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Other Criteria

The luminosity limits of 0.10x - 3.0x are the primary criterion for the Encyclopedia. However, there are some exceptions.

  • Luminosity OK, stellar properties not: There are a few stars which have the right luminosity to qualify for the Encyclopedia, but which are especially unlikely to have habitable or otherwise Earthlike planets around them.

    Subgiants: stars that are somewhere in between the main sequence and the red giant stage. Subgiants typically have luminosities around 2-5x solar, and so many are nominally acceptable. However, they generally started with much lower luminosities, and are brightening quickly. In other words, subgiants are frying (if they haven't already fried) any Earthlike planets orbiting them. There are four subgiants in the volume of the Encyclopedia and I have omitted them.

    Subdwarfs: stars that have a somewhat lower luminosity (by a factor of as much as 10) than ordinary main sequence stars of the same temperature and spectrum. Many subdwarfs have acceptable luminosities; however, they are almost invariably poor in "metals" (elements heavier than helium), often having less than 10% of the solar abundance of these elements. Because of this, they are unlikely to have Earthlike planets in orbit around them (see the section on "Metallicity" under "stellar properties" for more information on metallicity). There were only two subdwarfs in the volume covered by the Encyclopedia, and I have omitted them.

  • Luminosity a bit off, but otherwise very interesting: I have included one star outside the normal luminosity range of 0.10 - 3.0x solar. This is Upsilon Andromedae, whose luminosity is just outside the normal limits (about 3.5x solar). I have included it because it is the first more-or-less Sunlike star discovered to have more than one planet orbiting it (multiple Jupiter-sized ones, to be exact). Upsilon Andromedae is thus of particular interest, so I decided to bend the rules a bit for it.

  • Multiple stars: Multiple stars, two or more stars traveling through space together, are a possible problem. If one of the stars orbits the other at a distance similar to that an Earthlike planet would have, it may not be possible for an Earthlike planet to exist at all in that group of stars.

    I have generally included multiple stars in the Encyclopedia, since many actually consist of stars that are very close or very distant from each other, and hence unlikely to interfere with planet orbits. As a general rule, I have assumed that if the minimum star-companion separation is 10 astronomical units (Earth orbit radii) or more, then gravitational interference from the companion is unlikely to be serious, at least over the short term. (The long term stability of such orbits, and the extremely important issue of planet formation in a binary system, are still open questions; I have not attempted to resolve them.) In cases where companion orbits may pose a problem for planet orbital stability, I've noted this in the encyclopedia entry for those stars.

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