On the Detectability of Compact Multi-Planet Systems via Gravitational Microlensing
Authors: Stephen R. Kane
Abstract
Compact multi-planet systems orbiting ultracool dwarfs, such as TRAPPIST-1, may be a major population of terrestrial worlds with poorly constrained demographics. Individual close-in planets are expected to escape microlensing detection, but whether a compact system can produce a detectable collective signal remains to be quantified. We calculate the microlensing signal from each TRAPPIST-1 planet, and of all seven combined, if the system were a lens toward the bulge. For a 0.09 $M_\odot$ lens at 4 kpc, $R_E \approx 1.21$ AU, placing the planets at $s = 0.010$--$0.051~θ_E$, 12--63 times interior to the lensing zone. We measure each signal as the residual after fitting a single-lens model, which accounts for the barycentric dipole. Because the quadrupole term cancels between the two images to first order, the residual scales as the octupole ($q s^3$), reaching $|δA / A| \sim 10^{-6}$. The seven planets combined yield $Δχ^2 \sim 10^{-3}$ even at 1~mmag precision, five orders of magnitude below the standard detection threshold. Detection requires increasing all planet masses by a factor of $\sim$300, whereas the resonant chain loses its guarantee of Hill stability at a factor of $\sim$7. Only a lens within $\sim$15 pc of the source or the Sun places the outer planets in the lensing zone. The Nancy Grace Roman Space Telescope is therefore not expected to detect a TRAPPIST-1-like system through microlensing, whereas its transit channel in the same fields can detect such planets in large numbers, marking a sharp boundary in the Galactic census between microlensing and transit sensitivity.
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Detecting Multi-Planetary Systems with Gravitational Microlensing and the Roman Space TelescopeHossein Fatheddin, Sedighe Sajadian · 2023 · Earth and Planetary Astrophysics · cited 3 times · AI summary available
Detecting Multi-Planetary Systems with Gravitational Microlensing and the Roman Space Telescope examines whether Roman can detect planetary systems through microlensing.
Abstract
It is plausible that most of the Stars in the Milky Way (MW) Galaxy, like the Sun, consist of planetary systems, instead of a single planet. Out of the estimately discovered 3,950 planet-hosting stars, about 860 of them are known to be multiplanetary systems (as of March, 2023). Gravitational microlensing, which is the magnification in the light of a source star, due to a single or several lenses, has proven to be one of the most useful Astrophysical phenomena with many applications. Until now, many extrasolar planets (exoplanets) have been discovered through binary microlensing, where the lens system consists of a star with one planet. In this paper, we discuss and explore the detection of …
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Detection of Extrasolar Planets by Gravitational MicrolensingDavid P. Bennett · 2009 · chapter 3 of Mason, J., 2008, Exoplanets, Edited by John Mason. Berlin: Springer, 2008. ISBN: 978-3-540-74007-0 · cited 47 times · AI summary available
Detection of Extrasolar Planets by Gravitational Microlensing reviews microlensing physics and evidence for planets at separations of a few AU.
Abstract
Gravitational microlensing provides a unique window on the properties and prevalence of extrasolar planetary systems because of its ability to find low-mass planets at separations of a few AU. The early evidence from microlensing indicates that the most common type of exoplanet yet detected are the so-called "super-Earth" planets of ~10 Earth-masses at a separation of a few AU from their host stars. The detection of two such planets indicates that roughly one third of stars have such planets in the separation range 1.5-4 AU, which is about an order of magnitude larger than the prevalence of gas-giant planets at these separations. We review the basic physics of the microlensing method, and sh…
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Exoplanetary MicrolensingB. Scott Gaudi · 2010 · Earth and Planetary Astrophysics · cited 17 times · AI summary available
Exoplanetary Microlensing explains how planets perturb microlensing images, providing an overview of the method used to assess planetary signals.
Abstract
Gravitational microlensing occurs when a foreground star happens to pass very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the source star into two images, which are typically unresolved. However, these images are also magnified, by an amount that depends on the angular lens-source separation. The relative lens-source motion results in a time-variable source magnification: a microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will further perturb the images, resulting in a characteristic, short-lived signature of the planet. This c…
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EDEN Survey: Small Transiting Planet Detection Limits and Constraints on the Occurrence Rates for Late M Dwarfs within 15 pcJeremy Dietrich, Dániel Apai, Martin Schlecker et al. · 2023 · Earth and Planetary Astrophysics · cited 8 times · AI summary available
The EDEN Survey measures transit detection limits and occurrence rates for small planets around nearby late M dwarfs, complementing the paper’s microlensing sensitivity analysis.
