Multi-bandpass Photometry for Exoplanet Atmosphere Reconnaissance (MPEAR) with the Habitable Worlds Observatory (HWO) -- I. Differentiating Earth from Neptunes During Discovery

TL;DR

This study develops the MPEAR framework to optimize multi-bandpass photometry during the first discovery visit of Habitable Worlds Observatory (HWO), aiming to distinguish Earth-like planets from Neptune-like templates when only a single broadband flux at 500 nm is available. By combining Keplerian orbital calculations, PSG-based flux modeling with disk sub-sampling, and a Python-based exposure-time calculator (pyEDITH) that propagates realistic noise, the authors quantify how different pairs or trios of parallel bandpasses can differentiate Earth from warm and cold Neptunes. They find that at the baseline discovery signal-to-noise ratio ($S/N=7$), no 2- or 3-bandpass strategy reliably separates Earth from both Neptune classes; however, certain bandpass configurations, especially those including a near-infrared band around 1.1 μm and a second VIS band near 850–880 nm, offer qualitative discrimination for the cold Neptune and, with longer integration, potential reliable discrimination. The results underscore the potential of on-the-fly photometric triage to prioritize follow-up spectroscopy and optimize mission yield, while also highlighting model-dependent limitations and the need for higher $S/N$, channel-edge optimization, and multi-epoch analyses in future work.

Abstract

As the architecture for the Habitable Worlds Observatory (HWO) is being developed, it is crucial to optimize the observing strategies for a survey to detect and characterize Earth-like planets around Sun-like stars. Efficient target identification and characterization will help drive mission requirements that can be matched to the planned observations. Current HWO concepts allow simultaneous multi-bandpass observations with the coronagraph instrument, critical for performing a qualitative planetary reconnaissance to optimize observing time for deriving orbital constraints and prioritize characterization of promising targets. We describe a new algorithm designed to determine the best combination of broadband photometric observations for extracting maximum information from the first visit. It identifies degeneracies in the orbital configurations, fluxes, and noise, and determines optimal secondary photometry bands to reduce these. We demonstrate its application by comparing an Earth seen at quadrature with a cold and a warm Neptune at inclined orbits and varying phases, with comparable flux in the discovery bandpass centered at 500 nm (20\% bandwidth). Using the noise and exposure time calculator that we developed for the HWO coronagraph instrument, we find that the baseline $S/N=7$ (corresponding to 3.2 hours observing time for a planet at 10pc) is only sufficient to marginally differentiate the Earth from a cold Neptune-like planet assuming two parallel bandpasses (550 nm + 850 nm). However, increasing to $S/N=15$ (7 hours observing time) and using three parallel bandpasses (360 nm + 500 nm + 1.11 micron) would differentiate the Earth from either a warm or cold Neptune.

Figures (11)

• Figure 1: Some combinations of semi-major axes, inclination, and true anomaly that were considered for this study. Top panel: 3D scatter plot showing the combinations of parameters that would yield a plausible orbit that would match the target at (0,-0.1) arcsec. The colorbar shows the absolute difference of the coordinates from the target position. Configurations are considered to be "compatible" with the target position if the difference is less than 0.01 arcsec. Bottom panels: Individual orbit plots for specific points A through E labeled in the top panel, each with their projection in $\hat{X}-\hat{Y}$ and $\hat{Z}-\hat{Y}$ space. For each point, details on the orbital location in terms of semi-major axis, inclination, true anomaly and apparent phase are shown. In the various plots, special markers indicate visually the phase that we would expect to find the planet in.
• Figure 2: Histogram showing the percentage of orbits that satisfy the orbital condition with respect to the semi-major axis parameter. In green, the orbits whose semi-major axis is $\le1.75$ AU; in red, the orbits with $a > 1.75$ AU. Labeled at the top, the cumulative percentages of all orbits belonging to these subgroups. Gaps in the results are caused by the discrete sampling of the grid.
• Figure 3: Temperature and abundance profiles for template Earth and Neptunes atmospheres. Left: Earth atmosphere from kofman24Center: Cold Neptune atmosphere from 2025arXiv250813368PRight: Warm Neptune atmosphere from 2019ApJ...887..166H.
• Figure 4: Orbits (top left panel: $\hat{X}-\hat{Y}$ orbital projection; top right panel: $\hat{Z}-\hat{Y}$ orbital projection) and fluxes (bottom panel) of the warm Neptunes that are compatible with the assumed detection point, compared to the orbit and flux of an Earth at quadrature. In black, the orbit (top panels) and the flux (bottom panel) of the Earth on a face-on orbit. In yellow-green hues, the orbits (top panels) and fluxes (bottom panels) of all warm Neptunes that are compatible with the detected position (Condition 1), color-coded according to the inclination of their orbits. In red hues, the orbits (top panels) and fluxes (bottom panel) of the warm Neptunes that also yield a comparable flux at 500 nm (Condition 2). In the top panels, the projected distance is shown as a yellow circle. In the bottom panel, the photometric spectrum of the Earth template calculated at 0.5 $\mu$m with a 20% bandwidth (horizontal errorbar), and assuming $S/N$=7 (vertical errorbar), is shown in yellow.
• Figure 5: Orbits (top left panel: $\hat{X}-\hat{Y}$ orbital projection; top right panel: $\hat{Z}-\hat{Y}$ orbital projection) and fluxes (bottom panel) of the cold Neptunes that are compatible with the assumed detection point, compared to the orbit and flux of an Earth at quadrature. In black, the orbit (top panels) and the flux (bottom panel) of the Earth on a face-on orbit. In yellow-green hues, the orbits (top panels) and fluxes (bottom panels) of all cold Neptunes that are compatible with the detected position (condition 1), color-coded according to the inclination of their orbits. In red hues, the orbits (top panels) and fluxes (bottom panel) of the cold Neptunes that also yield a comparable flux at 500 nm (condition 2). In the top panels, the projected distance is shown as a yellow scatter plot. In the bottom panel, the photometric spectrum of the Earth template calculated at 0.5 $\mu$m with a 20% bandwidth (horizontal errorbar), and assuming $S/N$=7 (vertical errorbar) is shown as a yellow errorbar point.