The word of the day: Albedo.
When you look up information about planets, one of the bits of data given is the albedo of the planet. Albedo is one of the vocabulary words that introductory astronomy students have to learn. According to the textbook that we are using, Mars has an albedo of 0.15, Jupiter has an albedo of 0.44, and Venus has an albedo of 0.59. So, what is albedo? What do these numbers mean?
Put very simply, albedo is a measure of the reflectivity of a body. You compute the albedo by dividing the amount of reflected light by the amount of incident light. So, an albedo of 0.25 means that a body reflects 25% of the light that shines on it. Unless otherwise stated, albedo normally refers to visual light. Rocky bodies, such as Mercury or the Moon, have low albedos. They are gray, and they absorb more light than they reflect. Icy bodies, such as Pluto, reflect a lot of light, so their albedos are high. Venus is covered in clouds that are very reflective, so it has an albedo greater than 0.5. That means that it reflects more light than it absorbs.
We talk about the albedo of planets, comet nuclei, moons, and asteroids. Another related term is absolute magnitude. In astronomy, magnitude is a measure of how bright an object appears. My stellar astronomy students know the term absolute magnitude as being how bright a star would appear if it were located at a distance of 10 parsecs (32.6 light years) away from us. It is a way of differentiating how bright an object really is from how bright it appears. Such a thing would also be useful for planets, asteroids, and comets. It is a way to directly compare them to one another. A larger body will appear brighter, simply because it reflects more light because of its size. A smaller one, though, could appear just as bright if it were more reflective and had a higher albedo. So, we can define something analogous to stellar absolute magnitude for objects within the solar system. Unfortunately, to the consternation of hosts of astronomy students, the term used is the same term: absolute magnitude. Of course, when we are talking about the absolute magnitude of a planet or asteroid, we definitely do not mean how bright it appears if it were a distance of ten parsecs away. Instead, this planetary absolute magnitude is basically how bright a body would appear as seen from the Sun if it were at a distance of 1 AU from the Sun (that is the distance that Earth is from the Sun). The absolute magnitude of a body in this system can be computed using the equation:
The H stands for the absolute magnitude (to avoid confusing it further with the stellar absolute magnitude, usually referred to as M in equations). D is the diameter of the body in kilometers. Naturally, for irregularly shaped bodies, it would be the effective average diameter. A in this equation is the albedo. There are other factors, of course, that I am not considering. Some substances reflect light differently at different angles of incidence. And, of course, some objects reflect different colors of light differently. But, this is a pretty good approximation. It is as far as we get in the introductory classes.