Habitable Zones of the Universe

The Earth would only have to move a few million kilometers sunward—or starward—for the delicate balance of climate to be destroyed. The Antarctic icecap would melt and flood all low-lying land; or the oceans would freeze and the whole world would be locked in eternal winter.

—Arthur C. Clarke, Rendezvous with Rama, 1973

Location! Location! Location! The secret for producing great Hollywood films—and for selling real estate—is also life's secret for populating the Universe. Much of the Universe is clearly hostile to life, and only rare places offer even potential oases for its existence. Empty space, the interiors of stars, frigid gas clouds, the "surface" of gaseous planets like Jupiter—all must be lifeless. We cannot know for certain what the limits are for life's environments, but looking at what is needed to support Earth life provides a basis for estimating where in the Universe life might exist. We speculate in this manner with the understanding that we have a biased perspective—that of inhabitants of a planet that seems to provide a nearly perfect habitat.

One of Earth's most basic life-supporting attributes is indeed its location, its seemingly ideal distance from the sun. In any planetary system there are regions—distances from the central star—where a surface environment similar to the present state of Earth could occur. The favorable region or distance from the star is the basis for defining the "habitable zone" (referred to by astrobiologists as the HZ), the region in a planetary system where habitable Earth clones might exist. Since its introduction, the concept of habitable zone has been widely adopted and has been the subject of several major scientific conferences, including one held by Carl Sagan near the end of his brilliant career.

The defining aspect of the HZ is that it is the region where heating from the central star provides a planetary surface temperature at which a water ocean neither freezes over nor exceeds its boiling point (see Figure 2.1). The actual width of the HZ depends on how Earth-like we decide a planet must be to be deemed habitable. Extreme events, such as the loss of oceans or a deep planetary freeze, may seem totally preposterous to Earth-lings happily living in nearly ideal climatic conditions, but these events would surely occur if Earth were (on the one hand) slightly closer to or (on the other) slightly farther from the sun. Occupying the HZ, or planetary "comfort zone," is analogous to sitting next a campfire on a cold night. Imagine trying to survive a night in the Yukon when the temperature is 100°F below zero. You have a large campfire, but if you sleep too close to it you catch on fire, and if you are too far back you freeze.

Astronomers held the first discussions of the habitable zone in the 1960s. The range of the habitable zone was considered to be bounded by two effects: low temperature at the outer edge and high temperature at the inner edge. Our closest neighbors in space provide sobering examples of what happens to planets close to, but not within, the HZ. Closer to the sun than the HZ, a planet gets too hot. Venus is an example. The surface of this neighbor is nearly hot enough to glow. If Venus ever had an ocean, it has long since evaporated and been totally lost to space.

Venus

Venus

M0 star

50% of the Sun's mass 6% of the Sun's brightness lifetime - 50 billion years

Earth

M0 star

50% of the Sun's mass 6% of the Sun's brightness lifetime - 50 billion years

Earth

outer edge

lifetime - 10 billion years

inner edge

outer edge

lifetime - 10 billion years

inner edge

F0 star

1.3 times the Sun's mass 4.3 times brighter lifetime - 4 billion years

F0 star

1.3 times the Sun's mass 4.3 times brighter lifetime - 4 billion years

Figure 2.1 Estimates of the habitable zones (HZ) around stars that are slightly less and slightly more massive than the Sun (based on results of Kasting, Whitmore, and Reynolds, 1993). Two estimates of the cold outer edge of the HZ are based on the temperature where CO2 (dry ice) begins to condense in the atmosphere (inner limit) and the theory that Mars was in the Sun's HZ early in its history (the outer limit). The hot inner edge of the HZ is estimated both in terms of the belief that any oceans on Venus boiled away at least a billion years ago and in terms of estimates of the atmospheric conditions required to produce runaway greenhouse heating.

Outside of the HZ, temperatures are too low. Mars, for example, is frozen to depths of many kilometers below its surface. If Earth were moved outward (or if the sun reduced its energy output), Earth's atmosphere would cool to a point where the planet would become ice-covered. Eventually, carbon dioxide would freeze to form reflective clouds of "dry ice" particles, and ultimately, CO2 would freeze on the polar caps.

In 1978 the astrophysicist Michael Hart performed detailed calculations and reached a stunning conclusion. His work included the well-known fact that the sun becomes slightly brighter with time. About 4 billion years ago, the sun was about 30% fainter than at present. As the sun brightens, the HZ drifts outward. Hart called the small region wherein Earth would remain within the HZ over the entire age of the solar system the continuously habitable zone, or CHZ. His computations indicated that sometime during its history, Earth would have experienced runaway glaciation if it had formed 1% farther from the sun and would have experienced runaway greenhouse heating if it had formed 5% closer to the sun. Both of these effects were considered irreversible. Once frozen or fried, there could be no turning back. It is now considered possible that a frozen planet might become habitable with continued brightening of its central star. If the shape of Earth's orbit had been more elliptical, these limits would have been even smaller. Hart's work implied that the CHZ was astonishingly thin for the sun and that for stars of lower mass it did not even exist. This suggested that Earth-like planets with oceans and life were rare indeed.

