• 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.

VLTI, ESO



Larger and jumbo images.



Enriched, solitary stars like Sol,
that are smaller than Sirius A but
larger than Proxima Centauri, are
probably more hospitable to the
development of Earth-type plant
and animal life.

Of course, no star is exactly like the Sun, and so many astronomers and scientists in related fields with a professional interest in the search for habitable planets among nearby stars have been examining some of the most critical issues for habitability. In late September 2003, astrobiologist Maggie Turnbull from the University of Arizona in Tucson identified a shortlist of 30 stars (including Chara, 18 Scorpii, and 37 Geminorum), that were screened from around 5,000 that have been estimated to be located 100 ly of Earth, as the best nearby candidates for hosting complex Earth-type life, as part of a larger project to expand the stellar targets for the new Allen Telescope Array that will be completed in 2005 by the SETI (Search for Extraterrestrial Intelligence) Institute. This list was discussed with a group of scientists from NASA's space-observatories project, the Terrestrial Planet Finder (TPF), which will search for habitable planets by using visible light with the "signature" of water and/or oxygen from an Earth-type planet after its scheduled launch around 2013, and the ESA's Darwin project involving six space telescopes (Astrobiology Magazine). The stars examined were selected from a larger list of 17,129 (of which 75 percent are located within around 450 ly, or 140 parsecs, of Sol) that were assembled into a Catalog of Nearby Habitable Stellar Systems (HabCat) by Turnbull and Jill Tarter of the SETI Institute (see: Margaret C. Turnbull, 2002, in pdf). Selection criteria for the 30-star shortlist included: X-ray luminosity, rotation, spectral types or color, kinematics, metallicity, and Str�mgren photometry. (Update: The TPF-C's top 100 target star list is now available (Robert A. Brown, forthcoming; in pdf).)

JPL, CalTech, NASA


Larger illustration.


To find rocky inner planets
around nearby Sol-type stars,
astronomers hope to use NASA's
Terrestrial Planet Finder (TPF)
and the ESA's Darwin group of
space observatories.

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, possibly suitable stars -- i.e., spectral type F0-4 (Kasting et al, 1993; abstract) -- may only be able to support Earth-type life for around 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.

NASA Observatorium




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

On the opposite extreme, stars with less than half of Sol's mass (e.g., smaller spectral type M dwarfs like Proxima Centauri) are more 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. However, since low-mass M- and K-type stars so numerous, some astronomers and planetary scientists are continuing to model low-mass stars and possible planetary environments that may be potentially suitable for Earth-type plant and animal life, as well as for microbes (Helmut Hammer, 2007; Tarter et al, 2007; Scalo et al, 2007; Khodachenko et al, 2007; and Grenfell et al, 2007; and Kiang et al, 2007).

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 (Holman and Wiegert, 1999; Wiegert and Holman, 1997; and 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.

Tim Pyle;
Trilling et al, 2006;
SSC; NASA

Larger illustration.


Recent detections of
giant planets and
dust disks around
close-orbiting binary
stars and around
individual stars in
widely separated
binaries offers hope
that Earth-like
planets can be
found in such
systems (more).

On March 29, 2007, astronomers using NASA's infrared Spitzer Space Telescope announced their finding that planetary systems -- dusty disks of asteroids, comets, and possibly planets -- may be at least as abundant in binary star systems as they are around single stars, like Sol. In a study of 69 binary (and a few multiple) systems with primary stars of spectral type A3 to F8 -- which are younger and more massive than Sol -- between 65 and 320+ light-years (20 and 100 parsecs) from the Solar System, roughly 40 percent of the systems observed were found to have dust disks that should be made of the bits of asteroidal and cometary rock and ice that may become part of planets under formation. Hence, planetary systems appear to be at least as common around binary stars as they are around single stars. Moreover, dust disks were found even more frequently (about 60 percent) around both stars in the 21 tightest binaries in the study, with stars between zero and three astronomical units (AUs) -- the Earth-Sun distance -- apart. However, far fewer disks were found in 23 intermediately spaced binary systems, where stars were between three to 50 AUs apart, which appears to suggest that binary stars may have to be either very close to each other, or fairly far apart, for planets to form (NASA press release; and Trilling et al, 2006).

