Astronomers have discovered a body that is difficult to classify clearly as either a planet or a star. It was detected thanks to tiny fluctuations in the light of the star it orbits. In terms of mass, the object lies precisely on the boundary that separates true stars from bodies in whose interiors thermonuclear fusion never begins.
An Unremarkable Star and Its Companion
The star TOI-2155 is located 1,350 light-years from Earth. It is slightly larger, heavier, and hotter than the Sun, but otherwise does not stand out on its own.
A much more interesting object orbits it, as phys.org reports with reference to a study published in the peer-reviewed journal The Astronomical Journal. Scientists detected this body only through tiny dips in brightness as it passed in front of the star’s disk. Researchers are still not certain which category it belongs to.
The Boundary of Thermonuclear Ignition
For a gas cloud to become a star, the pressure in its interior must become high enough to sustain the stable thermonuclear fusion of hydrogen into helium. It is this process that provides the long-lasting radiation of heat and light that we are accustomed to considering a sign of a star. However, astronomers have been debating for decades how massive a gas cloud must be for this to happen.
If there is not enough mass, the object does not support stable hydrogen combustion and becomes a brown dwarf. It is quite hot at the beginning of its existence, but without continuous hydrogen burning, it gradually cools and emits only faint infrared light.
To understand why some gas clouds become stars while others remain brown dwarfs, astronomers deliberately search for objects in this transitional zone. It is there that the heaviest brown dwarfs and the lightest stars are found, allowing theory to be tested against real observations.
A Mass on the Edge of Theory
Using NASA’s TESS satellite and ground-based telescopes from different countries, the team refined the physical parameters of the object. TOI-2155b is almost the same size as Jupiter, but it is about 81 times more massive.
The density of TOI-2155b reaches approximately 110 g/cm³, meaning the object is almost ten times denser than lead. Despite its mass, which is about 81 times that of Jupiter, its radius is almost no larger than that of Jupiter. This is explained by the extremely strong gravitational compression of matter, which is supported inside the object by the pressure of the degenerate electron gas.
Traditionally, astronomers have placed the boundary between brown dwarfs and stars at 75 to 80 Jupiter masses. Modern models have shown that this boundary is also affected by the object’s age, chemical composition, and even atmospheric properties. For this reason, specialists still do not agree on exactly where the dividing line lies.
A Rare Discovery for Science
TOI-2155b may turn out to be one of the most massive known brown dwarfs or one of the lightest stars. Very few bodies are known in this transitional zone. Today, only a little more than over half a hundred transiting brown dwarfs have been confirmed, and only a small fraction of them come as close to the hydrogen-fusion boundary as TOI-2155b.
Astronomy has traditionally gained some of its greatest insights from the rarest discoveries. Each new example near the mass boundary helps test whether theoretical models agree with actual observations.
One discovery alone cannot determine the exact position of the boundary. Only after detailed study of a larger number of similar bodies will astronomers be able to clarify the conditions under which a star can steadily sustain thermonuclear reactions for billions of years.
