Demonstrating Exoplanet Transit Photometry from Space with a 15-mm Aperture Optical Navigation Camera on Hayabusa2

TL;DR

This study demonstrates that a 15 mm aperture space-based telescope (ONC-T on Hayabusa2) can detect and characterize exoplanet transits, addressing biases against long-period and low-SNR signals. Using 14 transits of WASP-189 b and MASCARA-1 b, the authors quantify per-transit and phase-folded SNRs, measure transit mid-times $T_C$ and planet–star radius ratios $R_p/R_*$ with high precision, and update planetary ephemerides. They validate the ONC-T results with contemporaneous TESS data, and show that Transit Least Squares can recover true orbital periods from such a small-aperture instrument, despite sampling limitations. The work provides a practical framework and a scaling relation for predicting transit detectability with miniature optics, underscoring the potential of CubeSat-scale surveys to uncover long-period, Jupiter-sized exoplanets and broaden the exoplanet census.

Abstract

Observations of exoplanet transits by small satellites have gained increasing attention for reducing detection biases. However, no unambiguous detection of an exoplanet has yet been demonstrated using optics with apertures smaller than 60 mm. Here, we investigated the detectability of exoplanet transits using the telescopic Optical Navigation Camera (ONC-T) onboard the Hayabusa2 spacecraft, which has an effective aperture of only 15 mm. We conducted transit observations of the hot Jupiters WASP-189 b and MASCARA-1 b, collecting data for ten and four events, respectively. The transit signal was detected with a signal-to-noise ratio (SNR) of 13 for WASP-189 b and 8 for MASCARA-1 b for each event. Stacking all events improved the SNR to 40 and 16, respectively. The transit mid-times of each event were measured with a precision of 6 minutes and were consistent with Transiting Exoplanet Survey Satellite (TESS) data to within 2 minutes. The planet-to-star radius ratio was determined with an absolute precision of 0.004 (6% relative) and agreed with TESS results to within 0.002 (3% relative). The recent ONC-T and TESS data enabled an update to the planetary ephemerides. We report a 4 sigma discrepancy between the updated orbital period of MASCARA-1 b and previously reported values. ONC-T sets a new record for the smallest-aperture instrument to detect an exoplanet transit from space, advancing the frontier of exoplanet science with miniature instrumentation. Our results suggest that optics as small as ONC-T may be capable of detecting transiting long-period Jupiters: a population that remains underrepresented in current surveys.

Figures (9)

• Figure 1: Population statistics of all 5,917 confirmed exoplanets as of 11 June 2025. (a) Orbital period vs. planet mass for all planets with direct mass estimates. Red filled symbols represent exoplanets with transit detections, while white open symbols denote those without. Marker shapes indicate the method of discovery. (b) Distribution of orbital periods for all known exoplanets and transiting exoplanets. The black solid line represents the transit probability, proportional to Period$^{-2/3}$, scaled to match the data at a 10-day period. This line represents the expected distribution of transiting exoplanets, under the assumption that planet occurrence is uniform in $\log_{10} (\mathrm{Period}$) space. Data are from the NASA Exoplanet Archive.
• Figure 2: ONC-T images of (a) WASP-189 (image hyb2_onc_20240918_073620_tif_l2b) and (b) MASCARA-1 (image hyb2_onc_20241130_093826_tif_l2b) captured during transit events 10 and 12, respectively. The left panels show a single reduced ONC-T image ($p^t (x,y)$) without hot pixel correction (i.e., data reduced assuming $p_\mathrm{hot}=0$ in Equation \ref{['Eq1']}). The center panels show the same reduced images with hot pixel subtraction. The right panels show the reduced ONC-T image after applying a 3 $\times$ 3-pixel median filter and stacking all frames co-aligned on the target star, allowing the stellar positions to be clearly visualized. The host stars are marked with circles.
• Figure 3: Pointing and temperature stability of ONC-T during each transit observation. (a) The top and bottom panels show the horizontal ($x$) and vertical ($y$) centroid positions of the host star, respectively, expressed as deviations from their median values. (b) CCD temperature measured during each observation. The temperature readings are quantized in steps of 0.52°C. The root mean square deviation (RMSD) from the median is indicated in the legend of each panel.
• Figure 4: Light curves for each transit of (a) WASP-189 b and (b) MASCARA-1 b observed by ONC-T. Left panels show the raw light curves ($\widehat{I^t}$), with blue curves representing the combined transit and noise model fit ($\mathcal{T}(t)+\mathcal{N}(t)$). Center panels display the noise-detrended light curves ($\widehat{I^t}-\mathcal{N}(t)$), with blue curves showing the fitted transit model ($\mathcal{T}(t)$). Right panels show the residuals: $\widehat{I^t}-[\mathcal{T}(t)+\mathcal{N}(t)]$. Light curves from transit events 1–10 for WASP-189 b and 11–14 for MASCARA-1 b are displayed from top to bottom in each panel. Each light curve is vertically offset for clarity by 0.03 in panel (a) and 0.05 in panel (b).
• Figure 5: Phase-folded light curves of (a) WASP-189 b and (b) MASCARA-1 b from ONC-T and TESS data. The top panels show the phase-folded detrended light curves ($\widehat{I^t}-\mathcal{N}(t)$), while the bottom panels display the residuals of the ONC-T data relative to the fitted transit model ($\mathcal{T}(t)$; solid blue curve). Gray points show the flux measured in individual ONC-T images, while blue circles represent 30-minute binned averages, with error bars indicating the standard error. Error bars are not visible when smaller than the marker size. The red curve represents the TESS light curve, baseline-corrected and binned over 4-minute intervals.