The discovery of planets around other stars is revolutionizing our notions of planet formation and is poised to do the same for planetary climate. Studying transiting planets is complementary to ...eventual studies of directly imaged planets: (1) we can readily measure the mass and radius of transiting planets, linking atmospheric properties to bulk composition and formation, (2) many transiting planets are strongly irradiated and exhibit novel atmospheric physics, and (3) the most common temperate terrestrial planets orbit close to red dwarf stars and are difficult to image directly. We have only been able to comprehensively characterize the atmospheres of a handful of transiting planets, because most orbit faint stars. The Transiting Exoplanet Survey Satellite (TESS) will discover transiting planets orbiting the brightest stars, enabling, in principle, an atmospheric survey of 102-103 bright hot Jupiters and warm sub-Neptunes. Uniform observations of such a statistically significant sample would provide leverage to understand-and learn from-the diversity of short-period planets, and would identify the minority of truly special planets worthy of more intensive follow-up. We argue that the best way to maximize the scientific returns of TESS is to adopt a triage approach. A space mission consisting of a ∼1 m telescope with an optical-NIR spectrograph could measure molecular absorption for nonterrestrial planets discovered by TESS, as well as eclipses and phase variations for the hottest jovians. Such a mission could observe up to 103 transits per year, thus enabling it to survey a large fraction of the bright (J < 11) hot-Jupiters and warm sub-Neptunes TESS is expected to find. The James Webb Space Telescope (JWST) could be used to perform detailed atmospheric characterization of the most interesting transiting targets (transit, eclipse, and-when possible-phase-resolved spectroscopy). TESS is also expected to discover a few temperate terrestrial planets transiting nearby M-Dwarfs. Characterizing these worlds will be time-intensive: JWST will need months to provide tantalizing constraints on the presence of an atmosphere, planetary rotational state, clouds, and greenhouse gases. Future flagship missions should be designed to provide better constraints on the habitability of M-Dwarf temperate terrestrial planets.
We analyze an ensemble of microlensing events from the 2015 Spitzer microlensing campaign, all of which were densely monitored by ground-based high-cadence survey teams. The simultaneous observations ...from Spitzer and the ground yield measurements of the microlensing parallax vector , from which compact constraints on the microlens properties are derived, including 25% uncertainties on the lens mass and distance. With the current sample, we demonstrate that the majority of microlenses are indeed in the mass range of M dwarfs. The planet sensitivities of all 41 events in the sample are calculated, from which we provide constraints on the planet distribution function. In particular, assuming a planet distribution function that is uniform in , where q is the planet-to-star mass ratio, we find a 95% upper limit on the fraction of stars that host typical microlensing planets of 49%, which is consistent with previous studies. Based on this planet-free sample, we develop the methodology to statistically study the Galactic distribution of planets using microlensing parallax measurements. Under the assumption that the planet distributions are the same in the bulge as in the disk, we predict that ∼1/3 of all planet detections from the microlensing campaigns with Spitzer should be in the bulge. This prediction will be tested with a much larger sample, and deviations from it can be used to constrain the abundance of planets in the bulge relative to the disk.
We analyze OGLE-2003-BLG-262, a relatively short (t sub(E) = 12.5 plus or minus 0.1 day) microlensing event generated by a point-mass lens transiting the face of a K giant source in the Galactic ...bulge. We use the resulting finite-source effects to measure the angular Einstein radius, theta sub(E) = 195 plus or minus 17 mu as, and so constrain the lens mass to the FWHM interval 0.08 < M/M sub(o) < 0.54. The lens-source relative proper motion is mu sub(rel) = 27 plus or minus 2 km s super(-1) kpc super(-1). Both values are typical of what is expected for lenses detected toward the bulge. Despite the short duration of the event, we detect marginal evidence for a "parallax asymmetry" but argue that this is more likely to be induced by acceleration of the source, a binary lens, or possibly by statistical fluctuations. Although OGLE-2003-BLG-262 is only the second published event to date in which the lens transits the source, such events will become more common with the new OGLE-III survey in place. We therefore give a detailed account of the analysis of this event to facilitate the study of future events of this type.
