Mar 13 2011
Jet Propulsion Lab scientists recently released calculations indicating that about 2% of Sun-like stars are expected to have “Earth-analog” planets. Joseph Catanzarite and Michael Shao base their estimate on the first 4 months of data (released February, 2011) on planetary transits of 150,000 FGK stars from observations by NASA’s Kepler mission. This is much lower than previous estimates.
The authors’ analysis informs planning for future missions that will study nearby Earth-analog planets, and it also highlights an important trend noticed by 21stCenturyWaves.com that is typical of approaches to 1960s-style golden ages of prosperity, exploration, and technology — e.g., the 2015 Maslow Window — over the last century+:
As we ascend toward another crescendo in human achievement — the 2015 Maslow Window … UFOs are being seen in China and around the world, potentially habitable planets are being discovered around nearby stars, and even the Vatican and the Royal Society are openly planning to properly greet intelligent interstellar visitors. One of the most important NASA missions ever flown — the Kepler spacecraft — will accelerate this ebullient trend in 2011.
Although a habitable zone (HZ) refers to the region where liquid water can exist on a planet’s surface, the fraction of Sun-like stars with Earth-analog planets is a strong function of the adopted HZ boundaries. Catanzarite and Shao define the scaled semimajor axis (mean planetary distance scaled to the square root of its star’s luminosity relative to the Sun) as between 0.95 AU to 1.37 AU (AU is Astronomical Unit = 1 Earth-Sun distance) from Kasting et al. (1993). Because Kasting et al. did not consider clouds (which can cool interior planets) and CO2 (which can warm distant worlds), the authors also consider the more optimistic scaled HZ boundaries of the Exoplanet Task Force Report (2008): 0.8 AU to 1.6 AU.
In addition to HZ boundaries, the JPL scientists’ Earth Analog region is defined by a scaled planetary radius (i.e., relative to Earth’s radius) from 0.8 to 2. The lower value corresponds to a mass of about 50% of Earth’s; the lower limit for retention of an Oxygen atmosphere. The upper value is adopted by the Kepler scientists and, assuming Earth-like parameters, implies a planet with twice the surface heat flow of Earth and half Earth’s lithospheric thickness. Active plate tectonics and volcanism is expected in these super-Earths.
Catanzarite and Shao fit the Kepler transit data to power laws for both the planet radius and the scaled planet distance; they judge that the power laws are excellent fits to the data for distances from 0.2 AU to 0.5 AU (inside the HZ limits) and planetary radii from 2 to 4 (just larger than the EA range). Using the power laws, the Kepler data set is then extrapolated into the Earth analog region defined above.
After removing probable false detections and correcting for the observational effect that not all planets’ orbit planes are in Kepler’s line of site (to produce an observable transit), the authors obtain their surprisingly low value of 2%, +1.6%/- 1.1%, for the fraction of Sun-like stars with an Earth-analog planet.
Although their estimate will become more accurate when the full 3.5 to 6 year Kepler data set is obtained, the authors comment on its surprising implications for planning future missions that will image and take spectra of Earth-analog planets,
Our result that Earths are relatively scarce means that a substantial effort will be needed to identify suitable target stars prior to these future missions.