Is it Time for a New Definition of “Habitable?”

Things tend to move from the simple to the complex when you’re trying to understand something new. This is the situation exoplanet scientists find themselves in when it comes to the term ‘habitable.’ When they were discovering the first tranche of exoplanets, the term was useful. It basically meant that the planet could have liquid water on its surface.

But now that we know of over 5,000 confirmed exoplanets, the current definition of habitable is showing its age.

The word ‘habitable’ and its definition is more than just pedantic word-play. The meaning of the word is intertwined with what we know about habitability and how we characterize it. And now that we know of thousands of exoplanets, characterizing habitability is more complex. For context, the term ‘habitable zone’ was first brought forward by American scientist Harlow Shapley more than 70 years ago.

In a new paper, researchers from France and the USA look at the current definition of habitable and how we can characterize habitable planets observationally with powerful new telescopes that are coming online. They also present a new and innovative method to assess habitability and inhabitation.

The paper is “Prospects for the characterization of habitable planets.” The first author is Stephane Mazevet. Mazevet is the Director of the Observatoire de la Côte d’Azur, Université Côte d’Azur, CNRS (French National Centre for Scientific Research.)

The notion of exoplanet habitability addresses one of humanity’s foundational questions: Are We Alone? When, or if, we ever find out we’re not alone, it likely won’t be because ETIs visit us or send us a clear signal. It’s far more likely that we find a planet with simple, single-celled life. After all, life on Earth consisted of no more than single-celled organisms for billions of years. Complex life only started to become prominent during the Cambrian explosion just over 500 million years ago. We don’t know how probable it is for simple life to develop into complex life.

But we do know that life affects a planet’s atmosphere. That’s what happened on Earth when organisms produced methane and oxygen. Detecting both those chemicals in an exoplanet’s atmosphere isn’t a sure sign of life, but due to the relationship between life and the atmosphere, the amounts of different chemicals in the atmosphere can indicate life. But there are over 5,000 confirmed exoplanets, and in all probability, there are vast numbers of them still waiting to be discovered.

It’s clear that we need detailed and robust methods to determine what planets might be habitable. “With thousands of exoplanets now identified, the characterization of habitable planets and the potential identification of inhabited ones is a major challenge for the coming decades,” the authors write. Their new method of identifying habitable worlds is focused on future missions and telescopes. For the authors, it’s too soon to think we can examine a single exoplanet closely and determine its potential habitability. Instead, we need to characterize the atmospheres of a large number of exoplanets to identify chemical trends in their atmospheres.

Exoplanet scientists work with the idea of both a conservative habitable zone and an optimistic habitable zone. The conservative habitable zone (light green) is bounded by the moist greenhouse limit and the maximum greenhouse limit. The optimistic habitable zone (dark green) is bounded by the current Venus and early Mars limits. Image Credit: Planetary Habitability Laboratory

One of the difficulties facing exoplanet scientists is the bewildering variety of exoplanets they’ve discovered. They range from Mercury-sized rocks to massive gas giants far more massive than Jupiter. Some exoplanets are separated from their stars by a vast distance, while others are in tight orbits and tidally locked to their stars. There are puffy, marshmallow planets and planets that might rain iron.

The extreme planets are pretty easy to dismiss, but what about all the rocky planets in the habitable zone? This is where the difficulty with the term habitable comes into focus. But it’s hard to redefine the word without methods that can flesh out a new definition.

<Click image to visit enlarged image> This graphic from the Planetary Habitability Lab shows known exoplanets and their relationship to habitable zones according to their distances from their stars, the type of stars, and the amount of energy they receive. Image Credit: Planetary Habitability Lab.

As most people who follow space news know, a key threshold for understanding habitable exoplanets better is the ability to characterize their atmospheres. The common definition of the habitable zone is based on how close a planet is to a star and how much insolation the planet receives. But for planets in the habitable zone to harbour liquid water, the atmosphere has to allow it, too. We’re in the early days of being able to understand exoplanet atmospheres. “Atmospheric characterization of exoplanets has only been obtained in a handful of cases,” the authors write. The James Webb Space Telescope was developed to address several issues in astronomy, and exoplanet atmospheres are one of them. It’s already made some progress.

Modern instruments on powerful telescopes are starting to make atmospheric characterization a reality. The JWST, the SPHERE and GRAVITY instruments on the ESO’s VLT and VLTI, the upcoming European Extremely Large Telescope, and the ESA’s ARIEL mission will all drive more progress.

“While addressing various questions along the way, such as the atmospheric composition of giant planets, the nature of ocean worlds, of sub-Neptunes and super-Earths, the ultimate and long-term goals of the next generation of instruments are to enable us to quantify the habitability of Earth-like planets and eventually identify inhabited ones,” the authors explain.

But there’s more to astronomy than instruments. Methods are equally important, and that’s where this research comes in.

