Wednesday is a travel day for me, and one with little chance to do any posting here. I’ll leave you, then, with a quotation, and get back to normal posting tomorrow.
Interstellar travel is incredibly difficult, perhaps as difficult to us today as a flight to Mars would have appeared to Christopher Columbus or other would-be transoceanic navigators 500 years ago. Indeed, the ratio of the distance from Earth to Mars compared to Columbus’ voyage from Spain to the Caribbean — 80,000:1 — is roughly the same as the ratio of the distance to Alpha Centauri compared to a trip to Mars. Thus, the key missions required to establish humanity successively as a Type I, Type II, and Type III civilization all stand in similar relation to each other, and if the 500 years since Columbus have sufficed to multiply human capabilities to the point where we now can reach for Mars, so a similar span into the future might be expected to prepare us for the leap to the stars. Actually, it should not take so long, because with its much larger population of inventive minds and better means of communication, the Type II civilization that will spread throughout our solar system over the next several centuries should be able to generate technological progress at a considerably faster rate than was possible by the emerging Type I civilization of our recent past.
I’m all for breakthroughs in physics that will give us capabilities as yet unknown. We may well get them someday. But even without such, methods can already be seen in outline by which currently known physics and greatly developed and refined versions of currently understood engineering can get us to the stars. That development and refinement will occur as part and parcel of the process of maturation of humanity as a Type II species.
Robert Zubrin, Entering Space (2000), pp. 188-189.
Cygnus X-1 is one of the strongest X-ray sources we can detect from Earth and the first widely thought to be a black hole. In fact, when Stephen Hawking bet against X-1 being a black hole back in 1975, he was more or less setting up a hedge, for black holes have been a crucial part of Hawking’s work. Hawking writes about his wager with Kip Thorne entertainingly in the first edition of his A Brief History of Time (1988):
This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse. When we made the bet in 1975, we were 80% certain that Cygnus was a black hole. By now, I would say that we are about 95% certain, but the bet has yet to be settled.
Not long afterwards, the bet would be settled. If you go on to look at Thorne’s Black Holes and Time Warps (1994), you’ll find that Hawking found a way to get into Thorne’s office at Caltech while the latter was overseas, signing the bet he found framed on the wall as a way of indicating he conceded. Thus the bet ended, Thorne received Penthouse and Hawking was out all those issues of Private Eye, although it would not be completely accepted — even by Thorne — that Cygnus X-1 was a black hole until the release of three new papers.
The new work draws on data from a wide variety of instruments. Optical observations of the unseen black hole’s motion around the massive blue companion star it orbits yield the most precise determination of the mass of Cygnus X-1 ever made — the asteroid-sized body is 14.8 times the mass of the Sun, making it one of the most massive stellar black holes in the galaxy. Moreover, data from the Chandra X-ray Observatory, the Rossi X-ray Timing Explorer, and the Advanced Satellite for Cosmology and Astrophysics reveal that the black hole’s event horizon is spinning more than 800 times per second, a spin as fast as any that have been analyzed.
Image: On the left, an optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box. Cygnus X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across. An artist’s illustration on the right depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. The black hole pulls material from a massive, blue companion star toward it. This material forms a disk (shown in red and orange) that rotates around the black hole before falling into it or being redirected away from the black hole in the form of powerful jets. Credit: Optical: DSS; Illustration: NASA/CXC/M.Weiss.
The precise spin and mass findings relied on new estimates of the distance of this object using the National Radio Observatory’s Very Long Baseline Array, which pegged the black hole at 6,070 light years from Earth. The relatively slow motion of Cygnus X-1 through the Milky Way implies, according to this Chandra news release, that the black hole was not produced by a supernova, but may have been the result of a massive star that collapsed without an explosion.
All of this is enough to convince Kip Thorne, who had still entertained doubts:
“For forty years, Cygnus X-1 has been the iconic example of a black hole. However, despite Hawking’s concession, I have never been completely convinced that it really does contain a black hole — until now,” said Thorne. “The data and modeling described in these three papers at last provide a completely definitive description of this binary system.”
