The “Just Right” Earth
• It is the perfect distance from both sun and moon to have a stable, predictable orbit. Twenty-four-
hour days ensure Earth’s entire surface is properly warmed and cooled every day.
• It is the perfect size and mass. If it were less dense, an atmosphere would not form and remain;
if it were more dense, its atmosphere would be uninhabitable. As it is, our atmosphere allows just
the right amount of radiation to keep us warm but not kill us.
• Earth’s metal core produces a magnetic field that protects the surface from radiation from space.
Radioactive heat from the core, mantle and crust creates plate tectonics, necessary for life for several reasons.
• Humans need 27 elements, including carbon, oxygen, nitrogen, phosphorus, potassium, sodium, iron and copper, to live. Earth has just the right mix of them.
• Earth has just the right amount of water in the proper ratios of liquid, solid and gas. A little more, and the continents would be drowned. A little less, and the planet would probably be too hot to host life.
“Just Right” Moon
• Our moon is huge: Its mass ratio compared to Earth is 50 times greater than that of any other known moon-planet combination. Yet it is the perfect size to stabilize the tilt of Earth’s rotational axis. Without our moon, the tilt of our planet would vary from zero to 85 degrees, producing catastrophic climate changes.
• Its gravity creates tidal movement. Thus, ocean waters do not stagnate—they clean the waters on the coasts from toxins and enrich them with nutrients. Yet these massive ocean movements never spill over across the continents. The ocean currents also regulate climate by circulating enormous amounts of heat.
“Just Right” Sun
• Most stars (two thirds to three quarters of them) are found in groups of two, three, four or more. If we had more than one sun, it would make life far less stable on Earth because of erratic gravitational effects, and/or possible dangerous eruptions of tidal gas passing between the stars.
• Ninety-five percent of all stars are less massive than the sun. A smaller, less dense sun would mean we’d have to be much closer to it to stay warm. The tidal locking would create synchronous rotation—where the same side of the Earth always faced the sun. Thus, half the planet would freeze.
• It has the perfect luminosity. Because it is a yellow star, its energy lies mostly in the visible part of the light spectrum—not even 10 percent of its energy is ultraviolet. If it was much hotter, producing mostly ultraviolet light, life would be impossible. If it was a smaller red star, the supply of visible light would be inadequate.
• The sun’s size and distance from Earth creates stable temperature fluctuations that allow Earth’s water to remain in perfect balance among its three phases, liquid, solid and gas.
“Just Right” Solar System
• Giant planets act as “comet and asteroid catchers.” Their gravity cleans up our solar system of space junk that might otherwise collide with Earth. Cosmic collisions can cause mass extinctions.
• Our solar system is unusually rich in metal content, necessary for advanced life.
“Just Right” Cosmic Location
• Evidence suggests that elliptical galaxies lack enough elements heavier than helium to host advanced life. Spiral galaxies like ours have enough.
• Within the Milky Way, we sit in the “galactic habitable zone”—far enough from the center that we’re not killed by radiation, but close enough that sufficient heavy elements needed for life are present.
• We’re nowhere near dangerous star clusters, quasars, nebulae, neutron stars or supernovas.
• “Real-estate brokers often say the key to property value is location, location, location,” “If this
principle applies to the cosmic scene, Earth’s location would be considered way beyond ‘prime.’ …
Earth appears to reside in the only neighborhood in the universe where humans can exist and thrive
long enough to enjoy a global, high-tech civilization and to discover how rare they are.”