A question about where the mass boundary of stars begins has led a Bangladeshi researcher to discover an unusual object more than 1,000 light-years from Earth. 

Md Redyan Ahmed, a Bangladeshi astronomer and third-year PhD candidate at the University of Sydney, is the lead author of a study describing an object that sits almost exactly at the boundary between a brown dwarf and a star.

The object, known as TOI-2155b, is about 1,350 light-years from Earth. It is roughly the size of Jupiter but has a mass of about 80.6 times that of Jupiter – placing it close to the mass range traditionally associated with the boundary between brown dwarfs and stars. 

“It might be a massive brown dwarf or a very low-mass star,” Redyan said. 

The distinction matters because stars and brown dwarfs form through broadly similar gravitational processes but follow different evolutionary paths. A star becomes a hydrogen-burning star when it has enough mass for its core to reach the pressure and temperature needed to sustain hydrogen fusion. Brown dwarfs never become massive enough to sustain hydrogen fusion over long timescales, so they gradually cool and fade as they evolve. They are sometimes described as “failed stars”—objects that formed through star-like processes but did not acquire enough mass to become sustained hydrogen-burning stars. 

Astronomers generally place the hydrogen-burning minimum mass at around 75 to 80 times the mass of Jupiter, although the precise boundary depends on factors such as age, chemical composition, atmospheric properties, and the underlying stellar models.  

TOI-2155b sits almost directly in this transition region. With a measured mass of about 80.6 Jupiter masses, it lies close to the theoretical boundary between the most massive brown dwarfs and the lowest-mass stars. Its well-constrained mass makes it a valuable object for testing models of the hydrogen-burning limit and asking whether the transition between brown dwarfs and stars can really be defined by a single threshold.

The discovery began with a change in starlight—the electromagnetic radiation emitted by stars.

Redyan and his fellow researchers identified TOI-2155b using NASA’s Transiting Exoplanet Survey Satellite (TESS). The spacecraft observes changes in the brightness of stars. When an orbiting object passes across the face of its host star from TESS's point of view, it blocks a small amount of starlight. 

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“We didn’t actually ‘see’ this object,” Redyan said. “We discovered it using transit—change in starlight—and radial velocity method.”

“We used ground-based telescopes in the United States, Europe and Russia to confirm and study the TOI-2155b. The satellite (TESS) as well as ground telescopes provide data; we extract the data and find the underlying physics,” Redyan added. 

TOI-2155b’s density is also unusual. It lies in a region where density begins to decrease as mass increases, adding to the puzzle of where brown dwarfs end and stars begin. 

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Astronomers have long sought objects in this transitional region because they can provide clues about how stars begin their lives and why some objects never reach the conditions required for sustained hydrogen fusion. One object cannot establish the exact boundary, but studying more objects like TOI-2155b could eventually allow researchers to refine the models used to describe stellar and substellar evolution.

Redyan’s discovery with his collaborators was published in The Astronomical Journal, placing his work at the centre of an investigation into one of astronomy's most persistent questions: where, exactly, does a star begin?

The answer may not come soon. More observations will be needed to determine what TOI-2155b truly is. For now, its importance lies in the questions it raises: about the elusive boundary between planets, brown dwarfs and stars, and how nature decides when a celestial object becomes a star.