner solar system. Instead, the object had to be out beyond the orbit of Neptune, and it looked planet-sized. The dot was named Pluto after the Roman god of the underworld. But Pluto is not the hoped-for Planet X: it is far too small to have the observed effect on Uranus and Neptune. Only in 1992 did new data, including a measurement of Neptune’s mass by Voyager 2, allow those apparent orbital anomalies to vanish. Rumours still persist, but Planet X is probably now fading into myth – having helped us find Pluto.
‘poof’ and disappear when they come into the inner solar system and are heated by the sun. Or maybe they never got created.” Counting craters of different sizes on Pluto gives an idea of how many iceballs are hitting it and how big they are. These scars should show us whether those kilometre-scale building blocks really are missing from the Kuiper belt – and whether we need a new theory of planet formation. One recent idea is called pebble accretion: little pebbles a few centimetres across gather into large groups which then suddenly collapse under gravity. “You go directly to objects 10 to 100 kilometres in size,” says Levison. Even if Pluto’s nitrogen craters have evaporated away, there should be a good cratering record on its outsize moon, Charon. Charon is so large that it and Pluto arguably make up a binary system (see “Charon’s secrets”, above right). Its probable origin in a giant collision could shed light on the violent events that shaped the young solar system once the planets had formed. Roughly 4 billion years ago, it’s thought, the young giant planets moved outwards through a dense disc of debris girdling the sun, in the process hurling comets in all directions. Some came our way, and huge impacts created the moon’s “seas” and scourged Earth in a trauma known as the Late Heavy Bombardment. Most models rely on Neptune and Uranus migrating out like this, but it’s not clear when and how fast, or how Jupiter moved around,
CHARON’s SECRETS plumes spouting from Saturn’s moon Enceladus, “but the only solid surface where we have identified ammonia in the entire solar system is Charon”, says Bill McKinnon of Washington University in St Louis, Missouri. New Horizons will find out whether there is enough to inject some activity into Charon’s landscape, perhaps lubricating geysers. “I want to see if there are flows of semi-solid ammonia water ice, or eruptions,” says McKinnon. Charon probably formed in a similar way to Earth’s moon, when something huge collided with the proto-Pluto and blasted out debris into a surrounding disc. Simulations show that Charon and the other moons, Nix, Hydra, Kerberos and Styx, could have grown from the disc. But in the simulations, Charon moves outwards after it forms and this destabilises the orbits of the small moons, says McKinnon. And the latest Hubble images suggest Kerberos is black as coal, while the other moons are significantly paler, hinting that Kerberos may have a different origin (New Scientist, 6 June, p 16). New Horizons should nail down the
or just what the original debris disc was like. The collision that formed Charon probably happened before all this, so working out exactly how it happened could tell us a little about that disc and how it became today’s Kuiper belt. Researchers had expected the Kuiper belt to be a settled, flattish disc of objects in nice circular orbits. Instead, it is a mess. “It looks like someone took the solar system, picked it up and shook it really hard,” says Levison. Again, the giant planets were probably responsible, but no existing models can match the real tangle of orbits. “Pluto’s formation and evolution is going to
Pluto’s companion, Charon, is almost too large to be a moon
Keck telescope
Pluto is an alien world in almost all respects, but in one way it resembles Earth: it has a large, close companion. In 1978, James Christy at the US Naval Observatory in Washington DC realised that a bump on some images of Pluto wasn’t a defect in observations, as had been assumed, but a giant moon. This was the largest moon to be discovered since 1846, when Neptune’s moon Triton was found a couple of weeks after its planet. As Pluto is god of the underworld, Christy named it Charon, after the ferryman who carries souls to the underworld across the river Styx. Rather than Pluto’s chemical tutti-frutti, Charon’s surface is mainly water ice. But it also has a dash of ammonia, which produces a distinctive dip in its infrared spectrum. Since the 1970s planetary scientists have thought ammonia might act as an antifreeze, enabling chilly moons such as Saturn’s Titan to have subsurface oceans, and explaining features on Jupiter’s moon Europa and elsewhere that look like frozen flows. Ammonia has been found in the atmospheres of giant planets and in the
orbits, sizes and compositions of these moons, and perhaps discover others. “Given that we keep finding these things we’re likely to find more,” says McKinnon. With a full picture of the satellite system, models can be refined. If it turns out that the collision couldn’t have created the small moons after all, then they might instead have been captured from the surrounding Kuiper belt – suggesting that other dwarf planets out there are likely to have multi-moon systems too.
