Neither Star Nor Planet

Imagine an object 68 times more massive than Jupiter orbiting a star barely larger than the giant planet it circles. Too massive to be a planet, too lightweight to sustain hydrogen fusion like a star. These in-between objects—brown dwarfs—occupy an awkward middle ground in the cosmic hierarchy.

Researchers have now characterized two such objects orbiting M-dwarf stars, the most common stellar type in the Galaxy. The discoveries reveal patterns in how these failed stars distribute themselves around their hosts. More importantly, the statistical analysis shows for the first time that M-dwarfs exhibit their own version of the "brown dwarf desert"—a curious absence of these objects at certain mass ranges and orbital distances.

The findings suggest brown dwarfs share formation pathways with stellar companions rather than planets, migrating inward through gaseous disks and accreting mass along the way. This challenges assumptions about where to draw boundaries between different classes of astronomical objects.

The Desert Problem

Brown dwarfs earned their reputation as cosmic rarities three decades ago. Surveys of Sun-like stars found that only about 1% have brown dwarf companions within 3 astronomical units—the region where most planets orbit. This scarcity became known as the brown dwarf desert.

The absence demands explanation. Giant planets appear commonly around these stars. Stellar companions show up frequently. But objects in the 13-80 Jupiter mass range—the brown dwarf regime—remain stubbornly scarce at short orbital periods.

Several theories attempt to explain the gap. Perhaps brown dwarfs form like stars through gravitational collapse but rarely end up in close orbits. Or maybe they do migrate inward but accrete enough gas to become low-mass stars instead of stalling in the brown dwarf mass range. The lack of examples has made testing these ideas difficult.

Most brown dwarf surveys focused on solar-type FGK stars. But M-dwarfs—red dwarfs comprising roughly 70% of all stars—remained undersampled. Do they show the same desert? Different formation conditions around lower-mass stars might alter the brown dwarf occurrence rate or mass distribution.

The new discoveries help fill this observational gap.

Finding the Failures

The Transiting Exoplanet Survey Satellite (TESS) monitors hundreds of thousands of stars, watching for the telltale dimming when an object crosses in front of its host. In 2020, TESS flagged two M-dwarfs showing deep transits—roughly 2-5% drops in brightness suggesting something much larger than a typical planet.

TOI-5389A exhibited transits every 10.4 days with approximately 4-5% depth. TOI-5610 showed transits every 8.0 days with 2-3% depth. These deep transits hinted at brown dwarf companions, but confirmation required additional observations.

Ground-based photometry from Wyoming's Red Buttes Observatory captured full transit events in 2023 and 2024, providing higher precision than TESS and confirming the periodic signals. High-resolution speckle imaging from the WIYN telescope ruled out nearby contaminating stars that might dilute or mimic the transit signals.

The crucial confirmation came from radial velocity measurements. The Habitable-zone Planet Finder spectrograph on the Hobby-Eberly Telescope in Texas measured how each star wobbles under gravitational influence from its companion. Over 10-12 observing nights spread across 2022-2023, the instrument tracked velocity changes of several kilometers per second—far larger than planetary perturbations.

These wobbles revealed the masses. TOI-5389Ab weighs 68 Jupiter masses. TOI-5610b comes in at 40 Jupiter masses. Both fall squarely in brown dwarf territory, too massive for deuterium to dominate their energy budget but below the 80 Jupiter mass threshold for sustained hydrogen fusion.

Extreme Pairing

TOI-5389Ab presents an unusual system architecture. The brown dwarf orbits an M1-M2 dwarf star with mass 0.43 solar masses—only about 450 Jupiter masses. This produces a companion-to-host mass ratio of 0.150, among the most extreme ratios known for any transiting brown dwarf system.

For context, Jupiter's mass ratio to the Sun is 0.001. Hot Jupiters around solar-type stars typically show ratios around 0.001-0.003. Even massive planets rarely exceed 0.01. TOI-5389Ab's ratio of 0.15 means the brown dwarf weighs 15% as much as its host star.

