Brown Dwarfs: The Middle Children of Space Objects
published during a waning crescent moon.
brown dwarfs

Credit: NASA/JPL-Caltech/UCB

Every story has a beginning, a middle, and an end. For the universe, the beginning was 13.82 billion years ago. The middle, we can say, is now. And the end, well, we’re still trying to figure that out. For exoplanet science, the beginning was in the early 1990s–and the end is certainly nowhere in sight.

Many of us can agree that the main goal of studying exoplanets is the search for extraterrestrial life. Compared to what we knew at the beginning of our story, our understanding of what we’re looking for has vastly improved. We know that life may not be in the form of aliens we see in the movies. We’re now looking for cells, microbes, or even for signs that life could exist in the form of specific chemical reactions only organic life could create. We also know that life need not necessarily exist on only rocky planets (as we had previously thought) because the vital molecules for life can exist as gasses and vapors in the atmosphere. We could find organic life on the rocks of an exomoon, or in the atmosphere of a gas giant exoplanet.

Previous and current searches for other habitable planets have alerted us to our technology limitations. Exoplanets are super faint, if visible at all. If an exoplanet is rocky, then it doesn’t output a substantial amount of light, but rather, it reflects a lot of light off of its host star. But reflected light isn’t the kind we’re seeking. Some gaseous exoplanets do shine a decent amount of light via chemical processes and gravitational energies, but it’s tiny compared to the light from its host star, and so it gets outshined.

So if there were objects that resembled gaseous exoplanets, but were more massive, and, therefore, brighter, we could see them better, right?

There are. They are called brown dwarfs, and they are substellar objects. In some ways, they resemble both stars and planets. These objects form similarly to stars and exist both in multiple-object systems and as free-floating objects. But they can resemble exoplanets in their chemical and physical atmospheric makeup. They are roughly 10-60 times more massive than Jupiter, but less than 8% the mass of our Sun, which is why they are sometimes called “failed stars.” Since you can fit 1,321 Earths in the volume of Jupiter, you’d expect to fit roughly 13,000-80,000 Earths in the volume of a brown dwarf. However, since brown dwarfs aren’t massive enough to sustain hydrogen fusion in their cores for their lifetime like stars do, they shrink and continually cool over their lifetimes. After their shrinking phase, they are about 10% larger than a Jupiter-sized radius, so still only about 1,450 Earths! In this way, they resemble gas giant exoplanets because of their size and their temperatures.

However, a brown dwarf’s mass is so immense compared to that of a gas giant exoplanet’s mass, that their subsequent brightness provides significantly more light for us to observe. Therefore, we can get substantially more spectral information when we peer through their atmospheres. By studying their light, we can see the similarities in the chemical makeup of the brown dwarfs and the gaseous exoplanets. For example, brown dwarfs and gaseous exoplanets like Jupiter have atmospheres of hydrogen, ammonia, methane, and water vapor, among other things. By studying these failed stars, or exoplanet analogs, we can bridge the gap between stars like our Sun, and gaseous worlds like Jupiter.

Brown dwarf science isn’t just a by-product of exoplanet science–it has its own exciting story, too! Think of the middle sibling in a family with three. In our case: stars, brown dwarfs, and exoplanets. Maybe the middle child wasn’t the first to experience cool things, and it isn’t the cute little one just entering the world, but Elite Daily says, “[Middle children] all know we’re part of the same family. Like Jan in “The Brady Bunch,” we may be unappreciated and constantly put down, but we’re the most badass.”

In a tweet, Dr. Mark Marley, astronomer at NASA Ames Research CenterMarley, cautioned scientists to prevent our baby sibling, the motivator, the plot-driver–a.k.a, the “MacGuffin”–to place blinders on us during our journey.

“I’m not saying that searching for Earth-like planets is wrong, or a bad idea, just that they serve as a goal that drives the science narrative of ‘what should we do next?’ Doing all those things will make for great science in themselves, whether or not we ever find the ‘MacGuffin’.” Marley wrote.

The MacGuffin is not the end of the story. If you interviewed each member of a family, you’d have N number of autobiographies about the same life. Stars, brown dwarfs, gas giant exoplanets, and Earth-like exoplanets all have their autobiographies, too.

* Marley speaks as an individual. His comments and opinions are not of NASA or Ames.