While Humans eventually may be able to colonize any star system by building space habitats, many people today prefer to dream about visiting an Earth-type planet or to communicate with intelligent Earth-type lifeforms. Currently, our only guide to what type of star is likely to host an Earth-type planet suitable for Human habitation without special environmental protection is our own Sun, Sol. A look at the map of nearby stars, however, quickly reveals that Sol is not like most stars in the Solar neighborhood. Indeed, Sol appears to have a few special characteristics:

• It's a solitary star although most stars have stellar companions, which is fortunate for life on Earth because stable planetary orbits like the Earth's are much more likely around single stars.

• It's among the most massive 10 percent of stars in its neighborhood so that it is not too cool and dim, but also not so massive that it burns out before life has time to develop, evolve, and manufacture an oxygen atmosphere to create an Earth-type planet.

Finally, it appears to have roughly 50 percent more "heavy" elements than other stars of its age and type, but only about a third of their variation in brightness, which is also fortunate because elements heavier than hydrogen are essential to make rocky planets like Earth and large stellar flare-ups can harm planetary life with hard radiation.

Of course, no star is exactly like the Sun, and so NASA's proposed Kepler Mission to search for habitable planets among nearby stars examined some of the most critical issues for habitability.

Possibly Suitable Stars

The range of star types that can support Earth-type life on planets may be limited to those lower mass stars that "live" long enough as stable luminous stars for planets to form and complex life to evolve. Although all main sequence stars generate luminous energy by converting hydrogen into helium through thermonuclear fusion, stars more massive than 1.5 times that of Sol (i.e., stars of spectral type O, B, or A dwarfs like Sirius) age too quickly to support the development of complex Earth-type life. Even the largest suitable stars (i.e., spectral type F) may only be able to support Earth-type life for about two billion years, and so planets in favorable orbits may not have sufficient time to develop complex life on land such as trees. Moreover, within a couple of billion years of a star's birth, cometary and asteroidal bombardment may still be so intense that living on such planets would be quite risky.

On the opposite extreme, stars with less than half of Sol's mass (e.g., smaller spectral type M dwarfs like Proxima Centauri) are likely to tidally lock planets that are orbiting close enough to have liquid water on their surface too quickly, before life can develop (Peale, 1977). Tidally locking (or synchronous rotation of the star and planet) may eventually cause the destruction of a life-sustaining atmosphere through condensation on the cold, perpetually dark side of the planet. Moreover, most M-type red dwarf stars would tend to sterilize life on a close-orbiting Earth-type planet regularly with large stellar flares. Therefore, NASA's proposed Kepler Mission will search for habitable planets at nearby main sequence stars that are less massive than spectral type A but more massive than type M -- dwarf stars of types F, G, and K.

Stable Orbits in Binary Star Systems

Over 65 percent of the main sequence stars in the solar neighborhood that are possibly suitable (i.e., with a stellar mass between 0.5 and 1.5 times that of Sol) for hosting Earth-type planets may be members of binary or multiple star systems (Duquennoy and Mayor, 1991). In binary star systems, however, a planet must not be located too far away from either one star or too close to two "home" stars or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet (Graziani and Black, 1981; Pendleton and Black, 1983; and Dvorak et al, 1989). Indeed, stable orbits may extend as far as one third of the closest separation between any two stars in a binary system, but according to NASA's Kepler Mission team, numerical integration models have shown that there is a range of orbital radii between about 1/3 and 3.5 times the stellar separation for which stable orbits around two stars are not possible (Donnison and Mikulskis, 1992). In star systems with more than two stars, the limits on stable orbital distance are so stringent that the presence of Earth-type planets in habitable orbits where surface water would be liquid are much less likely.

In addition to the question of stable orbital distances for planets in binary star systems, astronomers also have concerns about the plane of space around such stars that planets could safely orbit for the billions of years necessary to support the development of Earth-type life. For example, do binary stars tend to spin within the same equatorial plane? And is the presence of "coplanarity" or the lack of it related to binary separation distances and other variable characteristics of stellar systems?

