Brown dwarfs are some of the most mysterious objects in the universe. They are often described as “failed stars” because they are too small to shine like true stars but too large to be called planets. For many years, scientists struggled to understand how these strange objects form, what they look like inside, and how they change over time.
Today, thanks to a new and growing group of brown dwarfs known as transiting brown dwarfs, scientists finally have the tools they need to study these objects in much more detail. Transiting brown dwarfs pass in front of their host stars from our point of view. This simple motion—crossing the face of the star—allows us to measure their size and mass directly, something we cannot do for most brown dwarfs floating freely in space.
Recently, researchers developed new models that focus specifically on these transiting brown dwarfs. Their work shows that strong heating from nearby stars can dramatically change the interior, size, and even the nuclear reactions inside brown dwarfs. These effects are much stronger than anyone expected.
What Are Brown Dwarfs? A Quick and Simple Explanation
Brown dwarfs sit in the middle ground between planets and stars.
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They are bigger than giant planets like Jupiter.
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But they are too small to sustain hydrogen fusion, the process that makes stars shine for billions of years.
Because they cannot shine steadily like stars, brown dwarfs gradually cool and shrink as they age. Their temperatures, sizes, and brightness change over time in ways that depend on their mass and internal structure.
For decades, scientists could only study isolated brown dwarfs, which float alone in space. While helpful, these objects are hard to understand because we cannot directly measure their mass or radius. We can estimate these properties only by comparing observations with models—and that means we may introduce errors without realizing it.
This is why transiting brown dwarfs are so important.
Why Transiting Brown Dwarfs Are a Game Changer
Transiting brown dwarfs orbit close to their host stars, and occasionally they pass directly in front of the star from our viewpoint. When this happens, the star briefly looks dimmer, and the size of the dip tells us how large the brown dwarf is.
Meanwhile, the tug of gravity from the brown dwarf causes the star to wobble slightly. This wobble lets us measure the mass of the brown dwarf.
Together, these two measurements—mass and radius—give us a direct and reliable picture of the brown dwarf’s physical characteristics.
Even better, the number of known transiting brown dwarfs has increased dramatically in recent years thanks to modern space surveys and improved measurements of star distances. Scientists now have dozens of objects to study, covering a wide range of:
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ages
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masses
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temperatures
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orbital distances
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levels of stellar heating
This makes transiting brown dwarfs ideal for checking, correcting, and improving the models we use to understand substellar objects.
Why Strong Star Heating Changes Everything
For a long time, scientists believed that heating from a nearby star would not affect brown dwarfs very much. After all, brown dwarfs have strong internal heat, and they are much heavier than planets. It seemed reasonable that the star’s heat would warm only the outermost layers, leaving the deeper interior untouched.
But new models show that this assumption was wrong.
1. Heating from the star goes much deeper than expected
The outer layers of a brown dwarf can be divided into two regions:
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A radiative zone, where heat travels outward in the form of radiation
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A convective zone, where heat is carried by rising and sinking gas, similar to boiling water
In isolated brown dwarfs, the convective zone usually begins relatively close to the surface. But when a brown dwarf is strongly heated by its star, the radiative zone becomes much larger. This pushes the convective boundary deeper and deeper into the object.
This matters because the size and depth of these zones determine how quickly the brown dwarf cools and how its interior evolves.
2. Strong heating can make brown dwarfs puff up
When the incoming stellar energy becomes very large, the brown dwarf’s radius increases noticeably. This happens when the brown dwarf receives very intense radiation—more than roughly a thousand times the amount Earth receives from the Sun.
Under these conditions, the brown dwarf becomes inflated.
Surprisingly:
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This inflation happens at all masses, not just the lighter ones.
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Even older brown dwarfs remain inflated under strong irradiation.
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Younger irradiated brown dwarfs inflate even more than older ones.
This result challenges earlier beliefs that only planets or very low-mass brown dwarfs could show such strong radius expansion.
3. Heating changes nuclear burning inside the brown dwarf
Brown dwarfs can burn one type of fuel—deuterium, a heavy form of hydrogen—for a short time early in their lives. More massive brown dwarfs can even burn hydrogen for a brief period, though not long enough to behave like true stars.
The new models show that external heating from the star affects these nuclear reactions in surprising ways.
Strong irradiation:
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Speeds up some nuclear burning processes
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Allows burning to start at lower masses than in isolated brown dwarfs
This result means that the usual mass boundaries—between planets, brown dwarfs, and stars—are not fixed. They depend heavily on how much stellar radiation the object receives.
How Irradiation Shifts the Planet–Brown Dwarf–Star Boundaries
Traditionally, the categories are:
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Planet: Too small to burn deuterium (usually less than about 13 times Jupiter’s mass)
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Brown dwarf: Can burn deuterium but not hydrogen
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Star: Can burn hydrogen steadily
But under strong irradiation:
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A lower-mass object can burn deuterium.
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A lower-mass brown dwarf can burn hydrogen.
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These burning processes last longer than expected due to the extra heat.
