Astrobiology

The habitability of a planet is defined at a fixed time. A bigger challenge is to understand how that habitability is sustained over geological timescales, and how the underlying processes compare across different planetary bodies.

Modelling of Earth- or super-Earth-sized planets with a thick H–He atmosphere shows that the hydrogen collision-induced absorptions in the infrared wavelength can make the planet amenable to hosting surface liquid water for several billion years, thus creating a long-term potentially habitable environment.

Calculations of water activity reveal that this parameter can be a substantial barrier to habitability for clouds of Solar System planets. In particular, water activity within droplets of Venus’s clouds is more than 100-fold below the threshold for biotic activity of known extremophiles.

A map of the stability of brines on Mars, obtained by combining thermodynamic and climate modelling, shows that some brines can stay liquid for longer than previously thought, particularly at high northern latitudes. However, they are not habitable to known terrestrial life, and can be explored without risk of biological contamination from Earth.

In the event of accidental transmission of microbes to other planets, we must consider whether the local conditions would allow their proliferation. Whereas temperatures on Mars are usually hostile to life, liquid water is available from deliquescing salts.

Present-day Mars is thought to be unsuitable for life as we know it. However, a thin coating of silica aerogel on the Martian surface may be enough to induce local, potentially habitable subsurface environments.

A combination of laboratory experiments and numerical modelling shows that a 2–3 cm-thick layer of silica aerogel deployed over the temperate regions of Mars could maintain a surface environment conducive to liquid water all year round. Such an approach would create a habitable setting for photosynthetic life.

Although a major objective in Mars exploration is the search for life, there are many scenarios that could lead to the recovery of lifeless samples. What could lifeless samples tell us about Mars and its habitability?

Chemical disequilibrium is a known biosignature, and it is important to determine the conditions for its remote detection. A thermodynamical model coupled with atmospheric retrieval shows that a disequilibrium can be inferred for a Proterozoic Earth-like exoplanet in reflected light at a high O₂/CH₄ abundance case and signal-to-noise ratio of 50.

On Earth, technological advances required open-air combustion, which needs an oxygen partial pressure of about 18%. This threshold can help guide searches for detectable technospheres on other planets.

A low atmospheric carbon abundance can be a ‘habiosignature’ and indicate the presence of substantial surficial liquid water, tectonic activity and/or a biomass in temperate rocky exoplanets. It can potentially be detected by JWST at 4.3 μm in a few tens of transits.

An information-theory-inspired re-analysis of Cassini mass spectrometry data reveals the presence of HCN and partially oxidized organics within the plume of Enceladus. Ongoing redox chemistry may create a habitable environment.

A state-of-the-art machine-learning method combs a 480-h-long dataset of 820 nearby stars from the SETI Breakthrough Listen project, reducing the number of interesting signals by two orders of magnitude. Further visual inspection identifies eight promising signals of interest from different stars that warrant further observations.

A radio signal detected in the direction of Proxima Centauri in a Breakthrough Listen programme is analysed for signs that it was transmitted by extraterrestrial intelligent life, using a newly developed framework. However, the signal ‘blc1’ is likely to be terrestrial radio-frequency interference.

Any detection of potential biosignature molecules like oxygen and methane needs to be put into the planetary environmental context to understand its actual importance. Such a contextual approach is also essential when considering alternative or agnostic biosignatures on planets and exoplanets.

The China Exo-Ecosystem Space Experiment — hosted on the Tiangong Space Station — aims to explore the ability of microbes to survive in space.

A nested orbit-to-ground approach for microbial landscape patterns at different scales, tested in the high Andes, provides a machine learning-based search tool for detecting biosignatures on terrestrial planets.

An experiment designed by the Chinese Academy of Sciences — the Balloon-Borne Astrobiology Platform (CAS-BAP) — paves the way to conducting astrobiology research in Earth’s near space as a planetary analogue.

Early Martian surface and subsurface were probably habitable for methanogenic microorganisms with a hydrogen-based metabolism, according to an ecological model coupled with a geochemical simulation. Feedback effects of such a biosphere on the atmosphere might have driven strong global cooling.

What is the origin of the methane detected in Enceladus’s plumes? A Bayesian approach to the problem shows that abiotic serpentinization of rocks cannot explain the methane abundance by itself, and biotic methane production gets the highest likelihood—provided the probability of life emerging at Enceladus is high.

Cassini measurements suggest hydrothermal activity on Enceladus that could support methanogenesis. Bayesian analysis of models simulating an abiotic or biotic ocean indicates the latter is more probable so long as abiogenesis is sufficiently likely to occur.

Escherichia coli bacteria and yeast cultures (representative prokaryotes and eukaryotes) have been tested under laboratory conditions in a 100% H₂ atmosphere. They can reproduce normally, with lower growth rates, producing a range of biosignature gases. Exoplanets with a H₂-dominated atmosphere might thus not be totally hostile to some forms of life.

A series of pieces published in this issue highlights the breadth and depth of topics discussed in modern astrobiology, an exciting discipline that has come to the forefront of astronomy in recent years and promises to answer one of the most fundamental questions of humanity.

If advanced technological extraterrestrial lifeforms are out there, where are they? Thus goes the Fermi paradox. This Perspective reviews various solutions and proposes that they are either not there or are deliberately hiding from us.

Is the scientific status of astrobiology undermined by the lack of evidence for alien life, the problematic influence of science fiction, or the use of ‘astrobiology’ as a buzzword for attracting funding? Here we defend the emerging discipline.

The search for life elsewhere involves variables across multiple scales in time and space, often nested hierarchically. We suggest that the emergence of artificial intelligence learning systems offers critically important ways to make progress.

We have found many Earth-sized worlds but we have no way of determining if their surfaces are Earth-like. This makes it impossible to quantitatively compare habitability, and pretending we can risks damaging the field.

As scientists, the terminology we choose influences our thinking as it carries our messages to colleagues and the public. In the face of pressure to turn science into clickbait, maintaining precision in the language we use is critical to dispel misinformation and, more broadly, to enable scientific progress.