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Rich & Ancient Star Systems
Metal-Poor Star Systems
Metal-Rich and Ancient Star Systems
Table of Ancient and Metal-Rich Star Systems
Sky Map Locations of Selected Ancient, Rich Stars
PRE-REQUISITES
FOR HABITABLE PLANETS
Two pre-requisites for habitable planets involve the age and chemistry of their parent star. Though the Dole-Asimov book, Planets for Man, spoke at great length about stellar age as an important consideration, very little was known about the ages of individual stars back in the 1960's. For most of the twentieth century, only the maximum possible age for a lone star could be stated with any certainty. With the techniques used then, a star like our sun might be an infant or poised on the brink of old age. There was no way to know.
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Today's techniques allow us to gauge the approximate age of hundreds of individual stars. Stars with the right age and the right chemistry are prime candidates for our search. Below is a list of stars that exceed these two pre-requisites. But first let's take a look at why these are pre-requisites in the first place.
INFANT STAR SYSTEMS
Too young a star system (~2 billion years or less) would still be in its formative stages. Planets would still be suffering the bombardment of large meteoric material. If life were to start on such a world, there exists a very real danger that one large meteor would destroy that life, much as one is theorized to have done in destroying the dinosaurs on Earth.
IMPORTANCE OF STELLAR AGE TO PLANETARY LIFE
Based on the admittedly small sample of one, the development of life on our planet might be used as a rough measure for that in other star systems. The timeline chart, below, shows many of the significant events in the progress of life on Earth. It wasn't until long after the major bombardments had stopped that Earth's atmosphere gained a significant amount of oxygen. During the Mesozoic era (green), the dinosaurs emerged and lived. And the Cenozoic saw the rise of mammals to dominance. Based on the Earth model, a star system younger than 2.5 billion years would likely not have any habitable worlds.
METAL-POOR STAR SYSTEMS
A star system too poor in heavier elements would likely not have planets, or the planets would be made largely of hydrogen and possibly water. No Earth-like world is likely to be found in such a system. Throughout our galaxy, and surrounding it in what is called the galactic "halo," are stars that are extremely old, but very poor in heavier elements. After all, the earliest stars were made of pure hydrogen; the heavier elements came later, with the death of the first giants. One such "halo" intruder to the galactic disk is the star, Arcturus, in the constellation Bootes. The globular clusters orbiting our galaxy are also metal deficient, yet are thought to contain some of the oldest stars in the universe. Based on more recent studies, it may well prove that all of these metal-poor systems are devoid of planets.
METAL-RICH AND ANCIENT STAR SYSTEMS
The following list is by no means complete. This is but a sampling of stars within and near the Solar neighborhood. Each star is rich with heavier elements, and old enough to have mature, life-bearing planets. In fact, all of these stars are thought to be at least as old as our own sun — some far older. If any of these systems have a habitable world, and if life developed there in a sequence similar to that on Earth, there exists the possibility that such a planet would harbor a civilization far more ancient than ours.
For more information on the meanings of the column headings, simply click the appropriate heading. For more information on a specific star, click on the HD number for that star. Distances are in parsecs. "Low," "Age," and "High" give a range of star ages, from low, to nominal and finally high estimates (in giga-years).
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