<P> The spectral standards for M - type stars have changed slightly over the years, but settled down somewhat since the early 1990s . Part of this is due to the fact that even the nearest red dwarfs are fairly faint, and the study of mid - to late - M dwarfs has progressed only in the past few decades due to evolution of astronomical techniques, from photographic plates to charged - couple devices (CCDs) to infrared - sensitive arrays . </P> <P> The revised Yerkes Atlas system (Johnson & Morgan 1953) listed only 2 M - type spectral standard stars: HD 147379 (M0 V) and HD 95735 / Lalande 21185 (M2 V). While HD 147379 was not considered a standard by expert classifiers in later compendia of standards, Lalande 21185 is still a primary standard for M2 V. Robert Garrison does not list any "anchor" standards among the red dwarfs, but Lalande 21185 has survived as a M2 V standard through many compendia . The review on MK classification by Morgan & Keenan (1973) did not contain red dwarf standards . In the mid-1970s, red dwarf standard stars were published by Keenan & McNeil (1976) and Boeshaar (1976), but unfortunately there was little agreement among the standards . As later cooler stars were identified through the 1980s, it was clear that an overhaul of the red dwarf standards was needed . Building primarily upon the Boeshaar standards, a group at Steward Observatory (Kirkpatrick, Henry, & McCarthy 1991) filled in the spectral sequence from K5 V to M9 V. It is these M - type dwarf standard stars which have largely survived as the main standards to the modern day . There have been negligible changes in the red dwarf spectral sequence since 1991 . Additional red dwarf standards were compiled by Henry et al. (2002), and D. Kirkpatrick has recently reviewed the classification of red dwarfs and standard stars in Gray & Corbally's 2009 monograph . The M - dwarf primary spectral standards are: GJ 270 (M0 V), GJ 229A (M1 V), Lalande 21185 (M2 V), Gliese 581 (M3 V), GJ 402 (M4 V), GJ 51 (M5 V), Wolf 359 (M6 V), Van Biesbroeck 8 (M7 V), VB 10 (M8 V), LHS 2924 (M9 V). </P> <P> Many red dwarfs are orbited by exoplanets, but large Jupiter - sized planets are comparatively rare . Doppler surveys of a wide variety of stars indicate about 1 in 6 stars with twice the mass of the Sun are orbited by one or more Jupiter - sized planets, versus 1 in 16 for Sun - like stars and only 1 in 50 for red dwarfs . On the other hand, microlensing surveys indicate that long - orbital - period Neptune - mass planets are found around one in three red dwarfs . Observations with HARPS further indicate 40% of red dwarfs have a "super-Earth" class planet orbiting in the habitable zone where liquid water can exist on the surface . Computer simulations of the formation of planets around low mass stars predict that Earth - sized planets are most abundant, but more than 90% of the simulated planets are at least 10% water by mass, suggesting that many Earth - sized planets orbiting red dwarf stars are covered in deep oceans . </P> <P> At least four and possibly up to six exoplanets were discovered orbiting within the Gliese 581 planetary system between 2005 and 2010 . One planet has about the mass of Neptune, or 16 Earth masses (M). It orbits just 6 million kilometers (0.04 AU) from its star, and is estimated to have a surface temperature of 150 ° C, despite the dimness of its star . In 2006, an even smaller exoplanet (only 5.5 M) was found orbiting the red dwarf OGLE - 2005 - BLG - 390L; it lies 390 million km (2.6 AU) from the star and its surface temperature is − 220 ° C (56 K). </P>

Where are red dwarfs located on the hr diagram