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The K Dwarf Advantage for Biosignatures on Directly Imaged ExoplanetsOxygen 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 1D 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 ratio (S/N) possible in a given observation. For instance, a 50 hr observation of a planet at 7 pc with a 15 m telescope yields S/N = 9.2 near 1 m for a planet orbiting a solar-type G2V star, and S/N = 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.
Document ID
20190002449
Document Type
Reprint (Version printed in journal)
Authors
Arney, Giada N. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
April 11, 2019
Publication Date
March 1, 2019
Publication Information
Publication: The Astrophysical Journal Letters
Volume: 873
Issue: 1
ISSN: 2041-8205
Subject Category
Lunar and Planetary Science and Exploration
Report/Patent Number
GSFC-E-DAA-TN66717
Funding Number(s)
CONTRACT_GRANT: NNH12ZDA002C
CONTRACT_GRANT: NNA13AA93A
CONTRACT_GRANT: 80NSSC18K0829
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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