SPECULOOS-3 b

Lore & Background

SPECULOOS-3 b was detected using the transit method. The host star, SPECULOOS-3, was first observed by the SAINT-EX telescope in 2021 over five nights, yielding two light curves with planetary transits. Subsequent observations by the SPECULOOS-North observatory's Artemis telescope revealed a clear transit-like signature, and further monitoring with SAINT-EX and SPECULOOS-South telescopes confirmed additional transits with an orbital period of 0.719 days. The planet's radius, deduced from transit depth, is similar to Earth's. Its mass has not been measured, though the NASA Exoplanet Catalog estimated a value (while erroneously listing the system distance as 22,152 light-years). Measuring the mass is essential to determining if the planet is rocky and constraining its composition. The planet orbits at just 0.7% of an astronomical unit from its star, receiving high levels of radiation. Its equilibrium temperature is estimated at 280°C, assuming a null bond albedo, making its dayside likely composed of solid rock. The planet is tidally locked, with one hemisphere always facing the star and the other always in darkness. The host star, SPECULOOS-3 (LSPM J2049+3336, TIC 230741378), is an ultra-cool red dwarf with one-tenth the Sun's mass, 13% of its size, and 0.09% of its luminosity. It has an effective temperature of 2800 K and is one of the smallest known stars, second only to TRAPPIST-1 among stars with transiting planets. Red dwarfs like SPECULOOS-3 make up 70% of Milky Way stars and have lifespans exceeding 100 billion years.

Reader's Guide

SPECULOOS-3 b's significance lies in its potential for atmospheric characterization. Due to the planet's high temperature and the host star's infrared brightness, the dayside's emission spectrum may be measurable by the MIRI/RLS instrument aboard the James Webb Space Telescope. Such observations could reveal the presence of an atmosphere or, if none is detected, constrain the planet's mineralogical surface. The planet's discovery by the SPECULOOS project—a network of telescopes searching for faint stars and exoplanets, led by the University of Liège in collaboration with other universities—highlights the growing capability to detect Earth-sized worlds around the smallest and coolest stars. Its proximity at 55 light-years and rapid 17-hour orbit make it a prime target for follow-up studies. The planet's tidal locking and high radiation levels provide a natural laboratory for understanding the climates and surfaces of rocky worlds in extreme environments. The unresolved mass measurement and the discrepancy in the NASA Exoplanet Catalog's distance underscore the need for further observations to confirm the planet's composition and refine its properties.

Did You Know?

• SPECULOOS-3 b orbits its star every 17 hours, making its year shorter than an Earth day.
• The planet is tidally locked, with one side always facing its host star and the other in perpetual darkness.
• Its host star, SPECULOOS-3, is the second-smallest star known to host a transiting planet, after TRAPPIST-1.
• The planet's dayside equilibrium temperature is estimated at 280°C, similar to the surface temperature of Venus.