Twinkle Twinkle Little Star, Roman Sees Where You Are: Predicting Exoplanet Transit Yields in the Rosette Nebula with the Nancy Grace Roman Space Telescope

TL;DR

This paper assesses the exoplanet transit yield of a hypothetical Nancy Grace Roman Space Telescope survey of the Rosette Nebula, a ~10 Myr star-forming region. It combines Gaia-based membership, an IMF-based population, synthetic photometry, and a forward-model injection-recovery pipeline with a conservative 7σ detection threshold to predict 33±9 young transiting planets in a month and 29±8 in a two-week baseline, dominated by small planets around M dwarfs. The extended baseline enhances sensitivity to longer-period planets around FGK stars, while most M-dwarf detections are captured in the shorter baseline. The study demonstrates Roman’s ability to map early planetary evolution, constrain radius inflation, and identify key targets for follow-up with JWST, Rubin Observatory, and the Habitable Worlds Observatory.

Abstract

Young stars host only a small fraction of the known exoplanet population because their photometric variability, magnetic activity, and frequent placement in dense, poorly-resolved regions hamper exoplanet detections. Yet, measuring planets at these ages is crucial since these phases are when dynamical processes that drive planetary migration are most active. We assess the expected yield of a hypothetical Nancy Grace Roman Space Telescope transit survey of the Rosette Nebula, a $\sim10\,\mathrm{Myr}$ star-forming region with a dense and diverse stellar population. Using the Roman Exposure Time Calculator to quantify sensitivity to Rosette members, we establish detection thresholds for companions and evaluate yields via Monte Carlo injection-recovery simulations. We predict the detection of $33 \pm 9$ young transiting exoplanets orbiting stellar hosts in a month-long survey, and $29 \pm 8$ in a two-week survey. The extended baseline primarily improves sensitivity to longer-period planets orbiting FGK stars, while most M dwarf detections are well-sampled within two weeks. Irrespective of the temporal baseline, transit detections are dominated by of 1-2\,$R_\oplus$ super-Earths and sub-Neptunes with $P\lesssim8$\,days. Such a sample would substantially expand the census of only three detected planets younger than $20\,\mathrm{Myr}$, probing an age regime in which planetary radii remain inflated, the stability of close-in orbits is uncertain, and planetary migration may still be ongoing. This survey offers a path to constrain early planetary evolution and establish prime follow-up targets for the James Webb Space Telescope, Vera Rubin Observatory, and the Habitable Worlds Observatory.

Figures (8)

• Figure 1: Roman WFI footprint overlaid on an image of the Rosette Nebula taken from the Second Digitized Sky Survey lasker_guide_1990. Red squares outline the 18 individual WFI detectors. The physical scale bar assumes a distance of 1.4 kpc to the Rosette Nebula.
• Figure 2: Gaia DR3 sources overlapping the Roman/WFI footprint in the Rosette Nebula. Candidate members (salmon) and field stars (gray) are shown in their sky distribution (left), proper motions (center), and distances (right). We also show $1\sigma$, $2\sigma$, and $3\sigma$ proper-motion contour lines for field stars (center). To reduce foreground and background contamination, we applied a distance prior restricting the sample to sources within 1–1.9 kpc.
• Figure 3: We show our extinction and reddening corrected Gaia sample (circles) in absolute color-magnitude space and the baraffe_new_2015 evolutionary track for a $0.8 \: M_\odot$ star (red line) with points at 0.5 Myr, 1 Myr, 2 Myr, 3 Myr, 4 Myr, 5 Myr, 7 Myr, 10 Myr, and 20 Myr (red circles). The purple circles represent stars with interpolated masses above our completeness limit of $0.8 \; M_\odot$, whereas the gray circles represent members below this limit that we discard in our determination of the initial mass function normalization. We also plot a 10 Myr solar-metallicity PARSEC isochrone (black line) and show the spectral classifications corresponding to each color bin.
• Figure 4: We present a 10 Myr brown dwarf marley_sonora_2021 model spectrum (orange line). We also show the normalized transmission of the Roman broadband F146 filter (black line) and its convolution with the spectrum (orange shaded region), corresponding to its effective stimulus.
• Figure 5: We show our $7\sigma$ detection limit on companion radius as a function of host mass using the black solid line. The green shaded region shows the range of detectable exoplanets around stars whereas the gray shaded shows the range of detectable exoplanets around brown dwarfs. The gray circles show all exoplanets from the NASA Exoplanet archive and the purple markers indicate the planets that are $< 20$ Myr old in this parameter space.