Astronomers have long been puzzled by the unusually puffy nature of WASP-107 b, an exoplanet orbiting a star 200 light-years away. Despite being nearly as large as Jupiter, WASP-107 b weighs just 12% of the gas giant’s mass, equivalent to only 30 Earths. This extreme puffiness has challenged conventional theories of planet formation and raised questions about the composition and structure of WASP-107 b’s atmosphere.
Previous observations of WASP-107 b’s size, mass, and age suggested that the planet had a small, rocky core enveloped by a rich reservoir of hydrogen and helium gases. However, such a scenario failed to fully explain the planet’s remarkably bloated orb, which receives insufficient energy from its star to account for its cotton-like density.
Now, using data from the JWST combined with previous observations from the Hubble Space Telescope, two independent teams of astronomers may have solved the puzzle. They discovered that methane in the planet’s atmosphere is only one-thousandth of what would be expected for a world of its size and temperature. Because methane is unstable at high temperatures, the surprisingly low amount suggests that the gas is “mixing vigorously with cooler layers higher up” in the planet’s atmosphere.
The extra heat driving this mixing likely comes from WASP-107 b’s highly elliptical orbit, which stretches and contracts the planet’s profile as it zips around its star. This constant gravitational pull generates tidal forces that heat the planet’s interior. Additionally, the planet’s core is likely hotter than previously thought due to radioactive decay of elements within the planet.
The hot core and tidal heating are not only affecting the planet’s atmosphere but also altering the chemistry of the gases deep within the planet. This heat is causing the chemistry of the gases to change, specifically destroying methane and making elevated amounts of carbon dioxide and carbon monoxide.
In addition to shedding light on the enigmatic nature of WASP-107 b, these findings have implications for our understanding of exoplanet atmospheres in general. They show that planetary atmospheres can inflate to remarkable amounts without employing esoteric theories of planet formation. Instead, factors such as intense heat and tidal forces can play a significant role in shaping the atmospheres of these distant worlds.
