Weird alien world may be a planetary sauna

Stressful day at the office? Muscles need soothing? A trip to a newly described exoplanet—featuring a thick saunalike steam atmosphere—may be in order. Of course, it would be a long trip for a spa day, given the planet’s location 72 light-years from Earth.

Amadeo Castro-González, an astronomer at the Spanish Astrobiology Center who led a recent study describing the steamy planet, says the find could represent a new class of exoplanets, something between a rocky planet and a gas giant. “We propose these are inflated super-Earths,” he says.

The planet, TOI-244 b, was initially discovered in 2018 by NASA’s Transiting Exoplanet Survey Satellite (TESS), which looks for the telltale dimming in a star’s light when a planet crosses its face. Castro-González and colleagues used TESS data to estimate that the planet is 1.5 times bigger than Earth.

To find out the planet’s mass, they had to use a different technique. The researchers got time on the Very Large Telescope in Chile and used an instrument called ESPRESSO to look for shifts in starlight caused by tiny gravitational tugs from the planet. Based on those shifts, the planet is 2.7 times more massive than Earth, the team reports in a paper posted this month to the arXiv preprint server and accepted for publication in the journal Astronomy & Astrophysics .

That size and mass give the planet a density that’s two times lower than expected for a planet of its size. After ruling out explanations such as an ironless core, the team believes the low density can be explained by the presence of a 500-kilometer-thick “hydrosphere,” a steamlike atmosphere that sits above the planet’s rocky core and mantle.

The planet follows a tight 7-day orbit around its red dwarf star, and its temperatures are thought to rise to more than 2000°C—too hot for the water to condense into oceans, but not hot enough for it to evaporate out into space.

Instead, Castro-González says, the water exists in a supercritical state between a liquid and a gas. “It’s a mixture of the two,” he says. “It’s not a typical state that we see here on Earth. A sauna is the best analogy”—at least in the higher layers of the atmosphere. Closer to the rocky surface, where pressures and temperatures would exceed those on Venus, conditions would be deadly.

The team thinks TOI-244 b, along with a handful of similar exoplanets , might represent a new kind of swollen water world. “People have talked about water worlds and thick hydrosphere planets off and on for 20 years,” says Jonathan Fortney, an astronomer at the University of California, Santa Cruz, who was not involved in the work. “This planet is pretty compelling to have that explanation be right.”

The team is now trying to add to the list of water worlds by looking for similar planets around other stars. They also want to confirm the presence of water in the atmosphere of TOI-244 b. Castro-González says the planet will be an attractive target for follow-up studies with the JWST space telescope and other observatories capable of scrutinizing atmospheres. “The larger the atmosphere, the easier it is to characterize,” he says. “In this case, we have a huge one.”