In the early 1990s, one study found that coplanarity between the orbital and equatorial planes of nearby binaries (within 100 parsecs or 326 ly) that are composed of Sol-type stars (F5-K5 V) "exists" for binaries with orbital separations up to the average orbital distance of Pluto in the Solar System -- roughly 40 times the Earth-Sun distance or "astronomical unit" (AU). Differences in the spectral type of the host stars, orbital eccentricity (degree of elliptical deviation from perfect circularity), and stellar age did not appear to have significant correlations, but in hierarchial multiple systems, noncoplanarity may exist at small separations. If the planetary distances found in the Solar System are typical, there should be no reason to expect that extrasolar planets orbit their parent stars significantly outside of their equatorial planes. Finally, noncoplanarity between the component stars of a binary system should not have a significant impact on the stability of close-in planetary orbits around each star (Alan Hale, 1994).

Habitable Zone around Stars

In general, the conditions needed to support Earth-type life may exist for rocky planets (or sufficiently large moons) that are orbiting a star in its so-called "habitable zone" or "HZ" (James F. Kasting). Such zones are bounded by the range of distances from a star for which liquid water can exist on a planetary surface, depending on such additional factors as the nature and density of its atmosphere and its surface gravity. In terms of orbital distance, the HZ for our own Solar System currently extends from at least 0.95 AU to 1.37 AU (where one AU equals Earth's average orbital distance around the Sun).

NASA

The hot inner edge of an HZ is located at the orbital distance where a planet's water is broken up by stellar radiation into oxygen and hydrogen. In contrast to gas giants like Jupiter, the freed hydrogen would escape to space due to the relatively puny gravitational pull of small rocky planets like Earth. It has been hypothesized that massive disassociation of planetary water occurred on Venus (which has an average orbital distance 0.7 AU) via a runaway greenhouse effect. On the other hand, atmospheric carbon dioxide condenses at the cold outer edge of the HZ which eliminates its greenhouse warming effect.

Moreover, main sequence stars brighten as they age and so a star's HZ shifts outward as it brightens. A "continuously habitable zone" (CHZ) for a star would represent the overlap of HZs at two widely separated points of geological time. Over the past 4.6 billion years, Sol's CHZ has extended from about 0.95 AU to 1.15 AU.

Indeed, Sol is becoming hotter and brighter as thermonuclear fusion of helium "ash" at its core becomes more statistically common. Indeed, some astronomers calculate that Sol has gotten at least 30 percent brighter since the formation of Earth. Although Sol is not expected to become a red giant star for another five billion years or so, it is expected to become another 10 percent brighter over the next 1.1 billion years, and so Earth may become too uncomfortably hot for even microbial life in another 500 to 900 million years.

Galactic Habitable Zone

One of Sol's unusual features is its orbit around the center of the galaxy, which is significantly less elliptical ("eccentric") than those of other stars similar in age and type and is barely inclined relative to the Galactic plane. This circularity in Sol's orbit prevents it from plunging into the inner Galaxy where life-threatening supernovae are more common. Moreover, the small inclination to the galactic plane avoids abrupt crossings of the plane that would stir up Sol's Oort Cloud and bombard the Earth with life-threatening comets.

NASA (Galactic region around Sol)

In fact, the Sun is orbiting very close to the "corotation radius" of the galaxy, where the angular speed of the galaxy's spiral arms matches that of the stars within. As a result, Sol avoids crossing the spiral arms very often, which would expose Earth to supernovae that are more common there. These exceptional circumstances may have made it more likely for life and human intelligence to emerge on Earth. According to Guillermo Gonzalez (an astronomer at the University of Washington in Seattle), fewer than five percent of all stars in the galaxy enjoy such a life-enhancing galactic orbit. Other astronomers point out, however, that many nearby stars move with Sol in a similar galactic orbit.

