Twinkle, twinkle, little star
How I wonder what you are
Ancient astronomer Hypatia. Credit: NASA and A. Feild (STScI)
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 traveling 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.
An artist’s depiction of the relative sizes of the Sun, a low mass star, a brown dwarf, Jupiter, and the Earth. Credit: NASA/JPL-Caltech/UCB
Despite their name, brown dwarfs can be up to 70 times more massive than gas giants like Jupiter. Brown dwarfs form like stars do, by the contraction of gas that collapses into a dense core under the force of its own gravity, whereas planets form from the accumulation of leftover debris from these stellar births. However, brown dwarfs do not have enough mass for their cores to burn nuclear fuel and radiate starlight. This is why they are sometimes referred to as “failed stars.” They are smaller and cooler than the Sun, and have complex planet-like outer atmospheres, including clouds and molecules such as H2
O. Astronomers now disagree on whether some “free-floating” bodies detected in space – not orbiting a star, but also not shining like a star – should be called planets or brown dwarfs.
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.
Difficult to detect, brown dwarfs hint at the many undiscovered wonders the universe still holds, hidden for centuries beyond the bounds of visible light. Much of the mass that holds the universe together with its gravity has thus far been undetectable, and is known as dark matter. In 2021, the James Webb Space Telescope will continue the work of NASA’s Hubble Space Telescope and infrared Spitzer Space Telescope in probing the furthest and “darkest” regions of the universe. Webb will see farther and in higher resolution, with unprecedentedly powerful infrared cameras and spectrographs. When Hubble launched, the only planets we knew of were those in our own Solar System. There were no images of brown dwarfs. Webb will take a detailed look at the atmospheres of brown dwarfs and exoplanets, determining their temperatures and chemical compositions.
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.