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Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun 1 , 2 . Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars 3 – 6 . To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve 7 – 10 . Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point (latitude and longitude of 0 degrees)—possibly indicative of atmospheric circulation 11 . Here we report a phase curve measurement for the smaller, cooler exoplanet LHS 3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS 3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040 ± 40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres. Phase curve measurements for the small (1.3 Earth radii) terrestrial exoplanet LHS 3844b show absence of a thick atmosphere, in agreement with theoretical predictions.

Black hole binary systems with companion stars are typically found via their x-ray emission, generated by interaction and accretion. Noninteracting binaries are expected to be plentiful in the Galaxy but must be observed using other methods. We combine radial velocity and photometric variability data to show that the bright, rapidly rotating giant star 2MASS J05215658+4359220 is in a binary system with a massive unseen companion. The system has an orbital period of ~83 days and near-zero eccentricity. The photometric variability period of the giant is consistent with the orbital period, indicating star spots and tidal synchronization. Constraints on the giant’s mass and radius imply that the unseen companion is 3.3 − 0.7 + 2.8 solar masses, indicating that it is a noninteracting low-mass black hole or an unexpectedly massive neutron star.

The atmospheres of white dwarfs often contain elements heavier than helium, even though these elements would be expected to settle into the stars’ interiors; observations of the white dwarf WD 1145+017 suggest that disintegrating rocky bodies are orbiting the star, perhaps contributing heavy elements to its atmosphere. A disintegrating planet orbiting a faded white dwarf The atmospheres of white dwarfs often contain elements heavier than helium, even though such elements would be expected to settle into the stars' interiors once they have exhausted their nuclear fuel. Heavy-element abundance ratios in such atmospheres are similar to those of rocky bodies in the Solar System, suggesting that the 'extra' elements may derive from planetary material. This paper presents observations of one or more disintegrating planetesimals in transit across the white dwarf WD 1145+017, with periods ranging from 4.5 to 4.9 hours. The strongest transit signals, occurring every 4.5 hours, are indicative of a small object with a cometary tail of dusty effluent material. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets. Most stars become white dwarfs after they have exhausted their nuclear fuel (the Sun will be one such). Between one-quarter and one-half of white dwarfs have elements heavier than helium in their atmospheres 1 , 2 , even though these elements ought to sink rapidly into the stellar interiors (unless they are occasionally replenished) 3 , 4 , 5 . The abundance ratios of heavy elements in the atmospheres of white dwarfs are similar to the ratios in rocky bodies in the Solar System 6 , 7 . This fact, together with the existence of warm, dusty debris disks 8 , 9 , 10 , 11 , 12 , 13 surrounding about four per cent of white dwarfs 14 , 15 , 16 , suggests that rocky debris from the planetary systems of white-dwarf progenitors occasionally pollutes the atmospheres of the stars 17 . The total accreted mass of this debris is sometimes comparable to the mass of large asteroids in the Solar System 1 . However, rocky, disintegrating bodies around a white dwarf have not yet been observed. Here we report observations of a white dwarf—WD 1145+017—being transited by at least one, and probably several, disintegrating planetesimals, with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths (blocking up to 40 per cent of the star’s brightness) and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star has a dusty debris disk, and the star’s spectrum shows prominent lines from heavy elements such as magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets.

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.

The measurement of the rotational periods of 30 cool stars in the 2.5-billion-year-old cluster NGC 6819 allows the calibration of gyrochronology — the determination of a star’s age on the basis of its rotation period — over a much broader age range than hitherto, meaning that it might be possible to determine the ages of many cool stars in the Galactic field with a precision of roughly 10 per cent. Age determination for cool stars The standard methods of stellar age estimation are unreliable when applied to the most numerous star type — low-mass cool stars like our Sun and smaller stars. Here Søren Meibom and colleagues describe an empirical calibration and test of a method to determine cool star ages reliably from their rotation periods — gyrochronology. Using Kepler Cluster Study measurements of the rotational periods of 30 cool stars in the 2.5-billion-year-old cluster NGC6819, the authors calibrate the rate of loss of angular momentum with time — previously such measurements were possible only in clusters under a billion years old. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10% can be derived for large numbers of cool Galactic field stars. The ages of the most common stars—low-mass (cool) stars like the Sun, and smaller—are difficult to derive 1 , 2 because traditional dating methods use stellar properties that either change little as the stars age 3 , 4 or are hard to measure 5 , 6 , 7 , 8 . The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology 2 , 9 , 10 , 11 . To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years 12 , 13 , 14 , 15 , 16 , and gyrochronology ages for older stars have been inferred from model predictions 2 , 7 , 11 , 17 . Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.

