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The Scientific Context of WFIRST Microlensing in the 2020s

Yee, Jennifer C. and Akeson, Rachel and Calchi Novati, Sebastiano and Henderson, Calen B. and Shvartzvald, Yossi (2019) The Scientific Context of WFIRST Microlensing in the 2020s. Astro2020 Science White Paper, . (Unpublished)

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As discussed in Exoplanet Science Strategy (National Academies of Sciences Engineering and Medicine 2018), WFIRST (Akeson et al. 2019) is uniquely capable of finding planets with masses as small as Mars at separations comparable to Jupiter, i.e., beyond the current ice lines of main sequence stars. In semimajor axis, these planets fall between the close-in planets found by Kepler (Coughlin et al. 2016) and the wide separation gas giants seen by direct imaging (e.g. Lagrange et al. 2009) and ice giants inferred from ALMA observations (Zhang et al. 2018). Furthermore, the smallest planets WFIRST can detect are smaller than the planets probed by radial velocity (Mayor et al. 2011; Bonfils et al. 2013) and Gaia (Perryman et al. 2014) at comparable separations. Interpreting planet populations to infer the underlying formation and evolutionary processes requires combining results from multiple detection methods to measure the full variation of planets as a function of planet size, orbital separation, and host star mass. Microlensing is the only way to find planets from 0.5 to 5M⊕ at separations of 1 to 5 au. Fundamentally, the case for a microlensing survey from space has not changed in the past 20 years: going to space allows wide-field diffraction-limited observations that can resolve main-sequence stars in the bulge, which in turn allows the detection and characterization of the smallest microlensing signals including those from planets with masses at least as small as Mars (Bennett & Rhie 2002). What has changed is that ground-based microlensing is reaching its limits, which underscores the scientific necessity for a space-based microlensing survey to measure the population of the smallest planets. Ground-based microlensing has found a break in the mass-ratio distribution at about a Neptune mass-ratio (Suzuki et al. 2016; Jung et al. 2018), implying that Neptunes are the most common microlensing planet and that planets smaller than this are rare. However, ground-based microlensing reaches its detection limits at mass ratios only slightly below the observed break. The WFIRST microlensing survey will measure the shape of the mass-ratio function below the break by finding numerous smaller planets: ~ 500 Neptunes, a comparable number of large gas giants, and ~ 200 Earths (if they are as common as Neptunes), and it can detect planets as small as 0.1M⊕ (Penny et al. 2018). In addition, because it will also measure host star masses and distances, WFIRST will also track the behavior of the planet distribution as a function of separation and host star mass.

Item Type:Report or Paper (White Paper)
Related URLs:
URLURL TypeDescription Paper
Yee, Jennifer C.0000-0001-9481-7123
Akeson, Rachel0000-0001-9674-1564
Calchi Novati, Sebastiano0000-0002-7669-1069
Henderson, Calen B.0000-0001-8877-9060
Shvartzvald, Yossi0000-0003-1525-5041
Additional Information:Science White Paper submitted to the Astro2020 Decadal Survey.
Group:Infrared Processing and Analysis Center (IPAC)
Series Name:Astro2020 Science White Paper
Record Number:CaltechAUTHORS:20191119-114457740
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99934
Deposited By: Tony Diaz
Deposited On:19 Nov 2019 21:27
Last Modified:19 Nov 2019 21:27

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