Calculating the Odds of Habitable Worlds Nearby

Categories: Cosmic Evolution

This shows starlight on planets relative to sunlight on the Earth. Credit: Chester Harman

According to a recent news release by Penn State University, there could be more planets with the potential for life than astronomers had previously thought.

Some of those planets may be orbiting cool, M-dwarf stars that are relatively close to our own solar system. Such low-mass M-dwarfs are more common than stars like our Sun.

"Based on the actual planet candidate number from NASA’s Kepler satellite data, I estimated that nearly 48 percent of low-mass, cool stars should have Earth-size planets in the habitable zones," Ravi Kopparapu post-doctoral researcher in geosciences and author of a paper accepted for publication in Astrophysical Journal Letters, told Astrobiology Magazine. "That is a conservative estimate," he added. "There could be more."

The study used habitable zone limits calculated in 1993 by Jim Kasting, which determined the a region around a star where rocky planets are capable of sustaining liquid water and therefore life.

Click here to read the press release.

Astrobiology Magazine contacted both Ravi Kopparapu and James Kasting for additional comments about the new research. Below are their comments:

James Kasting:
referring to the fact that this study was based on previous research by a team at Harvard (Dressing &Charbonneau):

Jim Kasting. Credit: Penn State

“Dressing and Charbonneau (D&C) used the "first CO2 condensation" limit for the outer edge of the habitable zone (HZ). This is the distance at which CO3 starts to condense to form CO2 clouds in a planet’s atmosphere. That was the pessimistic limit suggested in my 1993 HZ paper (in ICARUS). But we’ve known for at least 15 years that this estimate was overly pessimistic, because CO2 ice clouds generally warm a planet’s surface, instead of cooling it, as we had assumed (See Forget and Pierrehumbert, Science,1997). So what D&C should have done is to use the "Maximum Greenhouse" limit for the HZ outer edge. It’s quite a bit farther out (1.67 AU for our Sun, compared to 1.37 AU), and so the chances that a planet will be inside it are correspondingly greater.

D&C also used an upper limit of 1.4 Earth radii to define an "Earth-like" planet. That, I think, is how the Kepler team defines them. But what the Kepler team calls "Super-Earths", which have a radii of 1.4 – 2.0 times Earth, should probably also be counted when evaluating Eta_Earth (the frequency of rocky planets with the HZs of their parent stars). A 2 Earth radius planet should be about 10 Earth masses, which we think is roughly the upper limit for a rocky planet. When Ravi uses 2 Earth radii as the upper limit, he finds several more potential habitable planets in the Kepler dataset.

The conservative estimates are for the theoretical “moist greenhouse” and “maximum greenhouse” HZ limits. The optimistic estimates are for the empirical “early Mars” and “recent Venus” limits. These are defined in my 1993 paper. Ravi’s estimates go from about 0.48 to 0.61, depending on what size range you choose for the planet and whether you use the conservative or optimistic limits. So, D&C are at 0.15, or roughly 1/6th, and Ravi is at roughly 0.5, or one-half. That’s a big difference when you’re trying to design a telescope like TPF to look for these planets.”

According to Kopparapu, the newer estimates are based on an updated model developed by him and his and collaborators, using information on water and carbon dioxide absorption that was not available in 1993.

Ravi Kopparapu:

Ravi Kumar Kopparapu, post-doctoral researcher in geosciences at Penn State University. Credit: Penn State

“Based on the actual planet candidate number from NASA’s "Kepler" satellite data, I estimated that nearly 48% of low-mass, cool stars ("M-dwarfs") should have Earth-size planets in the habitable zones. As an illustration of what I actually did, attached with this email is an image that shows the number of actual planet candidates that were detected and are in the habitable zones of their stars. The yellow objects have sizes between half to 1.4 times Earth-radius. Since I know how many planets were detected in the habitable zone around given number of stars, I could then calculate how many planet candidates are there per star. As an example, if I observe 10 houses uniformly distributed in a town and 4 of those houses have swimming pools, I estimate (without knowing anything else, like income etc.) that in general there are 0.4 swimming pools per house in the whole town.

In our new climate model, we updated how strongly greenhouse gases such as water & carbon dioxide absorb light coming from the star. No one has updated the old climate model (from 1993) to take into account the advances made in the absorption of these gases. Since it was not updated, the water and carbon dioxide gases in the old model were not absorbing the star radiation effectively. This will result in not estimating the habitable zones accurately (they will be closer to the star because the star radiation will not heat the surface as much). But after taking into account most recent absorption values in our new climate model for water and carbon dioxide, we found that the habitable zones moved farther away from the star (because now the star radiation is absorbed easily so the surface becomes warmer easily).

The Harvard team’s study (Dressing & Charbonneau) applied old model results and found two planets in the habitable zone (in the attached figure, they are numbered as 1422.02 and 2626.01). In fact, they used *very* conservative estimates of the habitable zone from the old model. The habitable zones are much wider than the ones they used. With our new habitable zones, there are two additional planets (total four, in the attached figure, yellow objects in the blue shaded region) in the habitable zone. Therefore, the occurrence rate of Earth-sized planets in the habitable zone increased."


For a bit more on habitable zones and life detection, here’s a very informative video from James Kastings:

Penn State distinguished professor of geosciences James Kasting discusses "How to Find a Habitable Planet" at Weber State University on March 10, 2011.


Now see if you can pass the Pop Quiz:

According to the new study, if you looked at 20 of the nearest small stars to Earth, how many ‘potentially habitable’ planets would you find?
8, give or take