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Rare Earth? Not Enough Data to Know

George Dvorsky takes on the ‘rare earth’ hypothesis in his Sentient Developments blog, calling it a ‘delusion’ and noting all the reasons why life in the galaxy is unlikely to be unusual. The post reminds me why the book that spawned all this was so significant. Rare Earth: Why Complex Life is Uncommon in the Universe (Copernicus, 2000) is Peter Ward and Donald Brownlee’s take on our place in the cosmos, concluding that complex life is rare because an incredibly fortuitous chain of circumstances must arise for it to occur. Indeed, the authors argue that large parts of our galaxy are composed of what they call ‘dead zones.’

The argument is complex and looks at factors ranging from a planet’s place in the galactic habitable zone (itself a controversial subject), its orbit around its own star, its size, its satellites, its magnetosphere, its plate tectonics, and more. I’m surprised to realize, looking through our archives here, that I haven’t managed to do a complete review of Rare Earth in the past five years (although we’ve certainly kicked its ideas around during that time). I’ll remedy that soon, because I think it’s a hugely significant work. Agree with it or not, the authors have laid out a strong case that has inspired spirited rebuttal, and the study of astrobiology has benefited from the insights they’ve brought.

Back to Dvorsky, who notes Charles Lineweaver’s work suggesting that planets began forming in our galaxy long before our own Sun ever ignited, and goes on to look at progress in exoplanetary science, which is uncovering so many new worlds. The point George is making is that there have been vast amounts of time for life to arise, and a huge range of galaxies within which it could evolve. He’s absolutely right, but I’ll part with him on the use of Gliese 581 c to make the case on life’s odds:

…shockingly, the first Earthlike planet was discovered in 2007 orbiting the red star Gliese 581. It’s only 20 light-years away, 1.5 times the diameter of Earth, is suspected to have water and an atmosphere, and its temperature fluctuates between 0 and 40 degrees Celsius.

If we are one in a billion, then, and considering that there are only 0.004 stars per cubic light-year, what are the odds that another Earthlike planet is a mere 20 light-years away?

But the odds are still where they were, Gliese 581 c now being generally considered too close to its star to be habitable. The next planet out, Gl 581 d, is possibly within the extreme outer edge of the habitable zone, and perhaps a better bet for life than what now seems to be a hellishly hot Gl 581 c, but in any case neither of these massive ‘super-Earths’ are truly Earth-like, and the question of whether they support life or not awaits further data. Recent work by Brian Jackson, Richard Greenberg and Rory Barnes suggests tidal heating for Gl 581 c that may be three times greater than what we see on Io, not a promising sign.

We still, in other words, don’t have a good read on how close the nearest habitable planet is, or how rare our Earth may be. Could Ward and Brownlee still be right? Absolutely. At this point we lack the data to know.

Alan Boss has much to say about all this, and George goes on to quote him to good effect. Author of the recent The Crowded Universe: The Search for Living Planets, Boss (Carnegie Institution) sees Earths as ‘incredibly common,’ going on to make this astounding statement: “…every solar-type star probably has a few Earth-like planets, or something very close to it.” Take that, rare Earthers!

I just finished The Crowded Universe the other day and will have more to say about it soon — we looked at its author’s ideas on habitable planets in a previous post. I tend to go along with Boss, as least in so far that I suspect life is quite common elsewhere in the universe, and probably complex life at that. But the question of sentient, technology-building life is quite another matter. It wouldn’t surprise me at all to find many living worlds in our galaxy, but of necessity we await the results of our explorations, one of which was so recently launched. One day Kepler’s successors will doubtless flag the spectroscopic signature of a living world, but barring a SETI breakthrough, the question will remain. Are there other civilizations? Do they too dream of traveling to the stars?

Comments on this entry are closed.

  • Administrator March 17, 2009, 18:11

    Larry wrote:

    “It is ironic, though, that so many people readily accept the concept of supernatural beings everywhere affecting every aspect of our daily lives, but the thought of “mortal” beings developing and living on worlds like we are is still met with skepticism and ridicule”.