All three of the papers on this work are in press at the Astrophysical Journal. The papers are Reid et al., “The Trigonometric Parallax of Cygnus X-1” (preprint); Orosz et al., “The Mass of the Black Hole in Cygnus X-1” (preprint); and Gou et al., “The Extreme Spin of the Black Hole in Cygnus X-1” (preprint).
Kelvin Long and Richard Osborne have seen to it that the British Interplanetary Society’s conference on the highly influential science fiction writer and philosopher Olaf Stapledon has gone off without a hitch. Here is their report from the event, a conference evidently as rife with speculation and far-future musings as anything the author himself ever penned.
by K.F. Long & R. Osborne, Symposium Chairmen
During the summer the British Interplanetary Society (BIS) played host to a symposium on World Ships, possibly the first such dedicated conference ever on these grand, long-term planning concepts. However, the most recent BIS symposium is on a topic that covers eons. There was no one who thought bigger and over longer timescales than the philosopher and writer Olaf Stapledon. Once again, the BIS has organized another first in history. On the 23rd of November members and visitors gathered to discuss the philosophy and literature of Stapledon in the context of today’s current space exploration activities. The session was organized for the purpose of facilitating wider exposure to his ideas and as a way to invite those who may never have heard of him to discover a gem in the literature of space exploration and science fiction.
William Olaf Stapledon was born in Seacombe, Wallasey, on the Wirral Peninsula near Liverpool, England, on the 10th of May 1886. He died on the 6th of September 1950. He spent much of his childhood growing up in Egypt. He obtained a BA Modern History, 1909, from Balliol College, Oxford and a PhD Philosophy University of Liverpool, 1925. His thesis was “A Modern Theory of Ethics”, later the subject of a book. He had worked as a teacher at the Manchester Grammar School and served with the Friends’ Ambulance Unit in France and Belgium during World War I. He was married to Agnes Zena Miller and together they had two children — their daughter Mary Sydney Stapledon and a son, John David Stapledon, whose nephew Jason Shenai attended the BIS symposium, much to the delight of all those present. Imagine having an actual Stapledon in the room. Many of the attendees felt elevated to a higher state of humanity this day. During the lunch Jason was presented with a small gift from the Society by Stephen Baxter, a well known science fiction author who has followed in the tradition of Stapledon and Clarke. Jason said he was really enjoying the day and found the experience moving. He said his entire family were appreciative for the event and the respect shown to his distant relative.
The BIS and Stapledon already have a long history together. On the 9th of October 1948 Stapledon gave a wonderful lecture to members of the society at the invitation of Arthur C.Clarke. It took place at St.Martin’s School, 107 Charing Cross Road, London. The BIS advert read: “In his opening lecture Dr.Stapledon will discuss the profound ethical, philosophical and religious questions which will undoubtedly arise from interplanetary exploration, the possibility of finding intelligent life on other worlds, colonization of planets, interstellar communication, and the possibility of telepathic communication”. Stapledon wrote many books in his life, including Odd John, Sirius, Worlds of Wonder, Darkness and Light. But it is for Last and First Men (1930) and Star Maker (1937) that he is most famous. From these incredible books sprang a range of ideas such as planetary terraforming, genetic engineering, human evolution, transcendence, Dyson Spheres, interplanetary genocide, and the cosmic mind, to name just a few. Arguably the daring works of Stapledon are as important an influence on our culture as the works of William Shakespeare, and yet Stapledon is not very well known throughout the world.
Image (top): Olaf Stapledon. The second image is of Jason Shenai, a Stapledon relative, accepting a gift from the British Interplanetary Society as presented by Stephen Baxter. Credit: Kelvin Long.