Hart's CHZ is now believed to be too narrow because of several effects that he did not take into account. One of these is the discovery of a remarkable chemical process known as the CO2-silicate cycle that, on Earth, acts as a regulating thermostat to keep the planetary temperature within "healthful" limits. This cycle can maintain habitable surface temperatures over a moderate range of solar heating effects. CO2 is a trace gas that constitutes only 350 parts per million of the atmosphere, but it is a "greenhouse" gas: Its infrared-absorbing properties retard the escape of heat back into space. This greenhouse effect warms Earth's surface about 40°C above the temperature it would otherwise have. As we will see later in the book, the thermostatic control of the CO2-silicate cycle (which is also known as the CO2-rock cycle) occurs because of the effects of weathering. If the planet warms, increased weathering removes CO2 from the atmosphere, and the loss of CO2 leads to cooling. When Earth is too cool, weathering and CO2 removal decrease, while the continual atmospheric buildup of volcanic CO2 leads to warming. This remarkable negative-feedback system widens the continuously habitable zone and also complicates efforts to determine its boundaries precisely, because the CO2-rock cycle is not perfectly understood on a planetary scale. Using this new information, astrobiologist James Kasting and his colleagues defined the HZ as "the region around a star in which an Earth-like planet (of comparable mass) and having an atmosphere containing nitrogen, water, and carbon dioxide is climatically suitable for surface-dwelling, water-dependent life." They estimated in 1993 that the width of the CHZ is from 0.95 to 1.15 AU (1 astronomical unit represents the distance from Earth to the sun, 93 million miles). This is much wider than Hart's estimate but still quite narrow.

The idea of a habitable zone is a very important concept of astrobiol-ogy, but being within an HZ is not an essential requirement for life. Life can exist outside the habitable zones of stars. Astronauts in an "ideally" supplied, powered, and designed spacecraft could survive almost anywhere in the solar system and (for that matter) almost anywhere in the vast, empty regions of the entire Universe. Furthermore, discovery of extremophiles requires that the HZ concept be viewed from a much different perspective than that of just a few years ago. The HZ as normally defined is really the animal HZ. Ex-tremophilic organisms that live deep underground and require only minute amounts of chemical energy and water might thrive outside the HZ in a wide variety of environments, including the subsurface regions of planets, moons, and even asteroids. A good example is Europa, the moon of Jupiter that probably has a subterranean ocean. Europa may provide a fine habitat for microorganisms, even though it lies well outside the HZ as conventionally defined.

We believe that the concept of habitable zones should be expanded to include other categories. For planets like Earth, the animal habitable zone

(AHZ) is the range of distances from the central star where it is possible for an Earth-like planet to retain an ocean of liquid water and to maintain average global temperatures of less than 50°C. This temperature appears to be the upper limit above which animal life cannot exist (at least animal life on Earth). Because water can exist on a planetary surface at temperatures up to the boiling point, a planet with liquid water on its surface (the original criterion of the habitable zone) might be much too hot to allow animal life. The AHZ is thus a far more restricted region around a star than the HZ as used by Hart, Kasting, and other astrobiologists. An even narrower type of HZ would emerge if we wanted to consider a zone where modern humans could live— say, a planet where enough wheat or rice could be cultivated to feed several billion people. A much wider and more readily determinable HZ is the microbial habitable zone (MHZ), the region around a star where microbial life can exist. It is nearly the entire solar system, and it extends temporally from soon after formation of the planets until the present day. HZs for other major categories of life could be defined as well: The HZ for higher plants would be wider than that for animals but narrower than the HZ for microbes.

Although the habitable zone is described in terms of distance from a central star, it must also be thought of in terms of time. In the solar system, the HZs have definable widths; and as the sun constantly gets brighter, they move outward. Earth will eventually be left behind as the greenhouse effect causes it to become more like Venus. This will happen between 1 and 3 billion years from now, and Earth will have had about 5 to 8 billion years in the HZ (see Figure 2.1). For more massive stars, the evolution is much faster. For these stars the HZ is farther out and has a much shorter duration. The lifetimes of stars 50% more massive than the sun would be too short for the leisurely pace at which animal life evolved on Earth.

Biological evolution requires vast periods of time to arrive at complex organisms—periods on the order of hundreds of millions to billions of years. The AHZ and the MHZ are therefore both spatial and temporal domains. Our newly defined AHZ is obviously the most highly restrictive, but paradoxically, it also allows for the greatest diversity of life. Earth is in this zone, whereas Venus (with its hellish surface temperature) and Mars (with its frozen surface and thin atmosphere) have been outside of it for billions of years. Relative to Earth's orbit, Venus is 30% closer to the sun and Mars 50% more distant. In terms of the intensity of sunlight, the solar illumination is twice as great at Venus and only half as great at Mars.

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