Life on other planets
Is there extraterrestrial life? No answer to this eternally fascinating question currently exists, but astronomers have gathered a significant amount of relevant information. Bruce Jakosky argues that we have every reason to believe that there could be life elsewhere in the universe. Reviewing the development of life on Earth, he considers the likelihood of comparable processes having taken place on Mars and Venus, on moons around Jupiter and Saturn, and on planets orbiting other distant stars.
The argument suggesting that there is life on other planets is very simple and straightforward. It begins with life on Earth as that is, of course, the only life that we know about. The fossil record and the genetic record tell us that life on Earth is very old, having originated somewhere before 3.5 billion years ago (b.y.a.) and possibly before 3.85 b.y.a. The oldest known life forms were at once both very simple and remarkably complex. The simple aspect is that they consisted of microscopic single-celled bacteria and archaea. These were much less complex than the macroscopic life forms that exist today and that have existed for around a billion years.
Even the oldest life forms that we can find in the fossil record, however, dating back to 3.5 b.y.a., are very complex--they are very sophisticated organisms that relied on DNA and RNA to transfer genetic information, on ATP to store energy in a usable and accessible form, and, for some of them, on photosynthesis to get access to energy. These organisms are much more complicated than we imagine the first life forms would have been.
Life did not exist, however, for one-half to one billion years following the formation of the Earth. The earliest half-billion years on the Earth were marked by the continued influx of impacting objects left over from the formation of the planets. Even today, these objects are capable of dramatically affecting the terrestrial environment when they collide with the Earth. Back then, when larger objects were present, they were capable of sterilizing the Earth completely. There were an estimated five or so impacts subsequent to about 4.5 b.y.a. that were capable of completely sterilizing the Earth's surface.
The early environment on the Earth, therefore, was not conducive to the continuing existence of life, and the earliest life may not have been able to grab a foothold until sometime after around 4.2 or 4.0 b.y.a. Thus, life may have taken as little as a hundred million years, and no more than about a half-billion years, to form once the environment became sufficiently clement to allow it.
The rapidity with which life originated on the Earth has important implications for the process of forming life. It tells us that the formation of life is not a difficult event, but, rather, it is a relatively straightforward consequence of natural events on the planet. This is consistent with our current view that life originated through chemical and geochemical processes, starting with organic molecules in a wet surface or near-surface environment and using energy from some chemical source to build them into more complicated molecules. Even though we do not yet understand the specific processes that led to the origin of life, we can easily see that simple chemical processes can lead to more complicated molecules and, eventually, to life.
The ingredients for life
On Earth, we imagine that life really needed only a few key ingredients to get started. Liquid water is one such substance. It is difficult for us to imagine that life could exist without liquid water. Even with this requirement, we still see that life could be widespread throughout the universe; if we allow for the possibility that a different liquid also could hold the key to life, then life could be even more widely distributed.
The second ingredient is access to the necessary biogenic elements, such as C, H, O, N, and so on. This is not a very limiting factor, though, since we expect these elements to be very widespread throughout the universe, to be incorporated into planets during their formation, and to be readily available at the surfaces of geologically active planets.
A source of organic molecules was required for the origin of life on Earth, and presumably would be required for life elsewhere as well. On Earth, organics could have come from one or more of several different sources. These include the Earth's atmosphere, where they could form from energetic processes such as lightning in a slightly reducing atmosphere (the so-called Urey-Miller process); hydrothermal vents at the bottoms of the early oceans, heated by the extremely active volcanism that would have been present then, where organics could form by a chemical slide toward equilibrium as very hot water cools off once injected into the oceans; or from organic molecules that were present in dust and planetesimals accreting onto the Earth. Most likely, all of these sources contributed to the prebiotic supply of organic molecules.
Finally, a source of energy is needed to power life. The energy causes the molecules in the environment to react, moving them out of their natural state of chemical equilibrium. As they move back toward equilibrium, they can release chemical energy to power other chemical reactions, thereby providing usable energy for biota.
