When Pluto was discovered in 1930, most astronomers believed there was life on Mars and perhaps on Venus. But Pluto was so cold that no-one seriously believed it was inhabited. One of the few astronomers who seriously entertained the idea was George Van Biesbroeck of Yerkes Observatory, who speculated “[If] there is a form of life on the new planet we can be sure it is totally different from that on the earth.”
Fiction writers, however, had been given a whole new world to play in, and a steady stream of Plutonians appeared in fiction. The first appearance of Pluto as an inhabited planet was in H P Lovecraft's The Whisperer in Darkness, written in September 1930 though not published until the next year. However, the creatures who lived on Pluto were not born there - Pluto was far too parochial for Lovecraft's monsters, and was just a stopping-off point : "Their main immediate abode is a still undiscovered and almost lightless planet at the very edge of our Solar System—beyond Neptune, and the ninth in distance from the sun. It is, as we have inferred, the object mystically hinted at as 'Yuggoth' in certain ancient and forbidden writings; and it will soon be the scene of a strange focussing of thought upon our world in an effort to facilitate mental rapport. I would not be surprised if astronomers became sufficiently sensitive to these thought-currents to discover Yuggoth when the Outer Ones wish them to do so." The creatures call themselves the Mi-Go, and are "a sort of huge, light-red crab with many pairs of legs and with two great bat-like wings in the middle of the back."
True Plutonians were introduced in 1931 by Stanton A Coblentz in his novelette Into Plutonian Depths. It is the first story to be set on Pluto, but there's nothing much to distinguish the setting from Earth; though dim, chilly and bleak, the air is breathable, the gravity is the same as Earth's and thick furs are all that is required to keep the two visitors from Earth comfortable. They reach Pluto by coating their vessel with a substance that cuts off gravity - the very same means that H G Wells used to get his visitors to their otherworldly destination in his 1901 novel The First Men in the Moon. In both books the travellers comprise a brilliant scientist-inventor accompanied by a man-in-the-street companion. In both, too, the inhabitants of the world they visit are frail and spindly underground dwellers (which makes good sense in a low-gravity lunar setting, but not on Coblentz's Pluto, with its Earth-normal gravity).
The Plutonians are equipped with natural lanterns on their heads which have been evolved to illuminate the dark tunnels they inhabit (a problem Wells solved by means of a luminous liquid flowing round a system of underground channels). The lamps also glow with different colours according to the emotions of the Plutonians. There are three genders on Pluto: male, female and neuter. The neuters are surgically produced and held in great esteem because only they have the necessary freedom from sexual impulses to become great scientists or poets.
Stanley Weinbaum's The Red Peri is the first story set on Pluto to take into account known conditions there - such as they were in 1935, when the story was written. All that was certain about Pluto were the size and shape of its orbit and its dimness - which meant that it could not possibly be a large, brightly reflective world like its neighbour Neptune. If it was as reflective as the other planets, it must be tiny (which turned out to be correct). If it were as large as Neptune - or even as the Earth - it must be a very dark world. This is what Weinbaum assumed: a coal-black planet, somewhat bigger than our own, with a gravitational pull on its surface about 20% higher than on Earth. And cold - as cold as anything that far from the Sun must be. Weinbaum gives a temperature of 10 "degrees absolute" (i.e., 10 kelvin), a temperature at which almost all gases would be frozen. Since helium would not be, Weinbaum assumes a thin atmosphere made of this gas. In fact, Pluto is slightly more hospitable than this, with a mean temperature of 44 kelvin, though with a much thinner atmosphere than Weinbaum guessed at (a maximum of 0.008 millimetres of mercury compared to his value of 5 millimetres - both so thin as to be practically un-noticeable to someone from Earth, where the sea-level air pressure is around 760 mm).
Designing an aggressive alien life form for such an unpromising environment must have been a challenge for Weinbaum, which he met by cleverly introducing a crystalline something on the borderland of the biological and the purely chemical, like a giant version of a virus. There are many kinds of these "crawlers", each with a particular kind of food, including sulphur, iron, and aluminium. Black crawlers eat carbon - and therefore human flesh. The crawlers make a distinctive crackling rustling sound as they move and, if stepped on, flash with blue sparks.
Although no-one's found any giant viruses yet, by a weird coincidence of names, plutonium does behave a little like a black crawler: if provided with oxygen, it grows larger and crackles and sparks as parts of it catch fire. And it's lethal, too.
Frank R. Paul, one of the greatest science-fiction artists of the 1930s and 1940s, painted a series of back covers for Fantastic Adventures pulp magazine. In February 1940, Pluto was the setting, occupied by creatures that were half-human, half-bat. According to the accompanying text, while these Plutonians might be highly intelligent, they might also be mad cannibals, attacking any visitors who approach the machines that provide them with heat and water from deep inside Pluto.
This last detail is spot-on : if there ever are any colonists on Pluto, they will have limitless supplies of both water and heat from underground.
E E "Doc" Smith's book First Lensman (1950) features a brilliantly weird alien on Pluto, with a constantly shifting appearance "now spiny, now tentacular, now scaly, now covered with peculiarly repellent feather-like fronds, each oozing a crimson slime." The alien actually originates on the extrasolar planet Palain Seven, which is as cold as Pluto. Palainians can live only on such cold planets, and are (at least) four-dimensional. They are also telepathic, and this particular alien, most unusually for the time the book was written, is female.
Larry Niven's 1968 short story "Wait it out", includes a giant land-dwelling amoeba. Unfortunately, we don't find anything much out about it and it is only briefly glimpsed.