A critical problem in our understanding of atmospheres and habitability is our overall lack of knowledge. Astronomers have updated the notion of the habitable zone with the idea of a conservative habitable zone and an optimistic habitable zone. But they’re both based on long-term climate models of the inner planets in our Solar System. Astronomers don’t have much else to go on.

This figure shows the conservative and optimistic habitable zones for a range of stars, with real exoplanets placed in the zones. Image Credit: Chester Harman.

The team proposes an integrated ecosystem-planetary model to help determine habitability with future missions and telescopes. It’s based on a few things that scientists know about planets.

Simple life can pave the way for complex life, as it did on Earth. Simple life can reshape the atmosphere by adding oxygen and other chemicals like methane. This can take a long time, and it likely also requires a carbon cycle to moderate the buildup of carbon in the atmosphere. On Earth, this means plate tectonics.

Methanogens play an important role in altering atmospheres. On Earth, they may have contributed to our planet’s long-term habitability, while on Mars, they helped drive it out of habitability if they existed. This illustrates the value of the team’s integrated method. “This quantitative modelling suggests a very different outcome between the Earth and Mars upon the potential apparition of methanogenesis, stressing the need for an integrated approach to address habitability and inhabitation on the same footing.” It highlights how life itself can contribute to habitability. “This also shows that habitability is, to some extent, determined by inhabitation.”

What it comes down to is a more holistic approach. “We support here the idea that the identification of habitability and inhabitation should be achieved by identifying trends in exoplanet populations rather than aiming at a precise characterization of a given planet.” The researchers point out how their idea affects the search for biosignatures.

“Considering the question of biosignatures, we propose that it should be approached by directly coupling the planetary environment with the potential biological activity to account for the viability of potential metabolisms and to include their impact on the atmospheric composition,” they explain. This is different than the predominant way of thinking about biosignatures. The search for biosignatures focuses on identifying individual chemicals in a planet’s atmosphere, something that we’re getting better and better at.

This figure from the study illustrates some of the researchers’ ideas. It’s a schematic representation of the Earth populated by a basic ecosystem involved in methanogenesis. The parameters that vary during the planet’s lifetime, ?, are indicated in red. ?vol c (?) stands for the volcanic outgassing, Wea(?) represents weathering, PCO2 (?) is the CO2 surface pressure, pH(?) is the ocean pH, and FXUV (?) is the stellar flux. (b) Shows the distributions of the CO:CH4 ratio for each plausible evolutionary scenario involving the basic metabolisms considered in the basic ecosystem. Image Credit: Mazevet et al. 2023.

The authors say we should be focusing on statistical trends to understand habitability, rather than planets on a case-by-case basis. Their proposed method has another upside: it reduces false positives. “This formally reduces the possibilities of false positives as it addresses the question of inhabitation of habitable planets as one of their global properties rather than the particularity of a single object.”

But to do all this, we need a telescope that can survey exoplanets. The team uses an example of Earth-like planets around G-type stars like the Sun to make their case. They developed a model of Earth that’s an integration of the ecosystem and the planet. It includes a carbonate-silicate cycle, a feature which is likely necessary for Earth-like planets to be habitable.

Earth’s carbonate-silicate cycle, also known as the inorganic carbon cycle, transforms silicate rocks to carbonate rocks over geological time by weathering and sedimentation. Carbonate rocks are then eventually transformed back into silicate rocks by metamorphism and volcanism.

“By demonstrating that a population of Earth-like planets follows the general concentration of CO2 anticipated would first validate the concept of a habitable zone,” the authors write. “It would secondly indicate that these planets may have liquid water on their surface as the carbonate-silicate cycle as we experience it on Earth depends on having liquid water at the surface.”

To accomplish this, the authors hope to influence the design of next-generation space telescopes. “This example illustrates how these studies can be used to design instruments and devise observational strategies aiming at identifying trends in populations of exoplanets,” they write.

New telescopes develop hand in hand with new methods. The identification of habitable planets is an enormous challenge facing astronomy in the next decades. “The design, as well as the future analysis of these experiments, require a new generation of models that integrate the coupling between the potential
biotic activity, the atmospheric and interior evolution of terrestrial planets that expand beyond the Earth’s size and density,” they explain.

An artist’s illustration of the LUVOIR-A telescope concept, a future space telescope that will be able to characterize large numbers of exoplanets. There are two conceptual designs for LUVOIR, one with an 8-meter mirror and one with a 15-meter mirror. Image Credit: NASA

The team’s model combines an Earth-based ecosystem and planetary modelling. In their study, they show how it can identify “… biotic activity and quantify its effect on habitability for Enceladus, primitive Earth and primitive Mars, as well as for a population of Earth-like planets around G-type stars.”

Upcoming space telescopes will advance our understanding of habitability, alongside better methods. NASA’s proposed LUVOIR telescope, a multi-wavelength platform, will be capable of characterizing large numbers of exoplanets, the exact type of survey needed. HABEX can do the same. Holistic methods like the one outlined in this study will be necessary to make sense of all of LUVOIR’s data.