SPECULOOS-3

SPECULOOS-3, also known as LSPM J2049+3336, is a red dwarf star (spectral type M6.5) located 54.6 light-years from Earth in the constellation Cygnus. It is one of the smallest known stars, and is much cooler, dimmer and smaller than the Sun, having 0.1 times the mass, 0.08% the Sun's luminosity, and an effective temperature of 2800 K, which is less than half of the Sun's temperature (5,772K). It is orbited by one known exoplanet, and is the second ultra-cool dwarf discovered to have a planetary system, after TRAPPIST-1.

Stellar properties
The age of SPECULOOS-3 is constrained at 6.6 billion years, 44% older than the Solar System, with significant margins of error. A bayesian analysis of the star derived a mass of, an effective temperature of 2800 K and a luminosity of. These characteristics classify SPECULOOS-3 as an ultracool dwarf, which are stars at the end of the main sequence, with low temperatures, low luminosites and sizes similar to Jupiter. It is spinning at a rotational velocity of 4.8 km/s and has a projected rotational period of 1.34 days.

The stellar radius, computed using the Stefan–Boltzmann law, is 0.134 solar radius. This makes SPECULOOS-3 the second-smallest star known to host a transiting planet, just marginally larger than TRAPPIST-1, and its size is similar to that of Jupiter. Its apparent magnitude is estimated at 17.8, which is too faint to be seen by the naked eye.

It was first discovered in 2005 as part of the LSPM-North catalog, whose objective was to map stars in the northern celestial hemisphere with proper motions larger than 0.15" per year and apparent magnitudes smaller than 21$$. Its trignometric parallax was first measured in 2014 at $10.867$, translating into a distance of 14.8 pc. Gaia Data Release 3 (2023) published a parallax of 59.7 milliarcseconds, translating into a distance of 16.75 pc. This make this star relatively close to Earth.

Red dwarf stars such as SPECULOOS-3 are the most numerous type of stars, making up 70% of all stars in the Milky Way galaxy. They are expected to live 10 times more than the Sun, with lifespans longer than 100 billion years.

Planetary system
In 2024, an exoplanet was found around the star using the transit method. Named SPECULOOS-3 b, it is an Earth-sized exoplanet that has a radius similar to that of Earth, equivalent to 0.98 Earth radii. It takes only about 17 hours to complete an orbit around SPECULOOS-3, and, because of that proximity, it receives very high levels of radiation and is likely tidally locked, meaning that one side of the planet always faces its host star. Its discovery was made using the SPECULOOS project, and was announced on 15 May 2024 in the academic journal Nature Astronomy.

The mass of SPECULOOS-3 b has been not measured, but it has been estimated by NASA's Eyes on Exoplanets at. Its equilibrium temperature is about 553 K, meaning that its dayside is likely formed by solid rock. The planet is an optimal target for characterization with the James Webb Space Telescope, giving more information about the planet's mineralogy and the possibility of hosting an atmosphere.