NASA’s Kepler Space Telescope Identifies 219 New Planet Candidates

Astronomers have identified 219 new planet candidates, 10 of which are near-Earth size and orbiting in their star’s habitable zone.

NASA’s Kepler space telescope team has released a mission catalog of planet candidates that introduces 219 new planet 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 which, 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.

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 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.

Figure 1

Figure 2

Kepler was the first NASA mission capable of detecting Earth-size planets, using the transit method, a photometric technique that measures the minuscule dimming of starlight as a planet passes in front of its host star. For the first four years of its primary mission, the space telescope observed a set starfield located in the constellation Cygnus (left). New results released from Kepler data today have implications for understanding the frequency of different types of planets in our galaxy and the way planets are formed. Since 2014 the Kepler telescope has been taking data on its extended second mission, observing fields on the plane of the ecliptic of our galaxy (right).

Credits: NASA/Ames Research Center/Wendy Stenzel

Studying the Stars with Kepler

Figure 3

This is an illustration of the different elements in NASA’s exoplanet program, including ground-based observatories, like the W. M. Keck Observatory, and space-based observatories, like Hubble, Spitzer, Kepler, TESS, James Webb Space Telescope, WFIRST and future missions.

Credits: NASA

Figure 4

There are 4,034 planet candidates now known with the release of the eighth Kepler planet candidate catalog. Of these, 2,335 have been confirmed as planets. The blue dots show planet candidates from previous catalogs, while the yellow dots show new candidates from the eighth catalog. New planet candidates continue to be found at all periods and sizes due to continued improvement in detection techniques. Notably, 10 of these new candidates are near-Earth-size and at long orbital periods, where they have a chance of being rocky with liquid water on their surface.

Credits: NASA/Ames Research Center/Wendy Stenzel

New Kepler Planet Candidates

Figure 5

In the final full survey of Kepler’s primary mission data, the entire data set was reprocessed using the mission’s most discerning analytical methods – a fully automated process represented here by the blue triangle. More than 4,000 planet candidates, including around 50 approximately the size of Earth and orbiting in the habitable zones of their stars, were identified in the data from observations of 200,000 stars. To be sure 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 really false signals, and checked how often the analysis mistook these for planet candidates. This process makes the eighth Kepler planet candidate catalog the best characterized set of high-confidence potential planets.

Credits: NASA/Ames Research Center/Wendy Stenzel

Figure 6

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.

Credits: NASA/Ames Research Center/Wendy Stenzel

Kepler Habitable Zone Planets

Figure 7

NASA’s Kepler mission is unique in its ability to detect small planets orbiting relatively far from their stars, with relatively long orbital periods of more than 100 days. Kepler’s data will now allow researchers to study the planet populations making up this group (indicated by the green oval) which may include Earth-size planets located in the habitable zone around stars like our sun.

Credits: NASA/Ames Research Center/Wendy Stenzel

Figure 8

This histogram shows the size distribution of planets discovered by NASA’s Kepler mission before the sizes were later refined with the help of the W. M. Keck Observatory. Smaller planets up to four times the size of Earth are more common than larger planets. The small planets originally appeared to span a range of sizes, but more precise size measurements from Keck revealed a scarcity of planets that are about 1.75 times the size of Earth.

Credits: NASA/Ames Research Center/CalTech/University of Hawaii/B.J. Fulton

Figure 9

Researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission have discovered a gap in the distribution of planet sizes, indicating that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earth-size and super-Earth-size (similar to Kepler-452b), and the mini-Neptune-size (similar to Kepler-22b). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth.

Credits: NASA/Ames Research Center/CalTech/University of Hawaii/B.J. Fulton

Small Planets Come in Two Sizes

Figure 10

This sketch illustrates a family tree of exoplanets. Planets are born out of swirling disks of gas and dust called protoplanetary disks. The disks give rise to giant planets like Jupiter as well as smaller planets mostly between the size of Earth and Neptune. Researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission discovered that these smaller planets can be cleanly divided into two size groups: the rocky Earth-like planets and super-Earths, and the gaseous mini-Neptunes.

Credits: NASA/Ames Research Center/JPL-Caltech/Tim Pyle

New Branch in Exoplanet Family Tree

Figure 11

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.

Credits: NASA/Ames Research Center/JPL-Caltech/R. Hurt

Assembly Line of Planets

Figure 12

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.

Credits: NASA/Ames Research Center/Natalie Batalha/Wendy Stenzel