SPECULOOS-3, also known asLSPM J2049+3336, is ared dwarf star (spectral type M6.5) located 54.6 light-years from Earth[4] in the constellationCygnus. It is one of thesmallest known stars, and is much cooler, dimmer and smaller than the Sun, having 0.1 times themass, 0.08% theSun's luminosity, and aneffective temperature of 2,800 K (2,530 °C), which is less than half of the Sun's temperature (5,772 K).[4] It is orbited by one knownexoplanet, and is the secondultra-cool dwarf discovered to have aplanetary system, afterTRAPPIST-1.[7][8]
The age of SPECULOOS-3 is constrained at 6.6 billion years, 44% older than theSolar System, with significant margins of error. Abayesian analysis of the star derived a mass of 0.101 M☉, aneffective temperature of 2,800 K (2,530 °C; 4,580 °F) and a luminosity of8.35×10−6 L☉. These characteristics classify SPECULOOS-3 as anultracool dwarf, which are stars at the end of themain sequence, with low temperatures, low luminosites and sizes similar toJupiter. It is spinning at arotational velocity of 4.8 km/s and has a projected rotational period of 1.34 days.[4]
The stellarradius, computed using theStefan–Boltzmann law, is 0.134 R☉ (93,000 kilometres).[4] This makes SPECULOOS-3 the second-smallest star known to host atransiting planet, just marginally larger thanTRAPPIST-1,[4] and its size is similar to that ofJupiter.[9] Itsapparent magnitude is estimated at 17.8,[3] which is too faint to be seen by thenaked eye.
It was first discovered in 2005 as part of theLSPM-North catalog, whose objective was to map stars in thenorthern celestial hemisphere withproper motions larger than 0.15" per year andapparent magnitudes smaller than 21m.[3] Its trignometricparallax was first measured in 2014 at67.5±1.7 mas, translating into a distance of 14.8 parsecs (48.27 ly).[6]Gaia Data Release 3 (2023) published aparallax of 59.7 milliarcseconds, translating into a distance of 16.75 parsecs (54.6 ly).[2] This make this star relatively close to Earth.[10]
Red dwarf stars such as SPECULOOS-3 are the most numerous type of stars, making up 70% of all stars in theMilky Way galaxy. They are expected to live 10 times more than the Sun, with lifespans longer than 100 billion years.[10]
SPECULOOS-3 hosts one exoplanet, discovered in 2024 via thetransit method.[11] NamedSPECULOOS-3 b, it is an Earth-sizedexoplanet that has a radius similar to that of Earth, equivalent to 0.98 Earth radii.[4] 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 likelytidally locked, meaning that one side of the planet always faces its host star.[9] Its discovery was made using theSPECULOOS project, and was announced on 15 May 2024 in the academic journalNature Astronomy.[12]
The mass of SPECULOOS-3 b has been not measured,[4] but it has been estimated by NASA'sEyes on Exoplanets at0.894 M🜨.[13] Itsequilibrium temperature is about 553 K (280 °C; 536 °F), meaning that its dayside is likely formed by solid rock.[4] The planet is an optimal target for characterization with theJames Webb Space Telescope, giving more information about the planet'smineralogy[12] and the possibility of hosting an atmosphere.[14]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | — | 0.007330(55) | 0.71912603(57) | — | 89.44±0.39° | 0.977±0.022 R🜨 |
