Its career as a planet last for less than half its year. Pluto was discovered late, in 1930, as our final planet, completing the Sun’s brood of nine. (In hindsight it had been seen, but not recognized, as early as 1909.) But it was always an odd one, the runt of the litter, banished to the far flung regions of the Solar System. By 2006 the International Astronomical Union had seen enough. It instigated a new class of objects, intermediate between planets and asteroids (or minor planets), and re-assigned Pluto to be the primary member of this class, called dwarf planets. There were good reasons to do so: Pluto was just too small, on too strange an orbit, and worst of all, similar objects were being discovered. If Pluto was a planet, perhaps as many as 50 other objects out there could claim planethood too. NASA astronomers objected vehemently. NASA had just launched a mission to Pluto, called New Horizons. US Congress would not be well pleased if its money, awarded to investigate the last unexplored planet of the Solar System, would end up at a dwarf planet instead. The objections were overruled by a convincing vote. 76 years after being discovered and named, having travelled only a third of its 248-years orbit around the Sun, Pluto’s brief summer as a planet came to an end.
Still, this newly-named dwarf planet, frozen beyond belief, is a fascinating object. It has everything: weather, enormous climate change, active geology and volcanoes, a large heart on its surface, a family of satellites, and a mysterious past. Maybe being a dwarf planet should be a badge of honour, worthy of every penny spent by NASA.
An interesting aside, the name Pluto was first suggested by Venetia Burney, 11 years old at the time and living in Oxford. Of course we don’t know who named the original planets but perhaps this was also the work of children. Children have curiosity and want to push their horizons. Space was made for them.
This post will present our knowledge of Pluto, and its place in the Solar System, prior to the New Horizons encounter. Other (later) posts will be about New Horizons itself, and what we learned from the fly-by.
Children of the Sun
Two decades ago, when everything was simpler, the Solar System had only two types of planets: the rocky or terrestrial planets (Mercury, Venus, Earth and Mars) and the gas giants (Jupiter, Saturn, Uranus, Neptune). Pluto as a solid body was classed with the rocky planets.
The four rocky planets all have iron cores, underneath a silicate mantle. They have only small amounts of the so-called volatiles: water, methane, CO2. There is a reason for this deficit. The planets formed from solid particles which came together. Each mineral has a temperature below which it can be a solid (there is no liquid phase in vacuum). For iron and silicates, this condensation happens at when temperatures drop below a balmy 800-1000 Kelvin, but water, methane and CO2 freeze out only below a chilly 200 K. The rocky planets formed at a distance from the young Sun where the temperatures were in the range 300 to 600 Kelvin. So water, and other volatiles, were in a gas phase and absent from the solid particles which made up these planets.
The 200-K temperature was reached somewhere within the asteroid belt. This is called the snow line. Further out, water became solid. Any ‘rocky’ planets forming out here would contain major amounts of water. In reality, the boundary wasn’t as sharp as the name ‘snow line’ implies, and the fraction of water in planets slowly increases further out, being very low for Earth, larger for Mars, and larger still for many asteroids.
The four gas giants are nowadays divided into two groups. Jupiter and Saturn, the monsters of the Solar System, are true gas giants. Uranus and Neptune, rather smaller but still 15 times heavier than the Earth, are considered water giants. They contain huge oceans in their mantles, above a rocky, Earth-like core.
The moons of the large gas giants, being outside the snow line, also are a mixture of rock and ice, with more ice than found in a typical asteroid. In all these objects, ‘ice’ is mainly water ice but it also contains ammonia, CO2, and methane.
How does Pluto compared to the rest of the family? If you like your information in numerical form, a Pluto factsheet is maintained by NASA. But the facts need interpreting.
Pluto’s mass may seem impressive, but it is positively minute compared to the planets: it is a midget, only 0.2% of the Earth, and 20 times less than the smallest planet, Mercury. Embarrassingly, seven moons in the Solar System are larger than Pluto. The orbit is also strange for a planet, very elliptical, with the distance to the Sun varying by 50%. For comparison, for Earth it varies only by 3.5%, and for Mars (the most elliptical orbit among the planets) the variation is 18%. Pluto is in a different league. The orbit is also well outside the plane of the ecliptic where all other planets are found. It really is an outcast.
