The discovery fills a curious gap in astronomers’ understanding of how stars and their companions interact. Across the galaxy, most planetary bodies circle their host stars in stable, detached orbits, like Earth around the sun. At the other extreme, astronomers have documented a handful of instances in which a star pulls a planet too close and swallows it whole — a fate that awaits Earth itself when the sun eventually swells into a red giant. ZTF J0440+2325 sits strikingly between these two extremes. Instead of a sudden engulfment or a peaceful coexistence, the system shows a star gradually feeding on its companion at a measured, sustainable pace. “When we think of stars interacting with planets or brown dwarfs, the picture is always that the star eventually swallows the other thing,” said Kevin Burdge, assistant professor of physics at MIT. “But here, we’ve found an alternative: Instead of swallowing the thing up, the star can gradually eat it, for billions of years.”

The story of the discovery began several years ago, when Burdge was combing through data from the Zwicky Transient Facility, or ZTF, a survey that uses a camera on a telescope at Palomar Observatory in California to scan the sky for rapid changes in brightness. Such flickers can signal supernovae, gamma-ray bursts, or colliding neutron stars. Light curves from supernovae typically resemble a bell curve, charting the gradual brightening and fading of an explosion. What Burdge noticed instead was something far stranger: a triangular waveform, rising and falling in a sharp, repeating pattern that no known stellar process seemed to produce. “I remember first looking at this and thinking, stars don’t make triangular waveforms like this,” he recalled.

At the time, Burdge and his colleagues were investigating a different class of objects known as black widow binaries, in which an extremely dense, rapidly spinning neutron star slowly consumes a much smaller companion star, much like the spider that gives the systems their name. Burdge wondered whether the triangular signal might come from a similar system. But the light from the source refused to cooperate. In black widow binaries, the light appears to wobble because a very light object whips around a much heavier one, and the amplitude of that wobble encodes the extreme imbalance of masses. “We weren’t seeing that whipping back and forth here,” Burdge said. “It didn’t make any sense. We couldn’t explain what this was.” The signal sat unresolved for years, a persistent puzzle in the team’s files.

The breakthrough came when Burdge and Aaron Householder, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, decided to revisit the mystery. From the original ZTF signal, they pinpointed the source’s location within the Milky Way, about 300 light years away, and trained multiple telescopes on the object. Their measurements revealed a wobble — but a far smaller one than any black widow binary would produce. “That was the real clincher for this system,” Householder said. “When we measured that wobble, we found we were not seeing a black widow. This was a low-mass star that’s orbited by a brown dwarf. The wobble was too small in amplitude to be anything else.” The insight that unlocked the puzzle was that a system with less total mass allows two objects to orbit each other gently, without the dramatic back-and-forth whipping seen in David-and-Goliath pairings.

The measurements established that the two objects are extraordinarily close, with the brown dwarf completing an orbit every 87 minutes. Both bodies are small by stellar standards: the star weighs roughly 85 times the mass of Jupiter, while the brown dwarf comes in at about 25 Jupiter masses. With two low-mass objects circling at such intimate range, the team suspected that one might be pulling material from the other — a process known as accretion. Accretion is most often observed around objects that are extremely massive yet physically tiny, such as black holes and neutron stars. When a black hole draws material from a nearby companion, the matter spirals inward through a disk. “The difference here is, the thing absorbing matter is not a tiny black hole but a star, which is relatively big in size,” Burdge explained. “So matter just pummels directly onto the surface, at very high speeds, like an asteroid hitting the moon.”

To test whether accretion was actually occurring, the researchers ran simulations of the system. Using the measured properties of the star and the brown dwarf, they modeled particles of matter on the brown dwarf’s surface and tracked how those particles would behave over time under the laws of physics and the equations of motion. The result was unambiguous: the test particles fell directly onto the surface of the star. “When we track those test particles, we see they indeed fall right onto the surface of the star,” Householder said. “This is the first time we’ve caught a low-mass star actively accreting from another low-mass object.” The finding establishes a new category of interacting binary, one in which the accretor is a full-fledged star rather than a compact remnant.

