Likelihood and appearance of life beyond the Earth: An astronomical perspective

TL;DR

This paper reviews the current state of exoplanet observations, discusses habitability criteria, and assesses the likelihood of discovering extraterrestrial life based on astronomical data and astrobiological considerations.

Contribution

It provides a comprehensive overview of exoplanet data, habitability definitions, and the probability of life beyond Earth from an astronomical perspective.

Findings

Approximately 1% of known exoplanets may be habitable.

Current technology faces challenges in detecting signs of alien life.

Billions of planets in our galaxy could potentially host life.

Abstract

As of 2025, over 6000 planets are known to orbit stars other than our Sun. We can measure their sizes and orbital periods, infer their masses and temperatures, and constrain their compositions. Based on these data, about 1% of extrasolar planets are potentially habitable for life as we know it, implying that of the billions of planets in our Galaxy, some may actually be inhabited, at least by microbes. However, recognizing signs of alien life forms is a major challenge for current technology, because of the wide range of conditions on extrasolar planets, and because of the wide range of forms that life may take. This chapter reviews observations of exoplanets and discusses astrobiological definitions of habitability and the likelihood of finding life beyond the Earth, both within and outside the Solar system.