Earth is a Lucky Planet, Part Three. Goldilocks’s Earth

In two previous essays, I discussed the Rare Earth Hypothesis of Peter Ward and Donald Brownlee. Earth is a lucky planet, first because the Sun is a stable star; and second because Jupiter and the Moon help to stabilize conditions on Earth. In this third essay, I explain how Earth itself is a mighty lucky planet.

Earth happened to form in the “habitable zone” of the new Solar System, in which temperatures were in the right range to allow water to exist in liquid form. No medium other than water is known in which lifelike processes can occur. Some scientists speculate that the liquid methane on Saturn’s moon Titan may be a medium for life. Even if this is the case, molecules move very slowly in liquid methane, and any metabolism of life forms on Titan would be very slow and the resulting life forms would be very simple. And Earth has plenty of water. The water may have been delivered to the young Earth by comets hitting it before 3.9 billion years ago. The ice in the comets melted and vaporized, creating a haze of steam, much of which was lost into space as new comets continued to rain from the sky. When the collisions became less frequent, and Earth cooled down, the steam became oceans, and water vapor saturated a hot, dense atmosphere of carbon dioxide and nitrogen gases. Mars, Earth’s little brother, also had oceans when it was a young planet.

The Earth is also just the right size. If the Earth were too large, its gravity would be so great that complex organisms (not to mention mountains or even continents) would not be able to stand up. If the Earth were too small, however, it would be unable to hold onto its atmosphere, partly due to a lack of sufficient gravity, and also because particles streaming from the Sun would have stripped the gases away. Mars, which is about half the size of Earth, has an atmosphere—just barely. Its atmosphere is about one percent as thick as ours. At first this seems strange, given that its gravity is at least one-quarter as strong as that of the Earth. But Mars is small enough that its core has cooled off and solidified. On Earth, the currents of molten lava produce a magnetic field that deflects much of the dangerous solar particle stream. Mars has no such protection. The solar particles have scoured away most of its atmosphere, as well as its surface water. When Earth and Mars first formed, they were both wet planets with carbon dioxide atmospheres. Earth kept its atmosphere, and evolved; Mars lost its atmosphere, and (apparently) died. The surface of Mars is without life; and if there is life on Mars, it is deep in the rocks and therefore microbial in size.

The fact that the Earth is not too large and not too small, and is just the right distance from the Sun, has been compared by some scientists to the story of Goldilocks, the Aryan girl who thought that she had the right to barge into somebody else’s house. She found that Baby Bear’s food and bed were “just right.” And if Earth were not “just right” in size and chemical composition, there would be no life on Earth more complex than microbes.

Our planet is also fortunate to have radioactive elements in its core. Without radioactivity, the core of the Earth might have cooled down and solidified, causing Earth to have a fate similar to that of Mars, though it would not have happened quite as quickly.

Our planet also happens to have plenty of carbon, which may be the only element from which life can be built and sustained. As any aficionado of Star Trek knows, silicon-based life forms such as the Horta are conceivable, since silicon atoms have several chemical similarities to carbon. However, silicon is probably too heavy to allow a silicon-based life form to participate in a silicon-based ecosystem. Silicon is a mineral; it is always a mineral. Carbon atoms can form carbon dioxide gas, which circulates through the atmosphere and is turned into complex molecules by photosynthesis. But silicon dioxide is quartz and just stays in the crust of any planet on which it is found.

The most obvious way in which Earth is lucky is that it has water—lots of it. Earth is not quite unique in this respect—Jupiter’s moon Europa is covered with oceans that are capped with ice. But on Earth, the water exists in all three states: ice, liquid water, and water vapor. Not only do life processes, as we know or imagine them, occur in liquid water, but water moves from oceans to continents and back in gaseous form. Europa has liquid water (kept from freezing not by any warmth from the Sun but by the pull of Jupiter’s gravitational field) but has no water cycle as does the Earth. Thus when you consider carbon and water, Earth is lucky not just in what it has but in what it can do with it: carbon and water can circulate around and around on the planet.

