Super-Earth LHS 3855b: Scientists Discover First Exoplanet With Confirmed One-to-One Tidal Locking

Generally speaking, each location on Earth experiences approximately half a year of daytime and half a year of nighttime. In places near the equator, the duration of day and night remain constant throughout the year, each lasting about 12 hours.

(Photo: Wikimedia Commons/ NASA/JPL-Caltech/R. Hurt (IPAC))

In a recent study, experts found the most compelling evidence of a planet having no half-measures of daytime or nighttime, a phenomenon known as tidal synchronization or 1:1 tidal locking.

What Causes Tidal Synchronization?

Tidal locking happens when a planet's rotation period matches its orbital period around its host star. As a result, one side constantly faces the star while the other turns away, stuck in total darkness. The gravitational forces acting between the planet and its st explain this phenomenon.

When a planet orbits very close to its host star, its near side experiences a much stronger gravitational pull than its far side does. Over time, the resulting imbalance, called a tidal force, slows the planet's rotation until it perfectly synchronizes with its orbit.

In such an occurrence, the time taken for the planet to rotate once on its axis is the same as the time needed to travel once around its star. Our moon is thought to have undergone this process, explaining why it has a 'far side' that does not face the Earth.

Many exoplanets are assumed to be 1:1 tidally locked due to their proximity to their host star. However, this claim is difficult to prove. Measuring the exoplanet's orbit is straightforward, but pinning down its rotation is much harder. This is especially true if the planet has an atmosphere that conceals its spinning surface from view.

Astronomers believe there are many tidally locked planets in the Milky Way galaxy. The James Webb Space Telescope is expected to provide further evidence as I study the rotation of exoplanets that orbit a great distance from their host star.

Compelling Case

To prove the tidal locking theory, the experts turned to a particular exoplanet that was its star, known as super-Earth LHS 3844b. In 2019, researchers measured the intensity of light coming off this planet using the Spitzer Space Telescope. The scientists realized that the measurements could tell them the planet's Earth-facing surface temperature because LHS 3844b probably has no atmosphere.

In the study "Super-Earth LHS3844b is Tidally Locked," the telescope data enabled the researchers to infer the surface temperature of LHS 3844b. Xintong Lyu of Peking University led the research, which collaborated with experts from the Max-Planck Institute for Astronomy, the Jet Propulsion Laboratory, and McGill University.

By measuring the intensity of light reflected by LHS 3844b, the research team discovered that the side facing the telescope was cool. This contradicts the fact that this planet orbits its host star at an extremely close distance and completes an orbit every 11 hours. The only explanation for this observation is that this surface is the dark side of a tidally locked planet.