A young star only half a billion years old has revealed a planet that offers a clear view into how small worlds can be reshaped by intense radiation in their early lives. The planet, TOI 5734 b, circles an orange dwarf located a little more than one hundred light years from Earth. It completes an orbit in just over six days and its size places it in a category that has produced some of the most puzzling examples in exoplanet research. It is roughly twice the size of Earth and about nine times as massive, yet its density is not consistent with the inflated envelopes often seen on planets of this scale. The relationship between these measurements points toward a compact body that has already been stripped of most of the gases it once held. This situation creates a straightforward look at a world caught in the final stages of losing its original structure, while the radiation from its young star continues to erode whatever outer layer remains. Since the discovery of planets around other stars became routine, astronomers have documented a wide spread of compositions and environments. Some worlds retain thick layers of hydrogen and helium that keep their radii large, while others contract into smaller, denser forms. These differences have led to the identification of a gap in the distribution of planetary sizes known to separate rocky planets from gas rich planets. TOI 5734 b sits directly on the upper edge of this divide. The location is significant because it suggests that the planet occupies a transitional state brought about by the combined effects of youth, proximity, and sustained radiation from the host star.
The star itself is a K type dwarf that has not yet reached the quieter stage of stellar evolution. At five hundred million years of age it rotates more quickly than an older star and exhibits stronger magnetic activity. These characteristics produce high levels of X ray and ultraviolet radiation and create surface features that brighten and fade as the star turns on its axis. Such conditions generate considerable noise in light curves and velocity measurements. The discovery and confirmation of TOI 5734 b required the combination of data from a space based photometric instrument and nearly one hundred precise spectroscopic observations from a ground based observatory. The timing of transits recorded by an orbiting telescope revealed the presence of a small planet crossing in front of the star at regular intervals. The radial velocity measurements from Earth tracked the subtle motion of the star as it responded to the gravitational influence of the planet. The difficulty arose from the fact that a young star can produce variations comparable to the signal of a close orbiting planet. To separate the two, the researchers applied statistical modelling designed to identify patterns caused by stellar rotation and starspots. Once these patterns were accounted for, the remaining signal aligned consistently with a single planetary orbit.
The measurements yield a radius of about 2.1 Earth radii and a mass of about 9.1 Earth masses. These values produce an average density close to the density of Earth. That density is unexpectedly high for a planet of this size. Many sub Neptunes, which is the informal term for planets between roughly two and four Earth radii, contain substantial volatile content. The large envelopes of hydrogen and helium give them lower densities and larger radii. TOI 5734 b fits a different profile. Its density points toward a structure dominated by rock and metal with only a very small atmospheric contribution. This is consistent with the expectation that planets close to energetic young stars will undergo significant mass loss, especially during the first few hundred million years. When a planet absorbs intense high energy radiation, particles in its upper atmosphere gain enough energy to escape the planet’s gravitational hold. If the atmosphere is not massive enough to resist this escape, it gradually thins until only a compact core remains. That core is largely composed of the same materials found in rocky planets, such as silicates and iron.
The environment around TOI 5734 provides all the necessary conditions for this process. The star emits far more high energy radiation than a star several billion years old. The planet lies close to the star, so the flux is strong enough to heat and accelerate the upper layers of any gaseous envelope. Over time the planet would lose much of this material. Models of atmospheric evolution applied to this system indicate that TOI 5734 b likely began with a much larger radius during the earliest stages of its development. As the atmosphere escaped, the planet contracted. The radius decreased from an initial size of more than three Earth radii to its present value of slightly more than two Earth radii. The simulations also indicate that the present envelope is extremely thin and that the remaining material will continue to dissipate. Within a few hundred million years the primary atmosphere is expected to vanish entirely. When that occurs, the planet will exist as a rocky world with a radius nearly identical to the radius of its solid core.
This view of planetary evolution creates a coherent picture of the system. The star is young and active. The orbit is close and circular. The density is high for the planet’s size. The inferred atmospheric mass fraction is extremely small. The projected loss rate over time fits the observed condition. The planet’s location in the size distribution of known exoplanets matches the boundary where rocky planets begin to dominate over volatile rich planets. These observations reinforce each other, allowing the system to be interpreted without the need for complex or uncertain assumptions. The planet is simply a case where the star’s early activity has already removed most of what once made the planet a sub Neptune.
The star’s rotation period provides additional context. It rotates in a little more than eleven days, which reflects a stage where magnetic activity remains strong. Observations of the star’s chromosphere and corona indicate elevated activity levels. Lithium measurements, which decline over time in stellar atmospheres, support the young age estimate. All these indicators confirm that the star is still in an energetic phase, consistent with the environment that would strip a close orbiting planet. The velocity data also show that the orbit of TOI 5734 b is essentially circular. If the planet had undergone past violent migration events or interactions with other bodies, the orbit might still be eccentric. A circular orbit at this age suggests that the planet reached its current position early, possibly during the existence of the protoplanetary disk. Once the disk dispersed, the orbit remained stable and the radiation from the star completed the shaping of the planet.
The temperature of TOI 5734 b is another important factor. With an equilibrium temperature approaching seven hundred Kelvin, the planet exists in an environment that further accelerates atmospheric loss. High temperatures increase the scale height of any remaining atmosphere and make it easier for particles to reach escape velocity when exposed to high energy photons. If any atmosphere remains today, it would be thin and likely composed of heavier elements that cannot escape as easily as hydrogen. The current observations do not measure the atmospheric composition directly. However, the physical parameters of the planet point toward a world where the dominant structure is the solid core rather than the gaseous exterior.
The future of TOI 5734 b is straightforward. As the star continues to age, its rotation will slow and its activity level will decline. The amount of high energy radiation will decrease. By that time TOI 5734 b is expected to have lost the last remnants of its atmosphere. The planet will settle into the category of compact rocky bodies that orbit close to their stars. Although the star will calm over time, the proximity of the orbit ensures that the planet will remain a hot and hostile environment. The surface, if exposed, would endure extreme temperatures. However, the existence of such a world provides a useful example of how planetary systems develop. It shows that even planets with initial volatile content can transform into rocky cores if they orbit young and active stars.
The system also presents an attractive target for future atmospheric studies. Even a thin atmosphere can imprint detectable features in the light passing through it when the planet transits the star. Space based infrared instruments planned for the coming decade could search for these features. While the atmosphere may be nearly gone, any detection would provide insight into the final stages of evaporation. It would also help researchers understand how quickly different types of planets cross the boundary from gas rich to gas poor environments. The number of required observations is large, but the potential scientific return justifies the effort. Nearby systems like TOI 5734 offer opportunities because the stars are bright enough for precise measurements.
The importance of TOI 5734 b arises from the simplicity of its physical scenario. It is young, close to its star, and compact. It fits the expectations of a planet that has undergone extensive mass loss. It occupies the region of parameter space where many planets appear to transition from large and gas rich to small and rocky. The system offers a live demonstration of how intense radiation shapes planetary interiors and atmospheres during the formative stages of a star’s life. Each component of the observation adds to the same story. The mass, radius, density, temperature, rotation of the star, and activity indicators all contribute to a consistent portrait of a world that is nearing the end of its atmospheric erosion. This makes TOI 5734 b a valuable example for understanding the range of outcomes for planets that form close to young stars.
Source:
Filomeno et al. 2026, “TOI-5734b: A hot sub-Neptune orbiting a relatively young K dwarf with an Earth-like density”, Astronomy & Astrophysics.
https://arxiv.org/abs/2602.18108
