Category Archives: Astronomy

Ice Worlds News: More Enceladus Ocean Chemistry

In the last two decades, humans have finally observed some of the most interesting places in our solar system—the ice worlds.   The Galileo spacecraft visited Jupiter and it’s moons, including Europa .  The Cassini spacecraft visited Saturn and its moons, including Titan and Enceladus.

These moons are as large as small planets and due to tidal forces from orbiting their giant planets, are likely to have liquid oceans with as much water (or liquid methane) as Earth’s oceans.

Titan has an atmosphere, with clouds, and possibly rivers and lakes.  Europa has a huge ocean of water under an icy crust.  Enceladus, too, seems to have a liquid ocean under ice.

Energy + liquid?  Sounds like life could happen.


Of course, the data from recent missions is still being analyzed, so even before the next visit we can learn more.

This summer an international team reported more findings from the Cassini spacecraft which measured particles as it flew near Enceladus (more than ten years ago now). [2]. Enceladus has a rocky core, a large ocean, and a thin ice crust.  Cracks in the crust emit water from below, and one ‘volcano’ is spewing vapor and ice crystals out into space.  Cassini flew through this plume and observed some of the particles.

Initial study identified the plume as mainly gasses and ice particles.   The new study extends the results, demonstrating that many of the observations appear to be ice crystals with traces of complex organic molecules embedded.  Given the probably origin of the plume, this strongly suggests that the ocean has a soup of organic compounds.  As the BBC confusingly put it, “a step closer to hosting life“.

The paper discusses the complexities of low temperature, low pressure ice, and argue that the hypothesized ‘dirty ice’ could only form from complex processes.  They offer a scenario involving a thin film of organics, which bubble up through a crack, becoming coated with ice, which is then ejected.  I didn’t follow all of the argument here, but there is an important point:  it is a mistake to assume that organic chemistry works in familiar ways in such a cold place.  Energy + water + complex chemistry does not mean “just like my back yard”.

Revisiting these ice worlds has become a top priority, at least for actual scientists (if not necessarily for funding agencies). If and when we visit them, we should find one of three possibilities:

  1. there is no sign of life, even though the environment likely could support life. This may tell us something about the probability of life emerging in the universe.
  2. there is recognizable life, and it is related to Earth. If we find some variation of DNA/RNA or whatever, that will open the questions of how a common ancestor got to two different places in the solar system.  Got Panspermia?
  3. there is recognizable life, but it is clearly not related to Earth. This “second example” will tell us something about what “life” is, and how it emerges.

(There is a fourth possibility:  there might be something so different we need new concepts. There may even be life, but we won’t recognize it, or may disagree about it.)

Any and all of these outcomes will be breathtaking!  We have to go there!

Editorial aside:  So, why are people wasting money on space tourism and suicide missions to Mars, when the most exciting discovery in the history of science is sitting right there, if we can just get our act together.


  1. Mary Halton, Saturn moon a step closer to hosting life, in BBC News Science & Environment. 2018. https://www.bbc.com/news/science-environment-44630121
  2. Frank Postberg, Nozair Khawaja, Bernd Abel, Gael Choblet, Christopher R. Glein, Murthy S. Gudipati, Bryana L. Henderson, Hsiang-Wen Hsu, Sascha Kempf, Fabian Klenner, Georg Moragas-Klostermeyer, Brian Magee, Lenz Nölle, Mark Perry, René Reviol, Jürgen Schmidt, Ralf Srama, Ferdinand Stolz, Gabriel Tobie, Mario Trieloff, and J. Hunter Waite, Macromolecular organic compounds from the depths of Enceladus. Nature, 558 (7711):564-568, 2018/06/01 2018. https://doi.org/10.1038/s41586-018-0246-4

 

Ice Worlds, Ho!

The Water Plumes Of Europa

Europa is interesting.  Very interesting.

It is one of at least seven “ocean worlds” in our solar system.

Orbiting Jupiter, Europa is warped and headed by the tides generated by Jupiter’s gravitation. This means that there is an ocean of liquid water, possibly larger than all of Earth’s oceans.

This ocean is covered by ice. The ice shell has not been conclusively measured, but it seems to be thin (10s of KM), and full of cracks through which tides drive ligquid water to the surface.