The team also calculated the rate of the transfer, and the numbers are remarkable in both directions. The brown dwarf is losing material at about one one-hundred-thousandth of an Earth mass per year — the equivalent of roughly 40 million dump trucks’ worth of material, or about 1.3 trillion one-pound burritos, every second. Yet even at that staggering everyday scale, the loss represents only a tiny fraction of the brown dwarf’s total mass. The transfer is slow and steady enough that, given the size of the system, the star could continue its leisurely snacking for hundreds of thousands of years, and possibly for billions. What would be a cataclysmic feast on human timescales is, cosmically speaking, a grazing meal.

The slow accretion also solved the mystery of the triangular light curve. The stream of material from the brown dwarf strikes the surface of the red dwarf, heating a large, glowing hot spot at the point of impact. Viewed from afar, the system’s brightness rises and falls as the brown dwarf and its stream circle the star, carrying the blazing impact zone in and out of view. “It’s like you’ve got this continuous fireball onto one of the objects, and as one orbits the other, that hotspot comes in and out of view, and the peak of the triangle signal is when you’re looking right at the fireball,” Burdge explained. The triangular waveform, once an inexplicable anomaly, is now a diagnostic signature of a star being pummeled by a steady stream of stolen matter.

With the puzzle resolved, the team is now turning its attention to finding similar systems elsewhere in the galaxy. Because the triangular light curve is so distinctive, it offers a ready-made search template for future surveys. “It’s inspiring a lot of new searches on our part,” Householder said. “I think we’re going to learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars.” The research, supported in part by the National Science Foundation, included MIT co-authors Aaron Householder, Kaitlyn Shin, Saul Rappaport, Joheen Chakraborty, and Emma Chickles, along with collaborators from Caltech, the University of Hawaii, the Instituto de Astrofísica de Canarias, the Universidad de La Laguna in Spain, and the Harvard-Smithsonian Center for Astrophysics. For now, ZTF J0440+2325 stands as proof that the drama between stars and their companions is not always a story of sudden destruction — sometimes, it is a meal savored over eons.

Subject of Research: Stable mass transfer from a brown dwarf onto a low-mass star in a close binary system

Article Title: Astronomers catch a star slowly snacking on a brown dwarf, 300 light years away

Article References: Astronomers catch a star slowly snacking on a brown dwarf, 300 light years away. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: brown dwarf, red dwarf, accretion, binary star system, ZTF J0440+2325, Zwicky Transient Facility, light curve, stellar evolution, mass transfer, MIT, Nature Astronomy, black widow binary

The finding, published in Nature Astronomy by a team led by Aaron Householder of the Massachusetts Institute of Technology, marks the first direct observation of stable accretion from a substellar donor onto a main-sequence star. Substellar objects — brown dwarfs and giant planets — have long been expected to remain detached from their host stars unless orbital decay or stellar expansion brings them into contact. When that contact occurs, the standard picture holds, the outcome is rapid engulfment and destruction. It is a fate long forecast for Earth and the other rocky planets of our own Solar System, which will be swallowed when the Sun expands into a red giant. The new observations demonstrate that this grim narrative is not the only possible ending: under the right conditions, a substellar companion can settle into a delicate equilibrium in which it loses mass gracefully, potentially surviving for billions of years as it is slowly consumed.

The key to this stability lies in the physics of Roche-lobe overflow, the mechanism by which material flows from one member of a close binary to the other. Every star in a binary system is surrounded by an invisible gravitational boundary called its Roche lobe — the region within which its own gravity dominates. When a star fills its Roche lobe, gas from its outer layers escapes through the inner Lagrange point, a gravitational saddle between the two stars, and streams toward the companion. Whether this process is stable or catastrophic depends on how the donor responds to losing mass. For most stars, shedding mass makes them shrink, pulling them back inside their Roche lobe and choking off the flow. But for objects near the substellar boundary, the opposite happens: brown dwarfs and low-mass stars are supported against gravity largely by electron degeneracy pressure, a quantum-mechanical effect that makes them grow larger, not smaller, as they lose mass. A degenerate donor that overflows its Roche lobe therefore expands further into the lobe as it sheds material, sustaining — and even enhancing — the mass transfer.

