The Optical Navigation Camera onboard Hayabusa2 sets a new record as the smallest-aperture optical system to detect an exoplanet from space

May. 25, 2026 | GATEWAY to Academic Articles

YUMOTO Koki / Department of Solar System Sciences, ISAS

Long-duration observations with nanosatellites have gained increasing attention as a method for detecting long-period exoplanets (planets outside our Solar System that orbit far from their star, taking many Earth years to make a single orbit) that are underrepresented in current surveys due to observational biases that cause more closely orbiting planets to be more easily detected. However, no successful detections have been reported using instruments with apertures smaller than 60 mm. Here, we report the successful detection of exoplanets using the 15 mm aperture Telescopic Optical Navigation Camera (ONC-T) onboard the Hayabusa2 spacecraft. The detections were made by observing the dimming of starlight caused by planetary transits when a planet passes across the star’s surface and obscures a small part of its light. This result sets a new record as the smallest-aperture optical system to detect an exoplanet from space. These results demonstrate the feasibility of exoplanet observations using nanosatellites and provide a benchmark for estimating the telescope aperture needed to achieve the observational requirements of future missions.

Research Summary

Nearly 8,000 exoplanets*1 have been identified to date, but many have shorter orbital periods than Solar System planets of comparable mass. For instance, “hot Jupiters”—gas giants with masses similar to Jupiter but orbiting extremely close to their host stars with periods shorter than 10 days—have been discovered in large numbers. In contrast, long-period giant planets with orbital periods exceeding 10 years, such as Jupiter in our Solar System, remain rare in current detections.
Does this imply that our Solar System is unusual? Not necessarily. This apparent discrepancy likely arises from observational biases in current detection techniques, which are more sensitive to large planets with short orbital periods. In the transit method, a planet is detected by measuring the slight dimming of a star as it passes in front of it (Fig. 1, left). Consequently, planets with shorter orbital periods, which transit more frequently, are preferentially detected.

To mitigate these biases, transit observations using nanosatellites, such as JAXA’s LOTUS mission*2, are gaining increasing attention. Although nanosatellites are less sensitive to small planets, long-term, continuous monitoring of the same star may enable detection of long-period giant planets.
However, successful transit detections using small-aperture instruments suitable for nanosatellites have been extremely limited. The smallest optical system to date that has detected an exoplanet transit from space is the 60 mm aperture telescope onboard NASA’s ASTERIA satellite*3. No successful detections have been achieved with smaller apertures, leaving the capabilities of such compact systems largely unexplored.

Results from Hayabusa2

In this study, we used the 15 mm aperture telescopic Optical Navigation Camera (ONC-T) onboard the asteroid explorer Hayabusa2 (Fig. 1, right) to perform exoplanet transit observations, setting a new record for the smallest-aperture space telescope used for this purpose.
After returning samples from asteroid Ryugu to Earth in 2020, Hayabusa2 is currently cruising toward a flyby of asteroid Torifune in 2026. ONC-T has contributed to scientific observations of Ryugu and the detection of faint scattered light from interplanetary dust, demonstrating high observational precision despite its small size. At the same time, the instrument has experienced some degradation due to more than 10 years of exposure to cosmic radiation. Together, these conditions provide an ideal testbed for evaluating the feasibility of long-term transit observations with nanosatellites.

Using ONC-T, we observed a total of 14 transit events of two hot Jupiters, WASP-189 b and MASCARA-1 b, over a two-year period. Each event involved approximately 21 hours of continuous observations, yielding about 10,000 images. We successfully detected a ~0.5% decrease in stellar brightness associated with the transits (Fig. 2). By combining all events, we achieved signal-to-noise ratios*4 of 40 for WASP-189 b and 16 for MASCARA-1 b (Fig. 3). The measured transit timings agree with those obtained by NASA’s TESS satellite*5 to within ~2 minutes, and the planet-to-star radius ratios to within ~0.2%. These results demonstrate that an instrument with an extremely small 15 mm optical aperture retains sufficient performance for transit observations of Jupiter-sized planets, even after more than 10 years of exposure to cosmic radiation in space.
Furthermore, comparison of our results with past ground-based observations suggests that the orbital period of MASCARA-1 b may differ between recent and earlier measurements. This discrepancy may be caused by perturbations of its orbit due to gravitational interactions with nearby planets or stars, highlighting MASCARA-1 b as an important target for future study.

Future Prospects

This successful demonstration is expected to advance the design of nanosatellites dedicated to exoplanet detection. Our results show that even telescopes with apertures small as that of ONC-T can, through continuous multi-year observations of the same star, enable the detection of long-period giant planets that have remained largely undetected. Such observations will help determine how common planetary systems similar to our Solar System are, providing insight into whether our system is typical or unique.

Terminologies

• ^*1 Exoplanet: A planet outside our Solar System that orbits a star other than the Sun.
• ^*2 LOTUS mission: “LOng-period Transiting exoplanet sUrvey Satellite (LOTUS)” is a proposed JAXA mission to detect long-period transiting exoplanets using a nanosatellite.
• ^*3 ASTERIA satellite: The “Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA)” was a 6U CubeSat (~10 × 20 × 30 cm) technology demonstration developed by NASA to demonstrate precision photometry that is needed to study phenomenon such as stellar activity and transiting exoplanets. The satellite was launched in 2017 and successfully conducted observations until the end of the mission in 2020.
• ^*4 Signal-to-noise ratios are a measure of the signal (desired observation) to the background noise (unwanted interference). A high signal-to-noise (SNR) indicates a clear signal and more certain detection.
• ^*5 TESS satellite: The “Transiting Exoplanet Survey Satellite (TESS)” is a NASA mission to perform an all-sky survey of the brightest stars to search for exoplanets. The satellite was launched in 2018 and continues active observations to this day.

Information

Author

YUMOTO Koki
March 2024　Ph.D., Graduate School of Science, The University of Tokyo
April 2024 to Present　Research Fellow (PD), Japan Society for the Promotion of Science (JSPS), Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA)
October 2024 to Present　Visiting Researcher, Paris Observatory.