This active, water rich geology appears to have all the prerequisites for life. Water, chemicals, energy, tidal action, gnarly geometry.  These are the signatures of fertile habitats on Earth.  Is there life under the ice?  I, for one, want to know.

To date, we have limited direct measurements of Europa.  Hubble and ground based telescopes have imaged it from afar. In the first wave of exploration, the Voyagers flew by, and the Gallileo probe obtained the best images so far of Europa, with old and only partly working technology [2].

It is obvious why Europa is a high priority for additional visits with much better instrumentation and eventually, landers.

In the mean time, we can pour over the data we have.

This month researchers report an analysis of data from the Galileo mission from 1997.  The data is measurements of the magnetosphere and plasma that the spacecraft encountered as it passed close to Europa (200 KM).  This data features a transient event, which was difficult to interpret.

The new study suggests that the measurements are related to Europa, and, in fact, represent traces of a volcanic plume of water spewed into space.

This study developed a detailed computational simulation of the plasma and ions near Europa.  Introducing a plume consistent with the telescope observations, and showed that the model closely aligns with the hitherto unexplained measurements from Galileo.  They conclude that the spacecraft did, in fact, fly through the traces of a plume of water from inside Europa.

In itself, this result isn’t too surprising.  It does support hypotheses that Europa not only has an ocean and an active icy crust, but like Enceladus, Titan, and other moons, is volcanically active.

Where there are volcanoes, there is the possibility of life as we know it.

We must go to Europa.

  1. Kenneth Chang, Europa, Jupiter’s Ocean Moon, May Shoot Plumes of Water Into Space, in The New York Times. 2018: New York. https://www.nytimes.com/2018/05/14/science/europa-plumes-water.html
  2. Richard J. Greenberg, \Unmasking Europa: the search for life on Jupiter’s ocean moon. 2008, Copernicus Books: New York.
  3. Xianzhe Jia, Margaret G. Kivelson, Krishan K. Khurana, and William S. Kurth, Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures. Nature Astronomy, 2018/05/14 2018. https://doi.org/10.1038/s41550-018-0450-z

 

New LIGO Data Analysis Technique

For many decades, one of the big questions for astrophysics has been how to confirm the behavior of gravity waves. Theoretically, they must exist, but they are awfully hard to detect amid all the “noise” of the rest of the universe going about everyday business.

In the last few years the Laser Interferometer Gravitational-wave Observatory (LIGO) project has successfully detected gravitational waves for the first time.  This heroic effort involves careful extraction of the interesting signal from a mass of noise.  Details are available in published papers and the data and code are available at the LIGO Open Science Center.


The basic goal of this software is to find needles in a messy haystack, which is actually a pretty generic task with lots of possible techniques that might work. Not surprisingly, other approaches have been applied.

The project has employed trendy “citizen science” techniques crowd sourcing “Gravity Spy” (via Zooniverse, naturally), and a variant of SETI@HOME (EINSTEIN@HOME).  The latter searches through masses of data to identify useful objects.  The former uses human perception to listen to the data rendered as sound, and classify it.  These human classifications are used as input to other software.

Over the last several years, a team at the National Center for Supercomputing Applications has applied machine learning techniques to this problem.  They are publishing their latest results this spring [2].

The project uses neural networks trained from the initial LIGO studies and the Gravity Spy classifications to train the nets, and then are able to create an unsupervised classifier that exceeds the capability of the original system.  I suspect that it probably exceeds the capabilities of the Gravity Spy crowd, as well.

Well done, all.


That this work is being done at a supercomputing center is a hint that it sucks a lot of CPUs, or, in this case, GPUs.  NCSA and other HPC centers have been developing ways to use GPUs for various numerical problems for a couple of decades (not even counting Illiac IV which was essentially a prehistoric GPU the size of a warehouse).

 

In one sense, this isn’t a surprising result.  Neural nets should be able to solve this problem, given enough training data.  But there is huge difference between should and really does.  Which goes to show you why you need the kind of multi discipline, full service HPC center that NCSA has pioneered for 35 years.