Consciousness and Convergence
To discuss his work some Stapledon thinkers came together on this special day. Kelvin Long (co-Chairman) discussed the concept of “universal mentality” and asked if it was at least credible. He pointed to possible physical limitations in the human brain due to the way neurons and axons were ‘wired’ and said this had been foreseen in Stapledon’s literature. Long argued that to become more intelligent we would converge further with technology, Homo Sapiens becoming Homo Electronicus, as Clarke had called it. Long said this would bring about a coupling to the extent that minds could join and the idea of a group mentality or cosmic union would become feasible. He discussed our own self-awareness that we are conscious and indeed aware of each other. He referred to work by the physicist Freeman Dyson who had argued in his book Disturbing the Universe that mind does appear to play a role in reality. This includes the observer dependence in the quantum description of reality and the potential for all our observations being represented by the analogue of a quantum wave function. He discussed the Hawking-Hartle wave function of the Universe. Long also talked about the various cosmic co-incidences in the universe, such as the many physical constants just being right for life, or even intelligent life to form, so-called anthropic reasoning. He ended with a discussion on the laws of physics and in particular the special theory of relativity, which demands the constancy of the speed of light. He said this would place fundamental limitations on any universal mentality or indeed the Star Maker, on how ‘it’ communicates with those that inhabit the universe. He said this law would have to be broken in order for the ‘Supreme Moment of the Cosmos’ (a term from Star Maker) to ever be feasible for all of the inhabitants of the universe simultaneously.
Andy Sawyer had visited from the Science & Science Fiction Library Special Collections and Archive of the University of Liverpool. He spoke about “The Future and Stapledon’s Visions” and quoted from Last and First Men directly: “The romance of the far future, then, is the attempt to see the human race in its cosmic setting, and to mould our hearts to entertain new values”. He talked about many of the books and ideas that had influenced Stapledon’s work in some way, such as The March of Intellect (1829) which depicted fantastic modes of transport such as balloons and steam transport. He referred to George Griffith’s images from The Angel of the Revolution (1893) and Olga Romanoff (1894). Charles Green had even set a major long distance record in a balloon by flying a distance of 480 miles, a record not broken until 1907. These sorts of developments would have found their way into Stapledon’s perspective on the world. Sawyer said that Stapledon showed that the idea of flight was linked to that of change. The culture of the First Men’s 24th-century World-State is based, technologically and spiritually, on aircraft. Sawyer impressed the audience by putting up a time chart that Stapledon had constructed for Last and First Men, complete with colored lines. Later, Andy would talk about the good work being done by the University of Liverpool Science Fiction Foundation, founded in 1971 and supporting 30,000 books and magazines in the fields of science fiction and related genres. The collection includes the Olaf Stapledon Archive and the Eric Frank Russell and John Wyndham Archives.
Patrick Parrinder, a former Professor of English at the School of English & American Literature, University of Reading, discussed “The Earth is My Footstool: Wells, Stapledon, and the Idea of the Post-Human”. Parrinder referred to Stapledon’s early life in Egypt and suggested that his mythical avatar was the Sphinx. His fiction was the portal to the mysteries of cosmic existence, unraveling the Sphinx’s riddle of the transformations of the human animal, and it does this with a Sphinx-like abstraction from domestic emotions and personal relationships. He said the Eighteenth Men whose outlook dominates in Last and First Men and Last Men in London are, we are told, both human and animal in nature, like the old Egyptian deities with animal heads. He said that later H.G.Wells had also taken the Sphinx as his symbol in a substantial work of fiction, The Time Machine, which stands alone among Wells’ novels for its unremittingly bleak view of human destiny. Stapledon apparently claimed not to have read this book when he wrote Last and First Men. The talk covered so much ground and in such a scholarly way it is impossible to do it justice in this brief article and the above is merely a snapshot of what was covered.
Kelvin Long presented a paper on behalf of Greg Matloff, Emeritus Associate Professor and Adjunct Associate Professor, New York City College of Technology. This talk was one of the most fascinating presentations of the day and was on the subject of “Star Consciousness: An Alternative to Dark Matter”. Matloff had looked for an alternative to explain the dark matter problem and proposed the hypothesis that stars may be conscious, as an exercise in philosophical speculation in the spirit of Stapledon’s literature. He pointed to models of stellar radiation pressure and stellar winds which failed to account for the anomalous stellar velocities and instead proposed psychokinetic action, a principle claimed to be now demonstrated in quantum fluctuations. He also pointed towards Parenago’s discontinuity to explain how stars can adjust their galactic velocity. Cool, red stars were said to move around the galaxy faster than hot, blue stars. Molecules were also said to be rare or non-existent in the spectra of hot, blue stars. If stars were ever found to be conscious, this would present a problem for the Search for Extraterrestrial Intelligence (SETI) in terms of how we communicate with them. The presentation ended by saying that Descartes argued in favor of a separation of consciousness from the physical world but possibly, the entire universe may be conscious.
