What is comparative planetology and why is it useful to astronomers?

the comparison and  contrasting the feature,  properties and behaviors of the different  planetary objects and the diverse worlds.

comprehend theories of the formation, origin of the different planets

Explanation:

The comparative planetology is the space science that deals with study of the the comparison and  contrasting the feature,  properties and behaviors of the different  planetary objects and the diverse worlds.

The comparative planetology  helps the astronomers to comprehend theories of the formation, origin of the different planets.

luster is not a good characteristic to use by itself because luster is the property of minerals that indicates how much the surface of a mineral reflects light. the luster of a mineral is affected by the brilliance of the light used to observe the mineral surface.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.