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TOI-2155 b: A Massive Brown Dwarf or a Very Low-Mass Star?

This paper presents the discovery and characterization of TOI-2155 b, a massive transiting companion with a mass near the hydrogen-burning limit that serves as a critical benchmark for testing evolutionary models at the boundary between brown dwarfs and very low-mass stars.

Original authors: Md Redyan Ahmed, Tansu Daylan, Theron W. Carmichael, Sarah L. Casewell, Anita Hafner, Jaime A. Alvarado-Montes, Allyson Bieryla, Samuel N. Quinn, Michael Calkins, Karen A. Collins, Cristilyn N. Watkin

Published 2026-05-27

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CC BY 4.0

Original authors: Md Redyan Ahmed, Tansu Daylan, Theron W. Carmichael, Sarah L. Casewell, Anita Hafner, Jaime A. Alvarado-Montes, Allyson Bieryla, Samuel N. Quinn, Michael Calkins, Karen A. Collins, Cristilyn N. Watkins, Keivan G. Stassun, Boris S. Safonov, Maria V. Goliguzova, Giuseppe Marino, Dennis M. Conti, Peter Tuthill

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "In-Between" Object

Imagine you are looking at a family photo album of the universe. On one side, you have planets (like Jupiter, which are big but don't make their own light). On the other side, you have stars (like our Sun, which are massive and burn bright with nuclear fire).

Usually, the line between them is clear. But then, there is a mysterious "gray area" in the middle called the Brown Dwarf. These are objects that are too heavy to be planets but too light to be true stars. They are the cosmic "teenagers" that never quite grow up to shine like a star, but they are too big to be just a rock.

TOI-2155 b is a new discovery sitting right on the fence of this gray area. It is a massive companion orbiting a star, and the scientists are asking a big question: Is it a super-heavy Brown Dwarf, or is it a tiny, failed Star?

How They Found It: The Cosmic Dance

The team found this object using a space telescope called TESS (Transiting Exoplanet Survey Satellite). Think of TESS as a very sensitive camera watching a star.

The Dip: Every few days, the camera noticed the star's light dimming slightly. It was like watching a moth fly in front of a porch light. The moth (TOI-2155 b) was passing in front of the light (the host star).
The Wobble: To confirm it wasn't just a glitch, they used a ground-based telescope called TRES. They watched the star "wobble" back and forth. Imagine a heavy person spinning a child on a rope; the heavy person has to lean and shift to keep balance. The star was wobbling because the massive object was tugging on it.

By combining the "dip" (to measure size) and the "wobble" (to measure weight), they could calculate exactly what TOI-2155 b is made of.

The Measurements: A Heavyweight Champion

The results were surprising and precise:

Size: It is about the same size as Jupiter (our solar system's biggest planet).
Weight: It is 80 times heavier than Jupiter.
Density: It is incredibly dense. If you could scoop up a cup of this object, it would weigh as much as a small car.

The host star it orbits is a hot, yellow-white star (an F-type subgiant) that is slightly older and larger than our Sun. The two are very close friends, orbiting each other every 3.7 days. That is a very fast dance; for comparison, Mercury takes 88 days to orbit our Sun.

The Big Mystery: Brown Dwarf or Tiny Star?

This is where the paper gets interesting. The object weighs 80.6 times the mass of Jupiter.

In the universe, there is a "magic number" (the hydrogen-burning minimum mass).

If an object is lighter than this number, it is a Brown Dwarf. It cools down over time and never starts a nuclear fire.
If an object is heavier than this number, it is a Star. It ignites hydrogen fusion and shines.

TOI-2155 b is sitting almost exactly on that magic number.

The Problem: The paper says that even with such precise measurements of its mass, size, and age, we still can't be 100% sure which side of the line it falls on.
The Analogy: Imagine a scale that is perfectly balanced. You put a feather on one side and a pebble on the other, but the scale is so sensitive that it wobbles back and forth. You know it's almost a pebble, but you can't say for sure if it's a feather or a pebble without a better scale.

The scientists found that its density is so high that it behaves like a "failed star" (supported by electron pressure, like a squeezed sponge), but it is so close to the limit that it might be a very small, dim star that is just barely burning fuel.

Why This Matters

The paper concludes that TOI-2155 b is a benchmark. It is a perfect test case for scientists trying to write the "rules" of how these objects form and evolve.

Formation: Because it is so heavy compared to its star (a mass ratio of about 6%), it likely didn't form like a planet (swirling dust in a disk). Instead, it probably formed like a star (collapsing gas clouds), just like a binary star system, but with one partner that is much smaller.
Tides: Because it orbits so closely, the star and the object have been pulling on each other for billions of years. This has smoothed out their orbit into a perfect circle, much like a dancer who has practiced a routine so many times that every step is perfectly synchronized.

The Bottom Line

TOI-2155 b is a cosmic "Goldilocks" object. It is too heavy to be a normal planet, but it sits right on the edge of being a star. The paper doesn't claim to have solved the mystery of exactly what it is; instead, it provides the most accurate measurements yet of this "in-between" creature, giving other scientists the data they need to finally figure out where the line between a planet and a star is truly drawn.