  1. Daniel George and E. A. Huerta, Deep Learning for real-time gravitational wave detection and parameter estimation: Results with Advanced LIGO data. Physics Letters B, 778:64-70, 2018/03/10/ 2018. http://www.sciencedirect.com/science/article/pii/S0370269317310390
  2. Daniel George, Hongyu Shen, and E. A. Huerta (2018) Classification and clustering of LIGO data with deep transfer learning. Physical Review D, https://journals.aps.org/prd/accepted/14078Q33Z9aEa21d90d88b77cee7844e90f7d512d

 

Many Black Holes at the Center

It has long been suspected that the center of our home galaxy (and probably many other galaxies) is a large black hole.  In the past couple of decades, observations have confirmed that the is very likely.

This spring an international team of astronomers report evidence that there is not one, but a cluster of black holes at the center of our galaxy [2].  This is consistent with theoretical calculations that a large black hole would generate more black holes, but it is the first observational data that seems to support the theory.

The study used data from the Chandra X-ray telescope which orbits ascend outside the Earth’s ion belts to get a clearer view of the X-ray universe.  (Chandra is approaching its twentieth year of operation!).

The research observed very close to the center of the Milky Way, to identify X-ray sources.  The analysis filtered the effects of other sources in the area, to identify a dozen X-ray sources that they attribute to black holes.

The researchers note that theory suggests that there could be many thousands of such stellar sized black holes surrounding the center.  These black holes are difficult to observe due to the strength of other sources there.  The big black hole and other mass at the center is sucking in huge amounts of mass, which is generating a lot of energy, stars, and, according to theory, black holes.

The data was collected over the past twelve years (!), which goes to show how hard it is to spot them.  And this also goes to show the importance of long term observations and data archives that enable data to be used and reused for long periods.

I don’t understand the astrophysics well enough to judge if these results will be controversial or have to be revised in the future.  The general finding certainly isn’t controversial or unexpected, but that doesn’t mean that these dozen sources really are black holes.


  1. BBC, Dozen black holes found at galactic centre, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-43648152
  2. Charles J. Hailey, Kaya Mori, Franz E. Bauer, Michael E. Berkowitz, Jaesub Hong, and Benjamin J. Hord, A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy. Nature, 556:70, 04/04/online 2018. http://dx.doi.org/10.1038/nature25029

 

Space Saturday

PS.  Wouldn’t “Density Cusps” be a great name for a band?

Jupiter Science from Juno Coming Out

The Juno spacecraft has been in orbit around Jupiter since July 2016, and will complete at least two more orbits under current funding (July, 2018).

One of the goals of the mission is to look in detail at the atmosphere of this gas giant.  From Earth, we can see the stripes, which are vast wind streams (in opposite directions ?!), and the Great Red Spot, the largest hurricane in the solar system. But what is going on under the cloud tops?

After more than a year of data collection, results are starting to come in.  Jonathan Fortney summarizes three new papers appearing this spring in Nature [2].  Fortney points out that Earth bound experiments and  theory have not been able to describe the complicated Hydrogen / Helium atmosphere below the surface we can see.

One study investigated the mass distribution of Jupiter by measuring the Doppler effects on the radio signals from the Juno spacecraft as it swooped past [4].  Fortney notes that this was a very finicky process, which had to account for tiny amounts of acceleration including the absorption and re-radiation of sunlight!  The researchers conclude that the bands we see extend quite deep into the atmosphere.

A second study extends this work to conclude that the strong winds decay slowly down some 3.000 kilometers [5].  I.e., the bands we see probably extend down some 3,000 kilometers into the atmosphere.

A third study finds that below that depth, the planet rotates as a solid [3]. At that depth, the pressure is such that the hydrogen ionizes and electromagnetic forces bind the material into a liquid. (This core is the source of the strong magnetic field.)  Obviously, there must be a very turbulent area at the boundary of these two regions, with huge bands of wind ripping East and West across an inner core.

These studies give a picture of a dense interior, with a deep atmosphere dominated by huge bands of strong winds.  An extremely stormy planet!

