The past century has witnessed extensive exploration of our Solar System. This exploration has virtually proven that there are no advanced forms of life, although the question still remains of whether or not simple life exists in places such as the Martian subsurface or the cold oceans of Europa. Astrobiologists have had few problems identifying sites to explore for evidence of past or present life within our Solar System. But what about locating sites for life beyond our Solar System?
This discussion will assume that at a minimum a planet is required for life, that life cannot develop on a small piece of rocky debris, in a nebula, or in the depths of interstellar space. Planets have a high concentration of the elements heavier than hydrogen or helium that are needed to set up complex chemistry needed for life. They also provide a stable platform on which life can exist. As a result, we need to ask the question: is our Solar System unique? Is planetary formation commonplace or mere happenstance?
There is a firm, physical basis that supports the ideas of accretion, condensation, and radiation that lead to planet formation. When our own Solar System was forming it was like a cosmic smelter. The overwhelming majority of the gas in the cloud that formed the Solar System collapsed into the newly formed Sun. However, the small amount of material left over had concentrations of elements heavier than hydrogen and helium that were hundreds or thousands times greater than the average values for the Sun. At close distances of a few AU, planets grew by accretion of rocks and metals, and then ices. We live on an icy cinder that is the residue from the formation process of the star that now warms us. It sounds like an implausible sequence of events. Yet theory and observation indicate that planet formation should be a common byproduct of star formation.
We now have an emphatic answer — yes, planets are ubiqitous around all types of stars. After a long and difficult search, astronomers confirmed the existence of the first extrasolar~planet in the early 1995. We now know of over 300 times as many planets outside our Solar System as we do inside! Although we have few actual images of these planets, observing the stars around which they orbit has proved their existence; planets exert a force on their central star that causes them to "wobble" and the wobble can be detected as a periodic Doppler shift. If the planet orbits such that it passes between us and the host star, we see an eclipse. These dramatic results complete over two thousand years of speculation. We have taken a dramatic new step in the Copernican~revolution by showing that planets are common throughout space and that Earth is not a unique vantage point from which to view the universe.
An upcoming generation of experiments will allow us to reveal Earth-like planets by direct imaging. Astronomers are already using special techniques to sharpen images of objects in space made with ground-based telescopes, compensating for the blurring effect of the Earth's atmosphere. They are also planning interferometers, linked telescopes that have the resolution of a single large telescope equal in size to the separation of the individual telescopes in the array. Interferometers in space could achieve a resolution sufficient to detect Earth-like planets around nearby stars. Finally, they are also developing coronagraphs, instruments that block the solar disk so that the region around a star can be seen, the region where we would hope to find planets.
Beyond imaging, astronomers hope to use new large telescopes to spread the feeble reflected light from extrasolar planets into spectra. We can then learn about the chemistry of the atmospheres of these remote planets. Oxygen is highly reactive and involved in many inorganic reactions. So when we see it in excess in a planet's atmosphere, it is a strong sign of a biological process; in other words, oxygen is continually replenished by photosynthesis or another life process. We might be able to infer life on other planets by the presence of oxygen (O2)along with ozone ()3) and water vapor (H2O). These spectral signatures of life are called biomarkers.
Identifying sites for life involves more than just locating planets. What about the requirements for a planet to be suitable for life? Since we know little about the diversity of planets we should try to make as few assumptions as possible. A basic assumption is that the planet's temperature must allow liquid water to exist. A liquid is by far the best medium for chemical and biological processes. The energy source to sustain an appropriate temperature does not have to be sunlight. Energy to maintain temperatures on a planet could also come from geothermal energy within a planet or from tidal flexing of a planet. Consequently, both planets and large moons of planets are potential sites for life. Unfortunately, we are currently unable to detect exomoons, but several research groups are trying to push the detection limits down to the size and mass of large moons.
Several conditions have been proposed as necessary to make a planet habitable. The star which the planet orbits should be a main-sequence star; evolutionary stages beyond the main~sequence are too short-lived or generate too little energy to shelter life. The star should also be no more than about 1.5 times the mass of our Sun. This upper bound allows enough time on the main~sequence for complex life to evolve, roughly a billion years. It also limits stars to about four times the Sun's luminosity; a value greater that this would provide too much damaging UV radiation, which is detrimental to organic molecules. The central star should be at least 0.3 times the mass of our Sun. (Notice that the lower bound is much more important than the upper bound, since the vast majority of stars in the universe are low in mass.) This lower bound corresponding to 1/100 of the Sun's luminosity allows the star to be warm enough for nearby planets to retain liquid water. The zone where liquid water could be present around cooler stars would be so close to the star that a planet at such a small distance would have its atmosphere ripped off. Moreover planets that close to a star would be tidally locked such that the same face always points to the star. About 25% of the 40 billion stars in the Milky Way are main~sequence stars of spectral types F, G, and K, spectral types that satisfy these first two conditions.
In addition to orbiting an appropriate star, it is necessary for a planet to have enough mass to have enough gravity to retain a substantial atmosphere. Also, the planet's orbit must be nearly circular, or at least stable enough to keep it at a proper distance and prevent drastic seasonal changes. Planet orbits are unlikely to be stable in binary star systems. So we must probably exclude them, except for very close or very widely separated pairs where the planets may be undisturbed by the stellar motions.
Astrobiologists conclude that sites for life may be found around a wide range of main-sequence stars on either planets or substantive moons of larger planets. Most sites for life may be variations on a familiar theme — Earth-like planets in orbit around a Sun-like star — but nature may also have developed an unexpected diversity of habitable places. With the great success of planet-hunting in the past decade, ovr a hundred extrasolar planets appear to be suitable for life. While we continue looking for an Earth clone, and improve our methods of detecting biomarkers, astrobiologists hope to learn more about the evolution of life by looking closely at the history of the place we call home.