We combine Spitzer and ground-based Korea Microlensing Telescope Network microlensing observations to identify and precisely measure an Earth-mass ( ) planet OGLE-2016-BLG-1195Lb at orbiting a ...ultracool dwarf. This is the lowest-mass microlensing planet to date. At kpc, it is the third consecutive case among the Spitzer "Galactic distribution" planets toward the Galactic bulge that lies in the Galactic disk as opposed to the bulge itself, hinting at a skewed distribution of planets. Together with previous microlensing discoveries, the seven Earth-size planets orbiting the ultracool dwarf TRAPPIST-1, and the detection of disks around young brown dwarfs, OGLE-2016-BLG-1195Lb suggests that such planets might be common around ultracool dwarfs. It therefore sheds light on the formation of both ultracool dwarfs and planetary systems at the limit of low-mass protoplanetary disks.
We present microlensing planet OGLE-2017-BLG-0173Lb, with planet-host mass ratio of either or , the lowest or among the lowest ever detected. The planetary perturbation is strongly detected, Δχ2 ∼ ...10000, because it arises from a bright (therefore, large) source passing over and enveloping the planetary caustic: a so-called "Hollywood" event. The factor ∼2.5 offset in q arises because of a previously unrecognized discrete degeneracy between Hollywood events in which the caustic is fully enveloped and those in which only one flank is enveloped, which we dub "Cannae" and "von Schlieffen," respectively. This degeneracy is "accidental" in that it arises from gaps in the data. Nevertheless, the fact that it appears in a Δχ2 = 10000 planetary anomaly is striking. We present a simple formalism to estimate the sensitivity of other Hollywood events to planets and show that they can lead to detections close to, but perhaps not quite reaching, the Earth/Sun mass ratio of . This formalism also enables an analytic understanding of the factor ∼2.5 offset in q between the Cannae and von Schlieffen solutions. The Bayesian estimates for the host mass, system distance, and planet-host projected separation are , , and , respectively. The two estimates of the planet mass are and . The measured lens-source relative proper motion will permit imaging of the lens in about 15 years or at first light on adaptive-optics imagers on next-generation telescopes. These will allow one to measure the host mass but probably will not be able to resolve the planet-host mass-ratio degeneracy.
We present the analysis of OGLE-2016-BLG-0613, for which the lensing light curve appears to be that of a typical binary-lens event with two caustic spikes but with a discontinuous feature on the ...trough between the spikes. We find that the discontinuous feature was produced by a planetary companion to the binary lens. We find four degenerate triple-lens solution classes, each composed of a pair of solutions according to the well-known wide/close planetary degeneracy. One of these solution classes is excluded due to its relatively poor fit. For the remaining three pairs of solutions, the most-likely primary mass is about , while the planet is a super Jupiter. In all cases, the system lies in the Galactic disk, about halfway toward the Galactic bulge. However, in one of these three solution classes, the secondary of the binary system is a low-mass brown dwarf, with relative mass ratios (1:0.03:0.003), while in the two others the masses of the binary components are comparable. These two possibilities can be distinguished in about 2024 when the measured lens-source relative proper motion will permit separate resolution of the lens and source.