See swirling cloud formations in the northern area of Jupiter’s north temperate belt in this new view taken by NASA’s Juno spacecraft. The color-enhanced image was taken on Feb. 7 at 5:42 a.m. PST (8:42 a.m. EST), as Juno performed its eleventh close flyby of Jupiter. At the time the image was taken, the spacecraft was about 5,086 miles (8,186 kilometers) from the tops of the clouds of the planet at a latitude of 39.9 degrees. Citizen scientist Kevin M. Gill processed this image using data from the JunoCam imager.

(Caveat:  these studies are based on the theory of gravitational harmonics which I don’t understand at all.)

Fortney suggests that Juno may be able to make further detailed observations of the Red Spot and other storms, which would be interesting details to have.  He also notes that data returned by the Cassini probe of Saturn should yield comparative measurements for the its less dense and probably deeper atmosphere.

Stay tuned. There is lots of other science coming.

The current funding ends in July, but the mission could continue for several more years if supported.


  1. Jonathan Amos, Jupiter’s winds run deep into the planet, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-43317566
  2. Jonathan Fortney, A deeper look at Jupiter. Nature, 555:168-169, March 7 2018. https://www.nature.com/articles/d41586-018-02612-y
  3. T. Guillot, Y. Miguel, B. Militzer, W. B. Hubbard, Y. Kaspi, E. Galanti, H. Cao, R. Helled, S. M. Wahl, L. Iess, W. M. Folkner, D. J. Stevenson, J. I. Lunine, D. R. Reese, A. Biekman, M. Parisi, D. Durante, J. E. P. Connerney, S. M. Levin, and S. J. Bolton, A suppression of differential rotation in Jupiter’s deep interior. Nature, 555:227, 03/07/online 2018. http://dx.doi.org/10.1038/nature25775
  4. L. Iess, W. M. Folkner, D. Durante, M. Parisi, Y. Kaspi, E. Galanti, T. Guillot, W. B. Hubbard, D. J. Stevenson, J. D. Anderson, D. R. Buccino, L. Gomez Casajus, A. Milani, R. Park, P. Racioppa, D. Serra, P. Tortora, M. Zannoni, H. Cao, R. Helled, J. I. Lunine, Y. Miguel, B. Militzer, S. Wahl, J. E. P. Connerney, S. M. Levin, and S. J. Bolton, Measurement of Jupiter’s asymmetric gravity field. Nature, 555:220, 03/07/online 2018. http://dx.doi.org/10.1038/nature25776
  5. Y. Kaspi, E. Galanti, W. B. Hubbard, D. J. Stevenson, S. J. Bolton, L. Iess, T. Guillot, J. Bloxham, J. E. P. Connerney, H. Cao, D. Durante, W. M. Folkner, R. Helled, A. P. Ingersoll, S. M. Levin, J. I. Lunine, Y. Miguel, B. Militzer, M. Parisi, and S. M. Wahl, Jupiter’s atmospheric jet streams extend thousands of kilometres deep. Nature, 555:223, 03/07/online 2018. http://dx.doi.org/10.1038/nature25793

 

Space Saturday

 

Early Light and Signs of Dark Matter?

These are the most interesting times in Astronomy.  In the last century, we have pushed back the history of our Universe to its beginnings, and observed traces of the big bang.  We have discovered black holes, pulsars, and any number of other weird and wonderful things.

Even more interesting, we have only recently discovered that the universe we have been studying is only about 4% of everything out there [3], most of it is “Dark Matter” and “Dark Energy”, which we haven’t figured out how to even see, let alone understand. Wow!  If you don’t think that is cool, I can’t really talk to you.

This month a research team report on observations of another trace of the early universe, from a time when stars were first forming [2]. The study examined radio signals averaged across large areas of the sky, looking for evidence of how the early microwave radiation interacted with the hydrogen in clouds and stars.  This is a pretty finicky measurement to make, but it is indirect evidence from a time that we cannot directly observe.

I am not an astronomer, but I gather that the basic idea is to detect an interaction between early stars, ubiquitous primordial hydrogen, and the cosmic microwave background. This should produce a distortion in the spectrum of the hydrogen, still detectable today (red shifted, and buried in all the other radio signals….).

“After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz.” ([2], p.1)

The study found just the predicted distortion, very clearly. However, it is twice as large as predicted by the theory, which suggests that there is a significant missing piece. The discrepancy indicates that the hydrogen was much colder than assumed.

