Is There Another Earth Out There?
(adopted from an article by Guillermo Gonzalez, Donald Brownlee and Peter D. Ward, Refugees for Life in a Hostile Universe, Scientific American, October 2001)
Is there life on other planets? Science has been trying to answer this question for a long time. In the article, the authors argue that it is highly improbable that complex life forms exist in our galaxy. To quote the authors: “Researchers are now casting a skeptical eye on musings about the prevalence of intelligent life throughout the Milky Way”. Furthermore, according to the article, not only may most of the solar system be unfriendly to multi-cellular or complex organisms, the same may be true of much of the galaxy. In fact, in recent years, astronomers have gained an appreciation of just how deadly our galaxy can be, filled as it is with exploding stars, radiation, and stellar close encounters.
Creating a Hospitable Planet
How does one determine the hospitality of a planet? Within a given planetary system, astronomers describe the optimal locations for life in terms of the cicumstellar habitable zone (CHZ). The CHZ is generally considered to be a region around a star where liquid water can persist on the surface of an Earth-like planet for at least a few billion years.
In 1999, there was a proposed concept of a galactic equivalent to the CHZ: the galactic habitable zone (GHZ). The GHZ defines the most hospitable places in the Milky Way-those that are neither too close nor too far from the galactic center. The boundaries of the galactic habitable zone are set by two requirements: a). the availability of material to build a habitable planet and b). adequate seclusion from cosmic threats.
a. In order for a planet to be created, sufficient building material must exist. Interestingly, metals are the building blocks of Earth-like planets. In fact, the abundance of metals affects the size of the planets that can form. Size, in turn, determines whether a planet can retain an atmosphere and sustain geologic activity. The abundance of metals is measured by metallicy-the ratio of the number of metal atoms to the number of hydrogen atoms. Particularly, the lower the metallicy of a planet, the lower is the abundance of metals. In fact, without enough metals, large planets can not form at all. On the other hand, too high of a metallicity can also be a problem. High metallicity increases the density of the protoplanetary disk and therefore induces the giant planets to shift position. The result of such shift is that such planet will throw any smaller, Earth-like bodies out of the system all together or push them into the sun.
b. Metallicy is not enough to determine hospitality of a planet. To contain life, a planet must also be kept reasonable safe from outside threats, such as the impact of asteroids and comets. Blasts of radiation also present a problem. Until a certain point, a planet’s magnetic field can fend off most particle radiation and its ozone layer can screen out dangerous electromagnetic radiation. However, sufficiently energetic radiation can ionize the atmosphere and generate nitrogen oxides in amounts capable of wiping out the ozone layer. Energetic radiation hitting the atmosphere can also let loose a deadly rain of secondary particles.
Effects on Earth
In many ways, the Milky Way is unusually hospitable: a disk galaxy with orderly orbits, comparatively little dangerous activity (comets and asteroids) and plenty of metals. Earth satisfies the requirement for optimal metallicity. The mix of land and sea on Earth is important for atmospheric temperature control and other processes. In our solar system, the frequency of asteroid impact depends on the details of Jupiter’s orbit and formation; the rest of the galaxy has no direct effect. As far as radiation is concerned, the nucleus of the Milky Way is currently relatively inactive. To quote the author: “If our ideas about the GHZ are correct, we live within an especially comfortable region of the Milky Way. Any civilization seeking a new world would, no doubt, place our solar system on their home-shopping list”.
The Possibility of Life on Other Planets
Given the above factors, can one determine if there is life on other planets? According to the article, such can be determined. The outlook on the possibility of extra-terrestrial life, however, is weak. Various reasons exist for author’s opinion.
The authors argue that only part of the Milky Way satisfies the requirement of optimal metallicity. Taking into account the disk metallicity, rough limits can be placed on the GHZ both in space and in time. In fact, few planets in our galaxy satisfy the requirement for optimal metallicity. The broad universe looks even less inviting than our galaxy. About 80 percent of stars in the local universe reside in galaxies that are less luminous than Milky Way. Because the average metallicity of a galaxy correlates with its luminosity, entire galaxies could be deficient in Earth-size planets.
As far as cosmic threats are concerned, the cometary threat is very sensitive to the galactic environment. The frequency of comet perturbation depends on the position in the Milky Way. As one goes toward the galactic center, the density of starts increases, so there are more close encounters. Comets are thought to reside in two long-term reservoirs: the Kuiper belts and the Oort cloud. Other stars probably have similar retinues.
All this suggests that complex life form is rare in the galaxy. The inner regions of our galaxy suffer from orbital instabilities, radiation bursts, and cometary perturbations. The outer regions are safer, but because of the lower metallicity, terrestrial planets are typically smaller there. Another effect concerns the dynamics of stars in a galaxy. Stars in elliptical galaxies have randomized orbits and are therefore more likely to frequent their more dangerous central positions.
Thus, as this paper has described, life on other planets is highly improbable. The authors attempted to explain why, in their opinion, it is difficult for complex life forms to exist in the universe. So for now, extra-terrestrials will remain in science-fiction stories. Interstellar travelers in books will continue to visit exotic locales in the Milky Way and meet with interesting aliens. As far as reality is concerned, however, the question of extra-terrestrial life still remains unanswered.