LHS 3844 b

Lore & Background

LHS 3844 b was discovered between July 25 and August 22, 2018, when TESS observed its host star and detected a transit. Follow-up observations on September 6, 2018, confirmed the transit, and additional detections were made by the El Sauce Observatory in Chile and the Cerro Tololo International Observatory. The star was also monitored by the MEarth-South telescope array. To validate the planet, several false-positive scenarios were ruled out: the signal was not an instrument artifact, not an eclipsing binary (as radial-velocity limits ruled out a companion above 0.96 Jupiter masses), not a binary star system, and not a background star with a transiting planet, given the star's high proper motion. In June 2023, the planet was named Kuakua and its star Batsũ̀, from Bribri words for 'butterfly' and 'hummingbird'.

The planet has a radius of about 1.286 Earth radii and a mass between 2.27 and 2.37 Earth masses, yielding a bulk density of approximately 6 g/cm³, consistent with a dry, Earth-like composition. It orbits at 0.00624 AU with a circular, edge-on orbit. Observations show no strong tidal heating, indicating the planet is tidally locked, making it the first exoplanet confirmed to have a permanent dayside and nightside. Its atmosphere is absent or extremely thin: transmission spectroscopy shows a flat, featureless spectrum, ruling out clear hydrogen/helium or water atmospheres above 0.1 bar, and also disfavoring carbon dioxide or sulfur dioxide down to 0.1 bar. The lack of atmosphere may result from formation inside the snow line, accretion of volatile-poor materials, or erosion by high-velocity impacts.

Thermal emission spectroscopy reveals an old, very dark surface with a Bond albedo of about 0.14, similar to the Moon or Mercury. The surface composition is consistent with ultramafic or mafic materials like basalt or olivine-rich rocks, ruling out felsic rocks such as granite. The surface is likely highly space-weathered. The absence of volcanic gases like carbon dioxide or sulfur dioxide suggests the planet has no geologic activity.

Reader's Guide

LHS 3844 b holds significance as the first exoplanet confirmed to be tidally locked, providing direct evidence of a permanent dayside and nightside. Its lack of a thick atmosphere makes it a unique laboratory for studying a bare, rocky surface, with thermal emission spectroscopy revealing a dark, basalt-like composition similar to the Moon or Mercury. The planet's ultra-short orbital period of 11 hours and its proximity to its red dwarf star place it among the most extreme known worlds. Its legacy includes demonstrating that some super-Earths can form without substantial atmospheres, possibly due to formation inside the snow line or atmospheric erosion. The planet also serves as a benchmark for understanding tidal locking and surface properties of exoplanets, and its naming as part of the NameExoWorlds project highlights cultural engagement with astronomy.

Did You Know?

• LHS 3844 b orbits its star every 11 hours at a distance of just 0.00624 AU.
• Its dayside temperature is about 1000 K, while its nightside is consistent with absolute zero.
• The planet's surface is old, dark, and composed of olivine-rich materials like basalt.