Dwarfing the Giants
What Makes Brown Dwarfs Unique?
Twinkle, twinkle, little star
How I wonder what you are
It’s not easy to tell a star from a planet when you look up at the night sky. Ancient astronomers noted that some lights moved across the sky, while others appeared to remain in a fixed position. The Greeks, picking up the work of these earlier scientists, called such a travelling point of light planēs – wanderer. We still call them planets today.
But other than orbiting around a star, what makes a planet a planet? As telescopes become more sophisticated and we learn more about the universe, the less some old definitions make sense. We now know that some planets are rocky, like Earth, while others are so-called gas giants, like Jupiter.
We also know that our middle-aged Sun is one type of a variety of stars, classified by their phase in a lifecycle we are still in the process of understanding. A star shines by producing its own light from nuclear fusion in its dense, hot core. Planets shine—to our eyes on Earth—by reflecting the light of stars.
These were the simple, sharp definitions of stars and planets until the discovery of a brown dwarf in 1995. Theorized as early as the 1960s, this new type of celestial body blurred the line between star and planet, requiring an exciting re-thinking of the universe.
A brown dwarf atmosphere is easier to study than that of an exoplanet, which is typically obscured in the blinding light of its parent star. But to study brown dwarfs you first have to find them. Their dim light makes this difficult, and eventually the visible light left over from their birth fades completely beyond the red end of the visible spectrum, and they emit only infrared light.
Do the traditional boundaries between planets and stars still make sense? Once purely philosophical, these questions now loom large in science. With infrared observations, the Webb Telescope will add to our understanding of the universe’s ongoing evolution, and the place of Earth and our Solar System within that bigger picture.