That runaway quality is precisely what makes the stability of ZTF J0440+2325 so remarkable. In a conventional cataclysmic variable, where a degenerate white dwarf strips material from a low-mass companion, the mass ratio between donor and accretor is extreme, and the transfer can persist for eons. In ZTF J0440+2325, the donor is a brown dwarf and the accretor is an M dwarf — a cool, low-mass red star — so the two objects are far more evenly matched in mass. Theory allows stability in such a configuration only within a narrow window of mass ratios and orbital periods, and the team’s modelling shows that this system sits squarely inside it. The brown dwarf is transferring mass at a rate that the M dwarf can accommodate, with the transferred gas forming a stream and accretion structure that heats up where it lands, producing the distinctive light variations the astronomers observed.

The discovery emerged from the Zwicky Transient Facility, or ZTF, a wide-field sky survey based at Palomar Observatory that scans the entire northern sky every few nights. Kaitlyn Shin and Kevin Burdge, both then at MIT, first identified ZTF J0440+2325 and a second object, ZTF J1444+4820, in the survey’s archive of periodic variable sources. What caught their attention was the extraordinarily short orbital period — 87 minutes for ZTF J0440+2325 and just 67 minutes for ZTF J1444+4820 — far shorter than any normal binary involving a main-sequence star could sustain without the components being in contact. At such tight separations, two Sun-like stars would be crushed inside each other; only a configuration involving a compact donor overflowing its Roche lobe can fit inside an orbit of this size.

To characterize the systems, the team assembled an extensive follow-up campaign. High-speed, multi-band photometry from HiPERCAM, a quintuple-beam imager on the 10.4-meter Gran Telescopio Canarias, captured the rapid flickering of the accretion process, revealing a hot spot where the mass-transfer stream strikes the accretion flow around the M dwarf. Phase-resolved spectroscopy from the Keck Observatory’s Low Resolution Imaging Spectrometer and Echellette Spectrograph and Imager dissected the light orbit by orbit, revealing transient Balmer absorption near peak brightness — the spectral fingerprint of an accretion-powered hot spot rotating into view — alongside H-alpha emission and the molecular absorption bands of the cool stellar components. By fitting the light curves and radial velocities with detailed models, the researchers constrained the masses, radii and temperatures of both objects in each binary, using Markov-chain Monte Carlo techniques to map the full range of allowed parameters.

The spectral analysis confirmed the extraordinary nature of the donor in ZTF J0440+2325. The object contributing the mass transfer is consistent with a brown dwarf — a failed star too lightweight to fuse hydrogen in its core — rather than an ordinary low-mass star. Its companion is an M dwarf of the kind that populates the vast majority of the Galaxy’s stellar census. The team also modeled the system’s evolution with the MESA stellar evolution code, showing that the binary’s present configuration is a snapshot in a long episode of stable mass transfer: as the brown dwarf loses mass, the orbit widens, the transfer rate adjusts, and the system settles into a self-regulating state. The brown dwarf will continue to be whittled down over billions of years, its substance gradually incorporated into the M dwarf, until it dwindles toward planetary masses.

The second system, ZTF J1444+4820, presents an equally intriguing picture with a twist. With its 67-minute period, it is an even tighter binary, and the team presents it as a strong candidate for a low-mass mass-transferring system — but with evidence pointing to a hierarchical triple architecture. The spectroscopic data are dominated by the molecular absorption features of an M4 dwarf that appears to be a tertiary companion orbiting the inner binary at a much wider separation, and the researchers suggest that gravitational perturbations from this third star — the classic Kozai-Lidov mechanism, in which a distant companion cyclically pumps up the eccentricity and inclination of an inner orbit — may have helped drive the inner pair into contact in the first place. Hierarchical triples have previously been implicated in producing other ultracompact binaries, including a 62-minute black widow pulsar system, and ZTF J1444+4820 adds a low-mass stellar example to that growing roster.

