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Kepler discovers 10 Earth-like exoplanets from 219 planet candidates

Kepler mission. Mini Planetary System

NASA’s Kepler space telescope team has identified 219 new planet candidates, 10 of which are near-Earth-size and in the habitable zone of their star. Image & Caption Credit: NASA / JPL-Caltech

NASA’s Kepler mission has released the most extensive list of exoplanet findings: a total of 219 planet candidates, of which ten are probably Earth-like and occupy their stars’ habitable zones – where temperatures allow liquid water to exist on the surface.

The eighth and final data release of the original, four-year Kepler mission brings the total number of exoplanet candidates found by the telescope to 4,034. Of these, 2,335 have been confirmed to be planets, and 50 are located in habitable zones and are roughly Earth-sized.

Launched in March 2009, Kepler spent four years observing stars in the constellation Cygnus by using the transit method, which involves searching for regular dimming of stars’ light as orbiting planets transit or pass in front of the stars. However, the failure of the spacecraft’s second of four reaction wheels in May 2013 put an end to that part of its mission, which had been expected to be extended until 2016.

Kepler’s latest findings – announced at a news conference on Monday, June 19, held at the NASA Ames Research Center in California – also revealed the existence of two divergent categories of small planets. The first are gaseous worlds with no known solid surfaces, dubbed “mini-Neptunes”, while the second are rocky planets sometimes described as “super-Earths”.

Kepler-452b / Kepler-22b histogram

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 Earths and super-Earths (similar to Kepler-452b), and the mini-Neptunes (similar to Kepler-22b). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. Image & Caption Credit: NASA / Ames / Caltech / University of Hawaii (B. J. Fulton)

Using the Kepler data, one group of scientists observed 1,300 stars and 2,000 planets that the telescope found with the W.M. Keck Observatory in Hawaii, with the goal of obtaining precise measurements of the planets’ sizes. Their studies confirmed the presence of the two distinct small planet types.

Rocky planets appear to have a size limit at around 75 percent larger than the Earth. For reasons not well understood, some small planets accumulate hydrogen and helium, swelling out to become gaseous worlds with heavy atmospheres and no known solid surfaces. The latter are not good locations to search for life.

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

Kepler mission. Planet factory assembly line graphic

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 & Caption Credit: NASA / Kepler / Caltech (R. Hurt)

Particular attention was given to one approximately Earth-sized world discovered orbiting in the habitable zone of its Sun-like star. Designated KOI-7711, this planet candidate resembles Earth in both its orbit and size, but the composition of its atmosphere and its ability to host liquid water on its surface is unknown.

In the Solar System, Venus, Earth, and Mars are all located in the habitable zone, yet only Earth is capable of supporting life, the scientists noted.

“There’s a lot we don’t know about this planet,” Kepler scientist Susan Mullally said, noting it is premature to refer to it as an “Earth twin”.

In the past, planets discovered by Kepler were initially thought to be habitable only to later be found inhospitable due to phenomena such as bombardment by stellar flares.

Kepler Habitable Zone Planets

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 & Caption Credits: NASA / Ames Research Center / Wendy Stenzel

To address the possibilities of both false positives and failure to identify actual planets, the Kepler team analyzed the data by combining it with software simulations that added false signals and deliberate misses of known planets. Mixing real data with simulated information accurately predicted both overcounts and undercounts.

“This carefully measured catalog is the foundation for directly answering one of astronomy’s most compelling questions – how many planets like Earth are in the galaxy?” said SETI research scientist and catalog study lead author Susan Thompson.

“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, a Kepler scientist in NASA’s Science Mission Directorate Astrophysics Division. “Understanding their frequency in the galaxy will help inform the design of future NASA missions to directly image another Earth.”

New Kepler Planet Candidates as of June 2017

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. Image & Caption Credit: NASA / Ames Research Center / Wendy Stenzel

None of the new data comes from the revamped K2 mission, for which the Kepler telescope is searching other parts of the sky beyond Cygnus. K2’s most recent discovery over 100 exoplanets was announced in July 2016.

The search for other exoplanets, in general, and another Earth, in particular, will continue with NASA’s Transiting Exoplanet Survey Satellite (TESS) and with the James Webb Space Telescope, both scheduled for launch in 2018.

Thompson, Fulton, Perez, and Courtney Dressing, a NASA Sagan fellow at Caltech, participated in the news conference.

All exoplanet candidates and confirmed planets are listed in NASA’s Exoplanet Archive online.

Kepler mission. Exoplanet Populations

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 & Caption Credit: NASA / Ames Research Center / Natalie Batalha / Wendy Stenzel



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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