Next time we will discuss the new evidence for planets around other stars. These recently discovered planets are massive worlds (like Jupiter), but there may also be undetected terrestrial planets or satellites in these systems (since other stars formed within the last 6 billion years or so have heavy atom compositions similar to the Sun's).

We will refer to the range in distance from the parent star where liquid water can exist as the HABITABLE ZONE (HZ). For the current day Solar System and a planet with a thick atmosphere (containing some greenhouse gases), define the inner boundary of the HZ where a runaway greenhouse effect would occur (0.84 AU). The outer boundary of the HZ occurs where all the water freezes (1.7 AU).

Stars slowly brighten as they age, so the boundaries of the HZ move outward with time. Define the CONTINUOUSLY HABITABLE ZONE (CHZ) as the safe distances for a long span of the star's life. Earth may leave the CHZ in several billion years.

Stars more massive than 1.5 x the Sun's mass are too short-lived for the long-term development of life. Stars less massive than 0.5 x the Sun's mass have an HZ very close to the star: planets are less likely to form there and those that do may get locked in synchronous rotation (where one side always faces the star).