Astronomy has a way of defying labels. Just when researchers think they’ve drawn a clear line between two cosmic categories, nature throws out something unexpected.
Take the stars and planets. A star is a huge, luminous ball of plasma powered by nuclear fusion. Whereas a planet is a much smaller, cooler and quieter companion, formed from the remains of a star’s birth.
But between these clear categories lies a class of objects that refuses to choose a path. These oddities, known as brown dwarfs, challenge our understanding of what it really means to be a star, a planet, or something in between.
How stars and planets are born
Stars begin their lives when huge clouds of interstellar gas and dust collide or are disrupted by external cosmic events, such as nearby supernovae. If parts of these clouds become dense enough, they begin to collapse under their own gravity, heating up and forming a few to a few thousand protostars, according to the Center for Astrophysics.
When a protostar accumulates enough mass (1,000 to 10 million times the mass of the sun), it becomes hot and dense enough to trigger nuclear fusion in its core. This intense gravitational fusion of hydrogen into helium sends enormous amounts of energy outward, precisely balancing the star’s inward gravitational collapse and causing it to shine brightly for millions or even billions. billions years, according to NASA.
Planets, on the other hand, usually form in the remnants of matter orbiting a newborn star, called a protoplanetary disk. Within these protoplanetary disks, clumps of dust and ice collide and stick to each other, gradually building up, according to the Astrophysics Center. As they build, they can transform into small rocky worlds, huge gas giants, and almost anything else.
So when it comes to stars and planets, how they form, nuclear fusion, and their overall mass are key differences. But there are also objects that don’t fit into either box, slightly blurring the seemingly simple line between stars and planets.
Learn more: A failing star can form brown dwarf stars, which host their own planetary systems
The Inbetweeners: What is a Brown Dwarf?
Brown dwarfs are often described as “failed stars.” But that’s only part of the story. According to NASA, they form like stars through the gravitational collapse of giant molecular clouds, but brown dwarfs never gain enough mass to support hydrogen fusion. Yet they are much more massive than ordinary planets, weighing about 13 to 75 times the mass of Jupiter, according to a study published in Astronomy and Astrophysics.
When brown dwarfs are young, they are still extremely hot from their formation and emit a lot of infrared light. And they can even fuse a heavy form of hydrogen called deuterium in their core, which can heat the brown dwarf internally for hundreds of millions of years. But once the deuterium fuel runs out, a brown dwarf is doomed to cool forever, according to a study from Astrophysics Journal.
Although they briefly underwent some form of merger, brown dwarfs also have varied and complex atmospheres, making them visually more similar to massive planets than tiny stars, according to NASA.
“I generally say that brown dwarfs form in the same way as stars like our sun, but have very low mass and are more Jupiter-like,” says Professor Kelle Cruz, an astrophysicist at Hunter College and research associate at the American Museum of Natural History who specializes in the study of brown dwarfs. “Planets, on the other hand, form as a byproduct of star formation.”
Why brown dwarfs are important
Being slight oddities, brown dwarfs occupy a strange but useful niche in astronomy. Unlike exoplanets, which are often difficult to study because they orbit bright stars, most brown dwarfs float freely in space because they are not bound to any star system. And that makes them much easier to observe in detail, according to a study published in PNAS.
“Currently, brown dwarfs are used to teach us more about planets,” says Cruz. “It’s a way to study planetary-mass objects without having to deal with the glare of a nearby host star.”
The atmospheres of brown dwarfs can also be strikingly similar to those of gas giant planets, with multi-layered clouds and powerful wind-driven storms. This makes them excellent windows into the atmospheric processes that shape the giant planets of our own solar system, as well as super-Jupiter exoplanets discovered outside our stellar neighborhood, according to a report from the University of California, Santa Cruz.
Learn more: The brown dwarf “Poster Child” is actually twins orbiting each other
Can a planet one day become a star?
You may have wondered if Jupiter could ever grow enough to trigger fusion and transform into a star. Although the answer is technically yes, this is also true for any object that suddenly accumulates around 80 times the mass of Jupiter. And this will not happen naturally in a stable, or even chaotic, star system. (Jupiter is already about 2.5 times the mass of all the other planets in the solar system combined, according to NASA.)
Small red dwarf stars also do not evolve into planets. As they reach old age, they briefly become hotter and brighter before exhausting their nuclear fuel and turning into white dwarfs, not planets. The transition from one category to another simply doesn’t happen, and brown dwarfs don’t seem to fit into either category, which makes them all the more interesting, according to the Center for Astrophysics.
The future of brown dwarf research
Fortunately, two major missions in particular are poised to advance brown dwarf research.
“I’m very excited about the LSST survey,” says Cruz, referring to the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, “which will allow us to discover more brown dwarfs based on their motions. Their ‘kinematics’ could give us insight into their formation mechanisms.”
Cruz is also looking forward to new observations from a recently launched space mission. “SPHEREx, a European mission, will also provide us with near-infrared spectra for many brown dwarfs,” she explains.
These precision measurements will help researchers model atmospheres and place constraints on the compositions of “planet-like” brown dwarfs.
Rethinking cosmic categories
Brown dwarfs remind us that space is not made up of neat boxes. It’s a continuum. And as we continue to explore the cosmos, we are bound to discover even more objects that challenge the way we think about classifying the universe around us.
But by studying objects that seem to slip through the cracks, we continually refine our understanding of taken-for-granted conclusions, like where planets end and stars begin.
Learn more: A lonely pair of mystifying space objects found traversing the void
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