Technology and Paradox
After lunch Richard Osborne (co-Chairman) and a member of the BIS Council, spoke about “Dyson Spheres”. These are hypothesized artificial habitats built around a star by a civilization with sufficiently advanced technology, able to capture as much as possible of the power output of the star. Osborne said the idea had originated in 1927 from J.D.Bernal but Stapledon had included a reference to the concept in his book Star Maker: “Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy.” The physicist Freeman Dyson, after whom the concept was named, worked on the idea in some detail in 1960, in a paper published in Science titled “Search for Artificial Stellar Sources of Infrared Radiation”. Osborne described the various other spin-off concepts that had evolved from the original idea, including Dyson Swarms, Dyson Statite Bubbles and Dyson Shells. Other astroengineering megastructure concepts were described including Matrioshka Brains, Shkadov Thrusters, Klemplerer Rosettes, Alderson Disks and of course Ringworlds, now made famous by Larry Niven’s excellent novel of the same name.
Image: Richard Osborne presenting his work on Dyson Spheres. Credit: Kelvin Long.
Stephen Baxter then took the stage for an interesting discussion on “Where was Everybody? Olaf Stapledon & The Fermi Paradox”. He opened with a quote from Stapledon’s ‘Interplanetary Man’ lecture: “If, by one means or another, man does succeed in communicating with intelligent races in remote worlds, then the right aim will be to enter into mutual understanding and appreciation with them, for mutual enrichment and the further expression of the spirit. One can imagine some sort of cosmical community of worlds”. Baxter said that Stapledon had communicated with both H.G. Wells and Arthur C. Clarke although it doesn’t appear that H.G. Wells had any influence on the work of Stapledon’s two key publications, Last and First Men and Star Maker. He described the Fermi paradox first presented by the Italian born physicist Enrico Fermi in the summer of 1950 and pointed out it is unlikely Stapledon heard of the paradox, as he sadly passed away in September of that same year. Baxter said that the Fermi Paradox had turned out to be a good organizing principle and a great plot generator for science fiction whilst also being a deepening paradox. The Search for Extraterrestrial Intelligence had seen the discovery of exotic biologies on Earth, habitable realms in the solar system, the discovery of many exoplanets, the invention of multiple contact strategies, and yet there had been 50 years of silence. He said that most of the Fermi Paradox solutions tended to fall into one of three categories; ETI is here; ETI exists but has not communicated; ETI does not exist. Baxter said that both Fermi and Stapledon were cosmic thinkers but the visions of Stapledon were still not found to be consistent with the paradox that Fermi had seen that one afternoon in 1950.
Finally, Ian Crawford, a Reader in Planetary Science & Astrobiology from Birkbeck College, London, gave a masterful exposition on “Stapledon’s Interplanetary Man: A ‘Commonwealth of Worlds’ & The Ultimate Purpose of Space Colonization”. Crawford described the three main futures that Stapledon had defined; speedy (self-inflicted) annihilation, creation of a world-wide tyranny (implied stagnation), and the founding of a new kind of world where every body works for the good of the common human enterprise. But he said there are other possibilities Stapledon had not considered, such as the creation of tyrannies that may not result in technological stagnation and may still be compatible with space exploration. He said space exploration can still proceed without the prior creation of social or political utopias and pointed to Project Apollo as an example of how nation state competition had still led to progress in space.
Crawford also said that Stapledon appeared to downplay the economic and scientific motivations for space exploration, yet the former is important for maximizing human well-being and the latter is a key component of human intellectual development. He spoke about the race we appear to be in now, between cosmic fulfillment and cosmic death. A situation echoed by our current dilemma, to become a spacefaring civilization or face stagnation and decay. Crawford made the important point that in thinking about space exploration we had to justify why we want another planet and what we are going to do with it, given that we already have a planet and have not treated the Earth very well. He asked whether before we consider this question, we should consider what man ought to do first with himself. Crawford ended by pointing towards the September 2011 publication of “The Global Exploration Roadmap” by the International Space Exploration Coordination Group and said that if Stapledon were here today he would have approved of this as a sign of positive progress that humanity is starting to work together as a global community in the exploration of space.
