  Oxygen and methane are considered to be the canonical biosignatures of modern
Earth, and the simultaneous detection of these gases in a planetary atmosphere
is an especially strong biosignature. However, these gases may be challenging
to detect together in the planetary atmospheres because photochemical oxygen
radicals destroy methane. Previous work has shown that the photochemical
lifetime of methane in oxygenated atmospheres is longer around M dwarfs, but M
dwarf planet habitability may be hindered by extreme stellar activity and
evolution. Here, we use a 1-D photochemical-climate model to show that K dwarf
stars also offer a longer photochemical lifetime of methane in the presence of
oxygen compared to G dwarfs. For example, we show that a planet orbiting a K6V
star can support about an order of magnitude more methane in its atmosphere
compared to an equivalent planet orbiting a G2V star. In the reflected light
spectra of worlds orbiting K dwarf stars, strong oxygen and methane features
could be observed at visible and near-infrared wavelengths. Because K dwarfs
are dimmer than G dwarfs, they offer a better planet-star contrast ratio,
enhancing the signal-to-noise (SNR) possible in a given observation. For
instance, a 50 hour observation of a planet at 7 pc with a 15-m telescope
yields SNR = 9.2 near 1 um for a planet orbiting a solar-type G2V star, and SNR
= 20 for the same planet orbiting a K6V star. In particular, nearby mid-late K
dwarfs such as 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be
excellent targets for future biosignature searches.
