In the past few years, interest has grown concerning the “habitable zone” around the sun and other stars. The habitable zone is the region in space where a rocky planet similar to Earth can maintain liquid water on its surface. We define it that way because life — all life that we know about, at least — requires liquid water during at least part of its life cycle.
This topic has heated up in the past few years because NASA’s Kepler Space Telescope has been counting planets in the habitable zones of other stars. Kepler monitored the brightness of approximately 160,000 stars continuously from 2009 to 2013, ending when it lost a critical reaction wheel (gyroscope) and could no longer point accurately.
During that time, Kepler detected more than 3,500 planets orbiting more than 2,600 stars. Kepler found planets by looking for the slight decrease in a star’s brightness when a planet passes in front of it. Of the 3,500 detected objects, about two-dozen are rocky planets orbiting within the habitable zones of their parent.
Based on the results so far, at least 10 percent of sun-like stars harbor rocky planets within their habitable zones. This rises to roughly 50 percent for stars much less massive than the sun, perhaps because Earth-sized planets are easier for Kepler to see.
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Our own research group recently has used one-dimensional climate models to re-evaluate the boundaries of the habitable zone, because of its importance to the Kepler estimates. The revised limits are shown in the figure.
The vertical scale shows the effective surface temperature of the star. Our sun is a G star with a surface temperature of about 5,780 Kelvin (9,945 degrees Fahrenheit). Stars that are more massive than the sun also are hotter, and stars that are less massive are cooler. Small, red dwarf stars (M stars) are much cooler than the sun and are also much dimmer. Orangish K stars lie somewhere in between.
The horizontal scale in the figure, labeled “effective solar flux,” represents the flux of light from the star relative to the flux at Earth’s orbit. Earth, which orbits the sun at a distance of 1 astronomical unit is thus located at “effective solar flux” of 1. Venus and Mars also are shown in the figure. The solar flux increases (or decreases) as one moves closer to (or farther from) the sun.
The colored curves show different estimates for the inner and outer boundaries of the habitable zone. The “moist greenhouse” and “runaway greenhouse” limits represent, respectively, the distances at which a planet would either begin to lose its water or at which the oceans would evaporate completely.
As one can see from the figure, the moist greenhouse limit for the sun appears to lie perilously close to Earth’s present orbit, suggesting that Earth’s climate is on the verge of becoming unstable. A recent three-dimensional climate calculation by Jeremy Leconte, at the Pierre Simon Laplace Institute in Paris, and his team (seen in the small red planet just to the left of Earth) shows, however, that this is not the case. Lower relative humidity in the 3-D model compared to the 1-D model helps to stabilize Earth’s climate against what’s known as the runaway greenhouse effect — when rising water vapor levels in the atmosphere cause temperatures to climb to extraordinarily high values.
The green “recent Venus” curve represents an inner edge to the habitable zone, based on the fact that Venus appears to have lost its water more than 1 billion years ago. The sun was about 8 percent less bright at that time, which is why that limit lies to the right of the present planet. The blue and orange curves represent theoretical and empirical limits on the outer edge of the habitable zone. The “maximum greenhouse” limit assumes that H2O and CO2 are the two main greenhouse gases, as on Earth. According to these calculations, Mars is within the habitable zone today. It is not habitable, though, because the planet is small and volcanically inactive; thus, it has no way to keep replenishing the CO2 in its atmosphere, as happens on Earth.
Finally, the two planets at the lower left of the diagram represent theoretical limits calculated by researchers including Jun Yang, at the University of Chicago, for tidally locked planets orbiting K and M stars. Tidally locked planets always show the same face to the star, as the moon does to the Earth. In that climate model, the starlit side of such a planet is perpetually cloudy, causing most of the incident starlight to be reflected into space.
So, planets orbiting K and M stars could conceivably be habitable even if they absorb considerably more starlight than does Earth. This hypothesis could conceivably be tested within the next 10 years using data from NASA’s James Webb Space Telescope, which is scheduled for launch in 2018. This telescope will be able to obtain spectra of transiting planets and may be able to determine if they are habitable and if they actually support life.