The Scorched Divide: How Starlight Carves the Exoplanet Radius Valley

Space is not the quiet, empty void we often imagine. It is a shooting gallery. For a planet orbiting close to its host star, existence is a daily battle against annihilation. The star is not merely a source of light; it is a nuclear furnace, spitting out torrents of high-energy X-rays and ultraviolet blasts. Consider the atmosphere of a young world. It is a fragile shroud of hydrogen and helium. It clings to the rocky core, held only by gravity. But the star attacks. The radiation slams into the gas particles, heating them until they move so fast they break free. They escape. The planet bleeds its air into the blackness. This is a slow, violent stripping. Over millions of years, a thick, gaseous envelope is flayed away, layer by layer. The planet shrinks. It transforms from a puffy sub-Neptune into a naked, scorched super-Earth. The star is the antagonist here, a sculptor that destroys as it creates, leaving behind a graveyard of stripped cores. There is no mercy in this proximity. The heat is relentless.

For years, astronomers stared at the aftermath of this violence without fully understanding the weapon used. They saw a statistical wasteland in the census of the galaxy. We found small rocky worlds. We found large gaseous ones. But we saw almost nothing in between. The intermediate sizes were missing.

Investigating the Exoplanet Radius Valley

To solve the mystery of this missing link, researchers turned to the sheer volume of discovery. They analysed 5,987 confirmed planets, looking for patterns in the chaos. The goal was to see if the intensity of the star's light correlated with the size of the planets that survived.

The measurements revealed a dynamic boundary. The gap, known as the exoplanet radius valley, is not a fixed line in the sand. It moves. The study found that at lower levels of stellar flux, the valley sits at roughly 1.5 Earth radii. However, when the incident flux screams to over 200 times what Earth receives, the valley migrates outward to 2.25 Earth radii. This is a massive shift.

The data suggests a clear cause and effect. The 49 per cent migration in the valley's location aligns with the physics of photoevaporation. In hotter environments, a planet must be significantly more massive to hold onto its atmosphere against the stellar wind. The researchers found that the valley's location scales with flux to the power of 0.37, a figure that brackets the theoretical prediction for energy-limited mass loss. While other theories like core-powered mass loss exist, this flux-driven migration offers direct empirical evidence that the star itself is responsible for stripping these worlds down to the bone.