Current instruments can directly image massive planets and brown dwarfs around only about 1% of stars, even in young systems where newly formed companions still emit residual heat and appear brighter than older objects. In most cases, these companions remain far fainter than their host stars and are lost in scattered starlight. OASIS addresses the problem of where to search by using space-based measurements to flag stars whose motion indicates the gravitational pull of unseen companions.
The survey draws on data from the European Space Agency's Hipparcos and Gaia missions to identify stars with accelerations that suggest orbiting massive companions. OASIS then observes these stars with Subaru's Coronagraphic Extreme Adaptive Optics (SCExAO) system to obtain high-resolution images and suppress starlight. SCExAO's performance allows observers to separate faint nearby companions from their host stars in regions where conventional imaging would fail.
One of the new discoveries, HIP 54515 b, is a planet orbiting a star 271 light-years away in the constellation Leo. The planet has nearly 18 times the mass of Jupiter and orbits its star at a distance comparable to Neptune's orbit around the Sun. From Earth, the angular separation between the star and planet is similar to the apparent size of a baseball viewed from 100 kilometers away, which illustrates the need for SCExAO's sharp imaging to resolve the companion.
The second object, HIP 71618 B, is a brown dwarf with a mass of about 60 Jupiters located 169 light-years away in the constellation Bootes. Brown dwarfs form in a star-like process but do not accumulate enough mass to sustain nuclear fusion in their cores. HIP 71618 B's orbit and host star brightness make it a strong candidate for testing advanced coronagraphs designed to image faint companions close to bright stars.
NASA's Roman Space Telescope will include a technology demonstration coronagraph system intended to validate techniques needed to directly image Earth-like planets that can be up to ten billion times fainter than their host stars. Before the identification of HIP 71618 B, there was no confirmed target that met all the constraints required for this demonstration. HIP 71618 B satisfies these criteria because its host star is bright, the brown dwarf's projected separation is well matched to Roman's instrument, and at Roman's operating wavelengths the brown dwarf will be faint enough relative to its star to provide a realistic test.
The OASIS discoveries illustrate the effectiveness of combining long-baseline space astrometry with ground-based direct imaging to uncover planets and brown dwarfs that would otherwise remain undetected. This joint approach improves the efficiency of high-contrast imaging campaigns by focusing on stars with strong evidence for massive companions. The results also highlight Subaru Telescope's role in exoplanet and substellar companion studies as new facilities and space missions begin operations.
Research Report:SCExAO/CHARIS and Gaia Direct Imaging and Astrometric Discovery of a Superjovian Planet 3 - 4 ?/D from the Accelerating Star HIP 54515
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