Kepler Team Releases Catalog with 219 new Exoplanet Candidates

Artist's concept of a mini-planetary system. Image credit: NASA/JPL-Caltech

The story of the Kepler space telescope is a saga of discovery, heartbreak, and redemption. Launched in 2009, Kepler's mission was to search for Earth-size and smaller planets orbiting nearby stars, and to estimate how many stars in the Milky Way have such planets. Within the first few weeks of observations, five previously unknown exoplanets were found orbiting close to their parent star. Over the next few years, thousands of planetary candidates were discovered.

In July of 2012, one of the telescope's four reaction wheels failed; these are a type of flywheel that keep the spacecraft pointed at its target, and the telescope needs three to function properly. In May of 2013 a second reaction wheel failed, ending new data collection for the original mission and putting the continuation of the mission into jeopardy.

In November of 2013, a new mission plan dubbed K2 "Second Light" was devised - by balancing "light pressure" from the Sun on spacecraft's solar panels to act as a virtual reaction wheel, and using the two remaining working reaction wheels, the spacecraft would be able to continue collecting data. In early 2014 this method was successfully tested, and in March of 2014 data collection resumed.

The conception illustration depicts how solar pressure can be used to balance NASA's Kepler spacecraft, keeping the telescope stable enough to continue monitoring distant stars in search of transiting planets. Image credit: NASA Ames/W Stenzel

On June 19, 2017, the Kepler team announced the discovery of 50 potentially habitable planets orbiting nearby stars, and the unexpected statistic that smaller planets seem to fall into two distinct size categories: Earth/Super-Earth size, and mini-Neptunes.

The K2 mission is funded through the end of 2019; on-board fuel is expected to be depleted by that time. Data analysis will likely continue for quite some time.

Kepler Space Telescope. Credit: NASA Eyes on the Solar System / Bob Trembley

NASA/JPL Press Release 2017-168:
NASA's Kepler space telescope team has released a mission catalog of planet candidates that introduces 219 new candidates, 10 of which are near-Earth size and orbiting in their star's habitable zone, which is the range of distance from a star where liquid water could pool on the surface of a rocky planet.

This is the most comprehensive and detailed catalog release of candidate exoplanets, which are planets outside our solar system, from Kepler's first four years of data. It's also the final catalog from the spacecraft's view of the patch of sky in the Cygnus constellation.

With the release of this catalog, derived from data publicly available on the NASA Exoplanet Archive, there are now 4,034 planet candidates identified by Kepler. Of those, 2,335 have been verified as exoplanets. Of roughly 50 near-Earth size habitable zone candidates detected by Kepler, more than 30 have been verified.

Additionally, results using Kepler data suggest two distinct size groupings of small planets. Both results have significant implications for the search for life. The final Kepler catalog will serve as the foundation for more study to determine the prevalence and demographics of planets in the galaxy, while the discovery of the two distinct planetary populations shows that about half the planets we know of in the galaxy either have no surface, or lie beneath a deep, crushing atmosphere -- an environment unlikely to host life.

The findings were presented at a news conference Monday at NASA's Ames Research Center in California's Silicon Valley.

"The Kepler data set is unique, as it is the only one containing a population of these near Earth-analogs - planets with roughly the same size and orbit as Earth," said Mario Perez, Kepler program scientist in the Astrophysics Division of NASA's Science Mission Directorate. "Understanding their frequency in the galaxy will help inform the design of future NASA missions to directly image another Earth."

The Kepler space telescope hunts for planets by detecting the minuscule drop in a star's brightness that occurs when a planet crosses in front of it, called a transit.

This is the eighth release of the Kepler candidate catalog, gathered by reprocessing the entire set of data from Kepler's observations during the first four years of its primary mission. This data will enable scientists to determine what planetary populations -- from rocky bodies the size of Earth, to gas giants the size of Jupiter -- make up the galaxy's planetary demographics.

To ensure a lot of planets weren't missed, the team introduced their own simulated planet transit signals into the data set and determined how many were correctly identified as planets. Then, they added data that appear to come from a planet, but were actually false signals, and checked how often the analysis mistook these for planet candidates. This work told them which types of planets were overcounted and which were undercounted by the Kepler team's data processing methods.

"This carefully-measured catalog is the foundation for directly answering one of astronomy's most compelling questions - how many planets like our Earth are in the galaxy?" said Susan Thompson, Kepler research scientist for the SETI Institute in Mountain View, California, and lead author of the catalog study.

One research group took advantage of the Kepler data to make precise measurements of thousands of planets, revealing two distinct groups of small planets. The team found a clean division in the sizes of rocky, Earth-size planets and gaseous planets smaller than Neptune. Few planets were found between those groupings.

This diagram illustrates how planets are assembled and sorted into two distinct size classes. First, the rocky cores of planets are formed from smaller pieces. Then, the gravity of the planets attracts hydrogen and helium gas. Finally, the planets are "baked" by the starlight and lose some gas. At a certain mass threshold, planets retain the gas and become gaseous mini-Neptunes; below this threshold, the planets lose all their gas, becoming rocky super-Earths. Image credit: NASA/Ames Research Center/JPL-Caltech/R. Hurt

Using the W. M. Keck Observatory in Hawaii, the group measured the sizes of 1,300 stars in the Kepler field of view to determine the radii of 2,000 Kepler planets with exquisite precision.

"We like to think of this study as classifying planets in the same way that biologists identify new species of animals," said Benjamin Fulton, doctoral candidate at the University of Hawaii in Manoa, and lead author of the second study. "Finding two distinct groups of exoplanets is like discovering mammals and lizards make up distinct branches of a family tree."

It seems that nature commonly makes rocky planets up to about 75 percent bigger than Earth. For reasons scientists don't yet understand, about half of those planets take on a small amount of hydrogen and helium that dramatically swells their size, allowing them to "jump the gap" and join the population closer to Neptune's size.

The population of exoplanets detected by the Kepler mission (yellow dots) compared to those detected by other surveys using various methods: radial velocity (light blue dots), transit (pink dots), imaging (green dots), microlensing (dark blue dots), and pulsar timing (red dots). For reference, the horizontal lines mark the sizes of Jupiter, Neptune and Earth, all of which are displayed on the right side of the diagram. The colored ovals denote different types of planets: hot Jupiters (pink), cold gas giants (purple), ocean worlds and ice giants (blue), rocky planets (yellow), and lava worlds (green). The shaded gray triangle at the lower right marks the exoplanet frontier that will be explored by future exoplanet surveys. Kepler has discovered a remarkable quantity of exoplanets and significantly advanced the edge of the frontier. Image credit: NASA/Ames Research Center/Natalie Batalha/Wendy Stenzel

The Kepler spacecraft continues to make observations in new patches of sky in its extended mission, searching for planets and studying a variety of interesting astronomical objects, from distant star clusters to objects such as the TRAPPIST-1 system of seven Earth-size planets, closer to home.

Ames manages the Kepler missions for NASA's Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

Highlighted are new planet candidates from the eighth Kepler planet candidate catalog that are less than twice the size of Earth and orbit in the stars' habitable zone – the range of distances from a star where liquid water could pool on the surface of an orbiting planet. The dark green area represents an optimistic estimate for the habitable zone, while the brighter green area represents a more conservative estimate for the habitable zone. The candidates are plotted as a function of their stars' surface temperature on the vertical axis and by the amount of energy the planet candidate receives from its host star on the horizontal axis. Brighter yellow circles show new planet candidates in the eighth catalog, while pale yellow circles show planet candidates from previous catalogs. Blue circles represent candidates that have been confirmed as planets due to follow-up observations. The sizes of the colored disks indicate the sizes of these exoplanets relative to one another and to the image of Earth, Venus and Mars, placed on this diagram for reference. Note that the new candidates tend to be around stars more similar to the sun – around 5,800 Kelvin – representing progress in finding planets that are similar to the Earth in size and temperature that orbit sun-like stars. Image credit: NASA/Ames Research Center/Wendy Stenzel

For more information about the Kepler mission, visit:

News Media Contacts

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.

Felicia Chou
NASA Headquarters, Washington

Michele Johnson
Ames Research Center, California's Silicon Valley

Further Reading:

Stellar Radiation Pressure:
NASA Exoplanet Archive:
Kepler Mission Homepage:
Kepler Mission on Wikipedia:
Educational Resources:
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