A review of exoplanetary biosignatures

Abstract

We review the field of exoplanetary biosignatures with a main focus upon atmospheric gas-phase species. Due to the paucity of data in Earth-like planetary atmospheres a common approach is to extrapolate knowledge from the Solar System and Early Earth to Earth-like exoplanets. We therefore review the main processes (e.g. atmospheric photochemistry and transport) affecting the most commonly-considered species (e.g. O₂, O₃, N₂O, CH₄ etc.) in the context of the modern Earth, Early Earth, the Solar System and Earth-like exoplanets. We consider thereby known abiotic sources for these species in the Solar System and beyond. We also discuss detectability issues related to atmospheric biosignature spectra such as band strength and uniqueness. Finally, we summarize current space agency roadmaps related to biosignature science in an exoplanet context.

Introduction

The science of biosignatures i.e. signals which suggest the presence of life, is a wide and interdisciplinary field which is currently undergoing rapid expansion due in part to advances in exoplanet science. In this context, it is therefore convenient to split current biosignature methods into two groups, namely ‘in-situ’ and ‘remote’. We define here in-situ biosignatures to include the study of fossil morphology and the direct examination of complex biotic molecules such as DNA and lipidic acids (see e.g. Horneck et al., [1] for a review). In-situ biosignatures are not our main focus here. We define remote biosignatures to refer to spectroscopic methods for detecting e.g. atmospheric species’ abundance, (surface) reflectivity etc. In recent years there has been some cross-over from in-situ to remote methods e.g. for biosignatures involving isotopic ratios, surface pigments etc. as we will discuss.

This paper reviews the field of remote biosignatures in an exoplanetary context with a main focus on atmospheric species. Due to limited data in Earth-like exoplanetary science, a common approach when studying potential exoplanetary biosignatures is to consider what can be learned from the modern Earth, Early Earth and the Solar System. Based on these, one then extrapolates knowledge gained (together with theoretical model studies) to the study of Earth-like exoplanets.

Literature reviews which cover subjects related to exoplanetary biosignatures but which are not discussed in detail in this paper are cited throughout the manuscript. For conciseness however, we list here briefly some of the main review papers relevant to our subject. A review of conditions affecting potential habitability and biosignatures beyond the Solar System is provided in the Special Issue “Planetary Habitability and Life” edited by Spohn et al. [2] and references therein. The potential habitability of terrestrial planets orbiting in the Habitable Zone of M-dwarf stars (important targets in exoplanet science) was reviewed in [3] and more recently by Shields et al. [4]. These are central objects in exoplanetary science and are targets of interest for forthcoming space missions. This is because firstly, M-dwarf stars are numerous in the solar neighborhood; secondly, planets orbiting smaller, fainter stars have stronger (planet/star) contrast ratio signals and thirdly a fainter star suggests a closer HZ where planets have faster orbits meaning more rapid collection of data. There are however possible caveats regarding the habitability of these objects such as possible planetary tidal-locking due to the close proximity of the star. This could lead to slower planetary rotation and a possible weakening in the planet’s magnetosphere hence potentially strong bombardment of the atmosphere with high energy particles as detailed in the Scalo and Shields reviews mentioned above. The extent to which these processes could be related however is generally not well-constrained. Lammer et al. [5] reviewed evolution of planetary environments towards habitable conditions. Regarding biosignatures, the recent NASA NExSS (N exus for Ex oplanet S ystem S cience) initiative has produced comprehensive reviews which discuss finding, assessing, detecting and planning observation programs for exoplanetary biosignatures, as detailed in [[6], [7], [8], [9], [10]]. Reviews by Seager [[11], [12]] summarize responses of gas-phase species in biosignature science. A review of biosignature-related photochemical responses on modern Earth is provided by e.g. Wayne [13]. Reviews of potential biosignature species on Mars (and Venus) are provided by e.g. Krasnopolsky [14] and Atreya et al. [15].

The paper is organized as follows. Section 1 discusses current definitions of life (1.1) and the steps involved when searching for life remotely (1.2). Section 2 discusses biosignatures for vegetation and biological pigments. Section 3 deals with spectrophotometry of gas-phase species which have been suggested as potential biosignatures. Section 4 summarizes the application of spectropolarimetry in remote biosignature science. Section 5 discusses reaction network properties in a biosignature context. Section 6 summarizes studies of Earth-like planets without life (“dead Earths”). Section 7 discusses current space agency roadmaps in astrobiology related to searching for remote biosignatures. Section 8 provides a brief summary.

There is currently no universally-accepted definition of life. Frequently cited [16] is the NASA definition of a “self-sustaining chemical system capable of Darwinian evolution”. Cleland and Chyba [17] review the challenges in defining life and its needs.

Assessing remote biosignatures is a multi-tiered process. The first step involves finding suitable targets for study. This requires prior information on the fraction of stars which host potentially Earth-like planets – the “eta Earth” (${\eta }_{\mathrm{earth}}$) parameter as discussed in e.g. Kane et al. [18]. The second step involves obtaining the measurement itself. Common methods discussed in the context of biosignature studies include transit spectrophotometry – for which there exists a large literature (see e.g. Rauer et al., [19]) and spectropolarimetry (see e.g. Stam [20]; Sterzik et al., [21]; see below). Deriving the atmospheric properties such as temperature, pressure and composition from the observed spectra usually involves dealing with degeneracies which may arise during the retrieval process. These can be addressed e.g. in the case of atmospheric spectroscopy by combining information from e.g. absorption band strength, shape and spectral slope as discussed in [22]. The third step involves a process of elimination to ensure that the biosignature candidate is really attributable to life. This requires characterization of the planetary environment and evolution in order to discount any abiotic sources which could be falsely interpreted as life. There now follows an overview of the various proposed biosignature approaches.