The first paper I will review is Habitable Zones Around Main-Sequence Stars: New Estimates (Kopparapu et al. 2013). In brief, this is a re-doing of the famous Kasting et al. (1993) paper which first developed the concept of the habitable zone, and made the first calculations of where this region would be around main-sequence (non-evolved) stars
So, for starters, let's break down some of this jargon to explain what's actually going on. A "habitable zone" is the region surrounding a star where you could put a planet somewhat like Earth, and it could have liquid water on its surface. Contrary to popular belief, being in the "habitable zone" does not necessarily mean that the planet has life, or could even support life as we know it. The title is just a statement about the planet's surface temperature. The reason that the habitable zone is a region rather than a single orbital distance is because water can exist as a liquid at a fairly broad range of temperatures: 273 K (0° Celcius) to 373 K (100° Celcius).
In the simplest possible scenario, the size of the habitable zone is determined solely by the planet's distance from its star. As in our own solar system (to first order), the closer a planet is to the Sun, the more radiation it receives, making it hotter. The boundaries of the habitable zone are set by the distances at which a planet is too close to its star, so its water would evaporate, or too far away, so its water would freeze.
But this obviously isn't the full story, because the average surface temperature of Venus is much higher than the average temperature of Mercury (735 K for Venus compared to ~300 K for Mercury with massive variations between its day and night sides). The difference, as the late Carl Sagan pointed out in his dissertation, is the atmosphere.
Venus' atmosphere, as we've learned from a series of probes and landers (Venera, Magellan, Pioneer Venus, and many more), Venus has a very thick atmosphere (about 90 times the mass of Earth's atmosphere). Further, that atmosphere is about 96.5% carbon dioxide. Carbon dioxide is already pretty infamous as a "greenhouse gas" courtesy of global warming here on Earth. So now imagine an atmosphere with about one hundred thousand times more carbon dioxide than we have here on Earth causing our warming. That's going to be pretty damn hot.
What exactly do greenhouse gases do? Below I have included a figure illustrating the physical processes that cause the greenhouse effect on Earth. The yellow beam represents the incoming radiation from the Sun, and the red beams show infrared radiation coming from various things on Earth. You may notice that some of the radiation is reflected by Earth's surface and even more is reflected from Earth's clouds (this is a very important process whose consequences I'll describe later). The reflectivity of the planet is also going to be important because it determines how much radiation is actually absorbed by the planet. This absorbed energy is what warms the planet in question (the Earth, in this case).
|Image credit: http://see-the-sea.org|
In summary, just through what we've talked about so far, we've identified some important factors in determining whether or not a planet is habitable.
1) Planets closer to their stars will be hotter.
2) Planets whose atmospheres (for whatever reason) have stronger greenhouse effects will be hotter.
3) Planets who reflect less radiation (absorb more radiation) will be hotter.
The last factor we really need to account for is the star around which the planet is revolving. Planets whose stars are brighter will have higher temperatures. A brighter star also makes the boundaries of the habitable zone farther away from the star, and makes the habitable zone itself larger. This effect is illustrated below. Be sure to note the logarithmic distance axis that appears to compress the blue stripe denoting the habitable zone for higher mass (higher brightness) stars. This is in appearance only. If you look at the numbers corresponding to the inner and outer edges, you can see that the habitable zone is wider around more massive/brighter stars.
|Credit: Chester (Sonny) Harman|
What Dr. Kopparapu did was create a model of a planet and the interactions that occur in that planet's atmosphere. Then, as part of the model, he included a stellar model and the ability to put the planet at various distances from the star. Fundamentally, this is the same as James Kasting's aforementioned 1993 paper. The main difference, however, is the atmospheric model. Kopparapu et al. (2013) used new absorption coefficients for the greenhouse gases in the atmospheric model (carbon dioxide and water vapor), along with new calculations Rayleigh scattering by water vapor and collision-induced absorption (as described in Section 2.1). Using the atmospheric model, they determined the distances of the inner and outer edges of the habitable zone around the sun (and around main sequence stars in general).
The inner edge of the habitable zone can be calculated in two different ways. The first is to find the "moist greenhouse limit", which is the distance at which the water vapor content in the stratosphere increases dramatically due to increased evaporation rates. When your stratosphere becomes water-rich, the planet can begin to lose its water to space (through a handful of interactions that I won't touch on here). This limit has gone from 0.95 AU to 0.99 AU for our Sun (keep in mind, Earth is, on average, 1 AU away from the Sun). The second way to determine the location of the inner edge is the "runaway greenhouse limit", at which point Earth becomes trapped in a runaway greenhouse-like scenario akin to that of Venus. This limit has gone from 0.84 AU to 0.97 AU, which is quite a change!
The outer edge of the habitable zone occurs where carbon dioxide in the atmosphere begins to condense out such that it can no longer contribute to greenhouse warming. The outer limit from the new models has moved outward a small amount, from 1.67 AU to 1.70 AU. It is noted, however, that this is a conservative limit, as it doesn't account for warming by carbon dioxide clouds.
Actually, this model doesn't really account for clouds, because it is a one-dimensional simulation (assumes spherical symmetry), and the phenomena that give rise to clouds require far more complicated three-dimensional simulations. Because clouds are the main contributors to a planet's reflectivity (albedo), they do have to be included in the initial calculations. This is solved by, as Dr. Kasting is fond of saying, "painting the clouds on the ground." This simply means the cloud albedo is assumed and just included in the general reflectivity calculations, but the amount of "cloud cover" remains constant.
Making cloud cover constant does have some important consequences on the range of the habitable zone. Specifically, not having dynamic clouds will shrink the habitable zone from both ends. On the inner edge, water vapor clouds will build up and reflect more incoming light, which will push the inner edge a bit closer to the Sun. On the opposite end, as stated above, carbon dioxide clouds would move the outer edge farther away from the Sun, also making the habitable zone larger overall.
Phew. That was an awful lot to write, and there is way more in the paper than I could possibly cover. If you're really interested in the other stuff, like habitable zones around other types of stars, read the paper (link provided above. It's well-written and, I thought, very easily readable). I will also shamelessly plug Ravi's habitable zone calculator applet on his website.
Tomorrow, I will write about the paper that was just released on the arXiv today, also by Dr. Kopparapu. It deals largely with some of the consequences of this paper's work, so I'll save that discussion for another time!