If you’re looking for life similar to Earth’s — based, that is, on carbon chemistry and water — you have to determine what kind of stars might have produced such planets. Certain factors weigh heavily in this analysis. The star must be a long-lived, solar-type star with constraints on its luminosity; it must offer an environment within which a planet with liquid water at its surface can exist. This Continuously Habitable Zone (CHZ) is defined this way in a new paper called “Astrobiologically Interesting Stars within 10 parsecs of the Sun,” now available on the arXiv site:

Also critical is planetary mass. A reasonable upper limit on mass seems to be a few Earth masses; planets larger than this are likely to be entirely covered with oceans, disrupting the carbonate-silicate cycle vital to atmospheric stability. Planetary mass is related to stellar metal content, and also to the mechanics of the planetary formation process, which leaves us with the uncomfortable realization that we know all too little about how Earth-like planets form.

The authors of this paper, Gustavo Porto de Mello, Eduardo Fernandez del Peloso and Luan Ghezzi (Observatório do Valongo, Brazil), argue that the Continuously Habitable Zone will be found around mid-K to late-F type stars. They go on to make some fascinating observations about how stellar type may affect Earth-like planet formation. M-type stars, for example, put their planets inside the tidal lock radius if they are to be warm enough to have liquid water at the surface, creating potential problems for climate stability. Mid-F type stars have wide habitable zones, but these zones move outward rapidly as the star evolves into subgiant status, leaving a time for development — less than 4 billion years — perhaps too short for the evolution of complex life.

Also considered is the orbit of stars around the galactic core. From the paper:

The stellar Galactic orbit is a rarely mentioned factor of stability which is drawing more interest as one regards how important it may be concerning the average interval between global catastrophes that a planet may undergo. The Sun lies very near the so-called co-rotation radius (Balázs, 2000, Lépine et al., 2001), where stars revolve around the Galaxy in the same period the density wave perturbations sweep across the Galactic disk. In such a situation the passages across the spiral arms, and consequently the potential encounters with star-forming regions and giant molecular clouds, are presumably minimized. The first are thought to present the danger of biologically lethal X- and gamma-ray irradiation episodes by supernova explosions (Gehrels et al., 2003); the latter, to trigger heavy cometary bombardment in the inner planetary system by perturbing the Oort cloud dormant population (Clube & Napier, 1982). Leitch & Vasisht (2001) have presented some evidence on the correlation of past significant mass extinctions with crossings of the spiral arms by the Sun.

All these criteria go into the paper’s analysis of nearby stars that are suitable for the development of complex life. Those stars that have a reasonable habitable zone (and one which is old enough to have maintained a habitable planet within the last few billion years) are likely to be interesting targets for upcoming interferometer studies in the infrared. Terrestrial Planet Finder is designed to be such a mission, and it is noteworthy that the New Worlds Imager concept discussed in an earlier post seems capable of making the initial spectroscopic studies of terrestrial planet atmospheres without the need for multiple spacecraft. Alternative proposals for the mission would require formation-flying spacecraft for such studies and would likely add a decade to the possible launch window.

The authors find 13 good candidates for nearby ‘biostars’ with three ranking especially high: HD 1581 (Zeta Tucanae), HD 109358 (Beta Comae Berenicis) and HD 115617 (61 Virginis): “We suggest these objects as high priority targets in SETI surveys and in future space interferometric missions aiming at detecting life by the ozone atmospheric infrared biosignature of telluric planets.” Future research should weigh more detailed models of planetary climate stability and (relatedly) a possible reassessment of M-class stars if tenable ecosystems seem to be allowed. And Centauri Dreams strongly seconds the paper’s emphasis on more intense study of galactic orbits, which may be the largest constraint of all on habitability.

A useful exercise: run the biostar candidates through the index of Notable Nearby Stars at the SolStation site, where the details of recent observational work are discussed.