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Transit timing variations of the four-planet system Kepler-223 are used to compute the long-term stability of the system, which has a chain of resonances; the results suggest that inward planetary migration, rather than in situ assembly, is responsible for the formation of some close-in sub-Neptune systems. Sub-Neptune quartet in harmony The mechanism of formation of systems of sub-Neptune exoplanets has been keenly debated since the detection of Kepler-11, the main question being whether they form in situ or migrate in. Sean Mills et al . have analysed Kepler-223, a planetary system in which four transiting sub-Neptune-mass planets are associated in a chain of resonances with each other as they orbit a Sun-like star. The authors report transit timing variations, model them as resonant-angle librations (or oscillations), and compute the long-term stability of the four planets. They conclude that the detailed architecture of Kepler-223 is too finely tuned for formation by scattering, and numerical simulations demonstrate its properties are natural outcomes of the migration hypothesis. Similar systems could be destabilized by any of several mechanisms, contributing to the observed orbital-period distribution where planets are near, but not in, resonances. Surveys have revealed many multi-planet systems containing super-Earths and Neptunes in orbits of a few days to a few months 1 . There is debate whether in situ assembly 2 or inward migration is the dominant mechanism of the formation of such planetary systems. Simulations suggest that migration creates tightly packed systems with planets whose orbital periods may be expressed as ratios of small integers (resonances) 3 , 4 , 5 , often in a many-planet series (chain) 6 . In the hundreds of multi-planet systems of sub-Neptunes, more planet pairs are observed near resonances than would generally be expected 7 , but no individual system has hitherto been identified that must have been formed by migration. Proximity to resonance enables the detection of planets perturbing each other 8 . Here we report transit timing variations of the four planets in the Kepler-223 system, model these variations as resonant-angle librations, and compute the long-term stability of the resonant chain. The architecture of Kepler-223 is too finely tuned to have been formed by scattering, and our numerical simulations demonstrate that its properties are natural outcomes of the migration hypothesis. Similar systems could be destabilized by any of several mechanisms 5 , 9 , 10 , 11 , contributing to the observed orbital-period distribution, where many planets are not in resonances. Planetesimal interactions in particular are thought to be responsible for establishing the current orbits of the four giant planets in the Solar System by disrupting a theoretical initial resonant chain 12 similar to that observed in Kepler-223.

A fully formed, Neptune-sized planet is observed orbiting a young star, demonstrating that planets can form in less than 10 million years and may also experience inward migration on these timescales. An inwardly migrating Neptune-like planet Trevor David et al . report the detection and confirmation of a fully formed, Neptune-sized planet orbiting a young (5–10-million-year-old) star every 5.4 days at an orbital separation of only about 8 stellar radii. Some models suggest that in situ formation of planets close to their host stars is unlikely and that the existence of such planets is evidence for large-scale migration. This work demonstrates that planets can form relatively quickly and may also experience inward migration on these timescales. Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and gas disk from which they formed 1 , 2 . The precise ages of meteorites indicate that planetesimals—the building blocks of planets—are produced within the first million years of a star’s life 3 . Fully formed planets are frequently detected on short orbital periods around mature stars. Some theories suggest that the in situ formation of planets close to their host stars is unlikely and that the existence of such planets is therefore evidence of large-scale migration 4 , 5 . Other theories posit that planet assembly at small orbital separations may be common 6 , 7 , 8 . Here we report a newly born, transiting planet orbiting its star with a period of 5.4 days. The planet is 50 per cent larger than Neptune, and its mass is less than 3.6 times that of Jupiter (at 99.7 per cent confidence), with a true mass likely to be similar to that of Neptune. The star is 5–10 million years old and has a tenuous dust disk extending outward from about twice the Earth–Sun separation, in addition to the fully formed planet located at less than one-twentieth of the Earth–Sun separation.

Analysis of the metallicities of more than 400 stars hosting 600 candidate extrasolar planets shows that the planets can be categorized by size into three populations — terrestrial-like planets, gas dwarf planets with rocky cores and hydrogen–helium envelopes, and ice or gas giant planets — on the basis of host star metallicity. Three regimes of exoplanet radius arising from host star metallicities Soon after the discovery of the first exoplanets, it was suggested that host star metallicity — the abundance of elements other than hydrogen and helium — has a role in the formation of planetary systems. Here Lars Buchhave et al . report the metallicity and other stellar parameters of more than 400 stars hosting 600 exoplanet candidates and find that the exoplanets can be categorized into three populations defined by statistically distinct metallicity regions and planetary radii. The three are terrestrial-like exoplanets, gas dwarf exoplanets with rocky cores and H/He envelopes, and ice/gas-giant exoplanets. Approximately half of the extrasolar planets (exoplanets) with radii less than four Earth radii are in orbits with short periods 1 . Despite their sheer abundance, the compositions of such planets are largely unknown. The available evidence suggests that they range in composition from small, high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick hydrogen and helium gas envelopes. Here we report the metallicities (that is, the abundances of elements heavier than hydrogen and helium) of more than 400 stars hosting 600 exoplanet candidates, and find that the exoplanets can be categorized into three populations defined by statistically distinct (∼4.5 σ ) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (radii less than 1.7 Earth radii), gas dwarf planets with rocky cores and hydrogen–helium envelopes (radii between 1.7 and 3.9 Earth radii) and ice or gas giant planets (radii greater than 3.9 Earth radii). These transitions correspond well with those inferred from dynamical mass estimates 2 , 3 , implying that host star metallicity, which is a proxy for the initial solids inventory of the protoplanetary disk, is a key ingredient regulating the structure of planetary systems.

Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b reveal that its mean density is similar to Earth’s, suggesting a composition of rock and iron. Like Earth — but a lot hotter A few exoplanets of about the size or mass of Earth have been discovered. Now, for the first time, both size and mass have been determined for one of them. Kepler-78b, first described in August this year, is close-in to its host star, which it orbits every 8.5 hours. Two groups have been able to exploit the closeness of planet and star to make Doppler spectroscopic measurements of the mass of Kepler-78b. The teams, led by Andrew Howard and Francesco Pepe, used different telescopes to arrive at mass estimates of 1.69 ± 0.41 and 1.86 +0.38/−0.245 Earth masses, respectively. They calculate the planet's mean density at 5.3 and 5.57 g cm −3 , very similar to Earth's and consistent with an Earth-like composition of rock and iron. Planets with sizes between that of Earth (with radius ) and Neptune (about 4 ) are now known to be common around Sun-like stars 1 , 2 , 3 . Most such planets have been discovered through the transit technique, by which the planet’s size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet’s mass—and hence its density, which is a clue to its composition—is more difficult. Planets of size 2–4 have proved to have a wide range of densities, implying a diversity of compositions 4 , 5 , but these measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star every 8.5 hours (ref. 6 ). Given a radius of 1.20 ± 0.09 and a mass of 1.69 ± 0.41 , the planet’s mean density of 5.3 ± 1.8 g cm −3 is similar to Earth’s, suggesting a composition of rock and iron.

Breakthrough Listen is the most comprehensive and sensitive search for extraterrestrial intelligence (SETI) to date, employing a collection of international observational facilities including both radio and optical telescopes. During the first three years of the Listen program, thousands of targets have been observed with the Green Bank Telescope (GBT), Parkes Telescope and Automated Planet Finder. At GBT and Parkes, observations have been performed ranging from 700 MHz to 26 GHz, with raw data volumes averaging over 1 PB day−1. A pseudo-real time software spectroscopy suite is used to produce multi-resolution spectrograms amounting to approximately 400 GB h−1 GHz−1 beam−1. For certain targets, raw baseband voltage data is also preserved. Observations with the Automated Planet Finder produce both two-dimensional and one-dimensional high-resolution (R ∼ 105) echelle spectral data. Although the primary purpose of Listen data acquisition is for SETI, a range of secondary science has also been performed with these data, including studies of fast radio bursts. Other current and potential research topics include spectral line studies, searches for certain kinds of dark matter, probes of interstellar scattering, pulsar searches, radio transient searches and investigations of stellar activity. Listen data are also being used in the development of algorithms, including machine-learning approaches to modulation scheme classification and outlier detection, that have wide applicability not just for astronomical research but for a broad range of science and engineering. In this paper, we describe the hardware and software pipeline used for collection, reduction, archival, and public dissemination of Listen data. We describe the data formats and tools, and present Breakthrough Listen Data Release 1.0 (BLDR 1.0), a defined set of publicly available raw and reduced data totaling 1 PB.

Multi-instrument detection of a nearby type 1a supernova shows that the exploding star was probably a carbon–oxygen white dwarf star in a binary system with a main-sequence companion. Identification of a supernova companion Supernova 2011fe in the Pinwheel galaxy, discovered by the Palomar Transient Factory on 24 August 2011, is the brightest type Ia supernova that's been seen from Earth for many years. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. Two new reports of observations of SN 2011fe narrow down the range of possibilities for the mystery companion. Nugent et al . present some of the earliest data ever obtained from a type Ia supernova. They find that the exploding star was probably a carbon–oxygen white dwarf, and conclude from the lack of an early shock that the companion may have been a main sequence star. Li et al . analysed pre-discovery images in the Hubble Space Telescope archives and find that no object was visible before the explosion. That rules out luminous red giants and the vast majority of helium stars as the mass-donating companion to an exploding white dwarf. Type Ia supernovae have been used empirically as ‘standard candles’ to demonstrate the acceleration of the expansion of the Universe 1 , 2 , 3 even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery 4 , 5 , 6 . There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance 7 from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon–oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper 8 uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

