An international research team led by Livingston, a graduate student at the University of Tokyo, Professor Tamura (the University of Tokyo, Astrobiology Center of National Institutes of Natural Sciences), extremely carefully analyzed raw data (extremely precise measurements of stellar brightness) from the K2 mission (Note 1) by NASA’s Kepler Space Telescope and ESA’s Gaia Space Telescope to select promising exoplanet candidates. They further conducted follow-up imaging and spectroscopic observations of these candidates from ground-based telescopes and newly verified 60 exoplanets. Combined with a previous announcement in August, this team has discovered a total of 104 exoplanets, breaking a new record for the most exoplanet discoveries in Japan. By this report the number of the verified exoplanets by the K2 mission greatly exceeded 300 in total.

This discovery has notable significance, in addition to reporting a large number of exoplanets within a short period of three months, for the bright host stars making detailed follow-up observations of these exoplanets feasibl, for successful discovery of three new “ultra-short period planets” (in total sevel combined with the previous discoveries) with orbital periods less than 24 hours that had been tough to discover, and for increasing the number of multiple planetary systems by 20. Understanding the formation and evolution of these attracting ultra-short period planets is crucial and this discovery of many of nearby, detailed-observable planets is an extremely powerful strength for future expansion of astrobiology in exoplanetary fields.

The research findings, first-authored by Livingston, a graduate student at the University of Tokyo, were published in the Astronomical Journal, a designated astronomy journal in the United States.

Key Points of the Announcement:

• 60 new exoplanets were discovered, totaling 104 exoplanets combined with the previous discoveries, breaking a new record for exoplanet discoveries in Japan.
• Among the discovered exoplanets are ultra-short period planets with orbital periods less than 24 hours and over 20 multiple planetary systems.
• 18 out of the 60 discovered exoplanets are rocky planets smaller than twice the size of Earth.

An international research team including the University of Tokyo and the Astrobiology Center has discovered 60 exoplanets based on observations from NASA’s K2 mission and ESA’s Gaia space telescope. From the K2 data, the team determined the properties and planetary system parameters of these candidate objects by analyzing 155 of them in detail (Figure 1). Due to the brightness of their primary stars, many of these planets are ideal for detailed studies of their compositions and atmospheres. This finding was obtained through precise time-series photometric observations with K2 and precise positional measurements with Gaia, which allowed a much better characterization of the planets and their main stars than previously possible.

This announcement was made by John Livingston, a graduate student at the University of Tokyo, who reported the discovery of 44 exoplanets in August of this year, and together with the 60 exoplanets reported this time, 104 exoplanets were reported in just two consecutive months. 44 was the most at the time, but 104 significantly broke the record for the most exoplanet discoveries in Japan. This was the largest number of exoplanet discoveries in Japan at the time, and the 104 discoveries significantly broke the record for the most exoplanets discovered in Japan.

The first Kepler space telescope (launched in 2009) was terminated in 2013 when the reaction wheel failed. The K2 mission was then launched, reusing the same space telescope and using a different observation strategy to search for exoplanets. The K2 mission also came to an end on October 30, 2018 due to fuel depletion, but it has discovered numerous exoplanets. Livingston said, “With the additional analysis of 227 candidate objects, we estimate that hundreds of exoplanets are still hidden in the K2 data.”

The newly discovered planets include more than 20 multi-planet systems and Ultra-Short Period (USP) planets with a year of less than 24 hours. These ultrashort-period planets have recently begun to attract attention because their formation is shrouded in mystery. Livingston says, “This planetary system provides important clues as to how these ultrashort-period planets formed.” He says. Also of particular importance are the nearby Earth-like small planets that have recently become available for closer examination, “18 of the 60 are less than twice the size of Earth and are likely to be rocky planets with little or no atmosphere,” Livingston says.

The team further confirmed that 18 of the 227 candidates were false detections due to eclipsing binary stars in transit (Note 3). To confirm this, in addition to the K2 and Gaia data, the team used high-resolution imaging observations using “adaptive optics” (Note 4) to cancel out atmospheric fluctuations and “speckle observations” (Note 5) to superimpose many short-exposure images, and high-dispersion spectroscopic observations (Note 6) to characterize the main stars in detail. Our sharp imaging observations can locate a companion star that is extremely close to the main star, and our high-dispersion spectroscopic observations can find the companion star even if it is hidden by the main star,” said Livingston. Livingston said. Such techniques will play an important role in characterizing new planets, and ongoing research will lead to the discovery of many more planets in the future.

Although the K2 mission has come to an end, its role has been taken over by the TESS mission, launched in April 2018, which has begun reporting on exoplanet discoveries based on TESS data. ‘The future is bright for transit planets,’ Livingston said. ‘We already have TESS, and we have JWST coming up soon. We look forward to many exciting planetary discoveries over the next few years.”

