The recent announcement of the detection of water vapor in the atmosphere of an exoplanet has (once again) raised many questions about how astronomers and planetary scientists should talk about their results. Results that are enormous technical accomplishments, but arguably represent only incremental advances towards the goal of identifying the presence of life elsewhere in the cosmos.
The insistence of some researchers that a result like this is directly to do with ‘habitable planets’, is awfully fraught. First, the term ‘habitable’ has insinuated itself into the scientific vernacular largely because of convenience rather than substance. The majority of astrobiologists and exoplanetary scientists use ‘habitable’ as shorthand, referring to both the orbital location and degree of stellar irradiation of a world, and the more complicated question of liquid water existing in a rather vaguely defined surface environment. But we also conflate ‘habitable’ with the size of planets (a gas giant is seldom thought of as habitable, but a rocky world with a modest atmosphere might be) and their suspected composition (more rock, less gas, rather than more gas and less rock).
Furthermore, ‘habitable’ is most certainly not assumed to mean inhabited, or even that a world has the right stuff to have initiated life on itself at any point, much less sustain that life. It is a bit like saying that a pond is ‘swimmable’ when you really mean that it has liquid that will provide a little buoyancy, won’t dissolve you (at least not immediately), and is probably not entirely toxic. Some efforts, like the development of the Earth-similarity index already try to add more realism to this.
Yet say ‘habitable’ to most people who don’t think about these topics all the time and – especially for non-scientists – the natural reaction is to imagine environments that are indeed awash in biology and even in sentient organisms.
All of which comes to a head in media reports which convey a misleading sense of discovery. Which is hugely unfortunate, because it makes it sound like science has already solved a cosmic puzzle that a) may still be a very long way from being answered, b) may or may not ever be a hundred-percent convincingly answered, and c) involves a lot more interesting puzzles and intricacies.
This also creates the very real risk that the science (and scientists) will be seen as untrustworthy because work is being over-hyped. All of which might also diminish our appreciation of the real deal of discovering life elsewhere, if and when that day arrives. Even worse these preemptive noises could trickle through to the halls of power and funding for science, where even well-informed politicians and administrators can be subtly swayed by what they’ve seen, or think they've seen.
I’m not quite sure we scientists can solve all of those issues – media will be media, no matter how much we wish it were otherwise, and it is human nature to pick up on all manner of cues and biases. But in the field of study itself we should probably come up with some better terminology and formalities of language use.
For instance, I personally think that ‘habitability’ could be retired. At this stage of the exploration process we are really concerned with bioprobability – the likelihood of living systems existing in particular environments. That doesn’t mean that we can yet assign numerical values of bioprobability in any specific cases. Or that all potential examples of life and its ecosystems can or should be treated equally (what about subsurface ocean moons, or subsistence biospheres, or possibilities we’ve not fully considered?) But by switching our thinking to bioprobabilities we can at least begin to separate out some of the critical underlying puzzles.
For instance, a planet might have a high probability of origin-of-life events, but a low probability that any of those stick. Or the opposite, where life has a slim chance of starting up, but once it does it rages on as a planet-sculpting catalyst. We don’t know the answers to those questions for any worlds (including Earth), but they would appear to be incredibly important.
Of course, bioprobability could also have its share of misuse. Given that most of us aren’t born to be good with probabilities and statistics, there is a barrier to its adoption for headlines and media soundbites. I don’t see major newspapers with ‘High bioprobability planet discovered’ as the banner. But most journalists do work incredibly hard to get things right, so the more science can show the way, and discuss the fascinating details, the better.
In fact, by introducing a new term we can lead the conversation in a richer direction. What does bioprobability mean? How is the origin of life possibly different than the persistence of life? What are the superficially different planetary environments that could still be equivalent in bioprobability?
Bioprobability also highlights some intriguing questions about the search for life. For example, we spend a lot of time thinking about how to characterize new worlds; hoping to sense so-called biosignatures of chemistry or other life-induced planetary changes. Yet we may not really know what an entirely uninhabitable, yet otherwise appealing world looks like. Even Mars and Venus in our own solar system cannot be said to be uninhabited now or at any point in the past with utter confidence. Either because of ancient climate conditions that were perhaps more conducive to life as we know it, or extreme possibilities for life on the harsh martian surface, or even in high-altitude Venusian clouds.
It may be time to let go of a catchy, sometimes useful term in 'habitable', for something better suited to the science taking place now. The truth is out there, so let’s not allow the pitfalls of human language stymie our efforts.