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Physics, Earth & Space Icon Physics, Earth & Space

For Fire, Our Planet Is Just the Right Size

Earth 1970
Photo: Earth from space, April 22, 1970, by NASA.

Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.

All the unique elements of fitness in nature for fire and the development of technology would be of no avail without rocky planets of the right size, like the Earth. 

If a planet is to possess the necessary stable hydrosphere and atmosphere fit for life, as discussed in previous posts in this series, it must be of approximately the size and mass and possess a gravitational field very close to that of the Earth1 and undergo a similar geophysical evolution. Its gravity must be strong enough to retain permanently the heavier gaseous elements such as nitrogen, oxygen, and carbon dioxide, but weak enough to permit the initial loss of the lighter volatile elements such as hydrogen and helium. Only on planets of similar mass and size to the Earth’s could there exist an atmosphere containing sufficient quantities of oxygen to sustain fire.

A Goldilocks Planet

But here is something even more remarkable: A “Goldilocks” planet like ours that is “just right” in size and gravity to provide an atmosphere supportive of fire and human respiration is also “just right” for the existence of carbon-based organisms of our size and design possessing an upright bipedal posture, i.e., organisms — as we shall see later in the series — of the right size and design to exploit the phenomenon of fire. 

Self-evidently, the gravity on the surface of a planet limits the maximum size of large terrestrial organisms. If the Earth had more than twice its current surface gravity, our weight would be more than doubled, necessitating radical compensatory changes in the design of the body that might well prohibit the existence of large upright bipedal creatures like humans. So planets of the Earth’s size and mass are coincidentally fit both for the design of a bipedal animal of the dimensions of a man capable of exploiting fire and for providing the right atmosphere to support combustion and respiration.2

Tomorrow, “For a Technological Civilization, We Must Have Metals.


  1. “Surface Gravity,” Wikipedia, May 12, 2016, accessed May 17, 2016,; Gravity = M/R2 where M = mass and R = radius of the planet
  2. NASA, “NASA’s Kepler Mission Announces Largest Collection of Planets Ever Discovered,” 16-051, Kepler and K2, May 10, 2016, accessed May 16, 2016,; In Nature’s Destiny Chapter Eleven, I wrote some time ago before the Kepler mission was launched: “If the cosmos is indeed uniquely fit for life as it exists on earth then the existence of [rocky, earth-like planets] capable of harbouring life should be relatively common.” (Michael Denton, Nature’s Destiny, 95.) Although to date no planet exactly like the earth has been detected (see Mike Wall, “1st Alien Earth Still Elusive Despite Huge Exoplanet Haul,”, May 11, 2016, accessed May 16, 2016, there is a widespread and growing belief that they will eventually be found (Sara Seager, “Searching for Other Earths,” The New Atlantis (Fall 2015), accessed May 16, 2016,