The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets

Abstract

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 µm 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.

A Limited Habitable Zone for Complex Life

“…This result is consistent with the seminal findings of Segura et al (2005), who showed that the low NUV radiation from stars with low effective temperatures would act to drastically reduce the generation of OH radicals in Earth-like atmospheres on planets orbiting these stars, with the consequence that trace gases like CH 4 otherwise destroyed by OH could build up to high abundances. This prediction has been replicated in a multitude of other studies focused on assessing possible biosignature abundances and detectability on terrestrial planets orbiting late-type stars (e.g., Grenfell et al 2013;Rugheimer et al 2015;Lincowski et al 2018;Meadows et al 2018;Arney 2019;Wunderlich et al 2019). An inescapable corollary to the prediction of high concentrations of biosignature gases, however, is that the concentrations of abiotic and biologically produced toxic trace gases like CO must also be high if their photochemical lifetimes are primarily set by interactions with OH radicals or more generally by NUV photons penetrating into the troposphere.…”

Dead zones of classical habitability in stellar binary systems

“…We limited our investigation to main-sequence stars with masses and luminosities between spectral types F and M. The justification for this restriction lies in the well-known initial mass function (IMF), illustrating that lower mass, and therefore lower luminosity, stars occur in higher frequency (Kroupa 2001(Kroupa , 2002Chabrier 2003, and subsequent work). Additionally, low-mass / low-luminosity stars appear to possess some preferential features to support environments favorable for life, possibly including advanced life, and may also more likely permit exolife detectability (e.g., Heller and Armstrong 2014;Cuntz & Guinan 2016;Arney 2019).…”

Chromospheric activity in 55 Cancri: II. Theoretical wave studies versus observations

“…Orange dwarfs are of particular interest to both astrophysics and astrobiology in consideration of various features, including those commonly considered favorable in support of exolife (e.g., Cuntz & Guinan 2016;Lingam & Loeb 2018, 2019Dvorak et al 2020). Those include the relative frequency of those stars (if compared to stars akin to the Sun), the relatively large size of their habitable zones (HZs) (if compared to M dwarfs), and their long mainsequence life times (i.e., 15 Gyr to 30 Gyr, compared to about 10 Gyr for solar-like stars; see, e.g., Pols et al 1998).…”

“…Those include the relative frequency of those stars (if compared to stars akin to the Sun), the relatively large size of their habitable zones (HZs) (if compared to M dwarfs), and their long mainsequence life times (i.e., 15 Gyr to 30 Gyr, compared to about 10 Gyr for solar-like stars; see, e.g., Pols et al 1998). Recent work on the significance of late G and K dwarfs for the possibility of supporting exolife has been given by, e.g., Arney (2019) and Schulze-Makuch et al (2020).…”