Again, there are several possible sources of energy, including sunlight (especially the energetic ultraviolet light that could have penetrated all the way to the Earth's surface in the early periods before there was significant ozone), lightning in the atmosphere, or geochemical energy obtained from geothermal heat in water circulating through hydrothermal vents. All of these energy sources were available, probably in abundance. There is no need for the energy sources that drove the earliest life to be the same as those that power life today; thus, the complicated chemical mechanisms that drive photosynthesis did not have to be present in the original life.
We expect that, under these conditions, the formation of life was relatively straightforward. We also expect that life could originate and continue to exist any place where similar environmental conditions are met. This could mean elsewhere in our solar system, or on planets around other stars. A search for life, therefore, is almost tantamount to a search for the basic environmental conditions in which life could exist.
Life on Mars
Elsewhere in our own solar system, we immediately think of Mars as a possible abode for life. There is abundant geologic evidence on the martian surface to indicate that liquid water has played an important role in shaping the surface throughout time. The evidence suggests that water was relatively stable at the martian surface during the first half-billion years recorded in its geology (from about 4.0 to 3.5 b.y.a.). If correct, this might suggest that life could have originated on Mars' surface at that time.
Subsequent to 3.5 b.y.a., however, there also is abundant geological evidence for the continued presence of water. At this later time, the water was not stable as a liquid at the surface, except perhaps intermittently. Rather, water was present deep within the crust and was released to the surface in catastrophic floods only occasionally. Within the crust, however, the water would have been available to support either an origin of life or its continued existence if it had originated earlier. In addition, within the crust there was an abundant source of energy from the volcanic activity that has persisted throughout most or all of martian history, and from chemical weathering of the minerals comprising the crust.
Life could have originated at the surface on early Mars or in the deep subsurface at any time, and life could exist today. If life is present today, it likely would be either deep beneath the surface where water could exist as a liquid (several kilometers deep, perhaps) or exposed at the surface in any transient vents where hot, volcanically heated water is released at the surface.
Although there is some evidence to suggest that there might be fossils from organisms within meteorites from Mars, this evidence is very controversial and is not yet generally accepted. Significantly, even if this meteoritic evidence is wrong, the basic argument regarding the possibility of life on Mars will not change. This is true even though the meteorite findings appear to have reinvigorated the interest in searching for life on Mars.
After Mars, other suggestions for an abode for life become more speculative. Life could have arisen on early Venus, when the Sun was dimmer, temperatures were lower, and the planet might not yet have undergone a transition to the present thick, hot, greenhouse atmosphere. Of course, any evidence of an early Venusian biosphere would have been long since obliterated.
The moons of Jupiter and Saturn
Life also could exist on Europa, a satellite of Jupiter, living in a possible ocean of water that may lie buried beneath the surface covering of water-ice. There, melting of the ice would result from tidal heating generated by Jupiter tides, triggered by gravitational interactions with Io as they both orbit around Jupiter. If there were an ocean, tidal heating and decay of radioactive elements would provide a substantial source of geothermal energy that might be tapped by living organisms.
Although there are exciting images from the Galileo spacecraft that suggest that liquid water has been present beneath the surface of Europa, there is no certain evidence for the existence of an ocean. Life conceivably could exist on Io, as well; there, abundant energy is available through the tidal heating, although there is no evidence for water of any sort.
Life also might have existed on Titan, a satellite of Saturn. This is much more speculative, because temperatures today are much too low to allow plausible life forms to exist. There might have been liquid water early in Titan's history, however, with the heat to melt the abundant water-ice being provided by large impacts during the end of the satellite's formation. Even without active biology Titan represents an interesting exobiological laboratory, where organic chemical processes occur even today in a manner similar to what might have occurred on the early, prebiotic Earth.
Life beyond the solar system
As we move outside of our own solar system, the prospects for finding environments suitable for life become still more speculative. As of today, we do not know of a single planet around another star that provides an appropriate habitat for life. However, this does not mean that we have no information on the subject.
A theory of how planetary systems form as a natural byproduct of the formation of stars has been developed. This theory is based strongly on the conditions that we see in our own solar system. However, it also is based on astronomical observations of star-forming regions in the galaxy, interstellar clouds of gas and dust, actual disks of gas and dust that occur around young stars, and, now, direct detections of giant planets and brown dwarfs around other stars. As a result, there is strong reason to believe that this theory might be more general than if it were based only on our own solar system.
