The media often brings news about newly-discovered planets in the “habitable zone” of some star beyond our solar system. They usually mean that scientists think the planet may be the right distance from the right kind and size of star for water to exist in liquid form.
Calling such a planet “habitable” may be misleading because there are many factors required to support life.
One of those factors is that the star also has to be extremely stable, as our Sun is. Consider the fact that a change in the Sun’s luminosity of only two percent would make Earth uninhabitable. That seems hard to believe, but scientists have calculated and modeled it.
If the Sun were only two percent dimmer and everything else the same, there would be more snowfall. Because snow reflects more of the Sun’s heat than land or water does, Earth’s surface would become cooler. The cooling would cause more snowfall, resulting in more reflection of the Sun’s rays and, therefore, more cooling. The result would be a runaway freezing of the surface water, and the entire Earth would become covered with ice and snow.
On the other hand, a two percent increase in the Sun’s brightness would cause greater evaporation of Earth’s surface water. The resulting water vapor would act as a greenhouse gas, trapping more of the Sun’s heat in the atmosphere. The increased heat would cause more evaporation resulting in more water vapor and an increased greenhouse effect. The result would be global warming on a massive scale.
Either way, life on Earth would not be possible. We don’t know if there are any other planets in the universe with all the factors required to support life. Our finely-tuned Sun is only one of many features that allow life on this planet. Some people would suggest that our just-right Sun is merely an accident, but we think this is another case of design by a wise Creator.
— Roland Earnst © 2021