    There may be newer polls that I’m not aware of, but a Roper poll from 2002 prepared for the SF Channel doesn’t indicate that fear of aliens, or ridicule of those who believe in them, is much of a factor. According to Roper, “The telephone interviews were conducted from August 23 through August 25, 2002, using a Random Digit Dialing (RDD) probability sample of all telephone households in the continental United States.” Among the results:

    “Three in Four (74 percent) Claim They Are At Least Somewhat Psychologically Prepared For an Official Government Announcement Regarding the Discovery of Intelligent Extraterrestrial Life.

    “Clearly, a majority of Americans are ready for the discovery of extraterrestrial life, with 42 percent saying they are “very prepared” and 32 percent saying they are “somewhat prepared.”

    The summary puts it this way: “Most Americans appear comfortable with and even excited about the thought of the discovery of extraterrestrial life. Three-quarters of the public claim they are at least somewhat psychologically prepared for the discovery of extraterrestrial life, and nearly half are very prepared.”

    On fear of aliens making people question their religious beliefs:

    “Very few Americans say that an official government announcement about the discovery of intelligent extraterrestrial life would cause them to question their religious beliefs. A full 88 percent say that such an announcement would have no impact on their religious beliefs.”

    And this is interesting: “Those who say that they are “very psychologically prepared” for the discovery of extraterrestrial life are more inclined to believe that their religious beliefs will not be affected by such a discovery.”


  • kurt9 March 17, 2009, 22:48

    There are only two SF movies that were any good. 2001, which was the most realistic depiction of space (and good sound track to boot), and Blade Runner, which is really a transhumanist, immortalist movie (also with a good sound track to boot).

  • Administrator March 18, 2009, 8:33

    Of these, ‘Bladerunner’ is my personal favorite, in whichever version. I watch it yearly to marvel at Ridley Scott’s work and only wish he would turn back to SF for future projects. Great script, too!

  • ljk March 18, 2009, 11:06

    I am glad to see that people may indeed be on their way to a
    better acceptance and understanding of alien life, especially the
    intelligent variety.

    Perhaps all these years of fictional aliens, skewered as their
    perceptions often have been, have helped to prepare the public
    for the idea of other intelligent species when the day comes that
    we do confirm their existence.

    As Frank White said in his 1990 book The SETI Factor, how we
    will respond to finding alien life depends on what kind of organisms
    we will encounter (microbes or Kardashev Type III types) and in
    what manner (an ancient signal from a very distant planet or a
    giant spacecraft hovering over New York City).


    Alan Tough also addressed these issues in his paper here:


    I agree that both 2001: A Space Odyssey and Blade Runner are
    among the best SF films ever made, with 2001 being unmatched
    on so many levels in 40-plus years.

    But there are other good SF films, of course, especially during the
    era from the late 1960s to the early 1970s. You can find a dozen
    or more SF films from that time period which did a good job with
    plots and meaning without either the big budgets and CGI special
    effects that have dominated SF cinema since the first Star Wars
    film in 1977 – most of them at the cost of good plots and meaning.

    Blade Runner, made in 1982, is a happy and relatively rare
    exception to the post-Star Wars era rule.

    Paul, I know you like 2010 better and I agree it is a good film as
    science fiction cinema goes (also made in the post-Star Wars era
    of 1984). The early scenes of Leonov’s arrival at Jupiter alone are
    worth the price of admission. But in some ways it seems more
    dated than 2001 and director Peter Hyams missed the whole point
    that Kubrick was trying to make by having the humans act like
    automatons compared to HAL 9000, a real machine.

    Instead the characters in 2010 talk and talk and talk and explain
    just about everything that is happening in the film, whereas Kubrick
    and Clarke only gave the audience just enough info in 2001 to make
    their own conclusions as to what was going on in the film. Kubrick
    even said if they answered all your questions in 2001, then he had
    failed with the film.

    A refreshing change of pace from most of today’s films, where even
    the comedies telegraph every joke to make sure the audience knows
    it is supposed to be funny.

    • Administrator March 18, 2009, 11:37

      Yes, I do like 2010 better, but I agree with your comment, Larry. The film isn’t as timeless as 2001 and misses some of the grand sweep of the earlier treatment. The point about the humans acting like automatons is exactly right — they were a fascinating contrast to HAL in 2001, with all the questions that raised. 2010 shows them in a much more ‘normal’ light, but on the other hand, that boosted the believability factor just a bit for me, and the aerobraking sequences were terrific, as you note.