give us hints to what happened in the entire outer planetary system,” says Levison. There’s only one chance to get things right, so just at the moment, the mission team is rather busy. “We are navigating by taking images and analysing them and computing rocket burn simulations,” says Stern. “We are looking for hazards, testing programs that will operate the spacecraft during the encounter, and preparing more than 150 software tools for data analysis. I’m not worried about anything we’ve thought of; I worry about what we haven’t thought of.” A first scan for hazards has come up clear.
discovery of pluto
Lowell Observatory Archives
A strangely mobile blob (arrows) identified in 1930 suggested the existence of a ninth planet
23 january 1930
29 january 1930
-228˚ Celsius Pluto’s surface temperature
And as Pluto gradually comes into sharper focus, new features already suggest a complex surface. Come 14 July, things will happen fast. Just a few minutes after closest approach, New Horizons will turn to scan Charon, then back to Pluto to map its southern polar regions, hidden on the approach. That side of Pluto is currently turned away from the sun, so the only light will be a faint glow reflected from Charon. Then, after this all-too-brief encounter it’s onwards and outwards, probably towards PT1 (“potential target 1”), a Kuiper belt object 1.5 billion kilometres further from the sun, and just a few tens of kilometres across. After that, the spacecraft will join the earlier Pioneer and Voyager probes as part of Earth’s interstellar flotilla, reporting on the state of the solar wind until its plutonium power fades away. But first and last things first. The first world of the Kuiper belt, or the last of the nine planets if you prefer, is coming into view. n See bit.ly/NSpluto for regular updates on Pluto and the New Horizons mission. Stephen Battersby is a science writer based in London 13 June 2015 | NewScientist | 33
COVER STORY
F
Marie Bergeron
It might now be a mere dwarf, but our first visit to Pluto could rewrite the solar system’s story, says Stephen Battersby
OUR and a half light-hours from Earth, a small planet is about to get a small visitor. On 14 July, NASA’s New Horizons craft will skim within just 13,000 kilometres of Pluto, entering a hitherto-unexplored zone of the solar system. When this probe was launched in January 2006, its destination was still the solar system’s ninth planet. Pluto was controversially demoted to a dwarf planet later that year, but its allure is undiminished. Above its surface, frosted with exotic ices, is a strange atmosphere that seeps constantly into space, and a complex system of moons, including the giant Charon. Pluto will be a dazzling destination in itself, and this first visit could also give us clues to how our own planet formed, and to the ancient upheavals that shaped the solar system. With the blobs of Pluto and Charon already swelling in New Horizons’s sights, these are exciting times at the solar system’s new frontier. Until recently, this frontier seemed a lonely one, with Pluto its sole denizen. Then, in the 1990s, astronomers began finding further icy bodies in this remote region of the sort that might make comets. More and more followed, forming a swarm of debris stretching far out beyond the orbit of Neptune – an area now known as the Kuiper belt. The Kuiper belt’s icy shrapnel is thought to be left over from the birth of the eight major planets, remnants long ago hurled out into their present dark domain. Many are dwarf planets on Pluto’s scale – a diverse bunch, with different colours and shapes and satellite systems. “The Kuiper belt is littered with small planets – and Pluto is the archetype,” says Alan Stern of the Southwest Research Institute in Boulder, Colorado, who is principal investigator on the New Horizons mission. None of this was known back in the 1980s when Stern began to push for a Pluto mission: he was mainly looking for a place to explore. “This was a chance for a new generation of scientists to mount a mission to a new place. There were no other new places to go.” Pluto’s realm – dark, cold and far from home – is tough territory. Solar panels are no use at this distance, so New Horizons runs on heat from the radioactive decay of a lump of
plutonium. Under such weak sunlight an unheated probe would cool to below -200 °C, so New Horizons is cosseted in a multilayer blanket that traps waste heat from its instruments and electrical systems, keeping its interior at about room temperature. Radio signals take 9 hours for a round trip to Earth, so the spacecraft must be highly autonomous. And with a 12-watt transmitter on board, the signal is so weak that it can carry only about a kilobit of information per second across 5 billion kilometres. Precious images and other discoveries from the fly-by will take 16 months to download. Just getting the spacecraft off the ground was a challenge. Several times a mission was proposed and studied, then ditched. “If the mission had been a cat it would have been dead long ago, because cats only get nine lives,” says Stern. “It was not just political intrigues, but the road to build and launch on time, with plutonium shortages and almost impossible schedules. There were so many problems.”
