Subaru's First OASIS Discoveries: An 18×Jupiter Exoplanet and a 'Failed Star' Ready for NASA's Roman Tests

Astronomers using Japan's Subaru Telescope on Mauna Kea have announced the first discoveries from the OASIS program: a massive exoplanet and a nearby brown dwarf that together demonstrate the power of precision imaging and astrometry.

OASIS (Observing Accelerators with SCExAO Imaging Survey) combines Subaru's high-contrast SCExAO imaging with astrometric data from ESA's Hipparcos and Gaia missions to find stars that show the tiny gravitational "tugs" of unseen companions. This approach makes it possible to pinpoint promising targets for direct imaging and follow-up characterization.

HIP 54515 b is the newly imaged exoplanet. Located about 271 light‑years away in the constellation Leo, it has an estimated mass of nearly 18 times that of Jupiter. The planet orbits its host star at a separation comparable to Neptune's distance from the Sun, placing it on a wide orbit that facilitates direct imaging with modern instruments.

HIP 71618 B is the brown dwarf discovery, roughly 169 light‑years away in Bootes. Brown dwarfs occupy the mass range between the largest planets and the smallest stars; they form like stars but lack sufficient mass to sustain long‑term hydrogen fusion, which is why they are often called "failed stars."

What makes HIP 71618 B especially notable is its suitability as a calibration and test target for NASA's Nancy Grace Roman Space Telescope. Roman will carry a technology demonstration of coronagraph systems designed to block starlight and image extremely faint planets—objects up to about 10 billion times fainter than their host stars. NAOJ says this brown dwarf meets Roman's tight requirements and will provide a valuable real‑sky target for testing those high-contrast instruments ahead of Roman's planned 2026–2027 launch window.

These initial OASIS results highlight how combining precise astrometry (Hipparcos and Gaia) with advanced high-contrast imaging (SCExAO on Subaru) speeds the discovery of massive exoplanets and substellar companions. Follow-up observations across different wavelengths and instruments will refine the properties of HIP 54515 b and HIP 71618 B and help prepare Roman and other telescopes to image ever-fainter worlds.

Bottom line: Subaru's OASIS has delivered two high-impact finds—a giant exoplanet and an ideal brown dwarf test target—that will help advance direct imaging technology and the next generation of space observatories.