[LINK] "Kepler Statistical Analysis Suggests Earthlike Planets Extremely Rare"

[rfmcdonald]

Space Daily's John Rehling shares news of a recent analysis of data from the Kepler space telescope, charting extrasolar planets by the thousands, which suggests that Earth-like worlds are--contrary to earlier analyses--relatively rare. The Kepler program, which basically detected planets via their eclipses of their stars, was biased towards the detections of close-orbiting worlds and of large worlds (larger than the Earth). It turns out that doesn't mean that Earth-like worlds, of the right size and in the right orbit, are common.

This release shows two favorable trends. As a statement from the Kepler team puts it, "With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant."

However, the fine details of this latest release favor more pessmistic projections. Although the release shows, in comparison to the previous release, one favorable trend regarding planet size and another favorable trend regarding planet distance from the star, these trends are, unfortunately, unfolding in an either-or respect: We see more Earth sized planets which are very close to their stars, and therefore likely very hot; and, separately, we see more giant planets which are located farther out from their stars.

This table shows the estimated frequency of planets per star in each bin [category]. The planet sizes are presented in terms of Earth radius, Re, while the orbital periods are in days (d). The bins are populated with frequency estimates based on the de-bias factor, times the observed candidate count, where that count was 4 or greater. In addition, more tentative numbers appear in parentheses in certain bins, based on a smaller n or the n that would have resulted from a single discovery. In each row, the maximum value appears in bold.

This shows that for each terrestrial planet size catgeory, we have observed the frequency max out at a very short period between 4 and 16 days, then exhibit decreasing frequency for longer periods.

[. . .]

For the bin corresponding precisely to the Earth, the projected frequency is 0.7%, a far cry from Traub's projection of 34%, owing in part to differences in the size of the bin: If we consider the 3x3 collection of bins that surround Earth's bin, the current projection rises to 7.8%. By including also the smaller Mars-sized planets, to 9.0%.

This does disprove some earlier optimistic estimates, although these estimates were made without any data at all.

In a memorable scene of Carl Sagan's Cosmos, Sagan considers the Drake Equation and estimates that perhaps 1/4 of all stars have planets and that each such system might have two planets suitable for the development of life. That estimate was recorded before any planet outside our solar system had been discovered.

With the current Kepler release, we find that the number of earthlike planets per star is likely to be considerably lower than the 0.5 implied by Sagan's estimate. Of course, the picture is considerably complicated: We may find habitable worlds, whether earth-like or Europa-like, orbiting giant planets. We may find earth-like worlds harbored as Trojans of giant planets in the habitable zone.

The frequency as a function of orbital period may have a second peak or flatten and fail to drop with still longer periods. Comfortable temperatures may also be found at planets close in to dim red dwarf stars, although that may likely result in the world being tidally locked, and therefore subject to one daylight hemisphere and one in eternal night. And if nothing else, the pessmistic characteristic of these results suggest that to find earth-like worlds elsewhere, we should prepare to look hard - and quite possibily very hard for decades if not centuries.