[Paper Review] White Paper: Exoplanetary Microlensing from the Ground in the 2020s

Jennifer C. Yee, Jay Anderson+26 more|arXiv (Cornell University)|Mar 21, 2018

Stellar, planetary, and galactic studies5 references3 citations

TL;DR

This white paper advocates for continued U.S. leadership in ground-based exoplanet microlensing during the 2020s by leveraging LSST and UKIRT surveys to map Galactic planet demographics, while proposing adaptive optics follow-up with ELTs to measure absolute planetary masses and separations. It demonstrates that combining high-cadence optical/IR surveys with post-event AO imaging can resolve lens-source blends and transform mass ratios into physical masses, enabling direct tests of planet formation theories across diverse Galactic environments.

ABSTRACT

Microlensing can access planet populations that no other method can probe: cold wide-orbit planets beyond the snow line, planets in both the Galactic bulge and disk, and free floating planets (FFPs). The demographics of each population will provide unique constraints on planet formation. Over the past 5 years, U.S. microlensing campaigns with Spitzer and UKIRT have provided a powerful complement to international ground-based microlensing surveys, with major breakthroughs in parallax measurements and probing new regions of the Galaxy. The scientific vitality of these projects has also promoted the development of the U.S. microlensing community. In the 2020s, the U.S. can continue to play a major role in ground-based microlensing by leveraging U.S. assets to complement ongoing ground-based international surveys. LSST and UKIRT microlensing surveys would probe vast regions of the Galaxy, where planets form under drastically different conditions. Moreover, while ground-based surveys will measure the planet mass-ratio function beyond the snow line, adaptive optics (AO) observations with ELTs would turn all of these mass ratios into masses and also distinguish between very wide-orbit planets and genuine FFPs. To the extent possible, cooperation of U.S. scientists with international surveys should also be encouraged and supported.

Motivation & Objective

• To extend ground-based microlensing surveys to map planet frequency across the Galactic disk and bulge, probing planet formation in diverse environments.
• To overcome the limitation of measuring only mass ratios by enabling absolute mass measurements through post-microlensing adaptive optics (AO) imaging of lens stars.
• To distinguish between wide-orbit planets and free-floating planets (FFPs) using AO flux constraints on lens stars.
• To support international collaboration by integrating U.S. assets with global microlensing surveys to maximize scientific yield.
• To optimize future AO campaigns for microlensing follow-up by reducing overheads through 'less-than-perfect fast' observation modes on 30-meter class telescopes.

Proposed method

• Utilize LSST’s high-cadence, all-sky optical survey to detect microlensing events across the Galactic plane (ℓ ∈ (−90°, 90°)) and anti-center fields.
• Combine LSST data with simultaneous high-cadence near-infrared (NIR) surveys (e.g., UKIRT) to improve source star flux measurements to ~1% precision, enhancing lens flux detection.
• Perform post-event adaptive optics (AO) imaging with 30-meter class telescopes (e.g., TMT, GMT) to resolve lens and source stars separated by ~1 mas, overcoming crowding in Galactic bulge fields.
• Measure the angular Einstein radius from microlensing light curves and combine it with lens flux measurements to derive absolute lens and planetary masses.
• Use AO constraints on unresolved flux to rule out host stars for free-floating planet (FFP) candidates, distinguishing them from wide-orbit planets.
• Implement a 'less-than-perfect fast' AO mode to reduce slew, settle, and acquisition overheads, enabling efficient systematic imaging of all microlensing planet hosts within 5 years of discovery.

Experimental results

Research questions

• RQ1 How does planet frequency vary between the Galactic bulge and disk, and what does this reveal about planet formation in different star-formation environments?
• RQ2 What is the true physical mass distribution of planets beyond the snow line, and how does it relate to the mass-ratio function measured by current surveys?
• RQ3 How do the planet mass-ratio functions derived from microlensing (biased toward M dwarfs) compare to those from radial velocity and transit surveys (biased toward FGK dwarfs), and what does this imply about host-star mass dependence?
• RQ4 Can AO imaging resolve lens-source blends in microlensing events to measure absolute masses and separations, and how efficiently can this be done with next-generation ELTs?
• RQ5 To what extent can a 'less-than-perfect fast' AO mode on 30-meter telescopes significantly improve the throughput of mass measurement campaigns without sacrificing scientific yield?

Key findings

• LSST and UKIRT microlensing surveys will enable the detection of planets across the Galactic plane (ℓ ∈ (−90°, 90°)), extending coverage beyond traditional bulge fields and probing planet frequency variations on a Galactic scale.
• High-cadence optical and NIR surveys (with ~1% flux precision) are essential to enable post-event flux measurements of lens stars, which, when combined with microlensing-derived angular Einstein radii, yield absolute planetary masses.
• Next-generation ELTs (e.g., TMT, GMT) can resolve lens-source blends in ~5 years post-event, enabling mass measurements for nearly all microlensing planets discovered by ground-based surveys.
• Adaptive optics imaging campaigns can distinguish between free-floating planets and wide-orbit planets by detecting or constraining flux from potential host stars, resolving a key ambiguity in FFP detection.
• A 'less-than-perfect fast' AO mode—prioritizing speed over optimal Strehl ratio—can reduce overheads and increase the efficiency of mass measurement campaigns by a factor of several, significantly boosting scientific yield at low additional cost.
• The break in the mass-ratio function near q ≈ 2×10⁻⁴, observed by microlensing at snow-line distances, can be physically interpreted only with absolute mass measurements, which will constrain planet formation theories more tightly than mass ratios alone.