A super-Earth with atmosphere 'Super-Earths' are extrasolar planets about two to ten times the mass of the Earth, too small to be considered 'Jupiters'. Observations from the MEarth Project — using two 40-cm (16-inch) telescopes that will eventually be part of an eight-telescope array — have now identified a super-Earth (GJ 1214b) transiting a nearby low mass star. GJ 1214b has a mass 6.55 times that of the Earth and a radius of 2.68 'Earths'. As the star is small and only 13 parsecs away, the planetary atmosphere is available for direct study with current observatories. A population of extrasolar planets has been uncovered with minimum masses of 1.9–10 times the Earth's mass, called super-Earths, but atmospheric studies can be precluded by the distance and size of their stars. Here, observations of the transiting planet GJ 1214b are reported; it has a mass 6.55 times that of the Earth and a radius 2.68 times the Earth's radius. The star is small and only 13 parsecs away, permitting the study of the planetary atmosphere with current observatories. A decade ago, the detection of the first 1 , 2 transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres 3 . Because such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies 4 , 5 , 6 and microlensing 7 have uncovered a population of planets with minimum masses of 1.9–10 times the Earth’s mass ( M ⊕ ), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b (refs 8 , 9 ), but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55 M ⊕ and a radius 2.68 times Earth’s radius ( R ⊕ ), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen–helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories.

The transit method of exoplanet discovery and characterization has enabled numerous breakthroughs in exoplanetary science. These include measurements of planetary radii, mass-radius relationships, stellar obliquities, bulk density constraints on interior models, and transmission spectroscopy as a means to study planetary atmospheres. The Transiting Exoplanet Survey Satellite (TESS) has added to the exoplanet inventory by observing a significant fraction of the celestial sphere, including many stars already known to host exoplanets. Here we describe the science extraction from TESS observations of known exoplanet hosts during the primary mission. These include transit detection of known exoplanets, discovery of additional exoplanets, detection of phase signatures and secondary eclipses, transit ephemeris refinement, and asteroseismology as a means to improve stellar and planetary parameters. We provide the statistics of TESS known host observations during Cycle 1 and 2, and present several examples of TESS photometry for known host stars observed with a long baseline. We outline the major discoveries from observations of known hosts during the primary mission. Finally, we describe the case for further observations of known exoplanet hosts during the TESS extended mission and the expected science yield.

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.

AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of 9.79 parsecs and with an age of 22 million years 1 . AU Mic possesses a relatively rare 2 and spatially resolved 3 edge-on debris disk extending from about 35 to 210 astronomical units from the star 4 , and with clumps exhibiting non-Keplerian motion 5 – 7 . Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic ‘activity’ on the star 8 , 9 . Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3 σ confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution. A transiting planet with a period of about 8.5 days and a radius 0.4 times that of Jupiter is reported within the debris disk around the star AU Microscopii.

An Earth-sized planet is observed orbiting a nearby star within the liquid-water, habitable zone, the atmospheric composition of which could be determined from future observations. Super-Earth rocks around cool star Planets cause a dip in the light received when they pass in front of their parent stars. M stars have masses less than 60 per cent that of the Sun, and account for three-quarters of our Galaxy's stellar population. Seven Earth-sized planets are known to transit such a star, TRAPPIST-1, at 12 parsecs from Earth, but their masses and therefore their densities are rather poorly constrained. Jason Dittman et al . report observations of LHS 1140b, a planet with a radius 1.4 times that of Earth that is transiting an M dwarf star 12 parsecs from Earth and receiving sufficient insolation to place it in the liquid-water, 'habitable zone'. They measure the mass to be 6.6 times that of Earth, which suggests a rocky bulk composition. M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy 1 . The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars 2 , 3 . Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away 4 . A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf 5 , but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away 6 , 7 , but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone 8 . With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth 9 , 10 . Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future 2 , 3 .