The most common objection I hear about what we call the ‘habitable zone’ is that it specifies conditions only for life as we know it. It leaves out, for example, conceivable biospheres under the ice of gas giant moons, examples of which we possibly have here in the Solar System. But there is another issue with defining habitability in terms of atmospheric pressures that can support liquid water on the surface. As Jason Wright and Noah Tuchow (both at Penn State) point out in a recent paper, the classic habitable zone concept does not take the evolution of both planet and star into account.
It’s a solid point. A planet now residing in the habitable zone could have remained habitable since the earliest era of its formation. Or it could have become habitable at a later time. Thus Tuchow and Wright make a distinction between what they refer to as the Continuous Habitable Zone (CHZ) and a class of planets they refer to as ‘belatedly habitable.’ These worlds may benefit from changes in the location of the habitable zone as stellar properties change, or they may enter the habitable zone through planetary migration. They may represent a substantial fraction of planets in the habitable zone. But are they truly habitable?
As the authors see it, there is not a single belatedly habitable zone (let’s refer to this as the BHZ), but rather two. The outer consists of the planets whose stars become more luminous over time, thus moving the habitable zone outward. The question here would be whether planets like this can successfully thaw and become habitable. I like James Kasting’s term for these worlds, coined as long ago as 1993. He calls them ‘cold start’ planets, and they represent a lively area of current research.
The inner belatedly habitable zone holds stars around which the habitable zone moves inward as the star dims. These inner BHZ planets are an intriguing lot because they orbit a wide range of lower-mass objects. Both brown and white dwarfs dim with time as they cool, making previously uninhabitable worlds more clement, though the authors note that these may lose many of their volatiles before achieving temperate conditions.
And because of their ubiquity in the Milky Way, we should pay special attention to M-dwarf planets. These worlds may spend millions of years in a greenhouse phase, with the possible loss of water, before their host star has finished the contraction that will eventually place it on the main sequence, dimming enough for habitability.
Given these distinctions, the liquid water habitable zone is actually a combination that includes the Continuous Habitable Zone as well as the inner and outer belatedly habitable zones, and as the authors point out, at any specific time in a star’s history, these regions will have different sizes and as the star evolves, may disappear entirely.
Image: This is Figure 1 from the paper. Caption: Habitable zone evolution for a 0.5 M⊙ M dwarf (left) and a 1.0 M⊙ solar analog (right). Continuous habitability is considered to start at the dashed vertical line, roughly representing the planet formation timescale. The green regions on the plots represent the continuously habitable zone, while the orange and blue regions represent the inner and outer belated habitable zones respectively. Credit: Tuchow & Wright.
To consider what the authors call ‘belated habitability,’ the star’s evolutionary history must be considered along with the presence of volatiles and their origins, the rates of cooling and outgassing as a young planet evolves, its related geophysical processes and more. Thus the complexity of the habitable zone deepens, taking the edge off quick claims for habitability in any given system. The fact that a planet is in the habitable zone today does not necessarily mean that liquid water exists on its surface:
A large portion of exoplanets that we find in the habitable zones of other stars will lie in the belatedly habitable zones, and future missions will greatly benefit by considering belated habitability and not assuming these planets are habitable. For example, in a search for biosignatures, the target stars and the search strategy will be affected by whether or not one considers the habitability of these planets. While the special circumstances of their habitability have been overlooked in the past, belatedly habitable planets could have major implications for future mission design and warrant future study.
I think these are useful distinctions that should come into play as our new generation telescopes come online. It’s certainly true that the press often exaggerates new discoveries of ‘habitable zone planets’ (and our friend Andrew Le Page is a shrewd judge of such claims), but from the standpoint of creating a catalog of best targets for further investigation, we need to be able to winnow the list efficiently and accurately. The study of ‘belated habitability’ should prove a productive research path.
The paper is Tuchow & Wright, “Belatedly Habitable Planets,” Research Notes of the AAS,” Volume 5, No. 8 (August, 2021). Full text.