This extreme ratio has implications for orbital dynamics and tidal interactions. The two objects orbit their common center of mass rather than the brown dwarf simply orbiting the star. Over time, tidal forces should circularize the orbit and spin down both objects. Yet the orbit shows slight eccentricity of 0.096, suggesting the system hasn't fully relaxed to its final configuration.

The brown dwarf's radius measures 0.82 Jupiter radii—smaller than the planet despite being 68 times more massive. This inverted mass-radius relationship characterizes brown dwarfs and low-mass stars. Unlike planets, which grow larger with added mass up to about 10 Jupiter masses, brown dwarfs contract under stronger self-gravity as mass increases. Electron degeneracy pressure supports them against collapse, leading to compact objects roughly Jupiter-sized regardless of mass.

TOI-5610b shows different characteristics. Weighing 40 Jupiter masses and measuring 0.89 Jupiter radii, it falls in the intermediate brown dwarf mass range. Its orbit exhibits moderate eccentricity of 0.354—much higher than TOI-5389Ab. This pronounced elliptical path suggests different formation or migration history.

The eccentricity might preserve information about how the brown dwarf arrived at its current 8-day orbit. Gravitational interactions with other objects in the system could pump up eccentricity. Or perhaps the brown dwarf formed in situ via disk instability, inheriting some eccentricity from the non-circular motions in the protoplanetary disk.

The Missing Intermediates

With two new brown dwarfs characterized, the researchers compiled a comparison sample from literature: 393 brown dwarf systems around main sequence stars, of which 131 transit their hosts allowing mass determination. The remainder show up only in radial velocity surveys, yielding minimum masses without knowing orbital inclination.

Plotting companion mass against host mass revealed striking patterns. Among transiting brown dwarfs, high masses near the 80 Jupiter mass upper limit appear preferentially compared to low masses near the 13 Jupiter mass lower boundary. This can't result from observational bias—lower mass brown dwarfs produce deeper transits due to slightly larger radii, making them easier to detect.

The abundance of massive brown dwarfs suggests an evolutionary process. Objects forming at lower masses might accrete additional material as they migrate through gas-rich disks, ending up near the stellar mass threshold. Only a fraction stall in the low-mass brown dwarf regime.

Focusing specifically on M-dwarf hosts revealed a new pattern. The team divided their 25 M-dwarf/brown dwarf systems into two groups: 13 with orbital periods under 13 days and 12 with periods between 13-2000 days. Statistical analysis using Kolmogorov-Smirnov and Anderson-Darling tests showed the two populations differ at 2.2-2.6 sigma significance.

The short-period systems strongly favor brown dwarfs more massive than 40 Jupiter masses. Only 15% of close-in brown dwarfs fall below this threshold. In contrast, 50% of brown dwarfs at wider separations occupy the 13-40 Jupiter mass range.

This marks the first evidence that M-dwarfs exhibit a brown dwarf desert similar to solar-type stars, with a specific dearth of mass ratios below q=0.1 (corresponding to roughly 40 Jupiter masses for typical M-dwarf hosts). The pattern suggests close M-dwarf/brown dwarf pairs form through similar mechanisms as more massive stellar binaries: disk or core fragmentation followed by inward migration and circumbinary accretion.

Brown dwarfs that migrate inward through gas-rich disks continue accreting as they spiral toward their hosts. Most either remain at wider orbits or accumulate enough mass to become low-mass stars. The minority that halt migration while still in the brown dwarf regime tend to be relatively massive, creating the observed distribution.

Formation Mysteries

How brown dwarfs form remains contentious. Two main pathways exist: core accretion and gravitational instability.

Core accretion—the dominant planet formation mechanism—involves dust grains colliding and sticking together in a protoplanetary disk. Kilometer-sized planetesimals merge into planetary embryos. Once reaching 10-30 Earth masses, the embryo's gravity becomes strong enough to rapidly accrete surrounding gas, potentially creating a gas giant.

But this mechanism struggles to explain brown dwarfs, especially massive ones. The process becomes inefficient at such high masses. Moreover, M-dwarfs possess lower-mass protoplanetary disks than solar-type stars, making it harder to assemble large cores and accrete massive gas envelopes.

