The Kepler team has released its analysis of the mission’s first three years of observations. The haul includes 10 Earth-size (and probably rocky) exoplanets in their stars’ habitable zones, and the stats show such planets are common.
Kepler can now officially boast finding more than 3,500 extra-solar planet candidates. The announcement came this week when the Kepler team released their analysis of data from the first three years of observation, adding the third year to data from years one and two (previously released).
According to this new analysis, researchers estimate about 70% of stars are host to at least one planet, making planets a common cosmic occurrence. There are now 1,750 candidates that are super-Earth-size or smaller, and 1,788 are Neptune-size or larger. Only 167 of the 3,538 candidates are confirmed to be planets, but Kepler has a good track record: the vast majority of these are probably real.
This plethora of information is in a public archive used not only by the Kepler team but also by others looking for planets, such as the crowdsourcing program Planet Hunters. From 2009 until its fateful failure earlier this year, the craft gathered observational information on 150,000 stars, yielding over 21 million light curves, the graphs that show how a star’s brightness changes with time. These light curves reveal the periodic dips caused by a transiting planet.
Since its beginning, the variety of Kepler’s discoveries (both confirmed and unconfirmed) has been astonishing. Some exoplanets have no equivalent in our own solar system. Kepler has found a world with the density of Styrofoam, worlds larger than Jupiter, and worlds around binary stars. There’s even a world with a size on par with Mercury which seems to be evaporating. These distant planets have orbital periods ranging from a few hours to many years.
The new analysis adds 838 planetary candidates to the existing Kepler catalog. Two dozen of these new candidates orbit in their parent star’s putative habitable zone — defined for simplicity’s sake as the region in space where there’s the right amount of starlight to create friendly temperatures on an Earth-like planet. Of those, 10 are less than double Earth’s size and possibly rocky in composition. With these additions, we now have a total of 674 Earth-size candidates.
The new work yields a 78% increase of Earth-size planet candidates compared with the team’s release from this past January, which only used observations from the first two years. Given the larger data set, researchers estimate that one in five Sun-like stars host at least one rocky planet orbiting in their habitable zone. That ratio is about the same as what some astronomers have calculated for the prevalence of rocky planets in orbits smaller than Mercury’s, as well as the prevalence of rocky planets around cool, small stars called M dwarfs.
When Kepler began, it was mostly detecting gas giants in tight orbits around their stars, simply because such planets are easiest to find. The reason for the recent increase in Earth-size candidates is due not only to having more observations in hand, but also to refinements in researchers’ vetting and analysis — in other words, researchers are getting better at detecting Earth-sized planets. For this reason, the team postulates that the final, fourth year of data (which is still unexplored) will be the most important in the search for small exoplanets.
While Kepler ended its fruitful planet-hunt in May 2013 when a second of the telescope’s four reaction wheels failed for good, all hope it not lost. The team is drafting plans for a new mission called K2, which would repurpose the space telescope in spite of its defects to continue the exoplanet hunt.
Since the problem is one of precision and stabilization, the plan is to point the telescope at the ecliptic, the plane drawn by Earth’s orbit. In this position, the pressure from solar radiation will act as a virtual third wheel, balancing out the remaining two reaction wheels to enable a pointing precision that (the team hopes) will be on par with what Kepler had in its full-functioning state. K2 would search for planets around smaller, cooler dwarf stars and in a range of stellar environments not in Kepler’s previous view, such as star clusters and nurseries. The team will submit their proposal in the next few months.