Technical Summary of "TOI-2155 b: A Massive Brown Dwarf or a Very Low-Mass Star?"

Problem and Context
The classification of substellar objects near the hydrogen-burning minimum mass (HBMM) remains a significant challenge in astrophysics. While brown dwarfs (BDs) are generally defined by a mass range of $\sim$ 13–80 $M_J$ , the precise boundary between massive brown dwarfs and very low-mass stars is not universal, depending on factors such as metallicity, age, and atmospheric conditions. Objects in this transitional regime may either sustain stable hydrogen fusion or cool and contract as brown dwarfs. Despite the discovery of over 1,000 solitary BDs, only $\sim$ 50 transiting BD systems have been identified, limiting the ability to test evolutionary models of stellar and substellar structure near the HBMM. This paper addresses the need for precise mass and radius measurements of companions in this critical mass range to distinguish between formation pathways and evolutionary states.

Methodology
The authors present the discovery and characterization of TOI-2155 b, a transiting companion to an F-type subgiant host star. The study integrates multi-faceted observational data:

Transit Photometry: Light curves were obtained from the Transiting Exoplanet Survey Satellite (TESS) across 13 sectors. The data underwent detrending using a biweight algorithm to remove systematics. Ground-based follow-up photometry was conducted using the Wendelstein Observatory (WST), Las Cumbres Observatory Global Telescope (LCOGT), and Wild Boar Remote Observatory (WBR) to confirm the transit signal and rule out background eclipsing binaries.
Contamination Analysis: The team performed rigorous aperture contamination analysis using TESS target pixel files and the tess-cont tool, confirming that the target star contributes $\sim$ 96.9% of the flux within the aperture. High-resolution speckle imaging from the Caucasian Mountain Observatory (CMO) was used to rule out close companions unresolved by TESS or Gaia.
Radial Velocity (RV) Measurements: To determine the companion's mass, 27 high-resolution spectra were obtained using the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5-meter telescope at Fred L. Whipple Observatory. These measurements spanned a baseline of approximately two years.
Stellar Characterization: Stellar parameters (effective temperature, metallicity, surface gravity) were derived using the Stellar Parameter Classification (SPC) software applied to TRES spectra. Stellar mass, radius, and age were estimated using EXOFASTv2 (fitting MIST isochrones) and independent empirical spectral energy distribution (SED) analysis.
Modeling: The system was modeled simultaneously using Allesfitter, a tool that combines nested sampling (DYNESTY) with light curve and RV modeling (including the ELLC model). This approach jointly fit the TESS photometry and TRES RV data to derive orbital and physical parameters.

Key Results

Companion Properties: TOI-2155 b is a short-period companion with an orbital period of $P = 3.7246950 \pm 0.0000014$ days. The derived mass is $M_b = 80.6^{+1.0}_{-1.1} M_J$ , and the radius is $R_b = 0.972^{+0.009}_{-0.008} R_J$ . This corresponds to a high bulk density of $\rho_b = 109^{+3.1}_{-3.3}$ g cm $^{-3}$ .
Host Star: The host star is an F-type subgiant with an effective temperature $T_{\text{eff}} = 6085 \pm 78$ K, a radius $R_\star = 1.705^{+0.066}_{-0.064} R_\odot$ , and a mass $M_\star = 1.33 \pm 0.008 M_\odot$ . The system age is estimated at $3.2^{+1.9}_{-0.9}$ Gyr.
Orbital Dynamics: The orbit is nearly circular ( $e = 0.013^{+0.13}_{-0.009}$ ), consistent with tidal circularization over the system's lifetime. The mass ratio ( $q \approx 0.061$ ) significantly exceeds the typical planet-star boundary ( $q \sim 0.01$ ).
Classification Ambiguity: The mass of TOI-2155 b places it extremely close to the theoretical hydrogen-burning minimum mass (which varies between $\sim$ 75–81.7 $M_J$ depending on models and metallicity). Consequently, the object lies in a transitional region where distinguishing between a massive brown dwarf and a very low-mass star is not straightforward based on mass, radius, or density alone.

Significance and Claims
The paper claims that TOI-2155 b serves as a valuable benchmark for testing evolutionary models of stellar and substellar structure near the hydrogen-burning limit.

Model Validation: The measured radius is broadly consistent with theoretical evolutionary models (specifically I. Baraffe et al. 2003 and Sonora Bobcat models) when uncertainties in age and metallicity are accounted for. The authors find no robust evidence for radius inflation, noting that the high density suggests an interior dominated by electron degeneracy pressure.
Formation Pathways: The high mass ratio ( $q \approx 0.061$ ) suggests a formation pathway more similar to stellar companions (e.g., gravitational collapse or disc fragmentation) rather than giant planets.
The "Brown Dwarf Desert": The discovery adds to the growing population of massive companions in the "brown dwarf desert" (short-period orbits), helping to fill the parameter space of known transiting systems.
Limitations: The authors modestly conclude that despite precise measurements of mass, radius, density, and age, and comparisons with theoretical models, they cannot unambiguously determine whether TOI-2155 b is a massive brown dwarf or an extremely low-mass hydrogen-burning star. The object remains in a regime where the distinction is inherently difficult due to the overlapping physical properties of the two classes.