Habitable Planets

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Now that you know what kinds of stars would be good to explore further and what criteria should be used for distinguishing lifeforms from other physical processes, let us hone in on the right kind of planet to support life. Unfortunately, our information about life is limited to one planet, the Earth, so the Earth-bias is there. However, scientists do know of the basics of what life needs and what sort of conditions would probably destroy life. With these cautionary notes, let's move forward.

The habitable planet should have:

• a stable temperature regime and
• a liquid mileau. Liquid water is best for biochemical reactions and could be very abundant but liquid methane and/or ethane, like what is found on Saturn's moon Titan might work. Since liquid water dissolves other compounds better than liquid methane/ethane and biochemical sort of reactions work better in liquid water than liquid methane/ethane, liquid water will probably be a requirement for a habitable planet. Also, water in some form (mostly either gas or solid) is actually quite abundant in the Galaxy so we are not limiting ourselves too much with the water bias. The liquid mileau is needed to mix...
• the essential building block elements together (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, and transition metals like iron, chromium, and nickel). Since the building block elements are only created in the stars, the best places to look for life is around stars formed from processed gas, ie., look at metal-rich stars.
• The planet should have a solid surface to concentrate the building block elements together in the liquid on top. The more concentrated the solution of water and molecules is, the more likely the molecules will react with each other. If the molecules were fixed in a solid, they would not be able to get close to each other and react with each other. If the molecules were in a gaseous state, they would be too far apart from each other to react efficiently. Though the reactions could conceivably take place, they would be rare!
• The planet should also have enough gravity to keep an atmosphere. An atmosphere would shield lifeforms on the surface from harmful radiation (charged particles and high-energy photons) and moderate the changes in temperatures between night and day to maintain a stable temperature regime.
• A relatively large moon nearby may be needed to keep the planet's rotation axis from tilting too much and too quickly. This prevents large differences in temperatures over short timescales (life needs sufficient time to adapt to temperature changes).
• Plate tectonics may be needed to: 1) regulate the surface temperature of the planet via its crucial role in the carbon cycle; 2) create a magnetic field to shield the planet from the deadly stellar winds; 3) create dry land on a water-covered world; and 4) promote a high level of biodiversity across the planet by creating new environments that organisms would have to adapt to.

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last updated: June 6, 2007