December 5, 2011 | David F. Coppedge

Earth’s Twin: What Does “Potentially Habitable” Mean?

The Kepler spacecraft has found dozens of “potentially habitable” planets around other stars, but this week announced one that some news sources are calling “Earth’s twin.”  The announcement provides an opportunity to study where empirical science ends and speculation begins.

The NASA press release announced only that it had found its first planet in the habitable zone of a sun-like star.  This implies that water could be liquid on its surface, provided other requirements are met.  A diagram shows the planet’s orbital radius compared with that of the sun’s rocky planets.  The Kepler team realizes that much remains unknown about the new planet, dubbed Kepler-22b, except that its radius is 2.4 times that of Earth.  “Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous or liquid composition, but its discovery is a step closer to finding Earth-like planets,” the article said.  Very little, therefore, is empirically known to justify the accompanying artwork of a watery world.

The scientific press took this as a cue to push the “Earth is not unique” angle.  NASA’s Astrobiology Magazine was somewhat restrained with the L-word life, noting just that water is a prerequisite.  Reporter Mike Wall at, however, used the word life three times, as in: “The discovery brings scientists one step closer to finding a planet like our own — one which could conceivably harbor life, scientists said.” The BBC News came just short of calling Kepler-22b Earth’s twin sister: “It is the closest confirmed planet yet to one like ours – an ‘Earth 2.0’.”

Last month, the BBC News presented a way of ranking which planets are most “liveable,” with Earth at a rank of 1.0.  The next nearest was Titan (0.64), then Mars (0.59), Europa (0.49), and an exoplanet named Gliese 581g (0.45).  It’s not known where the new planet Kepler-22b would rank, but the criteria included surface properties, the presence of a liquid solvent, and whether the world has an atmosphere and a magnetic field.

In addition, the life history of a planet must be taken into account.  Stephen Battersby in New Scientist wrote about “Earth’s wild ride: Our voyage through the Milky Way.”  Assuming that Earth is as old as scientists believe, the Earth would have orbited the Milky Way every 200 million years in its assumed 4.5 billion year lifetime, sending it into a veritable shooting gallery of other objects and exposing it many times to the lethal radiation of supernovae.  Battersby had to admit that much of this is speculation:

As yet, this is guesswork. We cannot retrace our path through the galaxy’s gravitational melee, still less calculate what incidents befell us where and when. Earth itself, its rocks constantly recycled by plate tectonics and remodelled by erosion, is remarkably forgetful of past assaults from space.

Maybe our sun was able to “surf” a spiral arm of the galaxy and avoid some of the dangers, he said, but admitted “This is mere circumstantial evidence.”  On the one hand he sees the exposure as a potential spark for life; on the other, it becomes a source of mass extinction.  Whether the moon holds clues to past events was offered as the only source of empirical evidence.

If scientists cannot even understand the history of the world under our feet, it seems premature at best to speculate about the habitability of Kepler-22b.  Even the diamond planets we reported on 8/25/11 would not be nice places to live, according to New Scientist.  They would be black, cold, lifeless worlds despite their riches. “The net result is that these diamond-rich planets will not have global carbon cycles nor plate tectonics – which in turn may have implications for ocean formation,” the article said.  Without oceans, forget life, even if such planets orbited in the habitable zone.

Habitability does not imply inhabited.  We have more habitable worlds in our own solar system than Kepler-22b:  Venus and Mars.  Look at them: Venus is a hell, and Mars a freezer, exposed to the ravages of cosmic radiation.  And whoever ranked Titan as the next most liveable world must be kidding himself: would you want to recline on a beach of Titan’s tarry lakes at 290 degrees below zero?

The Illustra documentary, The Privileged Planet, described at least 20 requirements for life: an oxygen atmosphere, a crust that can support plate tectonics, a global magnetic field, the right kind of crustal composition, a stable star, and much more.  Calculating the probability a planet in the habitable zone would meet all the requirements, given a conservative estimate of 1 in 1,000 for each, leads to a probability of a thousandth of a trillionth that a planet would be habitable.  In our own pages we have seen scientists constrain it further, describing other “habitable zones” than the usually-considered zone in which water could be liquid: the galactic habitable zone, the tidal habitable zone, ultraviolet habitable zone, the magnetic habitable zone, the temporal habitable zone, the chemical and thermodynamic habitable zone (2/26/2011).