9 hours for a signal to travel from Earth to Pluto and back
Progress since blast-off has by contrast been serene. In 2007 New Horizons flew by Jupiter to gain a gravitational kick, and tested its cameras and other instruments. They revealed lightning at Jupiter’s poles, the Tvashtar volcano erupting on the moon Io and signs of a recent impact in Jupiter’s rings – as well as a surprising lack of small moons. Since then, the trip has mostly been spent in hibernation mode. “We’ve been in cruise for so long you get used to the pace: ‘What’s new this month? The spacecraft is OK’,” says Bill McKinnon, a planetary scientist at Washington University in St Louis, Missouri. “Now after all this time, something’s really happening.” That “something” might yet be calamitous. When they were building the spacecraft, scientists expected the Pluto system to be > 13 June 2015 | NewScientist | 29
Destination in sight After a journey of nearly 10 years, the New Horizons probe will fly through the Pluto system in July
NEPTUNE KUIPER BELT January 2006 Launch from Cape Canaveral, Florida SUN EARTH
SATURN URANUS
JUPITER
February 2007 Jupiter fly-by
2007 -2014 Hibernation mode
December 2014 Probe woken up PLUTO
ORBIT OF PLUTO
New Horizons is the fastest solar system probe ever launched, but the record for distance travelled still rests with the earlier Voyager probes Voyager 1
Launched 1977
Voyager 2
1977
New Horizons
2006
19.7 billion km 16.2 billion km 4.8 billion km
relatively hazard-free. Then Pluto’s smaller moons were discovered: Nix and Hydra in 2005, Kerberos in 2011 and Styx in 2012. When another object from the Kuiper belt hits one of them, it could kick up a cloud of shrapnel that would escape the moon’s weak gravity. At the speed New Horizons is travelling, even a small pebble or ice shard could be catastrophic. The latest simulations suggest that the chance of a serious collision is slim, but right now the team are analysing images to look for new moons or rings that might increase the hazard. They have contingency plans to change course and avoid any danger zones, though that would mean a slightly more restricted view of Pluto. While New Horizons analyses dust and ionised gas around Pluto, most eyes will be on
248 years for Pluto to orbit the sun 30 | NewScientist | 13 June 2015
the dwarf planet itself. Already the probe has seen signs of a polar cap and other surface features, and as it closes in two imagers will scan Pluto’s surface in more and more detail. LORRI, equipped with a small telescope, will capture black-and-white images with a resolution of about 70 metres at closest approach; another, called Ralph, will furnish the colour pictures. Ralph can also analyse infrared light, and should reveal the detailed chemical make-up of Pluto’s landscape. From the way Pluto’s brightness changes as it spins, we know that methane, nitrogen and carbon monoxide ices form changing patterns on its surface, but what that landscape looks like remains a mystery. Probably the closest model is Neptune’s giant moon Triton, thought to be an ex-Pluto long ago captured from an independent life in the Kuiper belt. “Triton has very unusual landscapes,” says McKinnon. In 1989 NASA’s outer solar system probe Voyager 2 sent back pictures of Triton’s “cantaloupe terrain”, which looks like nothing in the solar system so much as the skin of a cantaloupe melon. Along with fresh lava flows, Triton’s relatively new terrain is a sign
of geological activity driven from within. “This is a mystery,” says Stern. “We don’t know how to power small worlds like this. Pluto will be a second data point.” McKinnon will be looking for signs that may betray a buried ocean of water. “We don’t have an ocean detector device, we can’t X-ray Pluto, so we’ll be looking for clues from its shape and landscape,” he says. Those include fracture patterns that might indicate the presence of a subsurface reservoir. “The holy grail would be evidence of past eruptions, or active vents,” says McKinnon. Perhaps, like Triton, Pluto has active geysers of nitrogen gas, powered by faint sunlight. It certainly has a tenuous nitrogen atmosphere with pressures a few millionths that of Earth’s. Without a supply of fresh gas the atmosphere would not last long, because under Pluto’s weak gravity it is leaking into space, even enveloping Charon. Nitrogen on the surface must be going direct from the frozen to the gaseous state; winds are then thought to blow from the day to the night side, where the gas refreezes. New Horizons will find out more about the >
Pluto and family PLUTO
HYDRA
1930
1978
2005
2005
2011
2012
Diameter
2370 km
1207 km
57-177 km
42-148 km
13-34 km
10-25 km
Surface gravity
6.7% of Earth’s
2.8% of Earth’s
—
—
—
—
Composition
70% rock, 30% ice
55% rock, 45% ice
—
—
—
—
Orbital period
—
6.3 days
38 days
25 days
32 days
20 days
Mean distance from system centre
—
19,600 km
65,000 km
49,000 km
58,000 km
43,000 km
HYDRA
STYX
EARTH’S MOON
CHARON
STYX
NIX
KERBEROS
CHARON
Discovered
14 July 2015 Pluto closest approach
KERBEROS
PLUTO
PATH OF NEW HORIZONS
2017-2020 Flies further into the Kuiper belt
NIX
PLUTO
Winds
Nitrogen atmosphere Ice DAY SIDE
NIGHT SIDE
Ocean? Rock core
Relative sizes
Dwarf or planet? Pluto’s elongated orbit is now taking it further from the sun. But even as it heads into colder, darker parts of space, the little world may be edging back into a more cosy position – at least in some people’s eyes – as a planet. Whatever that means. In 2006, competing definitions of the term were put to a vote at a meeting of the International Astronomical Union in Prague. The initial proposal included any star-orbiting object large enough that its own gravity has pulled it into a “nearly round” shape. That would have extended our solar system’s pantheon of planets to encompass not only Pluto but also the asteroid Ceres, along with Eris and Makemake in the Kuiper belt and probably dozens more planets-in-waiting beyond Neptune. A rival proposal added the criterion that to be a planet, an object should be massive enough that its gravity has cleared out its orbital neighbourhood of most debris. Pluto fails on this count as it is just part of the swarm of bodies criss-crossing one another
in the Kuiper belt. Under the final agreed definition, therefore, it became classed as a “dwarf planet”. Before the final vote, the astronomer Jocelyn Bell-Burnell had wielded an umbrella to clarify what would, without qualification, count as an planet. The umbrella extended over rocky bodies such as Earth and giants like Jupiter, but left dwarf planets out in the rain. The new definition was and is contentious. Some find it confusing or arbitrary. Others are sentimental about poor little Pluto, or dislike the terminology. “I coined the term ‘dwarf planet’ in 1991,” says Alan Stern, head of the New Horizons mission. “Some members of the public think it’s an insult, so when I use the term ‘small planet’, it is meant to be harder to misinterpret as pejorative.” Last year a debate at Harvard confirmed that the public, at least in the US, is pro planet Pluto. The IAU has no plans to reopen the debate, but perhaps “dwarf planet” might creep back in under the planetary umbrella, in both popular and scientific parlance.
Then dwarfs would be just one subspecies of planet, along with terrestrials, gas giants, ice giants, super-Earths and probably some other exotics we haven’t seen yet. New Horizons may well help to revive Pluto’s planetude. “When people see imagery of the Pluto system, I don’t know what else they will call it,” says Stern. “If you’re watching Star Trek, the moment you see the destination you know instantly if it’s a star or a planet or an asteroid or a comet or an alien spacecraft; you don’t have to do anything sophisticated to make a decision.” On the other hand, over the coming years more and more dwarf/small planets will be identified in the Kuiper belt. It might be natural enough for people to talk about the planet Pluto, the planet Eris and the planet Quaoar; a sterner test of terminology will be whether we blithely talk about the 73 known planets of the solar system (or is it 74 this week?). Usage will show whether people find this planetary proliferation tiresome or exciting. 13 June 2015 | NewScientist | 31
4.8 billion km Current distance of New Horizons from Earth composition of Pluto’s atmosphere using an instrument called Alice to analyse the spectrum of starlight filtering through it. The spacecraft will also send radio signals through the atmosphere and back to Earth, where mission scientists can work out temperature and pressure profiles from the signal distortion.
