This star is approximately a quarter the mass of the Sun[3] and has 35% of the Sun's radius.[10] Luyten's Star is at the maximum mass at which a red dwarf can be fully convective, which means that most if not all of the star forms an extended convection zone.[11] It has a stellar classification of M3.5V,[3] with the V luminosity class indicating this is a main-sequence star that is generating energy through the thermonuclear fusion of hydrogen at its core. The projected rotation rate of this star[a] is too low to be measured, but can be no greater than 1 km/s.[12] Measurements of periodic variation in surface activity suggest a leisurely rotation period of roughly 116 days (which would give a velocity of ~0.15 km/s).[6] The effective temperature of the star's outer envelope is a relatively cool 3,150 K, giving the star the characteristic red-orange hue of an M-type star.[5][13]
At present, Luyten's Star is moving away from the Solar System. The closest approach occurred about 13,000 years ago when it came within 3.67 parsecs.[14] The star is currently located 1.2 light years distant from Procyon, which would appear as a visual magnitude −4.5 star in the night sky of Luyten's Star's planets.[15] However, Luyten's Star would only have an apparent magnitude of 4.6 from Procyon's sky because it is much less luminous. The closest encounter between the two stars occurred about 600 years ago when Luyten's Star was at its minimal distance of about 1.12 ly from Procyon.[16] The space velocity components of Luyten's Star are U = +16, V = −66 and W = −17 km/s.[16][17][18]
In March 2017, two candidate planets were discovered orbiting Luyten's Star.[9] The outer planet, GJ 273b, is a super-Earth in its star's optimistic habitable zone. It has a minimum mass of 2.89 ± 0.26 Earth masses and orbits at a distance of 0.09110 ± 0.00002 AU, completing one orbital period in 18.650 ± 0.006 days. While the planet is on the innermost edge of the star's conservative habitable zone, the incident flux is only 1.06 S🜨, so it may be potentially habitable if water and an atmosphere are present; depending on albedo, its equilibrium temperature could be anywhere between 206 and 293 Kelvin. The inner planet, GJ 273c, is one of the lightest exoplanets detected by radial velocities, with a mass of only 1.18 ± 0.16 Earth masses. However, it orbits much further in, with an orbital period of only 4.7234 ± 0.00004 days.[9]
GJ 273b is one of the closest known planets in its star's habitable zone.[9]
Both planets are near 4:1 resonance; it is possible that, with still undiscovered ones, the entire inner part of this system is trapped in a single simple-mean-motion resonance chain like TRAPPIST-1.
In 2019, two more candidate planets were detected by radial velocity, making a potential total of four known planets in the system.[19] If all four planets are present, their true masses must be close to their minimum masses for the system to be stable, with upper limits of 3.03 M🜨 for b, 1.24 M🜨 for c, 11.35 M🜨 for d, and 9.70 M🜨 for e.[4]
In October 2017, "Sónar Calling GJ273b", a project by METI and the Sónar music festival in Barcelona, Spain[20] transmitted a series of radio signals towards Luyten's star from a radar antenna at Ramfjordmoen, Norway.[21] The signal consisted of a scientific and mathematical tutorial on how to decode the messages and was accompanied by 33 encoded musical compositions by various musicians. A second signal series was transmitted in May 2018. Were anyone listening, the soonest response would be received by 2042.
^This is denoted by v sin i, where v is the rotational velocity at the equator and i is the inclination to the line of sight.
^ abIn a solution with 2 planets around Luyten's Star for Luyten b and Luyten c, the system was unstable when the inclination was below 50° and the planets were on very eccentric orbits. However, with lower eccentricities, the 2 planet system tolerated the full range of inclinations.
^ abcdePozuelos, Francisco J.; Suárez, Juan C.; de Elía, Gonzalo C.; Berdiñas, Zaira M.; Bonfanti, Andrea; Dugaro, Agustín; et al. (2020). "GJ 273: On the formation, dynamical evolution, and habitability of a planetary system hosted by an M dwarf at 3.75 parsec". Astronomy & Astrophysics. 641: A23. arXiv:2006.09403. Bibcode:2020A&A...641A..23P. doi:10.1051/0004-6361/202038047. S2CID219721292. GJ 273 is a planetary system orbiting an M dwarf only 3.75 pc away, composed of two confirmed planets, GJ 273b and GJ 273c, and two promising candidates, GJ 273d and GJ 273e ... the system remained stable only for values of inclinations ranging from 90◦ to ~72◦
^ abSuárez Mascareño, A.; Rebolo, R.; González Hernández, J. I.; Esposito, M. (September 2015), "Rotation periods of late-type dwarf stars from time series high-resolution spectroscopy of chromospheric indicators", Monthly Notices of the Royal Astronomical Society, 452 (3): 2745–2756, arXiv:1506.08039, Bibcode:2015MNRAS.452.2745S, doi:10.1093/mnras/stv1441, S2CID119181646
^Luyten, W. J.; Ebbighausen, E. G. (September 1935). "A faint star of large proper motion". Harvard College Observatory Bulletin. 900 (900): 1–3. Bibcode:1935BHarO.900....1L.
^Lacy, C. H. (August 1977). "Radii of nearby stars: An application of the Barnes-Evans relation". Astrophysical Journal Supplement Series. 34: 479–492. Bibcode:1977ApJS...34..479L. doi:10.1086/190459.
^Delfosse, Xavier; Forveille, Thierry; Perrier, Christian; Mayor, Michel (March 1998). "Rotation and chromospheric activity in field M dwarfs". Astronomy and Astrophysics. 331: 581–595. Bibcode:1998A&A...331..581D.
^Tuomi, M.; Jones, H. R. A.; Anglada-Escudé, G.; Butler, R. P.; Arriagada, P.; Vogt, S. S.; et al. (2019). "Frequency of planets orbiting M dwarfs in the Solar neighbourhood". arXiv:1906.04644 [astro-ph.EP].
