The Earth’s ocean tides come from the Moon’s gravitational pull. As the Moon orbits the Earth, it pulls on the closer side of the Earth’s surface stronger than the far side, causing the oceans to bulge. The Earth rotates through this bulge, giving rise to high and low tides. Tides can do even stranger things in outer space.

Two independent studies showed that the tidal force imparted on a distant exoplanet by its sun explains the planet’s mysteriously low density. The planet, WASP-107 b, is roughly the size of Jupiter, but with only about 10% of the mass. Usually, planets with inflated radii are in close proximity to their sun, which causes extreme heating and inflation of the gaseous exterior. But WASP-107 b isn’t close enough to its star to cause such heating. Something else must be going on.

Scientists solved this mystery by combining spectroscopic data from the Hubble and James Webb Space Telescope to identify the gasses in WASP-107 b’s atmosphere. To their surprise, they found that its atmosphere had about 1000x less methane than expected. It’s well known in the field that missing methane signals that an exoplanet’s interior is very hot and mixes with its cooler outer atmosphere, suggesting that the interior of WASP-107 b is much hotter than previously proposed. So what’s heating the planet’s interior? It turns out that the planet’s orbit is not perfectly circular and as its distance from the star changes, the planet is stretched in different directions. This tidal force causes enough friction and heat to puff the planet up. This is fundamentally the same type of force that creates the Earth’s ocean tides. Tidal forces also make Jupiter’s moon Io the most volcanic object in our solar system and could someday provide clues about volcanism on more rocky exoplanets.

These studies showcase the power of the new James Webb Space Telescope. From discovering water on habitable exoplanets to imaging some of the most distant galaxies, the Webb continues to unravel the mysteries of our universe. 

These studies were led by David Sing, an astronomer at Johns Hopkins University, and Luis Welbanks, an astronomer at Arizona State University.

Managing Correspondent: Collin Cherubim

Press Article: Webb Cracks Case of Inflated Exoplanet (

Original Journal Article: A warm Neptune’s methane reveals core mass and vigorous atmospheric mixing (Nature); A High Internal Heat Flux and Large Core in a Warm Neptune Exoplanet (arXiv)

Image Credit: Andrew-Art/Pixabay

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