If you are searching for other inhabited worlds, almost any kind of star may do according to current findings, except perhaps for white dwarfs. As it turns out, planets orbiting these dying stars, as well as brown dwarfs, are probably very poor candidates for life.
In recent years, a mind-boggling variety of exoplanets have been discovered orbiting other stars, including ones which are potentially habitable. They’ve been found around various types of stars also; some of these stars are similar to our own Sun, while some are larger and others smaller. Depending on a star’s size, luminosity, and other factors, there is the so-called “habitable zone” where, for rocky-type planets like Earth which orbit within that zone, liquid water could exist. This then improves the odds of finding planets which are not only habitable, but actually support life of some kind.
New research just published in the November 2012 issue of Astrobiology, however, concludes that any planets orbiting white dwarf and brown dwarf stars would be very unlikely to have life, even if they orbit in the habitable zone. The findings come from Rory Barnes of the University of Washington and René Heller of the Leibniz Institute for Astrophysics Potsdam in Germany.
White dwarfs are the remaining tiny, condensed cores of stars which are in the last stages of their lives, while brown dwarfs are objects which are larger than gas giant planets, but were still too small to initially “ignite” via fusion reactions and failed to become actual stars. Both can still emit some heat, even enough to form a habitable zone, but there is a problem—they are both in the process of cooling off over time, which means that any habitable zone will keep gradually shrinking. That’s bad news for any habitable or potentially habitable planets. A planet in the habitable zone now may not have been when it first formed, and it may not be again in the future. This instability of the planet’s environment would make it difficult for any life to adapt and continue.
According to Barnes, lead author of the study, “These planets, if we find them today in a current habitable zone, previously had to have gone through a phase which sterilized them forever.”
Even if a planet was able to remain in the habitable zone longer by migrating in closer to a white dwarf, for example, it would then be subject to tidal heating if it got too close, meaning that any water would probably be evaporated by intense ultraviolet radiation, rendering the planet uninhabitable (at least by Earthly standards).
It may be premature to say that life would be impossible under these conditions, but there are much better places to look. Namely, planets similar in size to the Earth which have been in their stars’ habitable zones for a much longer time, or even exomoons, ones similar to Europa and Enceladus in our own solar system, which have subsurface oceans or lakes of liquid water. Such moons can even be well outside the normal habitable zone, as is the case here.
So while this news may seem discouraging, just remember that it is now estimated that there are millions of planets in our galaxy alone orbiting normal stars—a humbling and hopeful thought.