Chapter 6, Problem 56PP

Exoplanets. As planets with a wide variety of properties are discovered outside our solar system, astrobiologists are considering whether and how life could evolve on planets that might be very different from earth. One recently discovered extrasolar planet, or exoplanet, orbits a star whose mass is 0.70 times the mass of our sun. This planet has been found to have 2.3 times the earth’s diameter and 7.9 times the earth’s mass. For planets in this size range, computer models indicate a relationship between density and composition:

Density compared with that of the earth	Composition
2–3 times	Mostly iron
0.9–2 times	Iron core with a rock mantle
0.4–0.9 times	Iron core with a rock mantle and some lighter elements, such as (water) ice
< 0.4 times	Hydrogen and/or helium gas

56. Observations of this planet over time show that it is in a nearly circular orbit around its star and completes one orbit in only 9.5 days. How many times the orbital radius r of the earth around our sun is this exoplanet’s orbital radius around its sun? Assume that the earth is also in a nearly circular orbit.

1. A.    0.026 r
2. B.     0.078 r
3. C.     0.70 r
4. D.    2.3 r