A planet hugging its star is like an ice cube forgotten on a scorching skillet. The ice melts and evaporates, except instead of water, it's hydrogen and helium from the planet's upper layers. Sometimes the evaporation is so intense that the gas doesn't just drift off into space: it wraps around the star, forming a ring like an inflatable pool float. Astronomers call this ring a "torus," but it's basically a gas doughnut.
Scientists set out to see how this gas doughnut affects the star's light. They built a computer model and calculated that starlight passing through the doughnut leaves a noticeable mark in the spectrum: a helium line that becomes deeper and broader. A spectrum is like a rainbow where each gas leaves its own dark stripes, like a barcode. From that barcode, you can tell what the gas is made of.
The authors suggest looking for such doughnuts around stars with close-in planets. To do this, you need to examine two lines in the spectrum: one from helium, the other from calcium. If helium is enhanced but calcium is not, you're likely looking at a star wrapped in the evaporated gas of its planet. This search could help find planets that are losing mass and understand how they die.
as written by the authors arXiv:2602.16816
Close-in exoplanets with H/He atmospheres often undergo hydrodynamic escape. In extreme cases, it is hypothesized that the mass loss can be high enough for the escaping planetary material to wrap around the star, forming a long-lasting circumstellar torus. In this work, we develop a physical model of such circumstellar tori and use a ray tracing scheme to calculate the attenuation of stellar light passing through them. We show that the presence of a circumstellar torus significantly increases the equivalent width of the observed stellar He I 10830~\AA~line. When combined with observations of the star's Ca II H & K lines, these systems can typically be distinguished from field stars. Based on these results, we propose a survey of stars hosting close-in planets, combining observations of the He I 10830~\AA~and Ca II H & K lines to search for circumstellar tori generated from planetary mass-loss in these systems.
The helium line at 10830 angstroms is like helium's fingerprint in starlight. One angstrom is ten billionths of a meter, hundreds of thousands of times thinner than a human hair.
In the novel "The Expanse" by James S. A. Corey, an alien protomolecule launched onto Venus creates a gas ring around the planet, which astronomers notice by the change in starlight.