Tim Pyle,
SSC, NASA




Larger illustration.




In this star system,
planets are already
developing within a
dust disk around a
pair of stars in a
tight orbit (more).

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?

� Lynette Cook

(Artwork from Extrasolar Planets - Collection III, used with permission)


View from an asteroid of tightly orbiting red dwarfs with a tidally-locked planet in the system's liquid water zone. The planet's darkside ice cap is backlit from a white dwarf in the distance.

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; and Kasting et al, 1993). 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. 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 therefore metallicity) 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 "co-rotation 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 complex life and human intelligence to emerge on Earth. According to Guillermo Gonzalez (an astronomer at Iowa State University), 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.

Yeshe Fenner, STcI, AURA, NASA, ESA



Larger illustration.



Only around 10 percent of the Milky Way's
stars reside in one recent definition of a
galactic habitable zone with chemical
and environmental conditions suitable
for the development of complex Earth-
type life (more).

The Sun resides in a pancake region of the Galaxy called the "disk" with a strong concentration of stars (and gas and dust) within 3,000 light-years (ly) of the galactic plane, which includes the so-called "thin disk" that has more relatively younger stars within 1,500 ly of the plane (more on stellar population groups in our Milky Way Galaxy). This region contains relatively young to intermediate-aged stars that within around five billion years old with relatively higher average metallicity than other galactic regions located outside of the galactic core, in a circular band that broadens with time. Generated by the deaths of older stars, the greater availability of elements higher than hydrogen and helium in this galactic region favor the formation of rocky inner planets as large as Earth, or bigger (Gonzalez et al, 2001). Moreover, the galactic orbits of stars in this region tend to be relatively circular -- with low to moderate eccentricity. According to one recent definition of the galactic habitable zone, as much as 10 percent of all stars in the Milky Way may have experienced chemical and environmental conditions suitable for the development of complex Earth-type life over the past eight to four billion years for evolutionary development (press release; and Lineweaver et al, 2004, in pdf). (Further discussion of the different galactic regions and their distinctive stellar populations is available from ChView's "The Stars of the Milky Way.")

In recent millenia, the Sun has been passing through a Local Interstellar Cloud (LIC) that is flowing away from the Scorpius-Centaurus Association of young stars dominated by extremely hot and bright O and B spectral types, many of which will end their brief lives violently as supernovae. The LIC is itself surrounded by a larger, lower density cavity in the interstellar medium (ISM) called the Local Bubble, that was probably formed by one or more relatively recent supernova explosions. As shown in a 2002 Astronomy Picture of the Day, located just outside the Local Bubble are: high-density molecular clouds such as the Aquila Rift which surrounds some star forming regions; the Gum Nebula, a region of hot ionized hydrogen gas which includes the Vela Supernova Remnant, which is expanding to create fragmented shells of material like the LIC; and the Orion Shell and Orion Association, which includes the Great Orion Nebula, the Trapezium of hot B- and O-type stars, the three belt stars of Orion, and local blue supergiant star Rigel.

Pat Rawlings, NASA -- larger image


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, persisting up to a few hundred years around some stars in the Solar neighborhood (more from the Spitzer Space Telescope and Astronomy Picture of the Day).

George Rieke, SST/CalTech, NASA -- larger illustration.

Dusty disks observed around 266 stars suggest that planets are chaotically built up
over varying periods of collisions between increasingly massive, rocky bodies (more).

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).

NASA


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. On July 21, 2003, some astronomers provided evidence from recent discoveries of giant extrasolar planets in mostly inner orbits around host stars that planetary systems may be more common around stars whose spectra show an enriched abundance of elements heavier than hydrogen and helium -- also called high "metallicity" (exoplanets.org press release; and Gonzalez, 1999). Another indicator may be the relative scarcity of lithium through an early process of accelerated break down from internal mixing that was generated by past stellar rotational braking by an early, massive circumstellar disk (Stanley et al, 1998). On April 18, 2007, however, astronomers using the Spitzer Space Telescope announced survey findings that indicating that planetary dust disks located within 1.6 light-years of O-type stars are likely to be "boiled off" by superhot radiation and winds (see Spitzer news release; and Balog et al, 2007).

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.