A Planet Suitable for Life
Many planets formed, but one was especially suitable for life. The Earth unites all the special conditions we discuss below.
A Host Galaxy Rich with Dust
Making the complex molecules of life requires most of the elements. The heavy elements are found in star dust. Some galaxies have very little dust. Life requires a dusty galaxy.
A Galactic Location among New Stars
Galactic centers usually have old, hot, bluish-white stars. We must look in the galactic rim, where there are new yellow stars burning at a lower temperature. These stars incorporate carbon and oxygen nuclei to catalyze their nuclear reactions.
A Solitary Parent Star
The parent star must be a bachelor star. The planet that hosts life must have nearly uniform lighting and heating. Double, triple, or multiple stars would make planet orbits too complicated. Complicated orbits, with the planet sometimes near one star, sometimes near another, sometimes far from any star, will not do. Only a single star can have planets with simple orbits.
A Star of the Right Size
A star’s luminosity depends on its size: the bigger, the brighter.
A little parent star has very low light output, and a planet has to orbit very close to the star to get enough warmth. The star would fill up a great deal of the daytime sky. There would hardly be shadows. Sundials would not work. But that’s not the only problem. A planet that is too close to its parent star will become tide-locked, synchronizing its rotation with its orbital revolutions. The Moon rotates only once in its orbit around the Earth. Mercury, the planet closest to the Sun, rotates three times every two orbits. Venus, the planet between Mercury and the Earth, makes complete rotations very slowly. The cooling and heating of night and day on a tide-locked planet is so slow that there are big temperature swings. Every night most of the water freezes and every day most of the water boils. Nearly all life would have to migrate daily around the planet, keeping close to the sunrise or sunset terminator. On Earth relatively few species migrate and of them only a few migrate distances comparable to the diameter of the planet. None migrates daily.
A big parent star has tremendous luminosity. The light output from a big parent star is very great, and the planet must orbit very far from the star to obtain the right temperature. The star would look like a point of light, much smaller than our Sun’s disk. The star would still provide information. It would serve as a sign for timekeeping. However, the planet could have no intelligent inhabitants to read a sundial. Big stars burn up their fuel very rapidly. Plant life would not have sufficient time to oxygenate the atmosphere. We need not search any planets orbiting a big star for animals or more intelligent life.
Once stars burn all their hydrogen, they begin to collapse and rise in temperature. If they are big enough, the temperature rises until they can burn helium. Helium makes a much hotter nuclear fire. The outer layers of the star expand outward, perhaps engulfing any planet that was at the right distance during the hydrogen-burning phase. Whether or not some planets are engulfed, the helium also burns up rapidly, the fire goes low again, and the star starts to collapse again, raising the temperature still higher. If the star is only 40 percent bigger than the Sun, the temperature rises to the ignition temperature of all the remaining 90 elements at once. The star becomes a supernova. The resulting conflagration blows the star apart in a few days. That would be the end if there were any life on any planet in the vicinity.
A Star of the Right Color
There are stars of different colors and temperatures. Stars range from the hottest, the bluish-white stars, on down through the intermediate yellow stars to the relatively cool red giant stars. The constellation Orion contains the range of colors. Orion is a hunter who carries a sword or dagger that hangs from his belt. The star on his right shoulder, alpha orionis or Betelgeuse, is a red giant star, with a surface temperature of 3 000 kelvins or Celsius, 5 400º F. The star on his left foot, beta orionis or Rigel, is a bluish-white star, with a surface temperature of 25 000 kelvins or Celsius, 45 000º F. There are stars twice as hot as Rigel, but they are either too far away to be bright and easily recognized, or located too close to the southern celestial pole to be seen from the northern hemisphere. Our Sun, with a surface temperature of about 6 000 kelvins or Celsius, 10 000º F, is intermediate.
Some people call the Sun average or even mediocre. This is not so. A star can have a planet orbiting around it at practically any distance. If the planet orbits close to the star, the planet will be hot. If the planet is far from the star, the planet will be cold. But a planet may orbit any star at the appropriate distance to maintain an average temperature of about 280 kelvins, 7º C or 45º F.