The brilliant Robert Silverberg was the first author to introduce an alien with a fully thought-out physiology. In his 1978 novel World's Fair 1992, human explorers encounter a crab-like dweller on the shores of Pluto's methane seas, based on carbon chemistry, electrical energy, superconducting nerves and superfluid-filled veins. The same creatures appear in Silverberg's short story "Sunrise on Pluto" (1984).
(Superconductivity is a phenomenon which occurs in many materials when they become sufficiently cold. All resistance to the flow of electricity ceases, and electrical currents flow endlessly. Superfluidity is a rarer condition, in which extremely cold liquids flow, spin or slosh endlessly. Actually, even Pluto is far too hot for any material to be either superconductive or superfluid, with one exception : hydrogen sulphide. However, this only becomes a superconductor at extremely high pressures).
Gregory Benford's 1990 novel Sunborn is the first to be told (partly) from the point of view of creatures who live on Pluto. The Zand are intelligent walrus-like creatures who lead a precarious existence in the frigid marshes of Pluto, their lives focussed on obtaining enough warmth to survive the long Plutonian nights. There is a great deal more to their story than that, but I don't want to spoil a book well worth reading.
The latest story of Plutonian life (as far as I know) is Stephen Baxter's "Gossamer" (1995), which sounds like a fairy tale in summary : a cobweb spun between Pluto and its enormous moon Charon. But it's based on an actual feature unique in the Solar System : there really is a place on Pluto where, if you looked straight up, you would see a particular location on Charon. And you would always see that same spot, whatever time of day or day of the year you looked. The Sun and stars and planets and Pluto's other four moons would spin and wheel around you, but that one point would remain fixed - so, a rope ladder could join Pluto and Charon. Or a cobweb. We never meet the web-spinners, but their nests of icy eggs are found under Pluto's frosty surface.
So what about the real Pluto? Is life there possible?
Now that the New Horizons spacecraft has visited the dwarf planet, we know that the answer is yes.
As far as we know there are just four requirements for life to evolve from chemicals :
carbon, a source of energy, liquid water and a safe environment.
There is certainly plenty of carbon on Pluto : we have known that carbon dioxide is present on Pluto since 1992, when the first precise infrared observations were made. Carbon monoxide was found in 2011. Life on Earth is based on organic molecules, which contain hydrogen as well as carbon and oxygen, and these are also plentiful on Pluto. Ethane, the simplest organic chemical, was detected in 1999. This year New Horizons added acetylene and ethylene to this list.
Energy? The greatest single discovery made by New Horizons is that Pluto contains a rich source, originating deep below the surface. In other worlds, such internal heating is common, the result either of lingering heat from formation, the tidal effects of orbiting bodies, the decay of radioactive materials, or collision with meteorites or larger objects. None of these can fully account for Pluto's sub-surface heat, but the recent discovery of ice volcanoes show that there is (or was) at least enough underground power to melt parts of the underground ice layer, providing the third requirement for life. It may be that there is a whole ocean under Pluto's ice crust, as is known to exist on at least two moons in the Solar System (Enceladus and Europa).
Whether Plutonian water is to be found in isolated pockets or global oceans, such environments are safe ones, in that they are fully shielded from the solar ultraviolet radiation that illuminates Pluto each day.
Could life really evolve in Pluto's sunless depths? Possibly : on Earth, there are colonies of living creatures close to underwater volcanic "springs" called black smokers which need only the warmth and organic chemicals to survive, and the theory that all life on Earth evolved from another kind of deep-ocean spring is as well-regarded as any other.
So, life is possible - but do we have any direct evidence that it exists? The answer to that is "not yet." Methane is produced by all sorts of living things, from cows to cowries and from humans to humming birds (weirdly enough, the majority of the methane produced naturally on Earth comes from termites). And there is a great deal - billions of tonnes, probably - of frozen methane on Pluto. The reason that there have been no headlines screaming LIFE ON PLUTO! is that methane can be produced in many other ways too, without the involvement of living things. From a given sample of methane, one can't tell how it was made. But there is one key difference : when methane is produced by chemical reactions, almost invariably a lot of other organic chemicals form as by-products. You and I, on the other hand, while being very efficient methane factories, are not really in the business of making much else in the way of simple organic chemicals. So, if it turns out that Pluto is a smorgasbord of organics, the likelihood is that chemistry is the source. But if there's little but methane, a biological source would be a distinct possibility. The answer may lie in the reams of data making their sluggish way across the Solar System from New Horizons. It will be almost a year before all of it reaches Earth, and probably months more before the process of chemical auditing is complete. Then we might just see those headlines after all...
So, one more question : if there really are Plutonians, what might they be like? It depends mainly on how long they have been there. On Earth, all living things were single-celled for over three billion years. We don't know if that is typical but it would be risky to assume that complex creatures could evolve much faster. So, assuming Pluto does have a sub-surface ocean, has it been there long enough for life (if it developed there at all) to evolve beyond the simplest structures?
This takes us back to the reason for the ocean being there at all: Pluto's mysterious internal heat source. In the absence of evidence of any recent cause, the best guess is that, whatever that source is, it has been there since Pluto's earliest ages - perhaps four billion years ago. Plenty of time, therefore, for evolution to run its course.
And the outcome of that evolution? We can only guess, but we do know that the living things in the coldest of our own seas are very slow, very old - and very large.
If you'd like to follow this story over the coming months, keep an eye on http://newhorizonspluto.weebly.com/
And if You'd like to buy Dr Mike Goldsmith's book on Pluto, check here, where I linked it in the first part of this blog: