|MadSci Network: Astronomy|
Dear The Insane One, There are a few conditions to consider when determining the habitable zone around a star. 1) Since the more massive a star the shorter its lifetime, the mass of the central star should not be more than 1.5 times the mass of the Sun. This is because we want the star to live at least 2 billion years to allow life enough time to evolve. 2) On the other hand, the star needs to have a mass of at least 0.3 times the that of the Sun since the less massive the star the cooler it is. The star needs to be hot enough so that there is a region in which the orbiting planet will be warm enough to have liquid water. 3) The the radiation from the central star should be stable in that there should not be many X-ray or UV flares. This means that the star should lie along the "main sequence" which represents the stable phase of a star's life when it is simply burning hydrogen in its core. The previous temperature restrictions narrow down the choices of stellar spectral type to F, G and K. The sun is a G type star with a photospheric temperature of around 5700 Kelvin. F stars are a bit hotter and K stars are a bit cooler. 4) The location of the habitable zone will then be determined by the distance in which liquid water will exist and not be permanently frozen or evaporate. 5) The orbit of the planet should be close to circular so that the temperatures on the planet surface are fairly stable throughout the year. This could rule out planets existing in binary or multiple star systems. 6) The planet itself must be massive enough so that it retains enough of an atmosphere to support life. This list of criteria for a habitable zone was adapted from "Astronomy: A Cosmic Journey" by William Hartmann. As for your specific questions: Definition of habitable zones - Despite the above criteria, our definition of a habitable zone is indeed changing. For instance, new discoveries of unique lifeforms around hydrothermal vents in our oceans have already negated the requirement that all life depends on sunlight in some fashion. Many universities are now developing Astrobiology institutes which will combine biology, astronomy and planetary science research in order to better define the necessary ingredients to produce life on a planet or moon. So the above ideas about what defines a habitable zone should be broadened to consider all possible forms of life. There is agreement, however, that liquid water does play an important role in the development of life. A white dwarf or red giant - If there was a habitable zone around the star originally, the red giant (or possible a supernova) stage which occurs before the star becomes a white dwarf would probably wipe out any life which had originally formed on a planet or moon within the zone. Since our own Sun will eventually become a redgiant, much thought has gone into the fate of the Earth during this event. Besides being fried since we will be inside the atmosphere of the Sun, when the Sun's atmosphere reaches the Earth, our orbit will be affected as well. The Earth may fall into the Sun and be destroyed or it will be flung far from the Sun and we will freeze. Astronomers are still debating over which scenario is most probable. Brown dwarfs - Now that is a good question. Again, the important factor is the presence of liquid water. We can consider this possibility by looking within our own solar system since Jupiter is much like a brown dwarf in temperature and composition. There has been recent excitement about the possibility for life on Europa (one of Jupiter's moons) which has a frozen outer crust. Scientists are planning a mission to launch a space probe to Europa which will try to land on the crust and somehow cut through the crust to sample the liquid beneath. Read more about the Europa mission at: http://sse.jpl.nasa.gov/missions/jup_missns/europa.html Planets close to stars - The many recent discoveries of Jupiter-like planets around other stars have made us rethink the formation of other solar systems. Many of these planets are closer to their stars than the Earth is to the Sun. Having life on a planet or moon close to the star will probably require a cooler star than our Sun. For information on the search for exoplanets check out: http://cannon.sfsu.edu/~gmarcy/planetsearch/planetsearch.html Thanks for your question. I enjoyed thinking about it. Angelle Tanner UCLA Astronomy
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