Astronomers have discovered an atmosphere surrounding a lava planet, a surprising find that challenges current understanding of how these ultrahot worlds behave. Unlike most rocky exoplanets orbiting extremely close to their stars, TOI 561b – a world comparable in mass to Earth but twice its size – appears to retain a substantial gaseous envelope. This discovery, published in the Astrophysical Journal Letters, provides the strongest evidence yet that such planets can hold onto atmospheres despite intense stellar radiation and weak gravity.
The Unexpected Retention of Gas
Most small planets near their host stars struggle to hold onto atmospheres. Their low gravitational pull allows fast-moving gas molecules to escape into space, while powerful radiation from nearby stars strips away any potential atmosphere. TOI 561b, discovered by the TESS spacecraft in 2020, completes an orbit in less than 10 hours, making it incredibly hot and seemingly inhospitable to atmospheric retention.
“We’d expect that the atmosphere shouldn’t still be there,” explains Nicole Wallack of the Carnegie Earth and Planets Laboratory. “But we need to have an atmosphere to explain what we’re seeing.”
The planet’s unusually low density for its size – roughly four times that of water – was one early indicator that something unusual was happening. An atmosphere could inflate the apparent size of the planet, creating this density anomaly.
JWST Reveals Moderate Temperatures
Using the James Webb Space Telescope, researchers analyzed the temperature distribution on TOI 561b. Without an atmosphere, they anticipated extreme temperature differences between the day and night sides. The side perpetually facing the star should be scorching, while the dark side would be frigid. Instead, JWST data revealed surprisingly moderate temperatures on both sides.
The daytime side was measured at approximately 1,800° Celsius, significantly lower than the expected 2,700°C. This suggests an atmosphere with strong winds distributing heat across the planet.
Magma Oceans and Atmospheric Persistence
The planet’s molten surface may play a role in replenishing the atmosphere. Blazing currents could release gases faster than the star can strip them away, allowing the atmosphere to persist over time. This discovery raises questions about how common atmospheres might be on similar lava worlds.
“We’re just beginning to learn more about this,” Wallack notes. “We’ll see as more observations come out just how anomalous having atmospheres on these types of rocky planets are.”
Although TOI 561b is far too hot to support life, the finding offers valuable insight into planetary evolution. The study of extreme exoplanets like this can help researchers refine models of atmospheric loss and retention, which are essential for understanding habitability on other worlds.
The surprising discovery of an atmosphere on a lava planet demonstrates that our current understanding of exoplanetary atmospheres is incomplete. Further research is needed to determine whether this phenomenon is rare or widespread among similar worlds.