NASA

Most modern theories of planetary development begin with the agglomeration of small solid grains into "planetesimals" within circumstellar disks of dust and gas. These disks appear to develop around stars condensing out of huge molecular clouds (or nebulae). Their formation seems to be part of a normal process of star birth as disks have been observed around many stars known to be very young.

Within these disks, planetesimals collide and agglomerate into larger protoplanetary bodies that eventually form planets. In the colder outer areas of the disk, some substances that would otherwise be gaseous or liquid such as water and methane are available as solid ices to agglomerate with the dust grains. Hence, colder planetesimals can grow more quickly into larger protoplanetary bodies. Under one popular theory (Boss, 1995), if such protoplanets become sufficiently massive while there is still abundant amounts of hydrogen and helium gases remaining in the disk, then they may accrete substantial amounts of those gases and become so-called gas giants (like Jupiter, Saturn, Uranus, and Neptune in the Solar System). On the other hand, planetesimals in the warmer inner region of the disk would only form small rocky planets that lack the massive gas envelope of the gas giants.

Under modern theories, the formation of planets is believed to be a common occurrence. A broad range of planetary sizes and masses is possible, including rocky planets several times as massive as the Earth. However, astronomers find the formation of stars and planets to be a complex process that makes it difficult to predict the diversity of planetary systems that may can arise (Lissauer, 1995).

The specific characteristics of a particular planetary system appear to depend on the interaction of a variety of factors, including: the diffusion of stellar magnetic fields; the composition, turbulence, and viscosity of disk dust and gas and the stickiness of the small grains; and torques between the growing protoplanetary bodies and their surrounding disk regions, among others. Thus far, no one theory has been able to make definitive predictions of the frequency of planet formation nor of the distribution of planetary sizes and orbits. However, there is now some preliminary evidence from recent discoveries of giant extrasolar planets that planetary systems may be more common around stars whose spectra show an enriched abundance of elements heavier than hydrogen -- also called high "metallicity" (Gonzalez, 1999) -- or depletion of lithium (Stanley et al, 1998).

Numerical modeling of the accumulation of planetesimals during molecular cloud collapse have produced, on average, four rocky inner planets for models similar to the Solar System. The results included two, roughly Earth-sized planets and two smaller planets, where their orbital distance ranged between that of Mercury (0.4 AU) and Mars (1.5 AU). Hence, some astronomers expect to find rocky planets around other stars within that range of orbits. (George W. Wetherill: extra-Solar planets; Earth-like bodies; terrestrial planets; and Mars' smaller size).

NASA

NASA's Kepler Mission is defining the size of an Earth-type planet to be those that have between 0.5 and 2.0 times Earth's mass, or those having between 0.8 and 1.3 times Earth's radius or diameter. The mission will also investigate larger terrestrial planets that have two to ten Earth masses, or 1.3 to 2.2 times its radius/diameter. Larger planets, however, will be excluded because they may have sufficient gravity to attract a massive hydrogen-helium atmosphere like the gas giants. On the other extreme, those planets -- like Mars or Mercury -- that have less than half the Earth's mass and are located in or near their star's habitable zone may lose their initial life-supporting atmosphere because of low gravity and/or the lack of plate tectonics needed to recycle heat-retaining carbon dioxide gas back into the atmosphere (Kasting et al, 1993).

As suggested previously, however, the variation of stellar radiation over time and planetary orbital distance are as critical to the development of Earth-type planets as their mass. For example, the planet Venus in the our Solar System has about 81 percent of Earth's mass. Unfortunately, Venus is located just outside Sol's habitable zone, as derived from the Sun's current luminosity (or "brightness"). Four and a half billion years after its birth, the shrouded planet is much too hot to support the presence of liquid water on its surface because of its dense carbon dioxide atmosphere and sulfuric acid clouds, which retain too much radiative heat from the Sun through a runaway greenhouse effect. On the other hand, conditions on Venus may once have been more conducive to Earth-type life earlier in the Solar System's history when Sol was as much as a third less luminous than it is today.