This study was published in the November 26, 2018 issue of The Astronomical Journal.

*December 3: The number of exoplanet candidates and the number of planets with short orbital periods have been revised.

用語解説：

Note 1: the K2 mission of the Kepler Space Telescope
NASA’s Kepler Space Telescope, launched in 2009, has discovered over 5000 exoplanets and candidates within a part of the Cygnus constellation. Due to a malfunction in 2013, however, it was repurposed as the “K2” mission. The objects reported by this telescope are merely planet candidates and confirmation and validation through ground-based observations or other methods are necessary. The K2 mission has verified nearly 300 exoplanets to date, but there is a demand for validating more diverse planets.

Note 2: Gaia Space Telescope
EA space telescope launched by the European Space Agency (ESA) in 2013. The objective is to precisely measure the positions of stars and create a detailed three-dimensional map of our Milky Way.

Note 3 Transit method
When a planet passes in front of a star, the light of the star periodically dims. This method is used to find a planet by observing this change in brightness over a long period of time. It is unlikely that a planet will pass “just in front” of a star, so it is necessary to observe a large number of stars. On the other hand, the larger the planet, the greater the change in brightness. By observing many stars, Kepler was able to find thousands of planets.

Note 4 Compensating Optics
This is a technique to obtain sharp images of stars by canceling atmospheric fluctuations in the instrument.

Note 5 Speckle Observation
This is an observation method to obtain sharp images of stars by acquiring a large number of images with a short exposure time and precisely aligning them during data processing. Together with adaptive optics, it is one of the methods to compensate for atmospheric turbulence.

Note 6: Spectroscopic observation
A method of observing celestial objects by using optical elements such as prisms and diffraction gratings to separate light into various wavelengths in order to precisely study the “color (= wavelength)” of celestial light. It can be used to detect close binary stars that cannot be resolved by imaging observations, and in high-precision observations, it can also be used to detect exoplanets (Doppler method).

Research Group:

University of Tokyo, Center for Astrobiology, National Astronomical Observatory of Japan, etc.

Research Support:

This work was supported by Grant-in-Aid for Scientific Research (18H05442) and Grant-in-Aid for Specially Promoted Research (No. 22000005).

Related Links:

NAOJ Press Release

The post Discovery of More than 100 Exoplanets by Collaboration of Space and Ground-based Telescopes first appeared on Astrobiology Center, NINS.

Presentation Points:

• Graduate students at the University of Tokyo led the discovery (and demonstration) of 44 new exoplanets at one time. This is the largest number of discoveries in Japan. Another 27 are also promising planet candidates. The fact that graduate students were able to lead this extremely rare achievement of numerous discoveries, excluding the release of candidate objects in units of 1,000 from the dedicated exoplanet satellite project, shows that exoplanet research is a field in which young people can play an active role.。
• The exoplanets discovered are small planets with an average radius of 2 Earth radii around relatively bright stars, which can be studied in detail in the future. The planets include some smaller than Venus.
• The K2 mission using the Kepler space telescope increased the number of exoplanets discovered in various sky regions by about 10%, demonstrating the importance of collaboration between space telescopes and ground-based observations. In particular, the increase in the number of planets around nearby stars, unlike the planets discovered by the Kepler satellites, is important for future detailed observations.

Presentation Summary:

Launched in 2009, NASA’s Kepler Space Telescope has discovered more than 5,000 exoplanets and their candidates in one region of Cygnus. However, due to a failure in 2013, the telescope has since been utilized for a new mission, K2. The K2 mission has so far demonstrated fewer than 300 planets, but more and more diverse planets are needed to be demonstrated.

This time, an international research team led by Dr. Livingston, Prof. Tamura (Director of ABC Center, The University of Tokyo), and Dr. Narita (Assistant Professor, The University of Tokyo) carefully analyzed the raw data from the K2 mission (ultra-precise measurement of stellar brightness) to select more promising planet candidates, and followed up with ground-based imaging observations and spectroscopic observations of the candidate objects. Follow-up observations were conducted by ground-based imaging and spectroscopic observations of the candidate objects, and as many as 44 exoplanets were successfully identified at one time. This is the largest number of exoplanets ever discovered in Japan. This brings the total number of exoplanets demonstrated by the K2 mission to more than 300. It is noteworthy that graduate students were able to take the lead in this extremely rare achievement of a large number of discoveries, excluding candidate releases from the Kepler satellites, which were in the order of 1,000s.

The number of known and demonstrated exoplanets is approximately 4,000, but this discovery is significant in that it has increased the number of bright small planets. One of these planets is smaller than Venus and is located around a red dwarf. This will be an important target for understanding the formation and evolution of terrestrial rocky planets.

The results of this study were published in the Astronomical Journal, an American journal of astronomy, with a lead author, a graduate student at Tohoku University’s Livingston Graduate School of Astronomy.