      It’s interesting to speculate where we would be re the public perception of aliens if there had been no Carl Sagan. He made the prospect real for so many.

  • ljk March 18, 2009, 13:12

    The other amazing early scene in 2010 is when Leonov approaches
    the USS Discovery as it is tumbling end over end between Jupiter
    above and Io below. The US spaceship is tinged yellow from all
    the sulfur that has been belched upon it by the Io volcanoes far


    And of course Jupiter being turned into a star by millions of Monoliths.

    Another dated feature about 2010 are the computers. They used
    real computer graphics as opposed to the hand-drawn versions in
    2001, yet the 1968 ones look better.

    Yes, without Carl Sagan’s quite literally brave efforts, the concept
    of alien life in all its forms would still be in a big state of gigglehood
    and dismissiveness. David Grinspoon discusses this at length in his
    book Lonely Planets, where he reveals that Sagan was once asked
    to help a group of scientists create a fake ETI signal detection but
    declined as he felt the ramifications of the discovery of the hoax
    would far outweigh finding no real signals at all.

    • Administrator March 18, 2009, 13:43

      Wow, that reminds me that I had intended to read the Grinspoon book some time back and never got around to it! Thanks — it now goes onto the reading list (again). For those unfamiliar with it:


  • ljk March 18, 2009, 14:43

    I meant to ask this before regarding that Roper Poll: When the
    people who responded to the poll said they were “prepared” for
    the discovery of ETI, did the pollsters explain or find out what
    was meant by prepared?

    Are we talking just psychologically able to deal with the fact
    there really are aliens and get on with their lives, or did they
    mean they would set up a parade or barbeque or welcome
    signs? Or do they mean building spaceships to meet them or
    a landing pad for them to land their ships here, or do they mean
    they’re loading their shotguns and boarding up their doors?

    And what about the 26 percent who said they are NOT ready
    for aliens? What will they do with such news?

  • Administrator March 18, 2009, 15:29

    From the answers to the poll, I would assume ‘psychologically prepared’ means they’re interested and accepting of the idea that intelligent life is out there. I doubt it means they’ve stockpiled weapons. :-) The rest of the poll gives the context that shows whether they’re knowledgeable or not (or gullible or not), the respondents believe in a wide range of extraterrestrial possibilities and take a generally positive attitude toward them.

    Interesting. I just found that a 2006 National Geographic Survey said 72% of the American population would be “excited and hopeful” if life was found on other planets. And then there’s this from Space.com:

    “A National Geographic-commissioned telephone survey of 1,000 Americans found that 60 percent believe life exists on other worlds. About 90 percent of those who believe in extraterrestrial life thought the Earth should respond to any communications emanating from an alien planet.”

    Which is at the end of this story:


  • kurt9 March 19, 2009, 1:55

    Nick Lane wrote “Power, Sex, and Suicide”, which is the story about mitochondria.


    Peter Hyams’ 2010 was also an excellent movie. However, for me it simply does not compare to Kubrick’s 2001. 2001 was very philosophical compared to 2010. Also, I really like the scene with the space craft moving around to the Blue Danube waltz.

  • ljk February 10, 2010, 0:25

    Two guys make a toy galaxy and find life is confined to Earth-type planets:


    If this is true and only fifteen percent of solar systems are like ours, then
    we can make an even better estimate for how many ETI are in the galaxy.

    But I think it is also safe to say we have a long way to go before we get
    more accurate data to know the real numbers here.

  • ljk January 1, 2012, 15:31

    DECEMBER 31, 2011

    The Loneliest Planet

    Review of the book Alone in the Universe by John Gribbin


    Aliens invaded my life one Sunday night in 1957 when I was 6 years old. As I pulled close to watch Walt Disney’s “Wonderful World of Color,” the screen of our old black-and-white Admiral TV seemed to bleed into rainbow hues.

    Disney introduced the evening’s program: “Mars and Beyond,” a meditation—in retrospect, an entertainment—on the possibility of life on other worlds. “Will we find planets with only a low form of vegetable life?”

    Disney wondered, then nodded genially toward his towering, metallic co-host. “Or will there be mechanical robots controlled by super-intelligent beings?” To my juvenile mind, these questions were an invitation to a living-room ride into outer space.

    Full review article here: