In the last few years researchers have made numerous claims of habitable (or potentially habitable) exoplanet finds. Almost without exception, the term "habitable" simply refers to the presence of liquid water on the exoplanet (even though we lack the ability to make this measurement). Yet assessing a body's true habitability requires far greater knowledge of the exoplanet's composition, atmosphere, and history than we currently possess. A wise response to "habitable" planet announcements accounts for these research limitations and recognizes that life likely requires far more than just liquid water.
“INCONCEIVABLE!” So repeats the great Vizzini in The Princess Bride to his companions Inigo Montoya and Fezzik. Finally, Inigo replies, “You keep using that word. I do not think it means what you think it means.”
I see a similar pattern in repeated proclamations of habitable (or potentially habitable) exoplanet discoveries. The term habitable conjures images of serene meadows or lush islands surrounded by deep blue oceans. Even without the picturesque vision, habitable implies that life exists on these exoplanets—or could, if transported there.
But the cold, scientific reality is that we currently lack the ability to detect the presence of liquid water or continents or the atmospheric composition of any planet with a remote chance of hosting life. Astronomers continue making remarkable strides toward this capability, but it appears that the technology won't be available for another decade or two. Ultimately, scientists seek the ability to detect the signatures of life on other planets, an achievement that will require even more time.
What we do know
Understanding what we actually do and don't know about extrasolar planets makes it less difficult to know what habitable means. This matter was raised in a question I received regarding the discovery of Kepler 186f, an Earth-like exoplanet. Despite the emphasis on the similarities between Kepler 186f and our own home, how much do we really know about this exoplanet (and others)—and how much is conjecture?
The list of exoplanet characteristics astronomers can measure is relatively short at this point. Data found using the transit technique (see the Kepler Mission for example) tells us the planet's size, as well as its orbit (including period and semi-major axis and, occasionally, the eccentricity and inclination). If astronomers detect the same planet using the radial velocity technique, they can also learn its mass and eccentricity. Both of these techniques require that astronomers observe the planet's orbit multiple times. Utilizing other powerful telescopes permits partial measurement of the atmosphere for a few dozen of these planets (mostly the hot-Jupiter types).
The direct measurement technique allows astronomers to detect the light emitted directly from the exoplanet. While this light provides a great deal of information, only planets relatively far from their host star (similar to the distance of Neptune from the Sun) can be observed this way.
The habitable planet discussion
In light of this information, there are several things we must keep in mind when discussing a planet hailed as habitable. First, we haven't obtained the technology to judge whether a planet is truly habitable or not. Second, most people talking about habitability assume that having liquid water makes a planet habitable. Clearly, life requires liquid water, but studies of solar system planets indicate a much more stringent set of conditions for a planet to actually host life. Third, even the accessible information (mass, size, orbit, and possibly some atmospheric data) permits only a tentative statement on the possibility that liquid water exists on the planet. The tentativeness ties directly to the lack of detailed knowledge regarding the planet's composition and atmosphere.
Imagine judging athletes' NBA potential based on height and weight alone. Though you could immediately rule out candidates who were either too short or too light, any additional evaluation requires knowledge of body mass index, muscle mass, strength, work habits, agility, body control, and a whole host of less tangible factors. At this time in the study of exoplanets we are assessing habitability based on the equivalent of height and weight information only.1
Yes, simply finding exoplanets and determining the information currently available is tremendous progress. No doubt astronomers will continue to make more sensitive measurements. It is a fascinating, exciting, and fruitful field of scientific research. However, to paraphrase Inigo's statement, we must be careful to ensure that habitable means what we think it does.