Even so, given the number of stars, there could be some exoplanets somewhere that meet the requirements.  But then, as the film continues, there are the fine-tuning arguments that militate against life forming by chance, and argue positively for intelligent design.  And seeing how precisely placed Earth is for scientific discovery (between spiral arms, the right distance from the sun with the right sized moon for total solar eclipses, the only planet with a clear atmosphere, and much more), extends the argument that our world was created to be habitable on purpose.

That’s what the secular media refuse to consider: purpose.  It leaves them with nothing better to do than confabulating speculative stories about what might exist out there.  It’s fine for Kepler to be gathering data; that’s a noble purpose.  Better to know what stars and planets exist then have speculation completely unconstrained by data.  But the Kepler spacecraft is unable to tell whether all the requirements for habitability are met.  Such a goal is years away.  The Terrestrial Planet Finder and Space Interferometry Mission (SIM) was two missions that had been planned to try to at least detect water, before they were cancelled due to budget cuts.  Even those would only provide small steps toward describing the nature of the candidate Earth-like planets.  SETI, meantime, is a 50-year failure so far.  Purposelessness leaves science with nothing but particles in motion hoping to win some cosmic lottery.  That is not science.

If scientists and reporters were more rigorous in their practice, they would refrain from going beyond the data.  A planet has been found within the habitable zone of a sun-like star.  OK.  Fair reporting would constrain that fact, first and foremost, to what is meant by “habitable zone.”  Then a list of all that is not known would follow.  Cautions about illogical conclusions would be presented to the reader; e.g., habitable does not imply inhabited.  There would be no suggestive artwork of an atmosphere and clouds.  The reporter would respect good science by sticking to the data.  Why do the science reporters and authors of press releases never do that?  It can be done.  Hey, we just did it here.

Speaking of Kepler, be sure to read our online biography of Johannes Kepler, a stalwart Christian who, as a scientist, was a giant on whose shoulders Isaac Newton stood.

Exercise: “Potentially habitable” is an example of the power of suggestion.  Think of some other uses of this kind of phrase that are silly.  Here are some to get started:  Think of the average Joe as:

  • Potentially an Ironman triathlete.
  • Potentially President of the United States.
  • Potentially winner of the Powerball lottery.
  • Potentially married to a princess.
  • Potentially World Chess Champion.

It’s even more silly when 20 some odd potentials are combined: “potentially an Ironman triathlete President married to a princess holding the World Chess Championship who wins the Powerball lottery.”  Think of this when you hear someone say of Kepler-22b, “Well there could be life on it.”

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  • Coreysan says:

    Is the “photo” of Kepler-22b real, or an artist’s rendering?

  • Editor says:

    Kepler doesn’t take photos.  It measures transits: the dip in light as a planet crosses in front of its star.  All a scientist is going to see is a light curve.  From that data they can determine the size of the planet and its distance from the star, but not much else.  The artwork accompanying the press release, therefore, is pure speculation.

  • Shawn says:

    I find the exercise at the end of this article particularly humorous and helpful. It is easy to overlook the words “probably” and “potentially” without seeing what they really mean. It has become a joke of mine lately that evolutionary biologists and others of that ilk play the worst odds known to man, so it is surprising that they aren’t lining slot machines around the world, eager to win some “easy money”.

    Thanks for the nice article, I saw something about this earlier on the news and was curious to see your take. You do an excellent and thorough job!

  • says:

    I first saw this headline in the news…and I hate to say it, but astronomy is much, much less of a science than is biology, chemistry or geology…the chief disciplines we deal with in creation science.  When I read something like this, I think…what a nice fairy tale.  When you’re trying to do “observable” science at distances of light years, and trying to determine habitability, it’s like me (I’m an implant dentist) studying dental implants at 5 miles away with a telescope and binoculars ….only their distances make mine look like I’m really looking thru a microscope. I’m quite critical of Hugh Ross and other old earthers who are astronomers and not biologists;  I think biologists get an entirely different feel for creation because of our ability to observe closely….

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