Beyond their individual peculiarities, these systems carry broad implications for how astronomers understand the demography of close binaries and the ultimate fate of substellar objects. Ultracompact binaries with periods of an hour or so are expected to be numerous but have proved difficult to find, because most searches have focused on white dwarf pairs or accreting white dwarfs. The ZTF discoveries demonstrate that the short-period census extends down to the substellar regime, and they provide real-world calibration for the stability criteria that theorists have long computed from polytropic models. They also hint at a hidden population: systems like these are natural sources of low-frequency gravitational waves, and some may be detectable by the upcoming LISA space observatory, adding to the multi-messenger picture of the Galaxy’s tightest binaries.

Perhaps most strikingly, the observations give substance to a possibility that until now existed only in theoretical papers: that the boundary between destruction and survival for a doomed companion is not a cliff but a narrow ledge. The Earth, when the Sun expands, will almost certainly be engulfed whole. But a sufficiently massive, sufficiently degenerate companion in the right orbit can thread the needle, converting a catastrophic plunge into a patient, billion-year meal. ZTF J0440+2325 shows that somewhere in the Galaxy, that slow feast is already underway — a brown dwarf dissolving into a red dwarf, one 87-minute orbit at a time, proving that even the smallest stars can meet a fate stranger than simple annihilation.

Subject of Research: Stable Roche-lobe mass transfer from a brown dwarf onto an M dwarf in an ultracompact binary

Article Title: Stable mass transfer from a substellar object onto an M dwarf

Article References: Householder, A., Shin, K., Burdge, K. B., Marsh, T. R., Rappaport, S. A., El-Badry, K., Chakraborty, J., Chickles, E., Dai, F., Graham, M. J., Kulkarni, S. R., Rodríguez-Gil, P., Vanderburg, A., & Whitebook, S. (2026). Stable mass transfer from a substellar object onto an M dwarf. Nature Astronomy. https://doi.org/10.1038/s41550-026-02992-6

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02992-6

Keywords: brown dwarf, M dwarf, mass transfer, binary stars, Roche-lobe overflow, Zwicky Transient Facility, ultracompact binary, stellar evolution, accretion, hierarchical triple, time-domain astronomy, gravitational waves

The stakes are higher than they might appear. In the nozzle zone of an industrial spray dryer, freshly formed droplets collide with so-called fines, tiny dry particles smaller than about 100 micrometers that are recirculated from the fluidized bed or cyclone back into the chamber. When a wet droplet meets a fine, the particle merges completely into the liquid. As the droplet surface partially dries, collisions instead produce sticking, the desired outcome that builds agglomerates. Dry too far, and the droplet surface becomes glassy and rigid, so particles bounce off and are lost as waste. The researchers set out to determine exactly where that sticky window lies, and whether it can be predicted from first principles rather than empirical rules.

The team used a sessile single droplet drying platform, in which droplets of roughly 400 micrometers were dispensed onto a surface and dried by a controlled air stream. Maltodextrin with a dextrose equivalence of 38 served as the model material, chosen because its diffusivity and sorption behavior are well characterized, making it ideal for physical modeling. Glass beads in the 70 to 110 micrometer range acted as stand-ins for fines particles, an approach validated in earlier work showing that collision outcomes are governed primarily by the state of the drying droplet rather than the identity of the collision partner.

A central concept in the study is the locking point, the moment when a drying droplet stops behaving like an ideal shrinking sphere and begins to deviate from its spherical shape. This deviation signals the onset of skin formation, when evaporation at the surface outpaces the diffusion of solute from the interior, creating a steep concentration gradient and a mechanically distinct outer layer. The researchers found that the locking point shifts linearly with drying conditions: raising the relative humidity of the drying air from 0 to 15 percent at 80 degrees Celsius delayed locking from about 6.6 to 11.8 seconds, while raising the air temperature from 70 to 90 degrees Celsius at zero humidity shortened it from roughly 9.4 to 5.1 seconds. Crucially, the normalized droplet radius at locking remained statistically unchanged across all conditions, hovering near 0.85 of the initial radius, suggesting that a critical surface solute concentration triggers skin formation regardless of how fast the droplet dries.

