Dark spots on stars can trick us into thinking planets have atmospheres.

When a planet crosses in front of its star, astronomers study the starlight to find gases in the planet's air. However, dark, active regions on stars called stellar spots can confuse these measurements. Researchers wanted to see if a single light-absorbing spectral line found only inside a stellar spotstellar spotA dark, cooler region on a star's surface with different chemical properties. could mimic an atmosphere. To test this, they simulated an airless world crossing directly over a spot on a Sun-sized star. As the planet covers and uncovers this spot-only feature during its crossing, it distorts the light signal. The team found that a single spot line creates fake signals that can easily fool observers. For a big, close-in, Jupiter-sized world, the spot creates an emission-like bump that can weaken real atmospheric signals. For a distant, Earth-sized planet, it creates fake absorption lines that look like a real atmosphere.

Why it matters

Astronomers searching for exoplanet atmospheres might mistake stellar spot signals for real atmospheric features. This could lead to false discoveries about whether distant worlds are habitable.

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