Gravitational instability offers an alternative. If a protoplanetary disk becomes massive enough, it can fragment directly through gravitational collapse—similar to how stars form from molecular clouds. Dense clumps contract, heat up, and become brown dwarfs without requiring core assembly.

TOI-5389Ab's 68 Jupiter mass and TOI-5610b's 40 Jupiter mass strongly suggest gravitational instability origin. These masses strain core accretion models even around Sun-like stars. Around M-dwarfs with less massive disks, gravitational instability becomes the only viable formation route.

Yet this raises puzzles. If brown dwarfs form like small stars, why the desert? Stellar companions show no particular scarcity at any mass ratio or separation around M-dwarfs. The answer likely involves subsequent orbital evolution rather than initial formation conditions.

Age Mysteries

Determining brown dwarf ages proves challenging. Unlike stars that settle onto the main sequence with well-defined mass-luminosity relationships, brown dwarfs continuously cool and fade after formation. No single age indicator provides definitive answers.

For TOI-5389A, multiple independent age estimates yield conflicting results. Spectral energy distribution fitting using stellar evolution models suggests roughly 10 billion years. The system includes a white dwarf companion (TOI-5389B) separated by about 2,450 astronomical units—a distant companion unlikely to affect the brown dwarf orbit. White dwarf cooling models indicate it ceased nuclear fusion about 2 billion years ago, providing a firm lower limit on system age.

Gyrochronology—dating stars from rotation periods—suggests 0.5 billion years based on the slow rotation inferred from spectroscopic line widths below 2 km/s. But old M-dwarfs can spin slowly too, making this a weak constraint.

Brown dwarf evolutionary models comparing mass and radius indicate 8 billion years for TOI-5389Ab. These models assume cloud-free atmospheres with solar carbon-to-oxygen ratios—approximations that introduce systematic uncertainties.

Galactic kinematics place TOI-5389A in the thin disk population based on its orbital motion around the Galactic center. Thin disk membership allows ages from essentially zero up to the 10 billion year disk age, providing little constraint.

TOI-5610 shows similar age ambiguities. Different methods yield estimates from 0.5 to 10 billion years. Its galactic velocities suggest either thin or thick disk membership, consistent with intermediate age.

These age uncertainties highlight fundamental limitations in brown dwarf characterization. Unlike planets that retain formation heat signatures for millions of years or stars that maintain quasi-steady fusion for billions of years, brown dwarfs occupy an awkward intermediate regime where multiple physical processes contribute comparable amounts to the energy budget depending on age and mass.

What Brown Dwarfs Teach

Despite uncertain ages, these objects provide crucial insights into the planet-star transition. Their very existence in particular configurations tests formation and migration theories.

The extreme mass ratio of TOI-5389Ab demonstrates that disk fragmentation can produce nearly equal-mass binary systems even around low-mass stars. This challenges the idea that M-dwarfs exclusively form planetary systems via core accretion.

The statistical deficit of 13-40 Jupiter mass brown dwarfs at short periods around M-dwarfs mirrors patterns seen for solar-type stars, suggesting universal formation pathways independent of host mass. Whether an object forms through disk fragmentation depends primarily on disk properties—surface density, temperature, gravitational stability—rather than central star mass.

The moderate-to-high eccentricities of both brown dwarfs (0.096 and 0.354) indicate these orbits haven't fully circularized despite tidal interactions over potentially billions of years. This constrains tidal quality factors and internal structure of both brown dwarfs and their host stars.

Orbital eccentricity also affects observability of secondary eclipses—when the brown dwarf passes behind its host star. Both systems show favorable geometries for detecting these occultations despite non-zero eccentricity. Models predict eclipse depths around 500 parts per million in near-infrared K-band for TOI-5389Ab and 200 ppm for TOI-5610b.

These secondary transits would reveal brown dwarf temperatures and atmospheric properties. Current estimates place both objects around 1,050-1,110 Kelvin based on evolutionary models—cool enough that thermal emission dominates over reflected light but warm enough to show substantial near-infrared emission.