To probe stickiness directly, the team fired glass beads at droplets at controlled times during drying and classified each collision as merging, sticking or bouncing. The results were strikingly consistent: across relative humidities from 0 to 15 percent, sticking collisions clustered between 0.75 and 1.5 times the locking point time. Increasing humidity delayed the entire sequence of collision regimes but did not fundamentally change its structure. This confirms and generalizes earlier findings from experiments with different maltodextrin formulations and particle shapes, pointing to the state of the droplet skin, not the drying schedule itself, as the dominant factor determining collision outcomes.

The experimental work was paired with a one-dimensional droplet drying model that solves coupled heat and mass transfer equations in a Lagrangian reference frame, discretizing the droplet into 40 concentric shells. Water diffuses between shells while solids remain fixed, and the model tracks the evolving glass transition temperature at the droplet surface using the Couchman-Karasz relation, which captures how moisture depresses the glass transition. The model was validated against two independent measurements: the volume decrease of droplets up to the locking point, captured by automated image analysis of high-speed video, and the droplet temperature over time, measured with an infrared camera through a special AMTIR-1 window at 125 frames per second.

The thermography revealed a familiar but now precisely quantified pattern. Droplets initially hold a constant temperature set by evaporative cooling, which rises with ambient humidity because higher moisture in the air slows evaporation. Measured constant temperatures of 28, 33, 39 and 44 degrees Celsius at 0, 5, 10 and 15 percent relative humidity matched the corresponding wet bulb temperatures. Once the skin forms and evaporation slows, the droplet temperature climbs toward the air temperature. The model reproduced these trends well, with minor deviations attributed to the difficulty of characterizing the thermal contact between droplet and drying surface, a resistance the team captured with a single fitted parameter.

The key predictive insight emerged when the researchers plotted collision outcomes against the ratio of glass transition temperature to actual droplet temperature at the droplet surface, a dimensionless measure of how close the skin is to becoming glassy. Merging dominated below a ratio of about 0.6, when the droplet is still liquid-like. Sticking occurred between roughly 0.55 and 0.9, with the densest concentration of sticking collisions between 0.6 and 0.8. Bouncing took over above 0.8, as the skin approached the glassy state. Some overlap between regimes was expected and observed, since drying droplets develop spatially heterogeneous surfaces with softer and stiffer patches, and buckling after locking creates irregular geometries that influence individual collisions.

When the researchers normalized drying time by the locking point time, the curves for all tested humidities and temperatures nearly collapsed onto a single master curve, with the glass transition ratio reaching about 0.6 at locking. This collapse means the stickiness regime can be predicted for new drying conditions without running collision experiments for each one: model the droplet, find the locking point, and the sticky window follows as 0.75 to 1.5 times that time. The authors propose this as a general methodology for formulations where glass transition behavior is well defined, while noting limitations for materials such as globular proteins, for very high feed solids concentrations, and for the higher air temperatures used industrially, where the rapidly shortening locking time makes visual determination harder.

The practical payoff could be substantial. Better control of the nozzle zone means manufacturers can tune air temperature, humidity and fines recirculation to maximize agglomeration, improving powder solubility and reducing the fraction of product lost as unagglomerated fines. Because the framework rests on fundamental heat and mass transfer rather than formulation-specific calibration, it offers a route from single droplet physics to pilot and industrial scale spray drying. For an industry that dries everything from milk and coffee to enzymes and probiotics, knowing precisely when a droplet will stick, and being able to compute it in advance, turns a long-standing empirical art into a predictive science.

Subject of Research: Predicting the stickiness regime of drying maltodextrin droplets during spray drying using single droplet experiments and heat and mass transfer modeling

Article Title: Evaluating and predicting the stickiness regime of drying droplets across drying conditions

Article References: de Leeuw, S., Bekebrede, J., Schutyser, M., & van der Sman, R. (2026). Evaluating and predicting the stickiness regime of drying droplets across drying conditions. Current Research in Food Science, Article 101588. https://doi.org/10.1016/j.crfs.2026.101588

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101588

Keywords: spray drying, droplet drying, stickiness, agglomeration, glass transition, maltodextrin, locking point, thermography, mass transfer, food powders, collision regimes, relative humidity