Abstract
Earth-sized exoplanets that transit nearby, late spectral type red dwarfs will be prime targets for atmospheric characterization in the coming decade. Such systems, however, are difficult to find via wide-field transit surveys like Kepler or TESS. Consequently, the presence of such transiting planets is unexplored and the occurrence rates of short-period Earth-sized planets around late M dwarfs remain poorly constrained. Here, we present the deepest photometric monitoring campaign of 22 nearby late M dwarf stars, using data from over 500 nights on seven 1-2 meter class telescopes. Our survey includes all known single quiescent northern late M dwarfs within 15 pc. We use transit-injection-and…
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Astrometric exoplanet detection with GaiaMichael Perryman, Joel Hartman, Gáspár Bakos et al. · 2014 · Earth and Planetary Astrophysics · cited 208 times · AI summary available
Astrometric exoplanet detection with Gaia estimates detectable planetary systems using simulated Gaia observations, addressing exoplanet sensitivity with a different observing method.
Abstract
We provide a revised assessment of the number of exoplanets that should be discovered by Gaia astrometry, extending previous studies to a broader range of spectral types, distances, and magnitudes. Our assessment is based on a large representative sample of host stars from the TRILEGAL Galaxy population synthesis model, recent estimates of the exoplanet frequency distributions as a function of stellar type, and detailed simulation of the Gaia observations using the updated instrument performance and scanning law. We use two approaches to estimate detectable planetary systems: one based on the S/N of the astrometric signature per field crossing, easily reproducible and allowing comparisons wi…
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Microlensing Constraints on the Stellar and Planetary Mass FunctionsJennifer C. Yee, Scott J. Kenyon · 2025 · Earth and Planetary Astrophysics · cited 3 times · AI summary available
Microlensing Constraints on the Stellar and Planetary Mass Functions compares microlensing mass functions and planetary populations rather than detectability of compact systems.
Abstract
The mass function (MF) of isolated objects measured by microlensing consists of both a stellar and a planetary component. We compare the microlensing MFs of Gould et al (2022) and Sumi et al (2023) to other measurements of the MF. The abundance of brown dwarfs in the Sumi et al (2023) stellar MF is consistent with measurements from the local solar neighborhood (Kirkpatrick et al 2024). Microlensing free-floating planets ($\mu$FFPs) may may be free-floating or orbit host stars with semimajor axes $a\gtrsim 10~\mathrm{au}$ and therefore can constrain the populations of both free-floating planetary-mass objects and wide-orbit planets. Comparisons to radial velocity and direct imaging planet pop…
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ROME/REA: A gravitational microlensing search for exo-planets beyond the snow-line on a global network of robotic telescopesYiannis Tsapras, R. A. Street, M. Hundertmark et al. · 2019 · Publications of the Astronomical Society of the Pacific, 131:124401 (12pp), 2019 December · cited 11 times · AI summary available
ROME/REA searches for planets beyond the snow line with microlensing, whereas the paper evaluates signals from close-in terrestrial planets.
Abstract
Planet population synthesis models predict an abundance of planets with semi-major axes between 1-10 au, yet they lie at the edge of the detection limits of most planet finding techniques. Discovering these planets and studying their distribution is critical to understanding the physical processes that drive planet formation. ROME/REA is a gravitational microlensing project whose main science driver is to discover exoplanets in the cold outer regions of planetary systems. To achieve this, it uses a novel approach combining a multi-band survey with reactive follow-up observations, exploiting the unique capabilities of the Las Cumbres Observatory (LCO) global network of robotic telescopes comb…
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OGLE-2015-BLG-0845L: A low-mass M dwarf from the microlensing parallax and xallarap effectsZhecheng Hu, Wei Zhu, Andrew Gould et al. · 2024 · Solar and Stellar Astrophysics · cited 1 time · AI summary available
OGLE-2015-BLG-0845L analyzes parallax and xallarap in one microlensing event to measure a low-mass lens, rather than testing planetary-system detectability.
Abstract
We present the analysis of the microlensing event OGLE-2015-BLG-0845, which was affected by both the microlensing parallax and xallarap effects. The former was detected via the simultaneous observations from the ground and Spitzer, and the latter was caused by the orbital motion of the source star in a relatively close binary. The combination of these two effects led to a direct mass measurement of the lens object, revealing a low-mass ($0.14 \pm 0.05 M_{\odot}$) M-dwarf at the bulge distance ($7.6 \pm 1.0$ kpc). The source binary consists of a late F-type subgiant and a K-type dwarf of $\sim1.2 M_{\odot}$ and $\sim 0.9 M_{\odot}$, respectively, and the orbital period is $70 \pm 10$ days. OG…
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