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Register a new userOne or more bound planets per Milky Way star from microlensing observations
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Most known extrasolar planets (exoplanets) have been discovered using the radial velocity$^{bf 1,2}$ or transit$^{bf 3}$ methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17--30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing$^{bf 6rm{bf -}bf 9}$, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing$^{bf 10}$. These planets are at least as numerous as the stars in the Milky Way$^{bf 10}$. Here we report a statistical analysis of microlensing data (gathered in 2002--07) that reveals the fraction of bound planets 0.5--10 AU (Sun--Earth distance) from their stars. We find that 17$_{bf -9}^{bf +6}$% of stars host Jupiter-mass planets (0.3--10 $MJ$, where $MJ {bf = 318}$ $Mearth$ and $Mearth$ is Earths mass). Cool Neptunes (10--30 $Mearth$) and super-Earths (5--10 $Mearth$) are even more common: their respective abundances per star are 52$_{bf -29}^{bf +22}$% and 62$_{bf -37}^{bf +35}$%. We conclude that stars are orbited by planets as a rule, rather than the exception.
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Planet formation theories predict the existence of free-floating planets that have been ejected from their parent systems. Although they emit little or no light, they can be detected during gravitational microlensing events. Microlensing events caused by rogue planets are characterized by very short timescales $t_{rm E}$ (typically below two days) and small angular Einstein radii $theta_{rm E}$ (up to several uas). Here we present the discovery and characterization of two ultra-short microlensing events identified in data from the Optical Gravitational Lensing Experiment (OGLE) survey, which may have been caused by free-floating or wide-orbit planets. OGLE-2012-BLG-1323 is one of the shortest events discovered thus far ($t_{rm E}$=0.155 +/- 0.005 d, $theta_{rm E}$=2.37 +/- 0.10 uas) and was caused by an Earth-mass object in the Galactic disk or a Neptune-mass planet in the Galactic bulge. OGLE-2017-BLG-0560 ($t_{rm E}$=0.905 +/- 0.005 d, $theta_{rm E}$=38.7 +/- 1.6 uas) was caused by a Jupiter-mass planet in the Galactic disk or a brown dwarf in the bulge. We rule out stellar companions up to a distance of 6.0 and 3.9 au, respectively. We suggest that the lensing objects, whether located on very wide orbits or free-floating, may originate from the same physical mechanism. Although the sample of ultrashort microlensing events is small, these detections are consistent with low-mass wide-orbit or unbound planets being more common than stars in the Milky Way.
We present the analysis of a very high-magnification ($Asim 900$) microlensing event KMT-2019-BLG-1953. A single-lens single-source (1L1S) model appears to approximately delineate the observed light curve, but the residuals from the model exhibit small but obvious deviations in the peak region. A binary lens (2L1S) model with a mass ratio $qsim 2times 10^{-3}$ improves the fits by $Deltachi^2=181.8$, indicating that the lens possesses a planetary companion. From additional modeling by introducing an extra planetary lens component (3L1S model) and an extra source companion (2L2S model), it is found that the residuals from the 2L1S model further diminish, but claiming these interpretations is difficult due to the weak signals with $Deltachi^2=16.0$ and $13.5$ for the 3L1S and 2L2L models, respectively. From a Bayesian analysis, we estimate that the host of the planets has a mass of $M_{rm host}=0.31^{+0.37}_{-0.17}~M_odot$ and that the planetary system is located at a distance of $D_{rm L}=7.04^{+1.10}_{-1.33}~{rm kpc}$ toward the Galactic center. The mass of the securely detected planet is $M_{rm p}=0.64^{+0.76}_{-0.35}~M_{rm J}$. The signal of the potential second planet could have been confirmed if the peak of the light curve had been more densely observed by followup observations, and thus the event illustrates the need for intensive followup observations for very high-magnification events even in the current generation of high-cadence surveys.
The Microlensing Planet Finder (MPF) is a proposed Discovery mission that will complete the first census of extrasolar planets with sensitivity to planets like those in our own solar system. MPF will employ a 1.1m aperture telescope, which images a 1.3 sq. deg. field-of-view in the near-IR, in order to detect extrasolar planets with the gravitational microlensing effect. MPFs sensitivity extends down to planets of 0.1 Earth masses, and MPF can detect Earth-like planets at all separations from 0.7AU to infinity. MPFs extrasolar planet census will provide critical information needed to understand the formation and frequency of extra solar planetary systems similar to our own.
We perform a comprehensive study of Milky Way (MW) satellite galaxies to constrain the fundamental properties of dark matter (DM). This analysis fully incorporates inhomogeneities in the spatial distribution and detectability of MW satellites and marginalizes over uncertainties in the mapping between galaxies and DM halos, the properties of the MW system, and the disruption of subhalos by the MW disk. Our results are consistent with the cold, collisionless DM paradigm and yield the strongest cosmological constraints to date on particle models of warm, interacting, and fuzzy dark matter. At $95%$ confidence, we report limits on (i) the mass of thermal relic warm DM, $m_{rm WDM} > 6.5 mathrm{keV}$ (free-streaming length, $lambda_{rm{fs}} lesssim 10,h^{-1} mathrm{kpc}$), (ii) the velocity-independent DM-proton scattering cross section, $sigma_{0} < 8.8times 10^{-29} mathrm{cm}^{2}$ for a $100 mathrm{MeV}$ DM particle mass (DM-proton coupling, $c_p lesssim (0.3 mathrm{GeV})^{-2}$), and (iii) the mass of fuzzy DM, $m_{phi}> 2.9 times 10^{-21} mathrm{eV}$ (de Broglie wavelength, $lambda_{rm{dB}} lesssim 0.5 mathrm{kpc}$). These constraints are complementary to other observational and laboratory constraints on DM properties.
We analyzed Suzaku and Chandra observations of the soft diffuse X-ray background toward four sightlines with the goal of characterizing the X-ray emission from the Milky Way circumgalactic medium (CGM). We identified two thermal components of the CGM, one at a uniform temperature of $rm kT = 0.176pm0.008 ~keV$ and the other at temperatures ranging between $rm kT = 0.65-0.90~ keV$. The uniform lower temperature component is consistent with the Galaxys virial temperature ($ sim10^{6}~ K$). The temperatures of the hotter components are similar to that recently discovered ($rm sim 10^{7}~ K$; Das et al.) in the sightline to blazar 1ES1553+113, passing close to the Fermi bubble. Alternatively, the spectra can be described by just one lower-temperature component with super-solar Neon abundance, once again similar to that found in the 1ES1553+113 sightline. The additional hot component or the overabundance of Ne is required at a significance of $>4sigma$, but we cannot distinguish between the two possibilities. These results show that the super-virial temperature gas or an enhanced Ne abundance in the warm-hot gas in the CGM is widespread, and these are not necessarily related to the Fermi bubble.
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