This is an exciting finding, because one possible explanation for the discrepancy is some form of interaction between dark matter and the hydrogen.  If this can be confirmed, it is new information about dark matter, and also a new understanding of the universe at this primordial time.

Possibly most important of all, if this really is an interaction between dark matter and hydrogen, it is something we haven’t seen before.  This may be a hint that we can actually detect dark matter, and that would be H-U-G-E, huge.

The published report is notable for the fact that much of the text is careful analysis of possible errors and alternative explanations.  The team spent two years trying to eliminate possible sources of error, and to come up with possible theoretical explanations [1]. This includes using multiple instruments, multiple configurations, multiple data processing workflows, and many simulations of possible errors.  They also have thought hard about any possible alternative explanations for the findings.

This result is very significant, so there will be efforts to replicate and confirm the findings. The researchers note several projects that may be able to replicate the findings, as well as some future studies that may be possible.  One interesting suggestion is to repeat the measurements from the far side of the moon, using Luna as a shield from the cacophonous radio noise spewing from Earth.  This would be a moon mission that would be worth way more than sending humans to either the moon or mars.  (Are you listening, Elon Musk?)

Finally, I’ll note that this is yet another example of why we need to actually make observations and collect data.  All the theory in the world means very little without empirical evidence, and careful observation almost always finds unexpected things.  In this case, the facts seem to show a huge question mark for theorists to fill.

Cool.


  1. Jonathan Amos, Signal detected from ‘cosmic dawn’, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-43200277
  2. Judd D. Bowman, Alan E. E. Rogers, Raul A. Monsalve, Thomas J. Mozdzen, and Nivedita Mahesh, An absorption profile centred at 78 megahertz in the sky-averaged spectrum. Nature, 555:67, 02/28/online 2018. http://dx.doi.org/10.1038/nature25792
  3. Richard Panek, The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality, Boston, Houghton Mifflin Harcourt, 2011.
  4. The Dark Energy Survey. Home – The Dark Energy Survey. 2017, https://www.darkenergysurvey.org/.

 

Space Saturday

Life On Ocean Worlds

One of the great philosophical mysteries of our age is, in the words ascribed to Enrico Fermi (AKA, “the pope of physics”): “Where is Everybody?” [3]  (This is known as Fermi’s Paradox, though he didn’t originate it, nor is it really a ‘paradox’.  It’s still a Fermi-grade question, though.)

Humans have been watching the skies for millennia, and in the past century have looked ever wider and deeper into the universe, not to mention into physics and the biology. Everything we know indicates that there could very well be life and even technological civilizations everywhere in the vast universe.  But we have never seen evidence of life beyond Earth.

Where is everybody?

Coming up with answers to Fermi’s question is a great scientific parlor game.

In 2002, Stephen Webb describes 50 answers [2], and in his 2015 update he gives 75 (!) [3].  The “solutions” listed by Webb range from “they are already here”, through “they are so strange we don’t recognize that they are there”, as well as the possibility that life really is very, very rare.

Along the way, he points out many uncertainties in our estimates of how likely the development of life and “intelligent” life may be (e.g., we have only our own planet to extrapolate from), as well as unknowable hypotheses about the possible psychology or politics of putative non-human civilizations (e.g., just because we want to talk to everyone doesn’t mean anyone wants to talk to us).

There are also disturbing warnings that “civilizations” are likely to self-destruct before escaping their home planet, or, even worse, may be snuffed out in the nest by predators or catastrophes. (With this in mind, blasting our electromagnetic presence in all directions might not have been a healthy life style choice.)

Webb’s compendium of “solutions” is fun to read, but the game is hardly over.


At the 2017 Habitable Worlds workshop, S. Alan Stern proposes yet another solution to the Fermi Paradox: most life evolves in “interior water ocean worlds”, i.e., in oceans under thick ice covers [1].

most life, and most intelligent life in the universe inhabits interior water ocean worlds (WOWs) where their presence is cloaked by massive overlying burdens of rock or ice between their abode and the universe.

There are several such worlds in our own solar system, and at times in the past the Earth itself flirted with such conditions, covered over with a kilometer of ice.