Ward and Brownlee conclude that simple microbial life may be common in the universe. But for advanced life to evolve, it is necessary that planetary conditions remain within certain limits for a long time. Such long term stability, and the complex life and advanced civilizations that would require such stability, appear to be vanishingly rare in the universe.

This essay is adapted from chapter 1 of my forthcoming book, Life of Earth: Portrait of a Beautiful, Middle-aged, Stressed-out World, to be released soon by Prometheus Books.

Earth is a Lucky Planet, Part Two. Thank God for Jupiter?

In a previous entry, I introduced the Rare Earth hypothesis of Peter Ward and Donald Brownlee, which states that Earth-like planets on which complex life could have evolved are very rare in the universe. One reason was that Earth revolves around a stable star, the Sun.
Ward and Brownlee also point out that the Earth resides in a very lucky neighborhood of the Solar System. The two sources of luck are Jupiter and the Moon. First, consider Jupiter.

When the Solar System first formed, it was a disc of small asteroids. Many of these asteroids ran into each other and were crushed into planets by their own gravity. These planets continued to mop up asteroids until about 3.9 billion years ago. After that time, few asteroids remained that could crash into the planets. Most of the craters on the Moon (which, as large as some planets, also helped to clear away asteroids) are older than 3.9 billion years. The Moon, which has no wind or weather, has preserved an intact sample of the asteroid impacts that imperiled the early Solar System.

Another important component of the Solar System is comets. There are billions of these dirty balls of ice that orbit the sun just beyond the outer edge of the Solar System. Most of them remain at the edge of the Solar System, but some of them have very elliptical orbits, which bring them close to the sun. They whip around the Sun like a slingshot, and fly back out into the outer edges of the Solar System. While comets are near the sun, solar radiation vaporizes some of the water, creating the comet’s “tail” that everyone recognizes. Before 3.9 billion years ago, there were also a lot of comets, but they are now, like asteroids, comparatively rare.

The principal reason that asteroids and comets now only rarely fall from the sky is the planet Jupiter. Jupiter is so massive, and has such a powerful gravitational field, that it has sucked up most of the asteroids in the inner solar system, except for those in the asteroid belt, whose orbits have been stabilized by that same Jovian gravitation. Any asteroid or comet that happens to come within several million miles of Jupiter is drawn inevitably into its gaseous embrace. This is exactly what happened to the comet Shoemaker-Levy 9 in 1994. After whipping around the Sun and heading back into the outer reaches of the solar system, this comet slipped too close to Jupiter, whose gravity fractured it into pieces. Each piece created a huge flare of radiation as it fell into Jupiter’s dense atmosphere, and each of the black spots that remained visible for a few weeks was similar in size to the Earth. Therefore Jupiter continues to clear away asteroids and comets from the Solar System. Without Jupiter, asteroids and comets might be hitting Earth so frequently that life would not have a chance to exist for very long.

And then there is Earth’s closest neighbor, the Moon. Most planets have moons, but Earth is the only planet in the Solar System with a moon so large in relation to it. Mars has two tiny moons, Deimos and Phobos, named after the two horses of the war god’s chariot. Jupiter and Saturn have moons larger than ours, but tiny in relation to the planetary masses. Our Moon is large enough and just far enough away to profoundly influence our planet without severely disrupting it. Everyone knows that the tug of the Moon causes the tides. Were it not for tides, there would be no intertidal zone, the only home of thousands of species of organisms. But tides may be of relatively little importance to the planet as a whole, even though they are important to barnacles. The major effect of the Moon on Earth, crucial to the survival of life as a whole, is to stabilize its movement.

