Michael Strauss: are Earth-like planets common in the universe?

Christianity and the progress of science
Christianity and the progress of science

Physicist Michael Strauss is one of my favorite speakers and scholars – he has a real love of experimental science which I find lacking in many Christian philosophers who seem to treat science (as opposed to naturalism) as the enemy.

Here is his latest blog post:

When scientists say that an Earth-like planet may have been discovered, they actually mean one of three things. Either (1) the planet is in such an orbit around its central star that allows the temperature on the planet to possibly harbor liquid water, or (2) the planet is about the same size as the earth, or (3) the planet is solid and rocky rather than gaseous. Of course any one of these criteria, or even all three, does not actually give us a true Earth-like planet. We know that our moon is in the correct location to contain liquid water, but it is not “Earth-like.” We know Venus is about the size of the earth, but it is not “Earth-like.” We know that Mercury is rocky and not gaseous, but it is not “Earth-like.” So none of these criteria really give an Earth-like planet. Headlines and sound-bites are not meant to be precise but to draw attention, and it is much more exciting to proclaim an “Earth-like” planet has been found rather than a “Venus-like” planet (if even that could be claimed).

[…]An enlightening book on this subject is Rare Earth: Why Complex Life Is Uncommon in the Universe by Peter D. Ward and Donald Brownlee, published in 2000. One of my favorite chapters in the book is titled “The Surprising Importance of Plate Tectonics” which documents why plate tectonics is required in order for complex life to survive. Having lived 23 years of my life in California, I am well acquainted with the consequences of plate tectonics, but had no idea that such activity was crucial for my survival. Ward and Brownlee document how plate tectonics not only forms and maintains continents, but promotes biological diversity, regulates global temperature, and helps maintain a planetary magnetic field. They write, “It may be that plate tectonics is the central requirement for life on a planet and that it is necessary for keeping a world supplied with water,” (p. 220).

The astrophysicist Hugh Ross has done a rough estimate of the probability of finding a single planet that could support even simple unicellular life for a sustained period of time. Including correlations and longevity factors, and assuming there are 10 billion trillion planets in the visible universe, he concludes that the probability of finding a single planet that could support unicellular life for a prolonged period of time is 1 in 10556, (see Part B of this document). If this informed estimate is even close to being correct, then there are no other planets in the visible universe that can support life.

Previously, I blogged about a few of the minimum requirements that a planet must satisfy in order to support complex life.

Here they are:

  • a solar system with a single massive Sun than can serve as a long-lived, stable source of energy
  • a terrestrial planet (non-gaseous)
  • the planet must be the right distance from the sun in order to preserve liquid water at the surface – if it’s too close, the water is burnt off in a runaway greenhouse effect, if it’s too far, the water is permanently frozen in a runaway glaciation
  • the planet has to be far enough from the star to avoid tidal locking and solar flares
  • the solar system must be placed at the right place in the galaxy – not too near dangerous radiation, but close enough to other stars to be able to absorb heavy elements after neighboring stars die
  • a moon of sufficient mass to stabilize the tilt of the planet’s rotation
  • plate tectonics
  • an oxygen-rich atmosphere
  • a sweeper planet to deflect comets, etc.
  • planetary neighbors must have non-eccentric orbits
  • planet mass must be enough to retain an atmosphere, but not so massive to cause a greenhouse effect

That’s a lot of characteristics that need to be present. When you calculate the probabilities of each one and then multiply them (product rule) to get the odds of getting a planet with ALL of them present, you get a number far smaller than the maximum number of possible life sites in the universe. We are alone.

5 thoughts on “Michael Strauss: are Earth-like planets common in the universe?”

  1. Interesting post. I’ll play devil’s advocate here.

    Do we even know the conditions that allow for life? And I’m just talking about life as we know it.

    And, if what you are saying is true, that would seem to count against theism. God could easily make the whole universe habitable, but that’s not what we observe. What is it about theism that predicts a universe that is largely inhospitable to life?


  2. Theism nor the Bible requires us to believe for or against the universe being able to sustain life all over.

    As far as the Bible shows it only records what is relevant to us on earth and leaves other questions unanswered so we can appeal to the best unbiased science to make up theories to fill in the rest.

    But because of our study of the universe and all we find pointing to evidence that our solar system is not the norm it makes those that believe in the we are alone camp as the ones with the best evidence

    Liked by 1 person

  3. As an amateur scientist, my opinion has always been that Earth is the only place with intelligent (if you want to call it that) life in the universe. Probably any life at all. God truly created it as a unique place.


  4. Upon finding the scientist findings of other planets, none seem to be in the exact center of the zone of life. It seems that most are close to the inner or outer boundary for some reason. Even Earth wasn’t found to be in the exact center. As for Venus, we have no other evidence to see if a slightly smaller planet with a lack of abundance of greenhouse gases could survive at Venus’ orbit under the right conditions. Even Keplar 69C may be another Venus. The star is slightly dimmer than our sun and it should still be far enough away, but it is no guarantee. We may only find a planet like Jupiter, or Venus or Mars at best. We may be truly alone.


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