Announcement Description:

The Kepler Space Telescope has discovered numerous exoplanets and their candidates in one region of Cygnus. It captures exoplanets using a technique called the transit method. This method uses a wide-field camera to continuously image a region of the sky to capture changes in brightness as a planet crosses the front of a star. Space telescopes, unaffected by Earth’s atmosphere, are capable of precisely capturing minute changes in brightness as small Earth-size planets cast shadows on the photosphere of sun-size stars. However, a failure in 2013 forced the Kepler Space Telescope to abandon its original observations, and it has since been resurrected as a new mission, K2, to observe different regions on the ecliptic plane on a seasonal basis. Due to the low resolution of the telescope and the failure of the satellite’s attitude control system, the data analysis for selecting planet candidates requires the latest attention, and follow-up observations, such as high-resolution observations on the ground, are essential to select planet candidates based on the data. However, there has been a need to demonstrate a larger number and variety of planets.

This time, an international research team led by Livingston, Tamura, and Narita of the University of Tokyo successfully demonstrated 44 exoplanets at once based on data from the K2 mission. The team consisted of researchers and students from Italy, Germany, Spain, and the Netherlands, in addition to Japan. This brings the total number of exoplanets demonstrated by the K2 mission to well over 300. Twenty-seven other exoplanets were also promising candidates, and only one other was a false planet. The results were published in the Astronomical Journal, an astronomy journal, with John Livingston, a graduate student at the University of Tokyo, as lead author. Livingston analyzed the K2 satellite data from raw data, which requires complexity and precision. It was painstaking work, with better parameters being identified at each step of the analysis, and sometimes the entire analysis had to be redone. He also proposed various ground-based follow-up observations of the resulting promising planetary candidates, and carried out the execution and data analysis of these observations, while cooperating with the international team.

Follow-up observations were conducted primarily with the telescope at Kitt Peak Observatory in the United States. Seventy-two K2 planet candidates were observed. The technique called “speckle imaging” was used to “freeze” atmospheric fluctuations by short-time integrated observations to obtain sharp images. This is different from adaptive optics, which corrects for atmospheric fluctuations in real time, but it is relatively easy to obtain high resolution images. Although this high-resolution image does not directly image the planet itself, it is a major factor in eliminating the possibility that the planet candidates selected from the K2 data are false planets, such as the presence or absence of nearby stellar mixtures. Spectroscopic observations were also made using the Texas telescope, which allowed us to precisely determine the physical parameters of the main star and, as a result, more precisely determine the size and temperature of the planet. From these high-resolution images and statistical analysis of spectroscopic and transit data, the final planetary demonstration was made.

This study is important because it has increased the number of small planets around relatively bright stars. Many of the small planets discovered by the Kepler telescope are too far away and their main stars are too faint for follow-up observations. On the other hand, planets around bright stars are easier to follow up. With this observation, the number of 1-2-Earth radius super-Earths, 2-4-Earth radius small Neptunes, and 4-8-Earth radius small Saturns orbiting bright stars (J-band magnitudes of 8-10 mag at 1.2 μm wavelength) increased by 4%, 17%, and 11%, respectively, compared to the current number of known planets (including those of non-Keplerian satellites). Of these 44 planets, 18 are members of multi-planet systems. Four are ultra-short-period planets with periods of less than one day. Another is a smaller planet than Venus orbiting a red dwarf. These will be important targets for future observations to understand the formation and evolution of terrestrial rocky planets.

Data from the K2 mission will continue to be released, and the team plans to follow up quickly. Similar ground follow-up observations will also be important for data from NASA’s TESS satellite, which was successfully launched in April 2018, so the ground team is expected to play an active role。

Publication：

Journal：“The Astronomical Journal“（Publication date to be determined, publication date to be determined）

Title：44 Validated Planets from K2 Campaign 10

Authors：John H. Livingston, Michael Endl, Fei Dai, William D. Cochran, Oscar Barragan, Davide Gandolfi, Teruyuki Hirano, Sascha Grziwa, Alexis M. S. Smith, Simon Albrecht, Juan Cabrera, Szilard Csizmadia, Jerome P. de Leon, Hans Deeg, Philipp Eigmueller, Anders Erikson, Mark Everett, Malcolm Fridlund, Akihiko Fukui, Eike W. Guenther, Artie P. Hatzes, Steve Howell, Judith Korth, Norio Narita, David Nespral, Grzegorz Nowak, Enric Palle, Martin Paetzold, Carina M. Persson, Jorge Prieto-Arranz, Heike Rauer, Motohide Tamura, Vincent Van Eylen, and Joshua N. Winn

Abstract URL  https://arxiv.org/abs/1806.11504

Related Links：

University of Tokyo Press Release

The post University of Tokyo Graduate Student Discovers 44 Exoplanets at Once first appeared on Astrobiology Center, NINS.