Introduction and Background Earth is the only planet known to harbor life of any kind, past or present. As part of the search for evidence of life on other planets, both in our solar systems and in other planetary systems, we are looking not just for evidence of the living organisms itself, but for evidence of the conditions that might even be hospitable to life as we know it. One feature that astronomers consider to determine whether a planet MIGHT have the conditions necessary for life as we know it is whether the planet falls within a 'habitable zone' of its host star. (Note: This lab will explore circumstellar habitable zones only, not galactic habitable zones.) If you have already learned about habitable zones in another astronomy class or from your own general knowledge, great. If not, or if you'd like to get a stronger background before proceeding, please read more about habitable zones at astro.unl.edu, universetoday.com, astronomynotes.com, or space.com. 1. What is a "habitable zone"? Give a good, complete definition, in your own words. If you're not sure and need to look anything up, use a reliable resource (such as one of the four linked above), and be sure to reference the resource you use. Be sure to include in your definition: a) what is meant by "habitable", b) why it is a "zone" and not one specific location, c) what object it is surrounding, and d) what object(s) may be located within it. e) Use complete sentences. Part 1 of 5: The Habitability of the Earth To begin, load up the Habitable Zone simulator written by the University of Nebraska at the following URL in a new window: http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.swf You will need this Angel question window and the unl.edu habitable zone simulator window open simultaneously, as you will need to switch back and forth frequently. The flash simulator will show you a visual diagram of the solar system in the top panel, a set of simulation settings in the middle panel, and a timeline of the habitability of the Earth in the bottom panel. The timeline units will either be Megayears (Myr) which means millions of years, or Gigayears (Gyr) which means billions of years. To run the simulation, click run in the bottom panel. This button immediately becomes a pause button which will allow you to pause the simulation at any time. The simulation runs pretty quickly by default. To adjust the speed use the rate slider bar to the right of the run button. You can also manually advance the simulation forward or backward by clicking and dragging the upside-down dark grey triangle above the timeline. To restore the simulation to the original default settings, press the reset button at the very top of the simulation. 2. The simulation is currently set to zero-age - this is the Solar System as it was when it first formed, about 4.5 billion years ago. Which planet(s) were in the Habitable Zone at this time, if any? 3. The blue region marked on the diag.