The four gases needed for life are oxygen, carbon dioxide, water vapor, and nitrogen. The latter is needed for plant fertilizer. They are all transparent in a band of frequencies called the visible band. The visible band transmits the red and green colors that photosynthesis needs. A yellow star emits most of its light in the middle of the visible band. If the star were any other color, the atmosphere of the planet would block most of the star’s light. A planet could be at the right distance from a bluish-white star or a red star to get the right temperature, but daylight would be dim at the surface. That would hardly be useful to intelligent beings with vision. If the planet orbits a red star, there will not be enough blue and violet light for normal rates of photosynthesis. There is some photosynthesis with red light only, but the lack of blue and violet light is a limiting factor. If the planet orbits a bluish-white star there will be far too much ultraviolet radiation. Intense ultraviolet radiation destroys the complex biochemical compounds needed for life. Plant life has to thrive for millions of years to oxygenate the atmosphere of a planet. Free oxygen will dissociate into ozone in the upper atmosphere under the action of the more energetic and harmful ultraviolet rays from the parent star. The ozone layer must be present to protect life on the planet’s surface from these rays. If the planet’s atmosphere doesn’t have free oxygen, it can’t have an ozone layer in the upper atmosphere to protect its surface. Harmful ultraviolet rays would reach the ground and prevent complex compounds like chlorophyll from ever forming.
We need not search for life near either red stars or bluish-white stars. Yellow stars are special. Also, there are little parent stars and big parent stars. Our Sun is a parent star of intermediate size. Is that special? Yes! Who says our Sun is mediocre? It is very special. Are we just lucky, or did a benevolent, powerful intelligence choose the Sun for us?
A Bright Sun in a Dark Sky
Livable temperatures on Earth require a bright Sun in a dark sky. We have already seen that the expansion of the universe makes the night sky dark. Sunlight governs the day because it comes from one direction only. We can use the angle of sunlight to determine the time of day. When the sky is sufficiently unclouded to allow the Sun to cast a shadow, we can get this information from the sunlight with a sundial. Even when a heavy overcast blurs all shadows it is usually possible to determine the position of the Sun with some accuracy. The Sun therefore serves as a sign of the time of day.
The same physical arrangement, a bright source in a dark sky, makes sunlight useful for powering heat engines. One very important kind of heat engine is animals, including people. We will examine this way of interpreting ourselves in the chapter below about the thermodynamics of life.
The host planet’s orbit must be nearly circular to avoid extremes of temperature throughout the year. If the star has more than one planet, the orbits must nest neatly to prevent collisions. All the orbits must be nearly circular. Many extrasolar systems have one large planet in a highly elliptical orbit. Imagine Jupiter in a highly elliptical orbit that overlaps the Earth’s orbit. It would run around in our solar system like a bull in a china shop. The Earth would sooner or later suffer a fatal collision that would wipe out all known life.
A Court of Planets
A court of planets will collect elements not needed in abundance on the host planet, like the extra hydrogen found in the atmospheres of Jupiter, Saturn, Uranus, and Neptune. Large planets should go on the outside, to use their gravity to disrupt the orbits of comets and eccentric asteroids and defend the inner planets.
A Nearly Spherical Planet
The planet must be nearly spherical. Otherwise the planet might become tide-locked, that is, its rotation could slow until its day was comparable to its year. The Moon is elliptical and one side always faces the Earth. This makes a day on the Moon a month long. Mercury is elliptical like a football and on its closest approach to the Sun one of its ends points toward the Sun. This makes Mercury’s day equal two thirds of its year. The Earth’s more rapid rotation provides more uniform heating.
Moderate Orbital Inclination
The planet must not be so inclined that its poles are close to the orbital plane, or the day on most of the planet will be the same as the planet’s year. The North and South Poles of the planet Uranus point alternately toward the Sun. Since it takes Uranus 84 years to revolve around the Sun, one cycle of light and darkness there is also 84 years long. On the other hand, some orbital inclination is needed to produce seasons. The Earth’s spin axis is inclined 23 degrees from vertical relative to the Earth’s orbital plane. This makes noontime sunlight shine from high in the sky on a summer day. It shines from low near the horizon in winter. The variation in temperature this produces during the year corresponds to important cycles of renovation among plants and animals.
A Large Satellite
Preferably the planet will have a satellite of appreciable relative size. The Moon helps to defend the Earth from impacts by disrupting the orbit of any stray asteroid that approaches the Earth.
The Moon also stabilizes the Earth’s orbital inclination against disturbances from the other planets. Without the Moon, the Earth would periodically tip its axis so far that its poles would point toward the Sun, and the day would be equal to the year. All animal life on Earth would have to make a semiannual migration of 10 000 miles or 16 000 kilometers.
A large satellite will cause tides to scrub the continental shelves and increase a healthy interaction between the hydrosphere and lithosphere.
Animals can use the satellite for some illumination at night. Intelligent life can use the satellite to gauge the passing of time in units longer than days and less than years.
The Right Temperature
The orbit must be at the right distance from the parent star. The distance and type of star determines the temperature range. Water should be liquid most of the time to permit a wide variety of chemical reactions. This is also the right temperature range for making long hydrocarbon chains.
The old science-fiction idea of high-temperature life based on hydro-silicon chains does not work. Silicon does not form nearly the variety of complex molecules that carbon does.
