💫 The boundary between stars and planets increasingly blurred

The boundary between stars and planets appears increasingly indistinct. Objects such as brown dwarfs, neither fully stars nor fully gas giant planets, blur the well-established categories defined by astronomers and highlight a fuzzy aspect of celestial body formation.

To try to clarify this, a team recently examined a set of 70 objects, ranging from Jupiter-mass planets to nearly stellar brown dwarfs. By studying the relationship between their mass, the chemical composition of their host star, and the shape of their orbits, the researchers hoped to draw a clear demarcation between formation modes. The results, presented in The Astronomical Journal, indicate that reality is far more blurred than anticipated.


Stars, such as our Sun, are born when vast clouds of gas collapse under their own gravity. At the heart of these clusters, the pressure becomes so intense that atoms fuse, triggering nuclear reactions that release heat and light. This process, called gravitational collapse, gives birth to celestial bodies capable of shining for billions of years.

On the other hand, gas giant planets like Jupiter come into being through the accretion of matter in a disk surrounding a young star. Dust grains gradually clump together to form a rocky core, which then attracts large amounts of gas.

Brown dwarfs occupy an intriguing intermediate position. With a mass between 13 and 80 times that of Jupiter, they are too light to fuse hydrogen like a star, but massive enough to activate the fusion of deuterium, an isotope of hydrogen. This unique characteristic places them in a grey area where traditional classifications become indeterminate.

The study led by Gregory Gilbert and his colleagues analyzed how the mass of objects relates to the metallicity of their stellar system and the eccentricity of their orbits. They expected to observe a clear cutoff, but the data reveal a gradual transition. For example, the presence of heavy elements like iron does not allow distinguishing between objects formed by collapse and those born by accretion.

Thus, there seems to be a continuum where formation processes overlap, making it difficult to distinguish between a failed star and an oversized planet. Astronomers are now exploring other parameters, such as orbital dynamics or atmospheric composition, to refine their understanding. These observations are leading to a reassessment of models describing the birth of celestial objects.

The influence of the chemical composition of stellar systems

The metallicity of a stellar system, i.e., its content of elements heavier than helium, plays an important role in planet formation. These elements, such as carbon, oxygen, and iron, often come from previous generations of stars that scattered their matter into space. A metal-rich environment favors the accretion of dust and gas, facilitating the birth of giant planets.

When a star forms in a molecular cloud, the initial composition of that cloud determines the amount of material available to build planets. High-metallicity systems tend to produce more rocky and gaseous bodies, because dust grains clump together more easily. This explains why giant exoplanets are often detected around so-called "metal-rich" stars.

However, the relationship between metallicity and formation is not always linear. Some massive objects, like brown dwarfs, can appear in metal-poor systems, indicating that other factors come into play. Gravity, turbulence in the protoplanetary disk, or the presence of stellar companions can also influence the final outcome.

Astronomers use spectrometers to measure the metallicity of stars by analyzing the light they emit. This data helps reconstruct the history of planetary formation and understand why some systems host very diverse planets.