As planets revolve around their suns, they rotate on their axes. These rotational axes wobble, pointing in different directions at different times. Any planet with a large amount of wobbling would have unstable climatic zones, since sometimes the equatorial zone and sometimes the polar zones would directly face the sun. The part of a planet directly facing its sun will receive the most intense radiation and be warmest. How could tropical, temperate, and polar plants and animals evolve, if the climates of those zones are extremely variable? This appears to have happened with Earth’s less fortunate little brother, Mars. Earth, however, has not tilted more than about 20 degrees from the plane of its revolution. Even the little bit of wobbling that the Earth does experience has been enough to cause about twenty Ice Ages during the last two million years of Earth history. We have the Moon to thank for the relative stability of Earth’s movements.

Earth is mighty lucky to have neighbors like Jupiter and the Moon. Otherwise, complex life might never have evolved here.

I adapted this essay from part of chapter 1 of my forthcoming book, Life of Earth: Portrait of a Beautiful, Middle-aged, Stressed-out World, to be released soon by Prometheus Books.

Also do not forget the new YouTube channel that I announced in the previous post (see below).

Earth is a Lucky Planet, Part One. Thank Our Lucky Star

Much of the story of life on planet Earth has been due simply to luck. This is particularly true of the physical environment which has allowed evolution to produce such a diversity of species.

One day in 1950 over lunch with his scientific colleagues, physicist Enrico Fermi heard someone speculate about how many advanced civilizations there must be out in space. Fermi quipped, “So, where are they?” He meant that if there were many advanced civilizations, some of them must be more advanced than we are, and must have invented space travel—and at least some of them should have contacted us by now. This has come to be known as “Fermi’s Paradox.” One answer to this paradox is that there are so few advanced civilizations in the universe that they have not found us yet and probably never will. According to this view, Earth-like planets might be very rare. Very few planets have been as lucky as Earth. According to the “Rare Earth Hypothesis” of planetary scientists Peter Ward and Donald Brownlee, we can begin by thanking our lucky star, the Sun.

First, the Sun is a calm and stable star. Many stars fluctuate wildly in their energy output. Such pulsations in energy may prevent life from ever getting started on any planets that revolve around variable stars. In contrast, the Sun has been stable for billions of years. Not perfectly stable, of course. The Sun has had occasional “coronal mass ejections,” in which it propels energy and particles from its outer layer out into the Solar System. One of these mass ejections, on September 1, 1859, was strong enough that it shut down the telegraph systems in the United States and Europe and caused auroras to occur in the skies of even tropical regions. The Sun also has an 11-year sunspot cycle. These variations, however, have not had much effect on Earth. Coronal ejections have not been known to have ever harmed life on Earth (no organisms were harmed when the telegraphs shut down), and solar intensity shifts by only 0.1 percent during the sunspot cycle. The Sun has, in fact, changed its energy output over the billions of years of its existence. It has increased the intensity of its radiation by about 30 percent during that time—but it has done so very gradually.

Second, the Sun is an isolated star. Many stars have partners, forming multiple-star systems—most commonly, binary systems in which two stars swing around each other like dancers. If the Sun were part of such a close family of stars, the other stars would prevent planets from having stable orbits, which might prevent the evolution of life. The Sun is also far away from stars that emit so much energy that they would disrupt or destroy life. For example, a supernova anywhere within a few dozen light years of Earth would wipe out all of life—but there have been no supernovae in the Sun’s neighborhood for at least several billion years. Moreover, stars in the centers of galaxies may be so close together that they would disrupt the revolution of one another’s planets, even if they are not part of multiple-star systems. But the Sun is on a swirling arm far from the center of the galaxy. If we were near the center of the galaxy, many stars would be so close to us that night would not be very dark, and those stars would yank and tug us around and disrupt the stability of our planet’s conditions.

Therefore, it appears that complex life would not have had time to evolve on planets that revolve around most stars. Earth is not just a special planet in our solar system, but in the universe. The story of evolution, as presented in this blog, might be something quite rare in the universe.

The foregoing was adapted from a portion of my upcoming book, Life of Earth: Portrait of a Beautiful, Middle-aged, Stressed-out World, soon to be released by Prometheus Books.