The Census Continues

Only 10 confirmed transiting brown dwarfs orbit M-dwarf hosts prior to these discoveries. This tiny sample size limits statistical analyses of occurrence rates, mass distributions, and orbital properties. Every new discovery significantly impacts our understanding.

The Searching for Giant Exoplanets around M-dwarf Stars (GEMS) survey aims to characterize 40 giant exoplanet and brown dwarf systems around M-dwarfs within 200 parsecs. By systematically following up TESS candidates with ground-based photometry and radial velocities, the survey builds a less biased sample than previous efforts focusing on solar-type stars.

Many survey targets initially flagged as exoplanet candidates turn out to be brown dwarfs or even stellar companions once detailed characterization begins. Rather than disappointments, these "false positives" illuminate the demographics of M-dwarf companions across the planet-star continuum.

Future observations could measure spin-orbit alignment through Rossiter-McLaughlin effect during transits or characterize brown dwarf atmospheres via transmission spectroscopy. High-resolution infrared spectra during secondary eclipses might reveal atmospheric composition, cloud properties, and temperature structures.

Long-term radial velocity monitoring could detect additional companions. If TOI-5389Ab or TOI-5610b formed via disk fragmentation, other objects might exist in the same systems. Jovian planets might orbit both brown dwarfs, formed through similar disk processes on smaller scales.

Blurred Boundaries

These discoveries highlight philosophical questions about astronomical classification. At what point does a massive planet become a brown dwarf? When does a brown dwarf become a star?

Current definitions rely on mass thresholds. Objects above 13 Jupiter masses burn deuterium and qualify as brown dwarfs. Above 80 Jupiter masses, sustained hydrogen fusion begins and the object becomes a star. These boundaries seem clear.

But mass-based definitions ignore formation mechanism—arguably the more fundamental distinction. Objects forming via core accretion through planet formation processes might deserve classification as planets regardless of mass. Objects forming via gravitational collapse of clouds or disks become brown dwarfs or stars depending on final mass.

This formation-based taxonomy creates ambiguity for intermediate mass objects. A 15 Jupiter mass companion that formed via core accretion could be called a super-Jupiter. A 10 Jupiter mass companion that formed via disk fragmentation could be called a low-mass brown dwarf.

TOI-5389Ab and TOI-5610b almost certainly formed via gravitational instability rather than core accretion given their high masses and low-mass hosts. But many lower-mass brown dwarfs around solar-type stars occupy an ambiguous regime where both formation pathways seem possible.

The statistical patterns—the brown dwarf desert, the deficit of intermediate masses at short periods—provide clues about formation. If these patterns result from migration and accretion processes, they encode information about disk properties and timescales that shaped the final mass distribution.

What's Next

As TESS continues surveying the sky and ground-based surveys characterize candidates, the brown dwarf census will grow. Each new discovery refines occurrence rate estimates and tests theoretical predictions about formation and migration.

Upcoming facilities promise new capabilities. The James Webb Space Telescope can characterize brown dwarf atmospheres through high-precision spectroscopy. The Extremely Large Telescope will resolve brown dwarfs around nearby stars, measuring sizes and potentially detecting surface features.

These observations will test evolutionary models, constrain ages through multiple independent methods, and reveal how brown dwarf properties depend on mass, composition, and environment.

The Vera Rubin Observatory will discover brown dwarfs through variability and proper motion surveys, finding isolated objects as well as wide companions. The Nancy Grace Roman Space Telescope will detect brown dwarfs through microlensing and direct imaging, probing different populations than transit surveys.

Together, these efforts will map brown dwarf demographics across mass, separation, host star properties, and galactic environments. They'll reveal whether the patterns seen in these initial samples—the brown dwarf desert, the intermediate mass deficit—hold across the wider population or represent selection effects in current surveys.

The boundary between planets and stars will remain scientifically productive terrain for years to come. Brown dwarfs occupy this boundary, teaching us about both realms simultaneously. Neither purely planetary nor fully stellar, they illuminate the processes that govern formation of all objects from the smallest planets to the largest stars.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.3847/1538-3881/adbb54