A habitable zone can be defined in many ways, but for our immediate purposes, defining it with reference to liquid water on a planetary surface makes sense. Sure, we believe that life could exist beneath the surface on places like Europa, where surface water is out of the question, but the key issue is this: Are there atmospheric features that we could use to make the call on habitability? It’s an important issue because with our current and near-future technology, this is how we can plan to investigate life on planets around other stars. We can study exoplanetary atmospheres already and we’re getting better, but we can’t drill through exoplanetary ice.
A new paper from Lisa Kaltenegger and Dimitri Sasselov (Harvard Smithsonian Center for Astrophysics) gets into these questions by looking at how to evaluate habitability, studying different kinds of planetary atmospheres and developing model calculations. The intent is to apply these ideas to the habitable planet candidates, 54 in number, so far produced by the Kepler mission, but the work generalizes to planetary candidates from any transit searches. The smallest size objects in the current Kepler sample are clearly the most interesting, because they could have the kind of atmospheres we see from Venus through the Earth to Mars in our own system, a range of chemistries that points out the limits of this kind of habitable zone.
A habitable zone can be conceived as a ring — an annulus — around a star, and the key points are these:
… the inner edge of the HZ is defined as the location where the entire water reservoir can be vaporized by runaway greenhouse conditions, followed by the photo-dissociation of water vapor and subsequent escape of free hydrogen into space. The outer boundary is defined as the distance from the star where the maximum greenhouse effect fails to keep CO2 from condensing permanently, leading to runaway glaciation.
Not all planets in a habitable zone will be habitable, of course, but we can say many things about the potential for habitability. The width and distance of a habitable zone around a particular star depend on a number of parameters that we can hope to examine from Earth or space-based missions as our technology improves:
- Incident stellar flux. Here we’re looking at the luminosity of the host star, its spectral energy distribution, and the eccentricity of the planetary orbit.
- Planetary albedo, the reflecting power of the planetary surface.
- The concentration of greenhouse gases in the planet’s atmosphere.
- The energy distribution in the planet’s atmosphere.
The authors look at three models of Earth like planets, from planets with high concentrations of carbon dioxide in their atmospheres to those with atmospheres more or less like the Earth today and those with high values of water vapor, modeling these for stars with effective temperatures between 3700 K and 7200 K. The models thus represent planets on the outer edge of a habitable zone, in the middle of it, and on the inner edge. Examined in this way through a straightforward equation developed by the authors to measure potential habitability, many of the 54 planetary candidates thought to be in the habitable zone in the Kepler sample turn out to be outside of it because their temperatures are too high. 27 of the Kepler candidates show temperatures in the habitable range, three of them being candidates with radii smaller than two Earth radii.
Cloud cover is an intriguing variable. In fact, the authors point out that depending on cloud cover, the outer edge of the habitable zone in our own Solar System varies from 1.67 AU (cloud-free) to 1.95 AU and 2.4 AU (50% and 100% cloud cover respectively). Assuming cloud cover of 0%, 50% and 100%, we get three sets of results:
Applying our analysis to the whole Kepler planetary sample of 1235 transiting planetary candidates, assuming the maximum Earth-like Bond albedo for rocky planet atmospheres… results in 12, 27, 67 planetary candidates with Teq [equilibrium temperature] smaller than the water loss limit (Tsurf = 373K) for 0%, 50% and 100% clouds respectively, and 18, 43, 76 planetary candidates with temperatures lower than the runaway greenhouse limit respectively. Among those are 2, 3, 6 as well as 3, 4, 6 planets respectively, that have radii below 2 Earth radii consistent with rocky planets (KOI1026.01, 854.01, 701.03, 268.01, 326.01, 70.03).
We can use transit timing variations to calculate the density of rocky planet candidates occurring in multiple planet systems, but remember that the fact that a planet is rocky does not necessarily make it habitable, depending on the local abundance of water and other materials. That means a good terrestrial planet candidate has to have its atmosphere fully examined for signs of life, and here Kepler will not be enough. Its target stars are 500 to 1000 parsecs from us, making its findings extremely helpful statistically but less useful than a future targeted mission at finding small planets orbiting stars closer to the Sun whose atmospheres we can analyze.
The paper is Kaltenegger and Sasselov, “Exploring the Habitable Zone for Kepler planetary candidates,” submitted to Astrophysical Journal Letters (preprint).