Name_________________________

STUDENT INSTRUCTION AND ANSWER SHEET

Activity 5: Size and Mass Are Important!

Up to this point we have identified that the "just right" condition for life is the presence of liquid water on a planet's surface. This suggests we should first search for a planet that rests in orbit around a Sun-like star (classes F, G or K) within the star's zone of habitability. In addition to the distance from the star, the planet must also have a suitable atmosphere. This requires that the planet have a mass between 0.5 and 10 Earth masses, with a radius between 0.8 and 2.2 times that of Earth. Planets that are too small will not have enough gravity to hold an atmosphere, and larger planets will have an atmosphere too thick to support life as we know it. For example, photosynthesis cannot occur if the atmosphere is impenetrable, or a runaway greenhouse effect could occur such as on Venus today. A habitable planet would also need to have the right type of atmospheric chemistry. Nitrogen, carbon dioxide and oxygen would likely need to be present in amounts similar to what is found on Earth.

Consider the information listed in the table below.

A. On which of these hypothetical planets would you search for Earth-like life? Explain your reasoning for each planet.