About 1,350 light-years from Earth lies a star called TOI‑2155. The star is slightly larger, heavier, and hotter than the sun, so it's not particularly unusual in itself. What really stands out is a smaller object orbiting it, TOI‑2155b—its existence can only be inferred by observing subtle changes in the star's light as it passes in front of its parent star.
What exactly is TOI‑2155b? Is it a "mini star", a giant planet, or something special in between? As described in a recent paper published in The Astronomical Journal, researchers are not yet sure whether TOI-2155b is good enough to be called a star, but what is clear is that it appears to be in a very fascinating borderline: between the "real stars" that can ignite and sustain hydrogen fusion and shine in the universe, and those brown dwarfs that fail to ignite sustained hydrogen fusion and are called "failed stars."
How stars "fail"
Stars originate from huge gas clouds in space. So, how big and heavy does a gas cloud have to be to finally become a star? It sounds like a simple question, but it has caused debate in the astronomical community for decades.
This is because, inside a star, only when the pressure caused by gravity is large enough to allow hydrogen atoms to continuously fuse into helium atoms, can the star continuously produce intense heat and light, which is also the most significant feature of stars. If the mass of a celestial body is not large enough, the internal pressure is not enough to sustain this type of fusion for a long time, or the hydrogen fusion cannot really "start" for other reasons, then this mass of gas will become a "failed star", that is, a brown dwarf. Such objects will be relatively hot in their early stages of life, but due to the lack of sustained hydrogen fusion, the radiation will gradually weaken and the surface temperature will slowly drop, leaving only faint infrared radiation.
To figure out which gas clouds will become actual stars and which will stay in the brown dwarf stage, astrophysicists must look for samples of the "transition zone" - the heaviest brown dwarfs and the lightest stars. TOI‑2155b is a key example of this, with a mass of about 80.6 times that of Jupiter, almost on the theoretically critical boundary.
Quality boundaries are not “squeaky clean”
The scientific research team combined observation data from NASA's TESS (Transiting Exoplanet Survey Satellite) and observations from multiple ground telescopes around the world to accurately measure the volume and mass of TOI‑2155b. The results showed that the object was almost the same size as Jupiter but about 80 times more massive.
Intuitively, one might hope that there is a very clear "mass threshold". Once this value is exceeded, a celestial body will "transform" from a planet or a brown dwarf into a star. However, like many boundaries in nature, reality does not have a clean dividing line. Traditional theory had placed the mass boundary between planets, brown dwarfs and stars at around 75 to 80 times the mass of Jupiter. However, the latest theoretical models show that this transformation is not only determined by quality as a single factor, but also affected by many other parameters.
Research shows that a celestial body's age, chemical composition and even the properties of its atmosphere affect whether it can initiate and sustain hydrogen fusion. This is why astronomers still disagree on where exactly the mass boundary between brown dwarfs and stars should be drawn. Against this background, TOI-2155b, which is in the critical region, is particularly important, providing a valuable opportunity to test the "nuanced differences" between theory and observation.
Extremely rare "transitional object"
Based on current observations, TOI‑2155b may be one of the most massive brown dwarfs ever discovered, or it may be one of the least massive. The number of known objects within this narrow mass transition region is very limited, making TOI‑2155b an extremely valuable object for studying the boundary between brown dwarfs and stars. In the development of astronomy, many key advances have come from in-depth research on the rarest and most special celestial objects, and TOI-2155b is expected to become one of such "samples."
Of course, no single object can give the final answer to the boundary between brown dwarfs and stars. Only after more celestial bodies with similar masses located in the transition zone are discovered in the future and high-precision measurements and long-term tracking observations are made on them, scientists can further optimize the existing theoretical models. By then, we may be able to more clearly outline the conditions under which stars ignite and continue to burn for billions of years, and we may be able to better understand how these "stellar engines" shaped the universe into what it is today.