The population of planets smaller than approximately 1.7 Earth radii (R⊕) is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths. This picture is largely corroborated by radial velocity mass measurements for close-in super-Earths but lacks constraints at lower insolations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate 1.51 ± 0.04 R⊕ planet Kepler-138 d (Teq,AB=0.3≈350K) combined with new radial velocity measurements of its host star obtained with the Keck/High Resolution Echelle Spectrometer. We find evidence for a volatile-rich ‘water world’ nature of Kepler-138 d, with a large fraction of its mass $M_{\\rm{d}}$ contained in a thick volatile layer. This finding is independently supported by transit timing variations and radial velocity observations (Md=2.1−0.7+0.6M⊕), as well as the flat optical/infrared transmission spectrum. Quantitatively, we infer a composition of 11−4+3% volatiles by mass or ~51% by volume, with a 2,000-km-deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. Any hypothetical hydrogen layer consistent with the observations (<0.003 M⊕) would have swiftly been lost on a ~10 Myr timescale. The bulk composition of Kepler-138 d therefore resembles those of the icy moons, rather than the terrestrial planets, in the Solar System. We conclude that not all super-Earths are rocky worlds, but that volatile-rich water worlds exist in an overlapping size regime, especially at lower insolations. Finally, our photodynamical analysis also reveals that Kepler-138 c (with a Rc = 1.51 ± 0.04 R⊕ and a Mc=2.3−0.5+0.6M⊕) is a slightly warmer twin of Kepler-138 d (that is, another water world in the same system) and we infer the presence of Kepler-138 e, a likely non-transiting planet at the inner edge of the habitable zone.A comprehensive study of the Kepler-138 system reveals the twin nature of Kepler-138 c and d and the presence of a fourth planet. Remarkably, the warm-temperate planet Kepler-138 d is probably composed of 50% volatiles by volume, indicative of a water world, rather than a rocky world, despite its small ~1.5 R⊕ size.

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 transits of two Sun-like stars by small planets in an open star cluster are reported; such a stellar environment is unlike that of most planet-hosting field stars, and suggests that the occurrence of planets is unaffected by the stellar environment in open clusters. A global rate of planet formation Until now only four planets — with masses similar to Jupiter — have been found orbiting stars in old open clusters, compared with more than 800 — mostly Neptune-sized — orbiting 'field stars' outside clusters. Most stars and planets form in open clusters that break up within a few hundred million years as stars drift away to become field stars. Older open clusters survive because they were denser in stars when they formed, a stellar environment very different from that of other planet-hosting field stars. This paper, part of the Kepler Cluster Study, describes observations of the transits of two Sun-like stars by planets smaller than Neptune in the 1-billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in our Galaxy. Most stars and their planets form in open clusters. Over 95 per cent of such clusters have stellar densities too low (less than a hundred stars per cubic parsec) to withstand internal and external dynamical stresses and fall apart within a few hundred million years 1 . Older open clusters have survived by virtue of being richer and denser in stars (1,000 to 10,000 per cubic parsec) when they formed. Such clusters represent a stellar environment very different from the birthplace of the Sun and other planet-hosting field stars. So far more than 800 planets have been found around Sun-like stars in the field 2 . The field planets are usually the size of Neptune or smaller 3 , 4 , 5 . In contrast, only four planets have been found orbiting stars in open clusters 6 , 7 , 8 , all with masses similar to or greater than that of Jupiter. Here we report observations of the transits of two Sun-like stars by planets smaller than Neptune in the billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in the Galaxy.

Small planets, 1–4× the size of Earth, are extremely common around Sun-like stars, and surprisingly so, as they are missing in our solar system. Recent detections have yielded enough information about this class of exoplanets to begin characterizing their occurrence rates, orbits, masses, densities, and internal structures. The Kepler mission finds the smallest planets to be most common, as 26% of Sun-like stars have small, 1–2 R ⊕ planets with orbital periods under 100 d, and 11% have 1–2 R ⊕ planets that receive 1–4× the incident stellar flux that warms our Earth. These Earth-size planets are sprinkled uniformly with orbital distance (logarithmically) out to 0.4 the Earth–Sun distance, and probably beyond. Mass measurements for 33 transiting planets of 1–4 R ⊕ show that the smallest of them, R < 1.5 R ⊕, have the density expected for rocky planets. Their densities increase with increasing radius, likely caused by gravitational compression. Including solar system planets yields a relation: [Formula]. Larger planets, in the radius range 1.5–4.0 R ⊕, have densities that decline with increasing radius, revealing increasing amounts of low-density material (H and He or ices) in an envelope surrounding a rocky core, befitting the appellation ‘‘mini-Neptunes.’’ The gas giant planets occur preferentially around stars that are rich in heavy elements, while rocky planets occur around stars having a range of heavy element abundances. Defining habitable zones remains difficult, without benefit of either detections of life elsewhere or an understanding of life’s biochemical origins.