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The K2 mission will make use of the Kepler spacecraft and its assets to expand upon Kepler's groundbreaking discoveries in the fields of exoplanets and astrophysics through new and exciting observations. K2 will use an innovative way of operating the spacecraft to observe target fields along the ecliptic for the next 2-3 years. Early science commissioning observations have shown an estimated photometric precision near 400 ppm in a single 30 minute observation, and a 6-hr photometric precision of 80 ppm (both at V = 12). The K2 mission offers long-term, simultaneous optical observation of thousands of objects at a precision far better than is achievable from ground-based telescopes. Ecliptic fields will be observed for approximately 75 days enabling a unique exoplanet survey which fills the gaps in duration and sensitivity between the Kepler and TESS missions, and offers pre-launch exoplanet target identification for JWST transit spectroscopy. Astrophysics observations with K2 will include studies of young open clusters, bright stars, galaxies, supernovae, and asteroseismology.

The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets, all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star's radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form.

The Ultra-Violet/Optical Telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (1 min) UV and optical photons from the afterglow of gamma-ray bursts in the 170-600 nm band as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey-Chrétien design with micro-channel plate intensified charged-coupled device detectors that record the arrival time of individual photons and provide sub-arcsecond positioning of sources. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.[PUBLICATION ABSTRACT]

Whereas large planets, such as gas giants, are more likely to form around high-metallicity stars, terrestrial-sized planets are found to form around stars with a wide range of metallicities, indicating that they may be widespread in the disk of the Galaxy. Exoplanets around metal-poor stars A key discovery of the past decade in the field of exoplanet research was the realization that stars of high metallicity are those most likely to harbour giant exoplanets, supporting the model in which planets form by the accumulation of dust and ice particles. Whether the planet–metallicity correlation holds for terrestrial planets remained unclear, but the Kepler mission's discovery last year of hundreds of small exoplanet candidates provided an opportunity to find out. The spectroscopic metallicities of the host stars of 226 small exoplanet candidates have now been determined. The smaller ones, of less than four Earth radii, were found around stars with a wide range of metallicities, on average close to that of the Sun. Larger planets were more common around stars of high metallicity. These findings suggest that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation. The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a ‘fossil’ record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets 1 , 2 , 3 , 4 , supporting the model that planets form by accumulation of dust and ice particles 5 . Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets 4 , 6 , 7 , 8 , 9 . However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA’s Kepler mission 10 , including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.

An analysis of transits of planets over starspots on the Sun-like star Kepler-30 shows that the orbits of the three planets are aligned with the stellar equator; this configuration is similar to that of our Solar System, and suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Exoplanet trio in Solar-System-like orbits In our Solar System, the Sun's equator and the planets' orbital planes are almost in alignment. This probably reflects the way they formed, from a single spinning disk of gas. Many exoplanet systems do not display this arrangement, however, and isolated 'hot Jupiters' are often misaligned and even have a retrograde orbit. This paper reports an exoplanet system that features alignments similar to those in the Solar System. Analysis of planetary transits across starspots on the Sun-like star Kepler-30 shows that the orbits of its three planets are aligned with the stellar equator. These findings support the suggestion that high orbital tilts (obliquities) are confined to systems that have experienced dynamic interactions of the type that produce hot Jupiters and potentially rule out star–disk misalignments as a cause. The Sun’s equator and the planets’ orbital planes are nearly aligned, which is presumably a consequence of their formation from a single spinning gaseous disk. For exoplanetary systems this well-aligned configuration is not guaranteed: dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion 1 , magnetic interactions 2 or torques from neighbouring stars. Indeed, isolated ‘hot Jupiters’ are often misaligned and even orbiting retrograde 3 , 4 . Here we report an analysis of transits of planets over starspots 5 , 6 , 7 on the Sun-like star Kepler-30 (ref. 8 ), and show that the orbits of its three planets are aligned with the stellar equator. Furthermore, the orbits are aligned with one another to within a few degrees. This configuration is similar to that of our Solar System, and contrasts with the isolated hot Jupiters. The orderly alignment seen in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Should this be corroborated by observations of other coplanar multi-planet systems, then star–disk misalignments would be ruled out as the explanation for the high obliquities of hot Jupiters, and dynamical interactions would be implicated as the origin of hot Jupiters.