Pluto is just about large enough that during formation, the interior would have melted from the heat of the colliding fragments. The melting will have allowed the denser silicates to sink down, and Pluto is thus expected to have a rocky core, surrounded by a water mantle. There is also an atmosphere, mainly consisting of nitrogen, with a pressure of 10 microbar, comparable to the Earth’s atmosphere 100 kilometer above the surface. Temperatures at Pluto are around 40 Kelvin, or -243 C. (If you live in one of the seven countries following the Polish-born Daniel Fahrenheit, this is -387 F.)
Pluto’s giant moon is called Charon. ‘Giant’ is relative, but Charon is large enough that some consider it a dwarf planet in its own right. Whereas Pluto has a diameter of 2370 km, Charon is 1200 km across. (For comparison, our Moon is 3475 kilometer across.) Four more moons were discovered in recent years. They are very much smaller, and a bit further from Pluto than Charon. Interestingly, the orbits are in resonance: the orbital periods scale approximately as 1:3:4:5:6, which is the only way such closely spaced moons can stay in stable orbits. The four large moons of Jupiter have the same kind of resonance.
Charon takes 6.4 days to orbit Pluto (that is the ‘1’ in the 1:3:4.. sequence above). Pluto’s own rotation period is also 6.4 days, as is Charon’s day. In other words, they are tidally locked, both always showing the same face to each other, just like our Moon does to us (but the Earth does not to the Moon).
How did this complex system form? It seems unlikely that a dwarf planet so far out in the Solar System can pick up 5 satellites! There are too few free objects around there for such multi-dating. The circular orbits of the moons suggest that they formed in situ, around Pluto, and were not captured from abroad. The large size of Charon is most easily explained as caused by a major impact during the formation of Pluto, which effectively split proto-Pluto in two. This impact would have been even more devastating (relatively speaking) than the one that formed our Moon. The other moons could have formed from debris from this impact. But could such a major impact have happened in such an empty region of the Solar System? Isn’t it a bit like a world-class collision in a road-less region in the desert heart of Australia, traversed by one vehicle per month?
A clue comes from the densities of Pluto and Charon. These are around 1800 kg/m3, midway between ice and rock, meaning that both are very roughly equal parts ice and rock. This is very similar to Titan and Enceladus, moons of Saturn. The moons of Jupiter, in contrast, are denser and have more rock than ice. Remember that the further from the Sun a planet or moon formed, the higher fraction of ice it contains (Earth has essentially none). Pluto will thus have formed somewhere in the general region of Saturn, which is much closer to the Sun than it is now. During a chaotic phase in the early Solar System, a lot of smaller bodies were thrown out of this region by encounters with Saturn and Neptune. It explains the strange orbit of Pluto. The collision that formed Charon could also have happened during this period of chaos.
Long before New Horizons was launched, we knew that Pluto has an interesting surface. Its brightness changed, such that the dwarf was considerably brighter when the side facing Charon was pointed towards us. There was a more reflective material on that side, and the most likely culprit was ice; nitrogen ice, to be precise, which is white at very low temperature, around 40 K. (When ‘warmed’ to 60 K, it becomes transparent, and at 63 K it evaporates.)
The Hubble Space Telescope had a go at mapping tiny Pluto. It found a particular bright region, neighbouring a very dark area. The dwarf planet also appeared to be changing: over a decade, the northern (darker) hemisphere was brightening, and the southern hemisphere fading. Over this time, the density of the puny atmosphere had doubled. Pluto was showing seasons!
We now know that the bright region is the same as the ‘heart’ found by New Horizons. Discoveries often have history.
Seasons of the heart
Pluto’s seasons are funny. Of course, this far from the Sun’s warmth, the seasons change between winter and ice age; you would not expect to find anything resembling summer!
On Earth the seasons are determined purely by the tilt of the polar axis. The angle between the orbit around the Sun and the Earth’s rotation is 23 degrees: this tilt causes the varying length of day, and the varying height of the Sun above the horizon. Within 23 degrees of the equator, the Sun can get to the zenith: these are the tropics. Within 23 degrees of the pole, there are times when the Sun doesn’t set or doesn’t rise, depending on the season: these are the polar regions.