The Right Size for Just Enough Atmosphere
The inhabitable planet should be large enough to retain an atmosphere, but not too large, or the atmosphere will be too thick. The planet must be small enough to let excess hydrogen escape. The size for retaining an atmosphere is related to the average temperature.
A Molten Core
The planet should have a molten, electrically conducting core to permit currents that generate a magnetic field. This defends the planet from those cosmic rays that consist of charged particles moving very fast. The iron core of the Earth is very suitable. The core must contain radioactive materials to keep it hot and fluid.
Various Kinds of Rocks
There should be a good mix of rock materials, some of high density and some of low density. Then the low-density parts of the crust will be thicker than the high-density parts. Since the crust will float on the molten core, the thick parts of the crust will be continents. Their outer surface elevation will be higher than that of the thin parts of the crust, which will become the ocean basins. Such a mix will provide a variety of habitats. Otherwise, a planet with much water would be almost all ocean, and one with little water would be almost all continent.
Abundant Water, Not Other Liquids
The water molecule has an odd angle between the hydrogen atoms, not exactly 90 degrees. In water vapor the angle is 104 degrees 40 minutes. Water’s property of expanding when freezing is due to flexibility in the angle. When the molecules are warm enough to move past one another, dynamically they occupy less volume than they do when they are cold enough to have fixed positions and a random structure.
Let’s explain this point in more detail. The oxygen atom is strongly electronegative. This means that it holds all the electrons very close to itself. There are ten electrons in a water molecule but the nucleus of the oxygen atom has only eight protons. Therefore the oxygen atom in a water molecule has almost two unbalanced negative charges. The two hydrogen nuclei are left nearly bare, so each has almost one unbalanced positive charge.
The oxygen atom in one water molecule is attracted to the hydrogen atoms of other water molecules. At a low temperature this kind of “extra-molecular hydrogen bond” can attach one water molecule to four others. The latticework that builds up occupies a great deal of space. At higher temperatures, in liquid water, there is another structure in which one molecule is attached to three others. The latticework is unstable and occupies less space than the low-temperature latticework. Liquid water molecules do not fill up as much space as they do in ice. This is what makes ice float on liquid water. Here is a case where some disorder, or order less than perfect, is necessary for life.
Liquid water has a specific gravity of 1.000 by definition at 4º C. It expands when it freezes to a specific gravity of 0.92. This means that ice floats. If water had molecules that fit neatly together like almost all other molecules, lakes and oceans would be frozen solid from the bottom up. In summer there might be pools of chilly water on top of the ice. Aquatic life could not exist.
On land, expansion on freezing enables water to break up rock masses. Water creeps into crevices and then freezes, exerting pressure that opens cracks. Then when a thaw comes the water fills the new crack and is ready to expand the crack some more. It took many cycles of freezing and thawing before water broke up the rocky surface of the land or the volcanic shield into fine fragments. When water had done its work, the Earth’s surface was loose enough to permit roots to penetrate, and plants could grow.
Neither methane nor ammonia has this property of expanding when freezing. These chemicals are abundant in the atmospheres of Jupiter, Saturn, Uranus, Neptune, and Titan, a moon of Saturn. One or more of the planets we just mentioned may be gas all the way through, but we know that Titan has a solid surface. Titan does not have soil like the Earth because any surface water present is too cold to alternate between freezing and thawing. The surface is not like soil and cannot support plants.
The Right Balance between Gases
The balance between carbon dioxide, water vapor, and temperature is critical. If the temperature is low enough but not too low, the water will be liquid. The oceans will dissolve a great amount of the carbon dioxide. Water containing carbon dioxide is a weak acid that wears down limestone and makes material available for seashells. The air will not contain too much carbon dioxide, and some of the heat will escape to outer space. Thus a temperature balance can be maintained on a planet like the Earth.
But if the temperature becomes too high, the water will release the carbon dioxide, like a warm soft drink releases bubbles. This will trap heat under the atmosphere and raise the temperature higher. Venus has a surface temperature of 500 degrees and no liquid water on the surface because of this runaway greenhouse effect. For the same reason Venus is subject to violent storms. Winds there are constantly moving at double the speed of the most violent hurricane winds on Earth.
Surface Soil and Dissolved Gases
The surface soil must be loose but not too loose. If there is nothing to hold it down there will be constant dust storms. Volcanoes can make the soil loose and porous if the molten lava has dissolved gases in it. These are present on Earth because tectonic plates are always sliding over one another. This drags earlier ocean floors with their seashells and diatomaceous matter down into the mantle. The biological material, including dissolved carbon dioxide, later returns to the Earth’s surface in molten rock. If there were no dissolved gases the lava flows would produce a hard, impervious surface like that of Venus.
