Most known terrestrial planets orbit small stars with radii less than 60% 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 -possibly indicative of atmospheric circulation 11 . Here we report a phase curve measurement for the smaller, cooler planet LHS 3844b, a 1.3 R ⊕ world in an 11-hour orbit around a small, nearby star. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1040±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 unstable 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 1 arXiv:1908.06834v1 [astro-ph.EP] 19 Aug 2019 results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.We observed a light curve of the LHS 3844 system with the Spitzer InfraRed Array Camera (IRAC) 12 over 100 hours between UT 4 February 2019 and 8 February 2019 (Program 14204). We used IRAC's Channel 2 (a photometric bandpass over the wavelength range 4 − 5 µm), and read out the 32 × 32 pixel subarray in 2-second exposures. The observations began with a 30-minute dithering sequence to allow the telescope to thermally settle. Following this pre-observation, we employed Spitzer's Pointing Calibration and Reference Sensor (PCRS) peak-up mode to position the target on the detector's "sweet spot", a pixel with minimal variation in sensitivity. After the first 60 hours of observation, there was a 3-hour break for data downlink. The data collection recommenced with another 30 minute thermal settling period and continued in PCRS peak-up mode for 40 more hours. The telescope was re-pointed every 20 hours to keep the image centered on the detector sweet spot.We began our analysis with Basic Calibrated Data provided by the Spitzer Science Center (SSC) pipeline, and reduced it with a custom aperture photometry routine 13 . This routine upsampled each exposure by a factor of 5 in the X and Y dimension and fit a 2D Gaussian profile to determine the image center. We estimated the background from the median value in an annulus 7 to 15 pixels from the target center. Bad pixels were identified and masked based on iterative σ-clipping over groups of 64 exposures. We then summed the flux in a fixed aperture centered on the target. We varied the aperture size from 2 to 4 pixels in 0.5 pixel incremen...

Astronomers have discovered thousands of planets outside the solar system 1 , most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star 2 , but more distant planets can survive this phase and remain in orbit around the white dwarf 3,4 . Some white dwarfs show evidence for rocky material floating in their atmospheres 5 , in warm debris disks [6][7][8][9] , or orbiting very closely [10][11][12] , which has been interpreted as the debris of rocky planets that were scattered inward and tidally disrupted 13 . Recently, the discovery of a gaseous debris disk with a composition similar to ice giant planets 14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether the planets can survive the journey. So far, the detection of intact planets in close orbits around white dwarfs has remained elusive. Here, we report the discovery of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95% confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red-giant phase and shrinks due to friction. In this case, though, the low mass and relatively long orbital period of the planet candidate make common-envelope evolution less likely. Instead, the WD 1856+534 system seems to demonstrate that giant planets can be scattered into tight orbits without being tidally disrupted, and motivates searches for smaller transiting planets around white dwarfs. WD 1856+534 (hereafter, WD 1856 for brevity) is located 25 parsecs away in a visual triple star system. It has an effective temperature of 4710 ± 60 Kelvin and became a white dwarf 5.9 ± 0.5 billion years ago, based on theoretical models for how white dwarfs cool over time. The total system age, including the star's main sequence lifetime, must be older. Table 1 gives the other key parameters of the star. WD 1856 is one of thousands of white dwarfs that was targeted for observations with NASA's Transiting Exoplanet Survey Satellite (TESS ), in order to search for any periodic dimming events caused by planetary transits. A statistically significant transit-like event was detected by the TESS Science Processing Operations Center (SPOC) pipeline based

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Recent years have seen increasing interest in the characterization of sub-Neptune-sized planets because of their prevalence in the Galaxy, contrasted with their absence in our solar system. HD97658 is one of the brightest stars hosting a planet of this kind, and we present the transmission spectrum of this planet by combining four Hubble Space Telescope transits, 12 Spitzer/IRAC transits, and eight MOST transits of this system. Our transmission spectrum has a higher signal-to-noise ratio than those from previous works, and the result suggests that the slight increase in transit depth from wavelength 1.1-1.7 μm reported in previous works on the transmission spectrum of this planet is likely systematic. Nonetheless, our atmospheric modeling results are inconclusive, as no model provides an excellent match to our data. Nonetheless, we find that atmospheres with high C/O ratios (C/O0.8) and metallicities of 100×solar metallicity are favored. We combine the mid-transit times from all of the new Spitzer and MOST observations and obtain an updated orbital period of P=9.489295±0.000005, with a best-fit transit time center at T 0 =2456361.80690±0.00038 (BJD). No transit timing variations are found in this system. We also present new measurements of the stellar rotation period (34±2 days) and stellar activity cycle (9.6 yr) of the host star HD97658. Finally, we calculate and rank the Transmission Spectroscopy Metric of all confirmed planets cooler than 1000 K and with sizes between 1 R ⊕ and 4 R ⊕. We find that at least a third of small planets cooler than 1000 K can be well characterized using James Webb Space Telescope, and of those, HD97658b is ranked fifth, meaning that it remains a high-priority target for atmospheric characterization.

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The occurrence rate of hot Jupiters from the Kepler transit survey is roughly half that of radial velocity surveys targeting solar neighborhood stars. One hypothesis to explain this difference is that the two surveys target stars with different stellar metallicity distributions. To test this hypothesis, we measure the metallicity distribution of the Kepler targets using the Hectochelle multi-fiber, high-resolution spectrograph. Limiting our spectroscopic analysis to 610 dwarf stars in our sample with g log >3.5, we measure a metallicity distribution characterized by a mean of , and the samples come from different parent populations according to a Kolmogorov-Smirnov test. We refit the exponential relation between the fraction of stars hosting a close-in giant planet and the host star metallicity using a sample of dwarf stars from the California Planet Search with updated metallicities. The best-fit relation tells us that the difference in metallicity between the two samples is insufficient to explain the discrepant hot Jupiter occurrence rates; the metallicity difference would need to be ;0.2-0.3 dex for perfect agreement. We also show that (sub)giant contamination in the Kepler sample cannot reconcile the two occurrence calculations. We conclude that other factors, such as binary contamination and imperfect stellar properties, must also be at play.

Transiting planets in nearby young clusters offer the opportunity to study the atmospheres and dynamics of planets during their formative years. To this end, we focused on K2-25b -a close-in (P =3.48 days), Neptune-sized exoplanet orbiting a M4.5 dwarf in the 650 Myr Hyades cluster. We combined photometric observations of K2-25 covering a total of 44 transits and spanning > 2 yr, drawn from a mix of space-based telescopes (Spitzer Space Telescope and K2 ) and ground-based facilities (Las Cumbres Observatory Global Telescope network and MEarth). The transit photometry spanned 0.6-4.5µm, which enabled our study of K2-25b's transmission spectrum. We combined and fit each dataset at a common wavelength within a Markov Chain Monte Carlo framework, yielding consistent planet parameters. The resulting transit depths ruled out a solar-composition atmosphere for K2-25b for the range of expected planetary masses and equilibrium temperature at a > 4σ confidence level, and are consistent with a flat transmission spectrum. Mass constraints and transit observations at a finer grid of wavelengths (e.g., from the Hubble Space Telescope) are needed to make more definitive statements about the presence of clouds or an atmosphere of high mean molecular weight. Our precise measurements of K2-25b's transit duration also enabled new constraints on the eccentricity of K2-25's orbit. We find K2-25b's orbit to be eccentric (e > 0.20) for all reasonable stellar densities and independent of the observation wavelength or instrument. The high eccentricity is suggestive of a complex dynamical history and motivates future searches for additional planets or stellar companions.

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