Habitable Planets

We don’t [yet] know what makes a planet habitable

For the first half of the twentieth century, we had only a vague understanding of what made a planet in our solar system habitable. They were no longer mere points of of light, but the unkowns still dominated. Even as late as 1960, it was still reasonable to imagine Mars and Venus teeming with life. By 1975, our improved knowledge had shown that they were barren. (While there are still reasons to be hopeful about Venus’ upper atmosphere and Mars’ interior, any biomass would make the antarctic interior seem incredibly lush)

More a speck of light than a world

For exoplanets, we are at best at a 1950s level of knowledge, and often not even 1890s. In those best cases we have sizes, masses, recieved starlight, and vague approximations of their atmospheres. Technically maps of reflection and emission features exist, but these have resolutions worse than what Hubble achieved for Pluto, let alone the actually resolved features that Lowell had for Mars. In some cases (eg: K2-3d, CoRoT-7b), errors in mass and/or radius have resulted in large errors in densities. Even relatively modest errors (eg: TRAPPIST-1 over the past 2 years) can still change assumptions about atmospheres and compositions significantly. And all this ignores that for most planets, we don’t even have both mass and radius!

We do have simulations, but with our limited data, they should be taken as possibilites rather than what those planets are actually like. And, well, they’re simulations. As time goes on, they will become increasingly accurate, but it remains possible that we missed something. Compare present models of tide-locked planets with mid-late 20th century ones, where we though atmospheric collapse was inevitable, for instance.

A good general example of poorly known processes are clouds and hazes. We are still looking for patterns in the extent of clouds in hot jupiters, let alone in more difficult to study planets. A quick perusal of atmospheric studies circa 2014-2018 will also find an embarassing number of flat/featureless spectra because of possible clouds/hazes. But also for some targets we’re fighting the limits of the instrumentation. Especially Spitzer, and its limited photometry (2 bands that are broad and yet relatively noisy).

A good specific example of poorly known processes is the hot super earth 55 Cancri e. We know that there is a non-trivial amount of heat transfer from its day to night side, but the observations and models have been contradictory. Initially there was reason to expect little to no atmosphere, and a dayside lava ocean. More recently, the possibility of an earthlike (in terms of thickness though not composition) atmosphere has been raised. And the dayside temperature changes? Volcanism. This world may be like Io, where we found evidence of vulcanism with ground based IR shortly before the Voyagers flew by. Unlike Io, we are not going to be able to image 55 Cnc e’s surface in any detail any time soon, however. As such, resolving the atmosphere and lava (if any) will continue to be challenging.

We also don’t know where a planet is habitable

The “Habitable Zone” itself is an unfortunate piece of terminology, as it describes the locations where a planet with the right atmosphere can have liquid water at its surface. This is obviously influenced by how bright the star is, but more subtlely by the star’s temperature, the planet’s spin, and even the planet’s tectonic activity. All of these are subject to change over the lifetime of a system, sometimes in ways we still poorly understand. And even if we do understand the general history of a star’s habitable zone, there are details of feedback processes that we cannot yet observe.

Beyond the “habitable zone” proper, there are many places where there can still be liquid water (and therefore possibly life). The oceans of Europa, Enceladus, etc. remain unexplored. Perhaps supririsingly, the martian subsurface and upper atmosphere of Venus remain faint possibilities.

Even if we find a habitable world, we might not recognize it

Our observations of the martian atmosphere don’t completely rule out life, but it must be quite rare.. Ocean worlds (that is, ones with an extensive upper layer of water and no exposed land), suffer from a lack of nutrients that is likely to hamper life. I don’t know of any reason that this will stop hydrothermal vent communities, but those tend to be difficult to observe unless you are literally right on top of them. The situation is likely even worse if the bottm of the ocean is in contact with high pressure ice instead of rock, though that’s more of a problem for something Earth-sized.

The net effect is that if worlds sufficiently unlike our own harbor life, it may be too hidden and/or sparse to be found anytime soon. By mass, most life on earth is (land) plants, followed distantly by (crustal) bacteria and (oceanic) animals. In decending order of total mass, life is land based first, oceanic second, and crustal third. One would hope that this order holds elsewhere, as it is also least to most detectable. Yet in outer solar system moons and exoplanets that are not extremely earthlike, this may also be the order from least to most habitable.

We don’t know what we don’t know

Basically, don’t take the term “habitable zone” too literally. Our knowledge of these environments is poorly defined, and will be for a number of decades. Even in our ‘backyards’, mysteries remain. If you’re writing hard science fiction set on alien worlds, you can largely go wild. And if you somehow solve one or more of these things so we can easily find life (or lack thereof), please tell me?

%need update on TRAPPIST 1 b being airless, 1 c being questionable, and the rest also uncertain: https://arxiv.org/abs/2401.16490