Pluto’s inclination is more than double that of Earth, at 57 degrees. This makes the seasons much more extreme. (The tilt is actually 123 degrees: the planet rotates upside-down and the Sun rises in the West. But the effect is the same as a tilt of 57 degrees.) As the dwarf moves around the Sun, at a certain point in its 248-year orbit the Sun will be above the equator: this is the equinox when day and night are the same length, everywhere on Pluto. A quarter orbit along, the Sun is only 33 degrees from the north pole of Pluto: the southern quarter of the planet is now in perpetual darkness and the northern quarter in perpetual sunlight. Than the Sun moves back to the equator, and finally to close to the south pole, and the polar day and night are reversed.
On Pluto, the ‘tropics’ (this name could be seen as optimistic) and the polar regions overlap: there is a region at intermediate latitudes where during the summer the Sun gets to zenith, therefore it counts as the tropics, but this Sun never sets, and half an orbit later the same region has a perpetual polar night; so it is both tropical and polar. At the moment, the Sun is not far from the equator and all of Pluto gets some sunlight. Spring has begun in the North, and the South is heading towards its century-long winter night. Pluto’s climate was designed for Sleeping Beauty.
A second problem is that the distance to the Sun changes so much during the Pluto year. Closest approach occurs during the spring equinox in the North (or autumn equinox in the South). The northern summer and southern winter happen when Pluto is far from the Sun. The South therefore has more extreme temperatures – a colder winter and warmer summer. The South has climate and the North has weather. Also, note that Pluto moves much slower when further from the Sun, so that the seasons are of very unequal length.
With Pluto just coming out of the northern winter, the expectation was that at the moment there is a nitrogen ice cap in the north, in the process of evaporating, while the southern ice cap has not yet started to reform. The atmosphere is therefore denser than usual. As Pluto moves away from the Sun, much of it will condense as frost and ice, on the southern hemisphere. The ice cap resembles an anti-swallow, migrating from winter to winter.
Is there life on Pluto?
There are four groups of objects in the outer Solar System similar to Pluto, ranging from puny to Pluto-sized. Over a thousand objects are known, several of which are large enough to be dwarf planets. Together they are called the Kuiper belt.
The first, and largest, group contains the ‘classical objects’: they have mildly elliptical orbits, around 42 to 45 AU (1 AU is the distance Earth-Sun; Pluto is at 39 AU), and most are close to the plane of the ecliptic where the planets are found.
The second group have the same orbital period as Pluto but very elliptical orbits (each one different) and a variety of inclinations to the plane of the ecliptic. These are called the plutinos. Their orbits are in a 2:3 resonance to Neptune, meaning that for every three orbits of Neptune, they do exactly 2. Many cross the orbit of Neptune (as does Pluto), but because of this resonance they never come close or collide. There probably used to be more, but any that had different orbital periods were slowly removed by Neptune. Gravity is a bastard: it can be slow and weak, but it always wins.
The third group is the ‘scattered disk’: these objects are much further out and their orbits are highly elliptical. The dwarf planet Eris, almost as big as Pluto, is a member of this group. They have clearly been thrown out here after coming too close to one of the giant planets, probably Jupiter, and coming off worst. Now they are the cricket/baseball out-fielders: hanging around, not doing much, waiting to catch the odd ball coming their way.
Finally, a few of the moons of the giant planets are similar to Pluto, including Phoebe (orbiting Saturn) and Triton (orbiting Neptune).
Triton is in some ways the other Pluto. It orbits Neptune in the opposite direction to Neptune’s rotation. That indicates it hasn’t always been there but was captured by Neptune at some time. Triton is a bit larger than Pluto and has a little bit higher density: a bit more rock and a bit less ice; thus, it probably formed closer to the Sun, perhaps near Jupiter. Like Pluto, Triton has a nitrogen atmosphere. Triton was photographed during the Voyager encounter, in 1989. It showed a surface that is a mixture of dirt and nitrogen ice: the lack of impact craters suggests that the ice is quite young (or re-formed regularly).
Most excitingly, Voyagers found geysers on the surface, ejecting gas kilometers high. These were only seen where the sun was directly above Triton, in the zenith. The geysers are probably from nitrogen: phreatic nitrogen volcanoes. Nitro-thermal activity, if you prefer