We present the detection of five planets—Kepler-62b, c, d, e, and f—of size 1.31, 0.54, 1.95, 1.61 and 1.41 Earth radii (R ⊕ ), orbiting a K2V star at periods of 5.7, 12.4, 18.2, 122.4, and 267.3 days, respectively. The outermost planets, Kepler-62e and -62f, are super-Earth-size (1.25 R ⊕ < planet radius < 2.0 R ⊕ ) planets in the habitable zone of their host star, respectively receiving 1.2 ± 0.2 times and 0.41 ± 0.05 times the solar flux at Earth's orbit. Theoretical models of Kepler-62e and -62f for a stellar age of ~7 billion years suggest that both planets could be solid, either with a rocky composition or composed of mostly solid water in their bulk.

We report the detection of Kepler-47, a system consisting of two planets orbiting around an eclipsing pair of stars. The inner and outer planets have radii 3.0 and 4.6 times that of Earth, respectively. The binary star consists of a Sun-like star and a companion roughly one-third its size, orbiting each other every 7.45 days. With an orbital period of 49.5 days, 18 transits of the inner planet have been observed, allowing a detailed characterization of its orbit and those of the stars. The outer planet's orbital period is 303.2 days, and although the planet is not Earth-like, it resides within the classical \"habitable zone,\" where liquid water could exist on an Earth-like planet. With its two known planets, Kepler-47 establishes that close binary stars can host complete planetary systems.

Stellar data from the Kepler spacecraft are used to infer the existence of a sub-Mercury-sized exoplanet, the smallest yet discovered, in orbit around a Sun-like star. Mercury-like exoplanets in Kepler's sights When the Kepler spacecraft was launched in 2009 its brief was to search for rocky planets around Sun-like host stars in our Galaxy. Many of the hundreds of known exoplanets are large 'hot Jupiters' close-in to their stars. Last year it became possible to detect Earth-sized exoplanets, and now comes the discovery of a rocky planet significantly smaller than Mercury. Kepler-37b is orbiting the Sun-like star Kepler-37 in a system with at least two other planets. It is similar to our Moon in size and is likely to resemble Mercury: rocky, no atmosphere and no water. Since the discovery of the first exoplanets 1 , 2 , it has been known that other planetary systems can look quite unlike our own 3 . Until fairly recently, we have been able to probe only the upper range of the planet size distribution 4 , 5 , and, since last year, to detect planets that are the size of Earth 6 or somewhat smaller 7 . Hitherto, no planets have been found that are smaller than those we see in the Solar System. Here we report a planet significantly smaller than Mercury 8 . This tiny planet is the innermost of three that orbit the Sun-like host star, which we have designated Kepler-37. Owing to its extremely small size, similar to that of the Moon, and highly irradiated surface, the planet, Kepler-37b, is probably rocky with no atmosphere or water, similar to Mercury.

Two exoplanets of Earth’s size have been discovered in orbit around the star Kepler-20. Since the discovery of the first extrasolar giant planets around Sun-like stars 1 , 2 , evolving observational capabilities have brought us closer to the detection of true Earth analogues. The size of an exoplanet can be determined when it periodically passes in front of (transits) its parent star, causing a decrease in starlight proportional to its radius. The smallest exoplanet hitherto discovered 3 has a radius 1.42 times that of the Earth’s radius ( R ⊕ ), and hence has 2.9 times its volume. Here we report the discovery of two planets, one Earth-sized (1.03 R ⊕ ) and the other smaller than the Earth (0.87 R ⊕ ), orbiting the star Kepler-20, which is already known to host three other, larger, transiting planets 4 . The gravitational pull of the new planets on the parent star is too small to measure with current instrumentation. We apply a statistical method to show that the likelihood of the planetary interpretation of the transit signals is more than three orders of magnitude larger than that of the alternative hypothesis that the signals result from an eclipsing binary star. Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate. The outer planet could have developed a thick water vapour atmosphere.

Nature 494, 452–454 (2013); doi:10.1038/nature11914 In this Letter, the Centro de Astrobiología affiliation (number 11, associated with authors David Barrado and Jorge Lillo-Box) was listed incorrectly; the correct address is: ‘Departamento Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC campus, PO Box 78, E-28691 Villanueva de la Cañada, Spain’.