Imagine a world where the western hemisphere is forever shrouded in darkness while the eastern hemisphere basks in perpetual daylight. For Londoners to catch a glimpse of the sun, the only route would be a flight to Tokyo. Published on March 28th in the Astrophysical Journal, a study presented the most compelling evidence to date that a planet possesses this characteristic known as tidal locking, or a 1:1 tidal lock.
"This theoretical thing now feels real. This is actually what these planets look like," said Nicolas Cowan, an astronomer at McGill University in Canada and co-author of the study.
When a planet orbits very close to its star, its near side experiences stronger gravitational forces than its far side. Over time, this imbalance, known as tidal forces, is thought to slow the planet's rotation until it becomes fully synchronized with its orbit. This means the planet takes the same amount of time to rotate on its axis as it does to orbit the star. The moon is believed to have undergone this process, which explains why it has a "far side" that never faces Earth.
Many exoplanets are thought to be 1:1 tidal locked because they orbit very close to their host stars, but proving this state has been challenging. While measuring the orbits of exoplanets is straightforward, pinning down their rotation is much harder, especially when the planet's atmosphere obscures its surface rotation.
Scientists targeted a nearby exoplanet orbiting very close to its star, ultimately proving the tidal locking hypothesis. In 2019, researchers used the Spitzer Space Telescope to measure the light emitted by this planet, known as the super-Earth LHS 3844b. Cowan and colleagues realized that these measurements could tell them the temperature of the planet's surface facing Earth, as the planet likely lacks an atmosphere.
Due to the conflict between rotation and the immense tidal forces exerted by the star, planets without tidal locking experience temperature spikes. The research team found that the surface of LHS 3844b is relatively cool, as expected for a tidally locked planet.
Emily Rauscher, a theoretical astrophysicist at the University of Michigan in Ann Arbor, said, "This is the most convincing evidence one could gather with existing information or instruments."
More evidence is expected soon. "The James Webb Space Telescope (JWST) is really going to shine in this regard," Cowan said, noting that JWST will enable astronomers to study the rotation of exoplanets slightly farther from their stars than LHS 3844b. Astronomers now believe such planets can maintain a moderate atmospheric temperature, constituting much of the habitable space in the galaxy.
Cowan said if JWST discovers they are tidally locked like LHS 3844b, "There might be a sizable fraction of planets, certainly most habitable planets, that are tidally locked."
As for whether these planets are suitable for habitation in any sense, Cowan cannot speculate at present. He said these worlds lack "tidal, seasonal, or diurnal cycles." "Can you evolve life with the same diversity and complexity? I don't know."
Related Paper: https://doi.org/10.1038/d41586-024-00414-z