Artist’s concept of Europa’s frozen surface. Credit: JPL-Caltech

Stern notes that these worlds appear to be highly conducive to the development of life.  The ice cap protects and stabilizes the ocean environment, providing a nest for fragile life to develop over long, evolutionary periods of time.  Thus, however likely life is to develop, ice worlds are prime candidates for successful evolution.

However, Stern also makes the interesting inference that life that evolved under a deep ice cap would have no direct view of the universe.  The protective shield overhead would also block out most evidence of other stars and planets. An emerging civilization under the ice would not know about the universe, at least until technology develops that detects (indirectly) the space above the ice.  Even then, intelligent beings might have difficulty imagining life that does not live under ice, so they might not think to look for signals from us or send signals we could detect.

Stern also argues that life adapted to an ice-covered ocean would find space travel difficult, at least compared to species adapted to the surface under a gaseous atmosphere. In addition to the technical challenge of penetrating many kilometers of rock hard ice, life-support would be necessary to support a dense, liquid environment.

He combines these arguments to answer the “Where is everybody?” question:  if much life develops in ice covered oceans, and any civilizations in such environments unlikely to know or care about the wider universe, then this explains why we haven’t heard from them.

This is an interesting idea to think about.  It is certainly useful to break out of the parochial idea that an Earthlike planet is the only or ideal locus for life or “civilization”.  In fact, we know that life on Earth has just barely survived at least five major extinction events, and an ice world might well be a safer crèche.

I’ll also note that his comment that life on such a planet “either cannot communicate or are simply not aware that other worlds exist” works both ways.  It is difficult for us to detect such inhabitants, and we haven’t be looking until recently.  In our own solar system, there are several ice worlds, but we still have no idea if they are inhabited or not.


On the other hand, several aspects of Stern’s argument are less convincing to me.

An ice-covered ocean world might be a favorable site for life to start, but it might also be a closed system that is quickly exhausted.  Experience on Earth certainly indicates that a closed “ark” will rapidly be overgrown, clogged, and die out.   It is likely that only some ice worlds will be sufficiently “active” or open enough for life to persist.  But who knows until we actually check.

I have to say that I find the arguments about the supposed psychology of native to ice worlds highly speculative, to say the least.  It is true that life on Earth can directly sense the solar system and wider universe, and there are plausible arguments that this knowledge has strongly influenced the development of what we call intelligence.  But it is very difficult to guess the implications of not having an open sky.

I also think that, should a technological civilization develop under an icecap, it will surely develop undertanding of the outside universe. They’ll surely learn about gravity, and when they learn to detect and manipulate electromagnetism, they’ll soon notice a lot of interesting stuff coming in through their icy roof.  For that matter, no matter how difficult space travel might be, wouldn’t they deploy robot explorers and harvesters on the outer side of the ice.  And from that perch, who would not look up and see other worlds?

In short, I’ll buy the idea that ice worlds are good places for life to develop, though they may not be great places to sustain life for billions of years.  But I reserve judgement on questions of how the lack of a sky might influence the development of “civilizations”.


In this article, Stern describes yet one more case for why there could be some extraterrestrial civilizations that we have not seen or heard.  But this clearly isn’t “the answer”. He joins the roster of all the dozens of other hypotheses (Indeed, Webb has a solution called “Cloudy Skies Are Common” ([3], p. 183), which probably subsumes Stern’s solution as a sub case.).

On the other hand, this thesis is yet more reason why icy ocean worlds are really interesting and really need to be explored..  There very well could be life under the ice, and we really should find out what we can.

We have several such worlds close at hand in our solar system that we could visit and actually see what is down under the ice. (EuropaEceladus!  Titan!)

Let’s go, already!


  1. S. Alan Stern, An Answer to Fermi’s Paradox in the Prevalence of Ocean Worlds?, in Habitable Worlds 2017: A System Science Workshop. 2017: Laramie, Wyoming. https://www.hou.usra.edu/meetings/habitableworlds2017/pdf/4006.pdf
  2. Stephen Webb, If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, New York, Copernicus Books In Association With Praxis Pub, 2002.
  3. Stephen Webb, If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY? Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, New York, Springer, 2015.

 

Space Saturday