The Habitability of Planets
March 28, 2020
1 Background
The presence of liquid water is considered to be a prerequisite for life as we know it, which makes looking for water a practical way to begin our search for life beyond Earth. For water to exist on the surface of a planet, the planet must have the right temperature on its surface. The main driving force behind the surface temperature of any planet is the light it receives from its parent star. Around every star there is a region where the planet will receive just the right amount of light to give it temperatures that are conducive to liquid water – this region is call the star’s Habitable Zone. The orbit of the Earth currently falls within the Habitable Zone of our Sun.
2 The Habitability of the Earth
To begin, load up the Habitable Zone simulator written by the University of Nebraska by entering the following URL in the address bar of your web browsers:
http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html
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. To run the simulation, click the run in the bottom panel. This button immediately becomes a pause button which will allow you to pause the simulation at any time. To restart the simulation, press the restart button at the very top of the simulation.
The blue region marked on the diagram is the Habitable Zone around our Sun. Notice how there is both an inner edge and an outer edge – the planets interior to the habitable zone are too hot to support liquid water, while the planets exterior to it are too cold.
1) The simulation is currently set to zero-age – this is the Solar System as it was when it first formed, 5 billion years ago. Which planets were in the Habitable Zone at this time?
1http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html
2) Press the start button and watch the Habitable Zone change with time. Pause the simulation when it reaches an age of 5 billion years (you can keep track of the time by looking at the timeline marker in the bottom panel). This is the Solar System as it is today – which planets are in the Habitable Zone now?
3) Allow the simulation to run until the Earth is no longer in the Habitable Zone. At what age does this happen? How long from now until this happens? You can use the timeline bar in the bottom panel to determine your answers. .
4) After the Earth is no longer within the Habitable Zone, what do you think the condi- tions on Earth will be like?
5) Resume the simulation and let it run until the end. Which planets other than the Earth fell within the Habitable Zone at any point during the Sun’s life?
6) If you had to choose planets of our Solar System for future colonization based on their future habitability, which would you choose, and why?
3 The Habitability Different Kinds of Stars
Now that you’ve simulated the Habitable Zone around our Sun, we’ll run the same simulation for other stars. Astronomers classify stars with letters, O, B, A, F, G, K, and M. The O stars are the hottest and brightest, while the M stars are the dimmest and coolest. Every kind of star has a Habitable Zone, but the brighter the star the farther out the Habitable Zone. Imagine putting and extra log on a campfire – the campers all have back off a few feet to maintain the same comfortable temperature.