Habitability of K-type main-sequence star systems
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K-type main-sequence stars, also known as orange dwarfs or Goldilocks stars, may be candidates for supporting extraterrestrial life.[1] Due to their smaller mass, they remain stable in the main sequence longer than the Sun by burning their hydrogen slower,[1] whilst also maintaining sufficiently-low solar-flare activity and a habitable zone large enough to prevent tidal locking to sustain life.[2] The odds of complex life arising may be better on planets around K-type main sequence stars than around Sun-like stars, given the suitable temperature and extra time available for it to evolve.[2] Some known exoplanets orbiting these stars are possible candidates for hosting life.[1]
Habitable zone
A K-type star's habitable zone approximately ranges between 0.1–0.4 to 0.3–1.3 astronomical units from the star,[citation needed] from which exoplanets receive relatively low amounts of ultraviolet (UV) radiation; low UV radiation may result in surface temperatures dropping below the freezing point of water, whereas excessive UV radiation can harm developing or existing life.[3][better source needed]
K-type stars are stable in their main sequence phases, lasting for most of the K-type main-sequence star's main sequence phase and with little instability of luminosity during that phase.[4]
Radiation hazard
Despite K-stars' lower total UV output, in order for their planets to have habitable temperatures, they must orbit much nearer to their K-star hosts, offsetting or reversing any advantage of a lower total UV output. There is also growing evidence that K-type dwarf stars emit dangerously high levels of X-rays and far ultraviolet (FUV) radiation for considerably longer into their early main sequence phase than do either heavier G-type stars or lighter early M-type dwarf stars.[5] This prolonged radiation saturation period may sterilise, destroy the atmospheres of, or at least delay the emergence of life for Earth-like planets orbiting inside the habitable zones around K-type dwarf stars.[5][6]
Potentially habitable planets
The super-Earth HD 40307 g around the K2.5V star HD 40307 orbits in the circumstellar habitable zone (CHZ), although it has a reasonably elliptical orbit (e=0.22). There may be many more, and the Kepler space telescope (now retired) was one of the main sources of information of these exoplanets.[7] Kepler-62 and Kepler-442 are examples of discoveries by Kepler of systems consisting of a K-type dwarf with potentially habitable planets orbiting it.
HD 85512 b was originally thought to be a super-Earth with habitability potential orbiting a K-type main-sequence star,[8][9] but it is now considered to be a false positive detection,[10] an artifact caused by stellar rotation.[11]