medium · SAT Reading & Writing

Not every planet circles a star. Astronomers now estimate that vast numbers of worlds drift alone through the galaxy, either flung out of their original systems by gravitational encounters or never bound to a star at all. Detecting these "rogue" planets is difficult, since they give off almost no light of their own. Researchers find them indirectly, watching for the brief moment when such a body passes in front of a distant star and bends its light. Each detection hints that starless planets may be far more common than the familiar, star-orbiting kind. Which choice best states the main idea of the text?

1. Rogue planets give off almost no light of their own.
2. Planets adrift without a star may be common, even though they are difficult to detect.
3. Astronomers can now easily locate and study planets that do not orbit any star.
4. Most of the planets in the galaxy were once ejected from their original solar systems.

Sign up free to see the explanation and track your rank →