Wispy though it is, this atmosphere allows some outlandish possibilities. Liquid neon might be stable, suggests hydrologist Jeff Kargel of the University of Arizona in Tucson. Then neon could play the part of water on Earth and methane on Saturn’s moon Titan, and run in rivers across Pluto’s surface. McKinnon is doubtful. “I’m not expecting rivers of liquid neon, but you never know,” he says. “We are going to see something we haven’t seen before, and that will delight us.” His own suggestion is that liquid nitrogen could flow in places under Pluto’s ice, and that a large impact could inject enough heat to melt lakes of nitrogen on the surface, or even cause nitrogen rainfall. So there seems little chance that Pluto will turn out to be a dull, dead world, a disappointing space rock marked only by impact craters. “But I hope to see some craters,” says McKinnon. For one thing, Pluto’s craters could be a key to understanding how planets came into being. Our traditional picture of planet formation starts with small grains that collide and build into boulders, then larger and larger objects.
The New Horizons probe almost didn’t get off the ground
If this is right, the Kuiper belt should hold leftover planetary building blocks of all sizes. But that idea is undermined by Jupiter’s moon Europa. Europa’s surface is only a few tens of millions of years old, having been renewed perhaps by a process akin to plate tectonics. Being so new, it only shows recent comet impacts. There are relatively few small craters, meaning a lack of small comets hitting Europa. “We would expect many times more kilometre-sized things,” says Hal Levison, a colleague of Stern’s at the Southwest Research Institute who specialises in the dynamics of planetary systems. “Maybe those things go
The hunt for Planet X All the clues pointed to something big out there. In the late 19th century, astronomers suspected that anomalies in the orbits of Uranus and Neptune were caused by the gravity of a large, unseen planet. They spent decades, on and off, searching for “Planet X”, to no avail. Then, in 1930, Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona, spotted something. He was checking photographic plates of an area in the constellation Gemini, using a device called a blink comparator to flick between plates taken on separate nights. On 18 February he found a small dot that had moved between two dates in January (see pictures, right). Being so far away, stars are effectively 32 | NewScientist | 13 June 2015
NASA/JHUAPL/SwRI
Rivers of neon
stationary as seen from Earth, so it had to be something within our solar system. Its motion turned out to be too slow for an asteroid whirling around the inner solar system. Instead, the object had to be out beyond the orbit of Neptune, and it looked planet-sized. The dot was named Pluto after the Roman god of the underworld. But Pluto is not the hoped-for Planet X: it is far too small to have the observed effect on Uranus and Neptune. Only in 1992 did new data, including a measurement of Neptune’s mass by Voyager 2, allow those apparent orbital anomalies to vanish. Rumours still persist, but Planet X is probably now fading into myth – having helped us find Pluto.
‘poof’ and disappear when they come into the inner solar system and are heated by the sun. Or maybe they never got created.” Counting craters of different sizes on Pluto gives an idea of how many iceballs are hitting it and how big they are. These scars should show us whether those kilometre-scale building blocks really are missing from the Kuiper belt – and whether we need a new theory of planet formation. One recent idea is called pebble accretion: little pebbles a few centimetres across gather into large groups which then suddenly collapse under gravity. “You go directly to objects 10 to 100 kilometres in size,” says Levison. Even if Pluto’s nitrogen craters have evaporated away, there should be a good cratering record on its outsize moon, Charon. Charon is so large that it and Pluto arguably make up a binary system (see “Charon’s secrets”, above right). Its probable origin in a giant collision could shed light on the violent events that shaped the young solar system once the planets had formed. Roughly 4 billion years ago, it’s thought, the young giant planets moved outwards through a dense disc of debris girdling the sun, in the process hurling comets in all directions. Some came our way, and huge impacts created the moon’s “seas” and scourged Earth in a trauma known as the Late Heavy Bombardment. Most models rely on Neptune and Uranus migrating out like this, but it’s not clear when and how fast, or how Jupiter moved around,
