First discoveries from new Subaru Telescope program

Astronomers using the Subaru Telescope in Hawaiʻi have discovered a massive planet and a brown dwarf orbiting distant stars. The discoveries are the first results from OASIS (Observing Accelerators with SCExAO Imaging Survey), which combines space-based measurements with the Subaru Telescope’s advanced imaging to find hidden worlds. These discoveries in turn enable NASA’s upcoming Roman Space Telescope to test critical technologies for imaging Earth-like planets.

Only about 1% of stars host massive planets and brown dwarfs that can be photographed directly with current telescopes. Even in young planetary systems where these objects are still glowing hot with the energy of having just been formed, making them brighter and easier to detect, they’re still much fainter than their host stars and are easily lost in the stellar glare. The key question for astronomers has been: where to look for these objects?

That is where OASIS [Principal Investigator (PI): Thayne Currie / Deputy-PI: Masayuki Kuzuhara] comes in. The program uses measurements from two European Space Agency missions—Hipparcos and Gaia—to identify stars being tugged by the gravity of unseen companions. OASIS then targets these promising candidates with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system, which provides the exceptional precision and advanced technology needed to actually photograph these hidden companions.

The newly discovered planet, HIP 54515 b, orbits a star 271 light-years away in the constellation Leo. With nearly 18 times Jupiter’s mass, it circles its star at about Neptune’s distance from our Sun. But the star and planet appear very close when seen from Earth; roughly the size that a baseball seen 100 km away would appear. The SCExAO system produced extremely sharp images allowing us to see the planet.

The second discovery, HIP 71618 B, is a 60 Jupiter mass brown dwarf located 169 light-years away in the constellation Bootes. Brown dwarfs are sometimes called “failed stars”—because they form like stars but never become massive enough to sustain nuclear fusion.

What makes HIP 71618 B special is its highly suitable properties for observations with NASA’s Roman Space Telescope. Roman will carry out a technology demonstration to test coronagraph systems that future telescopes will need to photograph Earth-like planets around other stars—planets that are ten billion times fainter than their host stars. Before this discovery, astronomers didn’t have a single confirmed target meeting all the strict requirements for this demonstration. HIP 71618 B changes that, checking off the boxes for being a suitable target: its star is bright and the brown dwarf is in the right location. At the Roman Coronagraph’s operating wavelengths it will be faint enough compared to its star to validate these new technologies.

These discoveries from OASIS showcase how combining space-based precision star-tracking and ground-based direct imaging can reveal planets and brown dwarfs that would otherwise remain hidden. This type of tag-team observations leading to new discoveries shows that the Subaru Telescope will continue to be a world-leading observatory in astronomy even as new telescopes come online.