Physical, geological, and climatic conditions on the Earth are relatively tranquil. Everywhere else in the solar system we find violence. Scientists have puzzled over many “Goldilocks” coincidences. Looking at Venus, Earth, and Mars we see that the Earth is “not too hot, not too cold, but just right.” Earth is the only known place in the universe with liquid water.
The daily weather report for Venus features winds twice as fast as the most powerful hurricanes on Earth. The Red Spot, bigger than the Earth, that Galileo saw on Jupiter turned out to be a storm that has been raging more than 350 years and perhaps forever. In 1989 we discovered a similar storm, the Black Spot of Neptune.
There are volcanoes and lava flows on Earth, but ours are dwarfs compared to those of Venus and Mars. One of the Voyager space probes photographed a volcano in eruption on Io, the innermost large satellite of Jupiter. Io is smaller than our own Moon, but it is covered with volcanoes and fresh lava flows. In the farthest, coldest reaches of the solar system there may be no hot magma, but the surface of Triton, Neptune’s largest moon, may have arisen from ice volcanism.
We can’t see the surface of Jupiter, Uranus, or Neptune, and we’re not sure that Saturn even has a solid surface. We haven’t yet seen the surface of Pluto or its satellite, Charon. But the surfaces of all the other bodies in the solar system are pockmarked with craters. Mercury has so many craters that more won’t fit. Any new crater obliterates parts of old craters. Venus has a protective atmosphere much thicker than Earth’s, and new lava flows resurface Venus every million years. Even so, radar found hundreds of impact craters there. Mars has what many astronomers thought was a smooth plain, until careful measurements with the Mars Laser Altimeter showed it to be a crater big enough to hold Western Europe or the United States from the East Coast to the Rockies. The moons of Jupiter, Saturn, Uranus, and Neptune are all as heavily cratered as our own Moon. The Earth has a few impact craters, but nothing like the number we see everywhere else. How did we escape?
An asteroid impact supposedly destroyed the dinosaurs and opened up a niche for the mammals at the end of an age about 65 million years ago. The asteroid needed to be big enough, at least 10 km in diameter, to destroy most large life forms. If it had been too big, say 30 km or more in diameter, it would have destroyed all forms larger than bacteria. That would have put the Earth back to pre-Cambrian conditions, and the progressive population of the Earth with plants, animals, and people would have had to start over. A really big one has never yet hit us.
There are an estimated 700 asteroids larger than 1 km whose orbits come as close to the Earth’s as 40 million km (30 million miles or 30% of the distance between the Earth and the Sun). The Asteroid Belt has about 1000 objects bigger than 50 km in diameter. Happily for us the known big ones are nicely shepherded by Jupiter’s enormous gravitational field and kept in the asteroid belt beyond Mars.
NASA has undertaken a survey to see if the Earth is at risk any time soon from asteroids whose orbits come close to or cross the Earth’s orbit. How long must they continue to survey the solar system until they can be sure they’ve seen most of the big ones? As an example we will take Halley’s Comet. Its nucleus is an irregular “potato shaped” object about 15 km long and 10 km in diameter. It spends most of its time out near the orbit of Neptune in the dark and cold where it can’t reflect enough light to be seen. We get to see it only once every 76 years when it ventures close to the Sun within the orbit of Jupiter. Every time a comet comes close to the Sun some of its material boils off and streams away in a long tail. Halley’s Comet will not last more than another thousand round trips. That means it will not last more than 76 000 years. Its present age must likewise be of the order of thousands, not millions, of years. If the solar system is about 5 000 million years old, why are there still comets? New objects are constantly being bumped down into lower orbits by close encounters with the outer planets. How many other big chunks are out there? In the newly discovered Kuiper belt, beyond Neptune, there are perhaps 100 000 objects bigger than 50 km in diameter.
From all of this we know that the Earth should have received as many hits as the Moon. Somehow we have escaped. Is the Earth just the luckiest planet in the universe? Or is there a better explanation?
In Isaiah 45:18 the prophet says, For this is what the LORD says—he who created the heavens, he is God; he who fashioned and made the earth, he founded it; he did not create it to be empty, but formed it to be inhabited—he says: “I am the LORD, and there is no other.” Can the reason the Earth is so well adapted for life be that God designed it that way? Does His invisible, protecting hand defend us from disastrous asteroid impacts?
One believes either in God or in the goddess Lady Luck. Our solar system and home planet seem to have “benevolent creative design” written all over them.
 Schwarzschild, Bertram, “Survey Halves Estimated Population of Big Near-Earth Asteroids,” Physics Today, 53 (Number 3, March 2000), pp. 21–23.