CHARON’s SECRETS plumes spouting from Saturn’s moon Enceladus, “but the only solid surface where we have identified ammonia in the entire solar system is Charon”, says Bill McKinnon of Washington University in St Louis, Missouri. New Horizons will find out whether there is enough to inject some activity into Charon’s landscape, perhaps lubricating geysers. “I want to see if there are flows of semi-solid ammonia water ice, or eruptions,” says McKinnon. Charon probably formed in a similar way to Earth’s moon, when something huge collided with the proto-Pluto and blasted out debris into a surrounding disc. Simulations show that Charon and the other moons, Nix, Hydra, Kerberos and Styx, could have grown from the disc. But in the simulations, Charon moves outwards after it forms and this destabilises the orbits of the small moons, says McKinnon. And the latest Hubble images suggest Kerberos is black as coal, while the other moons are significantly paler, hinting that Kerberos may have a different origin (New Scientist, 6 June, p 16). New Horizons should nail down the
or just what the original debris disc was like. The collision that formed Charon probably happened before all this, so working out exactly how it happened could tell us a little about that disc and how it became today’s Kuiper belt. Researchers had expected the Kuiper belt to be a settled, flattish disc of objects in nice circular orbits. Instead, it is a mess. “It looks like someone took the solar system, picked it up and shook it really hard,” says Levison. Again, the giant planets were probably responsible, but no existing models can match the real tangle of orbits. “Pluto’s formation and evolution is going to
Pluto’s companion, Charon, is almost too large to be a moon
Keck telescope
Pluto is an alien world in almost all respects, but in one way it resembles Earth: it has a large, close companion. In 1978, James Christy at the US Naval Observatory in Washington DC realised that a bump on some images of Pluto wasn’t a defect in observations, as had been assumed, but a giant moon. This was the largest moon to be discovered since 1846, when Neptune’s moon Triton was found a couple of weeks after its planet. As Pluto is god of the underworld, Christy named it Charon, after the ferryman who carries souls to the underworld across the river Styx. Rather than Pluto’s chemical tutti-frutti, Charon’s surface is mainly water ice. But it also has a dash of ammonia, which produces a distinctive dip in its infrared spectrum. Since the 1970s planetary scientists have thought ammonia might act as an antifreeze, enabling chilly moons such as Saturn’s Titan to have subsurface oceans, and explaining features on Jupiter’s moon Europa and elsewhere that look like frozen flows. Ammonia has been found in the atmospheres of giant planets and in the
orbits, sizes and compositions of these moons, and perhaps discover others. “Given that we keep finding these things we’re likely to find more,” says McKinnon. With a full picture of the satellite system, models can be refined. If it turns out that the collision couldn’t have created the small moons after all, then they might instead have been captured from the surrounding Kuiper belt – suggesting that other dwarf planets out there are likely to have multi-moon systems too.
give us hints to what happened in the entire outer planetary system,” says Levison. There’s only one chance to get things right, so just at the moment, the mission team is rather busy. “We are navigating by taking images and analysing them and computing rocket burn simulations,” says Stern. “We are looking for hazards, testing programs that will operate the spacecraft during the encounter, and preparing more than 150 software tools for data analysis. I’m not worried about anything we’ve thought of; I worry about what we haven’t thought of.” A first scan for hazards has come up clear.
discovery of pluto
Lowell Observatory Archives
A strangely mobile blob (arrows) identified in 1930 suggested the existence of a ninth planet
23 january 1930
29 january 1930
-228˚ Celsius Pluto’s surface temperature
And as Pluto gradually comes into sharper focus, new features already suggest a complex surface. Come 14 July, things will happen fast. Just a few minutes after closest approach, New Horizons will turn to scan Charon, then back to Pluto to map its southern polar regions, hidden on the approach. That side of Pluto is currently turned away from the sun, so the only light will be a faint glow reflected from Charon. Then, after this all-too-brief encounter it’s onwards and outwards, probably towards PT1 (“potential target 1”), a Kuiper belt object 1.5 billion kilometres further from the sun, and just a few tens of kilometres across. After that, the spacecraft will join the earlier Pioneer and Voyager probes as part of Earth’s interstellar flotilla, reporting on the state of the solar wind until its plutonium power fades away. But first and last things first. The first world of the Kuiper belt, or the last of the nine planets if you prefer, is coming into view. n See bit.ly/NSpluto for regular updates on Pluto and the New Horizons mission. Stephen Battersby is a science writer based in London 13 June 2015 | NewScientist | 33