Image: Ian Crawford discussing the possibilities in Stapledon’s fictional futures. Credit: Kelvin Long.
Kelvin Long rounded up the day with two quotes that he thought Stapledon would have approved of. The first was from Carl Sagan: “The Universe is not required to be in harmony with human ambitions”. The second was from William Hartmann: “Space exploration must be carried out in a way so as to reduce, not aggravate, tensions in human society”. In a foreword to an Orion Books reissue of Star Maker, science fiction writer Brian Aldiss said of Stapledon: “He is too challenging for comfort. The scientifically minded mistrust the reverence in the work; the religious shrink from the idea of a creator who neither loves nor has need of love from his creations”. It is well known that Arthur C. Clarke was influenced by Stapledon’s Last and First Men and he said: “No other book had a greater influence on my life…[It] and its successor Star Marker are the twin summits of [Stapledon’s] literary career”. Clarke’s work had embraced hard science fiction but with an almost mystical tone to some of his stories. Long speculated that perhaps this is a good place to be for a writer, at the boundary between what we know to be true and what we can only speculate may be possible, the boundary between reality and imagination. As C.S.Lewis once said of Stapledon, he was a corking good writer. Now is the time for others to discover the literature of Olaf Stapledon, and the imagination that sprang from the dreams within his grand philosophy. It is fitting and proper to end an article on Olaf Stapledon by giving him the final word:
“Is it credible that our world should have two futures? I have seen them. Two entirely distinct futures lie before mankind, one dark, one bright; one the defeat of all man’s hopes, the betrayal of all his ideals, the other their hard-won triumph.”
Dr. William Olaf Stapledon, Darkness and the Light (1942)
A full article on the BIS symposium will be submitted to the society’s magazine Spaceflight. The papers from the symposium will appear in a special issue of the Journal of the British Interplanetary Society. Stapledon’s work, especially Star Maker and Last and First Men, remains widely available.
Voyager 2 received commands in early November to switch to the backup set of thrusters that control the roll of the spacecraft. I keep close tabs on the Voyagers because, still operational, they constitute our first interstellar mission, headed beyond the heliosphere and still returning data. Launched in 1977, they’re an obvious example of long-term survival in space, an issue that will become increasingly visible as we plan for longer and deeper missions beyond our Solar System. We got word on November 5 that Voyager 2 has accepted the new commands.
Let’s talk about this first in terms of engineering. Behind the switch is the need to reduce operating power, for using the backup thruster pair that controls roll motion will let engineers turn off the heater that warms the fuel line to the primary thruster, saving about 12 watts of power. With Voyager 2’s power supply providing about 270 watts, finding savings like this can help the spacecraft remain operational. It’s remarkable to consider that the thrusters involved here have fired more than 318,000 times, while the backup pair has not yet been used in flight. Voyager 1 made a similar change in 2004 and is now using all three sets of its backup thrusters.
Sometimes when I read the relatively dry language of the status reports on Voyager my thoughts turn to ancient journeys that once defined out thinking. Pushing deep into the unknown evokes Homer to me, the journey of Odysseus and his crew on a ten year attempt to find their way home while running into all manner of mysteries, but of course there are mythic links to man’s innate urge to explore in many other cultures. Just making such connections seems like a romantic view of hard science, but why not? I just read Athena Andreadis’ short interview in SF Signal in which she talks about the uses of intuition in science, coupled with a ‘type of rigor and dedication usually associated with monastic orders.’ She goes on to liken scientists to wizards and ‘astrogators who never sleep,’ a direct nod to speculative fiction and its influence.
Andreadis knows all about hard science, of course. She’s a researcher in molecular neurobiology as well as being a cross-genre writer of considerable talent. We’re just coming off the Thanksgiving holiday here in the States and with the weekend approaching, I’m in a reflective mood anyway, so what Athena says about science has a fine resonance for me this morning, wrapping itself around the Voyager story and its interplay with the human need for journeying. Later in the interview, Charles Tan asked Andreadis whether the exploration of space was essential to the human future. The answer is a qualified yes, but one that takes into account our frequent over-estimation of our own destiny and the things we are capable of:
Space is inherently hostile to humans. People argue that humans have managed to overrun Earth and hence we can do the same beyond Earth, given advanced enough technology. However, we evolved here and even now, despite our technology, we are helpless before major planetary upheavals. The concept of going beyond our planet has a powerful hold on our imagination, for a good reason: we have a deep-rooted urge to explore, which is both a blessing and a curse. The challenges of crewed space expeditions are mind-boggling.