We analyze the single microlensing event OGLE-2015-BLG-1482 simultaneously observed from two ground-based surveys and from Spitzer. The Spitzer data exhibit finite-source effects that are due to the ...passage of the lens close to or directly over the surface of the source star as seen from Spitzer. Such finite-source effects generally yield measurements of the angular Einstein radius, which when combined with the microlens parallax derived from a comparison between the ground-based and the Spitzer light curves yields the lens mass and lens-source relative parallax. From this analysis, we find that the lens of OGLE-2015-BLG-1482 is a very low-mass star with a mass or a brown dwarf with a mass , which are located at and , respectively, where is the distance between the lens and the source, and thus it is the first isolated low-mass microlens that has been decisively located in the Galactic bulge. The degeneracy between the two solutions is severe ( ). The fundamental reason for the degeneracy is that the finite-source effect is seen only in a single data point from Spitzer, and this single data point gives rise to two solutions for , the angular size of the source in units of the angular Einstein ring radius. Because the degeneracy can be resolved only by relatively high-cadence observations around the peak, while the Spitzer cadence is typically , we expect that events for which the finite-source effect is seen only in the Spitzer data may frequently exhibit this degeneracy. For OGLE-2015-BLG-1482, the relative proper motion of the lens and source for the low-mass star is , while for the brown dwarf it is . Hence, the degeneracy can be resolved within from direct-lens imaging by using next-generation instruments with high spatial resolution.
Using gravitational microlensing, we detected a cold terrestrial planet
orbiting one member of a binary star system. The planet has low mass
(twice Earth's) and lies projected at ∼0.8 astronomical ...units (AU) from
its host star, about the distance between Earth and the Sun. However,
the planet's temperature is much lower, <60 Kelvin, because the host
star is only 0.10 to 0.15 solar masses and therefore more than 400 times
less luminous than the Sun. The host itself orbits a slightly more
massive companion with projected separation of 10 to 15 AU. This
detection is consistent with such systems being very common.
Straightforward modification of current microlensing search strategies
could increase sensitivity to planets in binary systems. With more
detections, such binary-star planetary systems could constrain models of
planet formation and evolution.
In the course of conducting a deep (14.5 r 23), 20 night survey for transiting planets in the rich ~550 Myr old open cluster M37, we have measured the rotation periods of 575 stars, which lie near ...the cluster main sequence, with masses 0.2 M M 1.3 M . This is the largest sample of rotation periods for a cluster older than 500 Myr. Using this rich sample we investigate a number of relations between rotation period, color, and the amplitude of photometric variability. Stars with M 0.8 M show a tight correlation between period and mass with heavier stars rotating more rapidly. There is a group of four stars with P > 15 days that fall well above this relation, which, if real, would present a significant challenge to theories of stellar angular momentum evolution. Below 0.8 M , the stars continue to follow the period-mass correlation but with a broad tail of rapid rotators that expands to shorter periods with decreasing mass. We combine these results with observations of other open clusters to test the standard theory of lower main-sequence stellar angular momentum evolution. We find that the model reproduces the observations for solar-mass stars, but discrepancies are apparent for stars with 0.6 M 1.0 M . We also find that for late K through early M dwarf stars in this cluster, rapid rotators tend to be bluer than slow rotators in B - V but redder than slow rotators in V - IC . This result supports the hypothesis that the significant discrepancy between the observed and predicted temperatures and radii of low-mass main-sequence stars is due to stellar activity.
Microlensing is a powerful and unique technique to probe isolated objects in the Galaxy. To study the characteristics of these interesting objects based on the microlensing method, measurement of the ...microlens parallax is required to determine the properties of the lens. Of the various methods to measure microlens parallax, the most routine way is to make simultaneous ground- and space-based observations, i.e., by measuring the space-based microlens parallax. However, space-based campaigns usually require "expensive" resources. Gould & Yee (2012) proposed an idea called the "cheap space-based microlens parallax" that can measure the lens-parallax using only two or three space-based observations of high-magnification events (as seen from Earth). This cost-effective observation strategy to measure microlens parallaxes could be used by space-borne telescopes to build a complete sample for studying isolated objects. This would enable a direct measurement of the mass function including both extremely low-mass objects and high-mass stellar remnants. However, to adopt this idea requires a test to check how it would work in actual situations. Thus, we present the first practical test of this idea using the high-magnification microlensing event OGLE-2016-BLG-1045, for which a subset of Spitzer observations fortuitously duplicates the prescription of Gould & Yee (2012). From the test, we confirm that the measurement of the lens-parallax adopting this idea has sufficient accuracy to determine the physical properties of the isolated lens.
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