A Sea of Blue Planets?

This week saw the second Kepler Science Conference, which took place at NASA's Ames Research Center in Moffett Field, California, where the latest analyses of data from the Kepler Space Telescope were discussed. The meeting, comprising of nearly 400 scientists from 30 different countries, produced some seriously exciting findings, including the prevalence of planets around Sun-like stars with the potential to host life.

Scientists from the University of California and the University of Hawaii have for the first time statistically determined that 22±8% of Sun-like stars possess Earth-sized planets within the habitable zone, from compiling data from Kepler and the W. M. Keck Observatory. The estimate that one in five stars similar to the Sun host temperate planets further suggests that the closest one could reside just 12 light-years from us, according to Erik Petigura, a UC Berkeley graduate student who led the analysis of Kepler and Keck data. This could mean there are billions of Earth-like worlds in the Milky Way, according to the report published on November 4th in the Journal Proceedings of the National Academy of Sciences.

The overall count of Kepler candidate planets from the first three years now stands at 3538, a number which has increased by 29% since January 2013. This rapid increase in planets has allowed a new comprehensive investigation into habitable zone bodies, which was a primary mission objective of Kepler.

Of the 833 new candidate planets announced this week by the Kepler team, ten are less than two Earth radii and reside in the habitable zone of their parent stars. This adds to the list of five Kepler planets already confirming this criteria, and brings the grand total to 12 confirmed and 38 candidate planets that may be potential habitable worlds.

Top of the list of possible Earth-like planets is Kepler-62 e, which was discovered earlier this year and has been issued with an Earth Similarity Index (ESI) value of 0.83. Residing 1200 light-years away in a system of five planets, it is believed to orbit the inner part of the habitable zone. Neighbouring body, Kepler-62 f, is ranked as #10 on the Planetary Habitability Laboratory's list of most habitable planets with as ESI of 0.67, and is thought to orbit the outer edge of the zone. With a radius of 1.6 RE and estimated surface temperature of 31 degrees Celsius, many researchers have postulated Kepler-62 e to possess a silicate-iron composition similar to the Earth's and possible oceans present on the surface. A recent paper submitted by Kaltenegger et al. discusses the potential environments of both bodies in detail: http://arxiv.org/abs/1304.5058.

Of course, just because a world lies within this zone it doesn't necessarily mean that it's habitable. The interior and atmospheric composition of the planet might not possess the key elements required to create or sustain life, or conversely could contain chemicals that prevent it from flourishing. There is also a long list of other factors that affect habitability, such as geochemistry, atmospheric structure and dynamics, orbital dynamics, evolution and variability of the host star, the evolutionary path of the planet itself, and also external factors such as bombardment. On the other hand, we also know that habitability cannot simply be restricted to the environment that applies to us, and that life may exist below the surface, or on moons orbiting large gas planets, far beyond the edges of the habitable zone.

Another exciting topic covered at the recent Kepler Science Conference was the future of the instrument itself. Without enough reactor wheels operating to continue detailed observations over a long time period, Kepler's future was thought to be bleak, but a recent evaluation suggests it may not have seen the last of its planet hunting days. The instrument is still able to point accurately at any patch of sky, but without its precision steering capabilities is unable to control its own rotation. The disturbance causing Kepler to spin is solar pressure, which falls unevenly on the craft and so gradually influences it to turn. The Kepler II team have found that when correctly positioned, the instrument can receive almost even amounts of sunlight on both sides and remain relatively stable for 2-3 months at a time, before the Sun obscures its view. As Kepler must detect at least three orbits of a planetary candidate, this means it will now only be searching for planets with much shorter orbital periods of 20-30 days. These periods correspond to bodies that reside much closer to a star than that of Mercury to the Sun; however, temperate planets can still be discovered within this constraint around dimmer red dwarf stars, where the habitable zone lies much nearer. This means that Kepler may continue its search for hospitable worlds but around a different target list of stars where we could discover an even more exotic array of planets. The proposal will be put to NASA soon for the funding evaluation process to begin, meanwhile tests have already begun on the new steering method. 

On top of this potential new endeavour, there is still over a year's worth of collected Kepler data yet to be analysed, from which hundreds more candidates are expected to arise.