2 July 2016
When the Handbook for Space Pioneers was written in 1978, there was no physical evidence of planets in any star systems except our own. The choice of the handbook’s star systems with habitable planets (now also on GAILearth.com) was based in part on speculation by Stephen Dole of the Rand Corporation published in popular form as Planets for Man with Isaac Asimov as co-author in 1964.
Since then, the knowledge of planets in other star systems has exploded with more than 3,285 confirmed. How have these discoveries affected what we know about the systems where we imagine human settlements might exist in the future?
It turns out that none of the new evidence found to date has conclusively ruled out habitable planets in any of the handbook’s original star systems. Nobody, for example, has found a giant planet orbiting in the habitable zone of any of the eight stars with future human colonies that would preclude an Earth-sized planet. However, the presence of newly discovered large planets in two of the systems and of companion stars in two more reduces the odds of finding earth-like worlds in stable orbits in those systems that could allow life as we know it to evolve.
Nonetheless, since there is no evidence that the star systems on this website could not have a planet habitable by Earth people, we have decided to leave them where they are. We have updated Figure 3.1.001 to show the new stars and planets found in some of the systems. We also updated estimates of the habitable zones surrounding each star using the most current data.
The original star data was taken from the Catalog of Bright Stars by Dorrit Hoffleit published by Yale University in 1964. Since then, properties of some stars, such as the spectral type, have changed slightly, and much more data have been gathered on their physical properties such as mass, luminosity, effective temperature, and size.
Today one of the best sources of information is the NASA Exoplanet Archive compiled by the California Institute of Technology under contract with the US National Aeronautics and Space Administration. This website includes not only information about stars with known exoplanets but also other nearby stars that might be candidates for having exoplanets.
By typing the designation of a star from the Bright Star Catalog (or any other common star compendium) into the archive search box, one can get physical properties of the star and any known exoplanets. Not all possible exoplanets are listed in the NASA archive because Caltech’s policy “is to only include planets from accepted, refereed publications.” For this reason, it’s also important to check Wikipedia, as it sometimes refers to papers published by university researchers that haven’t been accepted by peer-reviewed journals.
In modifying figure 3.1.001, we recalculated the habitable zones using a web-based Habitable Zone Calculator from the University of Washington astronomy department. The calculator takes as input the star’s effective temperature and luminosity and returns the inner and outer radii of the habitable zone in astronomical units. We input the most recently published data for each of GAILE’s eight stars from the NASA Exoplanet Archive, calculating the luminosity ratio from each star’s estimated radius.
The results produced habitable zones that in five cases are farther out than those of the original handbook. So we had to increase the planets’ orbits to keep them in their habitable zones. These more distant orbits in turn produced longer solar years for some planets as shown in Table 3.1.001. The calendar year for Wyzdom, for example, stretched from 341 days to almost 526.
In future updates we’ll discuss the exoplanets and companion stars that have been discovered in some of the GAILEarth systems and what these might mean for the probability of habitable planets. For background on how exoplanets have been discovered to date, see our update “Why have no human-habitable exoplanets been discovered yet?”