Planets are thought to form from the collapsing gas and dust that eventually become a star. As the cloud collapses due to the pull of its own gravity, it will begin to spin faster due to the conservation of angular momentum. Because it is spinning, not all of the material can collect into a single central ball that becomes the star. Some of the matter will stay behind as a disk around the protostar; this disk consists of dust grains and gas, in orbit around the newly formed star.
The dust will begin to accumulate into larger objects, first by sticking together due to electrostatic forces, and later by gravitationally attracting other nearby objects. Eventually, these planetesimals become large enough to accumulate into a small number of individual protoplanets, each of planet-sized proportions. Only the rocky material can accumulate at the relatively high temperatures that occur close in to the star.
Farther out, where temperatures are cooler both from the lesser compression of the protoplanetary disk and from the greater distance from the central star, water-ice also will condense and accumulate. The greater mass available due to the presence of water-ice allows more massive planetary cores to accumulate. These then can begin to attract the gas that also resides in the disk. The gas accumulation then allows giant planets, similar to our own Jupiter and Saturn, to form.
These processes are thought to be relatively general, allowing the formation of planetary systems that might look much like ours. Numerical simulations of the formation of rocky planets suggest that our inner solar system might be typical, consisting of a small number of planets in well-spaced orbits.
If so, this suggests that habitable planets might be relatively common--there will be a significant likelihood of finding a planet at just the right distance to allow liquid water to exist. Moreover, a relatively wide range of distances from the central star would allow this. In our own solar system, the habitable zone might extend from almost as close in to the Sun as Venus to almost as far away from the Sun as Mars. There is at least one habitable planet in our own solar system, and possibly as many as three or four more that might have been habitable at one time or might still be habitable today.
Planets beyond the solar system
Significantly, we are now able to detect planets that are orbiting other stars, and we are finding that they are relatively abundant. For the most part, we cannot yet detect Earth-sized planets, only gas-giants. It is hard to estimate what fraction of stars might have planets. While as many as half of the young stars have protoplanetary disks that may lead to planets, less than 10 percent of the more mature stars that have been examined seem to have gas-giant planets. Unfortunately, the statistics for Earth-like planets are not known and cannot be determined from the available information; even for gas-giants, such a small number of stars have been examined that the statistics may not be reliable yet.
The planets that we are detecting are providing new information on how planetary systems evolve. For example, gas-giant planets have been discovered that are much closer in to their star than was expected. These almost certainly would have to have migrated in toward the star from farther out, a process that would have devastating results for any terrestrial planets.
Given that planets do exist, however, we imagine that there must be abundant rocky planets, and that many of these will be within their star's habitable zone. This means that liquid water probably will be abundant on planets in our galaxy. If life really is able to form as easily as we think it can, under the proper conditions, then it is likely that life is rampant throughout the galaxy. Of course, life is much more likely to take the form of bacteria-like organisms than of larger, more complex organisms.
While we expect that evolution will occur on other planets, and that more complicated forms of life could exist, it seems most likely that life on other planets will be like the simplest life forms on Earth--those that have existed for the longest time and in the most varied environments, and those that may be the most abundant forms of life on Earth--bacteria.
Does this mean that intelligent life does not exist elsewhere in the galaxy? It is hard to say. On the one hand, some scientists suggest that increased intelligence offers such a tremendous benefit to an organism that it must be a highly likely outcome of evolution on any planet, given sufficient time. On the other hand, intelligent life on Earth is the outcome of a random series of evolutionary processes, and there appears to be no natural imperative either toward more complex organisms or toward more intelligent organisms. So it may be that intelligence is a rare phenomenon.
Whether we are speaking of bacterial life or intelligent life, however, we have insufficient evidence today to know for certain whether extraterrestrial life exists. Although the present discussion is based on solid observations of life on Earth and the nature of the universe, the application to the question of life elsewhere so far is purely theoretical. It is only through the continued exploration of our home planet, our solar system, and our universe that we can hope to find fundamental solutions to the questions surrounding the existence of life.