by Richard Peachey
"The highest heavens belong to the LORD, but the earth he has given to men." (Psalm 115:16)
"In the 15 years since the discovery of the first planet around a star other than our Sun, we have found more than 400 such exoplanets — but there is still no place like home." — Debra Fischer (Yale University astronomy professor). 2010 (Apr 29). "Signatures of life on other worlds." Nature 464:1276.
". . . the NASA spacecraft Kepler, which watches for a dimming in a star’s light as a planet crosses its face . . . was launched in 2009, and data from its first two years of operation have revealed about 3,000 candidate exoplanets. . . . Surprisingly, the cumulative data set does not contain a single Earth-sized planet at a habitable distance from its star . . . ." — Eric Hand. 2012 (Oct 18). "The exoplanet next door: Earth-sized world discovered in nearby α Centauri star system." Nature 490:323.
• Earth has a large supply of water, and our optimum distance from the Sun allows the water to exist in three forms — vapour, liquid, and ice. (Other planets have virtually no liquid water.)
• The two major constituents of our atmosphere, oxygen (21%) and nitrogen (78%) are balanced to make up the ideal medium for the support of life.
• Water vapour and carbon dioxide in our atmosphere produce a "greenhouse effect" moderating temperature extremes and allowing survival of a variety of life forms.
• The nearly circular orbit of Earth and its relatively short period of rotation also limit temperature variations.
• Earth is provided with multiple safety shields to screen out the harmful components of the Sun's radiation. In addition to our distance from the Sun, shielding factors include the Earth's magnetic field and atmosphere, in particular the ozone layer.
• Elements essential to life (e.g., carbon, nitrogen, oxygen, phosphorus) are found in Earth's crust, hydrosphere, and atmosphere in quantities much greater than the average abundance observed in other parts of the universe.
• The inclination of Earth's axis of rotation (tilted 23.5° from a line perpendicular to the plane of Earth's orbit) provides for the seasons, permits all places on Earth to receive sunlight regularly, and probably prevents the locking of excessive amounts of water in polar glaciers.
• Our Moon's gravitation produces important tidal circulation effects in the oceans, making conditions much more suitable for sea life in the shallow zones along shores and in estuaries. The Moon's size is adequate (larger than most moons compared to their planets), and the Earth-Moon distance is small enough (unlike that of many moons from their planets) to produce tides of appropriate size, and to provide a good-sized night light.
As for other planets within our solar system: — None of them have any liquid water or atmospheric oxygen to speak of.
The surfaces of Mercury and Venus are hot enough to melt lead. Mercury is heavily cratered and has virtually no atmosphere. Venus's atmosphere is mostly poisonous carbon dioxide, with clouds made of sulfuric acid droplets.
Mars has only a thin atmosphere of carbon dioxide, nitrogen, and argon.
The gas giants Jupiter, Saturn, Uranus, and Neptune all have very low temperatures, accompanied by intense storms of radiation and magnetism; their atmospheres consist of hydrogen, helium, and methane.
Be it ever so humble — there's no place like home!
"Rare Earth Factors"
The late astronomer and popularizer of science, Carl Sagan, described planet Earth as an unremarkable "pale blue dot" floating in the vastness of space. He suggested there might well be billions of planets suitable for life, even within our own galaxy. But more recently, two scientists have presented a case that, in light of the known evidence, our planet is very special. Peter Ward, an evolutionary geologist, and Donald Brownlee, an evolutionary astronomer, both at the University of Washington, have written a book titled Rare Earth: Why Complex Life is Uncommon in the Universe (2000. New York: Copernicus [Springer-Verlag]). This book argues that a variety of factors make the Earth "rare" as a home for complex life — in particular, animal life. For example:
- right position in galaxy (not near the centre with its high star density, dangerous supernovae, and lethal radiation)
- right mass of star (not too much ultraviolet radiation)
- right distance from star (temperatures suitable for life)
- solar system has stable planetary orbits (giant planets do not create orbital chaos)
- Jupiter-like neighbour at right distance, to clear out comets and asteroids
- large supply of liquid water near planet's surface
- right kind of ocean, in terms of pH, salinity, temperature, and volume
- right atmospheric properties (to maintain adequate temperature, composition, and pressure for plants and animals)
- right planetary mass to retain atmosphere and ocean
- right amount of carbon to sustain life
- right tilt (seasons not too severe)
- large moon at right distance; stabilizes tilt
Note: Both of the Rare Earth authors accept macroevolution, and a few of the "factors" quoted in their book reflect this frame of reference (their summary, presented on pages xxvii-xxviii of the book, is shown below). Most of their points, however, are acceptable from anyone's point of view.
|Right distance from star
Habitat for complex life.
Liquid water near surface
Far enough to avoid tidal
Right mass of star
Stable planetary orbits
|Right planetary mass
Retain atmosphere and
ocean. Enough heat for
Clear out comets and
asteroids. Not too close,
not too far.
Small neighbor as
possible life source to
seed Earth-like planet
Build up land mass.
Enhance biotic diversity.
Enable magnetic field.
|The right tilt
Seasons not too severe.
Few giant impacts.
No global sterilizing
impacts after an initial
|The right amount
Enough for life.
Not enough for
Maintenance of adequate
and pressure for plants
pathway to complex
plants and animals
|Evolution of oxygen
Invention of photo-
synthesis. Not too much
or too little. Evolves at
the right time.
|Right kind of galaxy
Enough heavy elements.
Not small, elliptical, or
|Right position in galaxy
Not in center, edge
Snowball Earth. Cambrian