In the new models:
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The mass required for deuterium burning decreases by around 16% under intense stellar flux.
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The mass needed for hydrogen burning decreases by around 13%.
This means some objects we call “planets” might behave like brown dwarfs if they orbit extremely close to hot stars, and some objects we call “brown dwarfs” might behave more like small stars.
In short, mass alone cannot classify these objects anymore—we must also consider the environment they live in.
Testing the New Models: What Real Observations Show
To test their ideas, researchers compared their models with the real measured radii of 46 known transiting brown dwarfs.
The results were very clear:
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For brown dwarfs receiving strong irradiation, the new models matched the observed radii much better than older models that ignored stellar heating.
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For weaker irradiation, both old and new models gave similar results, meaning irradiation matters most for objects close to their stars.
However, not everything matched perfectly.
About ten brown dwarfs still have radii that disagree strongly with the model predictions—by more than three times the expected uncertainty. This means some physical processes are still missing from the current models.
Possible missing ingredients include:
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unexplored atmospheric chemistry
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winds and circulation patterns
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magnetic fields
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unusual cloud behavior
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unexpected heavy-element composition
These mismatches will guide the next generation of research.
Do Metal Cores, Clouds, or Migration Matter?
Although stellar heating plays the biggest role, researchers also examined other factors to see how much they influence evolution.
1. Metal cores
Some brown dwarfs may contain large rocky cores, but these cores are small compared to their total mass. In the models:
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Cores have only a small effect on the radius
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Their influence is strongest during the brief period of deuterium burning
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In general, irradiation dominates over the core’s effects
2. Clouds and atmospheric metals
Clouds and high metallicity have long been considered important. They do affect the cooling rate and brightness of brown dwarfs, but:
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Their impact is much less than that of irradiation, especially for highly heated objects
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They cannot explain radius inflation by themselves
3. Changes in irradiation over time (migration)
Some brown dwarfs may have moved inward toward their stars long after they formed. This could change their irradiation exposure.
But the models show that:
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Migration has only a minor impact
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The long-term radius evolution is not strongly affected by gradual changes in irradiation
Again, this reinforces how dominant continuous strong heating is.
What Transiting Brown Dwarfs Can Teach Us About Hot Jupiters
One of the biggest puzzles in exoplanet science is the “hot Jupiter radius anomaly.” Many hot Jupiters are much larger than they should be based on their age and mass. Several explanations have been proposed, but none fully solve the mystery.
Brown dwarfs can help.
Strongly irradiated brown dwarfs, especially those with masses below about 40 Jupiter masses, respond to stellar heating differently from planets. By comparing the two groups, scientists can test which inflation mechanisms are universal and which are planet-specific.
For example:
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If both planets and brown dwarfs inflate the same way under similar heating, then the cause may be a shared physical process.
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If only planets inflate strongly while brown dwarfs do not, then the cause likely depends on planetary-specific physics such as composition or atmospheric dynamics.
This comparison may help solve one of the longest-standing questions in exoplanet research.
What We Still Need to Learn
While the new models are powerful, they are not the final answer. To fully understand irradiated brown dwarfs, several kinds of observations are needed.
1. Atmospheric measurements
Studying their atmospheres through spectroscopy can tell us:
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internal heat levels
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chemical makeup
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cloud properties
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how heat is transported across the object
This information will greatly improve models of brown dwarf cooling and evolution.
2. Better age estimates
The size of a brown dwarf depends strongly on its age. Better age measurements for their host stars—and therefore for the brown dwarfs—will reduce uncertainties.
3. More discoveries
A larger sample of transiting brown dwarfs across many different environments will make it easier to see patterns and refine models.
4. More advanced interior physics
Future models may need to include:
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magnetic fields
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3-dimensional heat flow
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updated equations of state
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more detailed cloud physics
These improvements will help solve the remaining mismatches between theory and observation.
Conclusion: A New Era in Understanding Substellar Worlds
Transiting brown dwarfs are transforming our understanding of objects that exist between planets and stars. Thanks to their unique alignment, we can measure their sizes and masses directly and test theoretical models with unprecedented accuracy.
The newest models reveal that:
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Stellar irradiation plays a far larger role than previously believed
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It can inflate radii across all masses
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It alters nuclear burning thresholds
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It shifts the traditional mass boundaries between planets, brown dwarfs, and stars
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It explains the behavior of many observed brown dwarfs better than older models
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Yet, some brown dwarfs still defy explanation, pushing us to explore new physics
These discoveries mark an exciting turning point. With more observations and improved models, transiting brown dwarfs will continue to help us understand how substellar objects form, evolve, and interact with their environments.
They remind us that the universe is full of surprises—and that sometimes, the smallest stars can teach us the biggest lessons.
Reference: Sagnick Mukherjee, Jonathan J. Fortney, Theron W. Carmichael, C. Evan Davis, Daniel P. Thorngren, "The Impact of Irradiation on the Radius and Thermal Evolution of Transiting Brown Dwarfs", ApJ, 2025. https://arxiv.org/abs/2512.08249
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