Based on these data, what is the most likely composition of this planet?

(a) Mostly iron;

(b) Iron and rock;

(c) Iron and rock with some lighter elements;

(d) Hydrogen and helium gases.

As planets with a wide variety of properties are being 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 the planet’s 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

6 0.4 times .................................................Hydrogen and/or helium gas