How true, and how often understated! But Andreadis believes in the attempt as part of that same urge for exploration that has seen ships embarking for ports unknown throughout our history:
Even so, I think it is indeed essential that we take to space at some point. Not for fortune or glory, but because we yearn for the next horizon. At the same time, we need to be deeply aware that we can never “conquer” space. The self-serving inanities of the Strong Anthropic Principle aside, triumphalism will avail us naught in a universe that is supremely indifferent to us and our aspirations.
In the poem ‘Mid-Journey,’ Andreadis writes in a way that calls up Homeric venturing and echoes (for me at least) Tennyson’s own Homeric reflections on getting older in ‘Ulysses’:
How plucked and gutted is our bright youth!
The gates of heaven stood open back then.
Now, fatigue and demons track our trail.
Within us and behind us, blood and darkness
And for those who loved us, ruins and flames.
Warmth and comfort are yokes for us.
We chose thorns, shoals and starlight.
We vowed ourselves irrevocably to battle.
We will die exiles, mercenaries to strangers,
Having seen and dreamed imperishable beauty.
You can hear the poem read aloud here. Stephen Pyne works nicely with the mythic nature of our spacecraft in his Voyager: Seeking Newer Worlds in the Third Great Age of Discovery (Viking, 2010), mindful of the need to relate what we do with science to the great themes of exploration as they have played themselves out in fact and in myth throughout history. We do well to remind ourselves, as Athena does, of both the rigor of science and the informed intuition that breeds the magic of discovery. I think about both, and about long voyages on wine-dark seas, when I imagine our Voyagers, still alive, being prepared for still deeper wanderings.
Our notions of habitability are built around environments like our own, which is why the search for planets with temperatures that support liquid water at the surface is such a lively enterprise. But as we saw yesterday, it is not beyond possibility that many places in our Solar System could have sub-surface oceans, even remote objects in the Kuiper Belt. And that raises the question of how we assess astrobiological environments, an issue studied by Dirk Schulze-Makuch (Washington State University) and Abel Mendez (University of Puerto Rico at Arecibo), working with an international team of researchers in a paper suggesting a new approach.
Schulze-Makuch makes the situation clear:
“Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star. For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”
To avoid overlooking potentially habitable worlds as we discover more and more exoplanets, the authors propose two indices that help to provide a quantitative look at a given exoplanet’s chances for habitability. The first is an Earth Similarity Index that screens exoplanets in relation to all the factors that make our planet hospitable to life. The second is a Planetary Habitability Index, which describes chemical and physical parameters that may allow life to exist under conditions that vary markedly from Earth. Think Enceladus, or Europa. Think the exomoon of a gas giant. Think, in other words, as speculatively as possible.
The Planetary Habitability Index is based on ‘the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent,’ as the paper’s abstract notes. But it’s also based upon hypotheses about life’s viability in extreme environments that we’re as yet unable to test. Acknowledging this, the authors see their index as an ongoing work that can be updated as technology and knowledge about astrobiology advances. Interestingly enough, they apply their metrics to the provocative Gliese 581 system, finding that both GJ 581c and GJ 581d show an Earth Similarity Index comparable to that of Mars, and a Planetary Habitability Index somewhere between that of Europa and Enceladus.
Future space instrumentation should be able to tell us whether an Earth-class planet shows the signature of life, but how do we use those instruments to size up an icy exomoon when we can’t make the call on far closer worlds like Europa? What will change the game is finding proof in our own Solar System that life can occur in just this kind of extreme environment. Such a demonstration would make the Planetary Habitability Index far more interesting — and accurate — telling us that life can adapt to places utterly unlike our own planet. Until that occurs, constructing the PHI seems like an intriguing but premature exercise.
The paper is Davila et al., “A Two-Tiered Approach to Assessing the Habitability of Exoplanets,” accepted by Astrobiology (abstract).