An international team of astronomers led by UT San Antonio researchers has discovered two substellar companions — astronomical objects orbiting young and previously unexplored stars.
Their work provides the first target for NASA’s Nancy Grace Roman Space Telescope, which is scheduled to launch in May 2027.
These discoveries are the first results of a new program known as OASIS (Observing Accelerators with SCExAO Imaging Survey) that finds and characterizes massive gas giant planets like Jupiter.
The discoveries were made using a novel and powerful combination of precision measurements of stars from the space telescope called Gaia and adaptive optics imaging with the Subaru and Keck Telescopes in Hawaii to photograph faint companions whose gravity is causing the stars’ motions to change.
OASIS is an international collaboration involving astronomers from institutions across the United States, Japan, Canada, Chile, and Europe, and is supported by the National Science Foundation for its scientific merit and NASA as Key Strategic Mission Support for the Roman Space Telescope.
“With OASIS, we are able to find, weigh and track the orbits of massive planets around stars we never thought of looking at before,” said Thayne Currie, OASIS Principal Investigator and UT San Antonio associate professor.
Super-Jupiter buried in its star’s glare
The first discovery, HIP 54515 b, is a gas giant planet with a mass just under 18 times that of the planet Jupiter, orbiting at a Neptune-like separation from a star twice the mass of the Sun.
At about 275 light-years from Earth, HIP 54515 b appears very close to its star in the sky, pushing the boundaries of what current direct imaging technology can achieve.
“Its separation is how long a football would appear to us if it were over 200 miles away, so we need extremely sharp images to be able to see it,” Currie said, co-lead author of the study with University of Michigan postdoctoral researcher Yiting Li.
One thing that makes HIP 54515 b interesting is its eccentricity— a measure of how much an orbit deviates from a perfect circle. Gas giant planets with Jupiter-like masses and orbital separations tend to have nearly circular orbits. In contrast, HIP 54515 b is among a growing number of massive planets on Saturn-to-Neptune-like orbits with moderately eccentric orbits.
These larger eccentricities suggest a dynamic history or formation history that may differ from gas giants more similar in orbit and mass to Jupiter.
Brown dwarf crucial for NASA’s Earth-imaging plans
The second discovery, HIP 71618 B, also orbits a two-solar-mass star but is a brown dwarf, an object that formed like a star and is intermediate in mass between gas giant planets and stars. HIP 71618 B is about 60 times more massive than Jupiter, orbits its star with an average distance slightly larger than Saturn’s orbit around the sun, and has a very high eccentricity.
While not a planet itself, HIP 71618 B may play an important role in future searches for Earth-like planets around other stars because of its suitability for the Roman Coronagraph Technology Demonstration due to be carried out sometime in 2027.
This experiment will be the first to test advanced planet imaging technologies in a space telescope, the kind that will be needed on a future NASA telescope to suppress the glare of Sun-like stars to see rocky, Earth-like planets 10 billion times fainter.
Before the discovery of HIP 71618 B, there was not a single system demonstrably suitable for these critical observations with Roman, said Mona El Morsy, lead author of the discovery and postdoctoral fellow at UT San Antonio working with Currie.
“The HIP 71618 system checks off the boxes for what we need for the technology demonstration,” El Morsy said. “Its star is suitably bright. Its companion HIP 71618 B is located within a region where these new technologies from Roman will work. At the wavelengths where the Roman Coronagraph will operate, we predict that HIP 71618 B will be faint enough compared to its host star that a strong detection with Roman will validate these technologies.”
The OASIS survey builds upon the first planet discovery jointly achieved through astrometry and imaging, HIP 99770 b, published by Currie and collaborators in Science in 2023.
HIP 54515 b is now the third planet discovered this way; HIP 71618 B is also one of a small but growing number of brown dwarfs found through by astrometry and imaging.
Both discoveries showcase how combining astrometry and direct imaging can reveal planets and brown dwarfs that would otherwise remain hidden. The OASIS program continues to survey dozens of additional candidate systems, with more discoveries expected in the coming years.
“These are the first two of what we hope will be many planet and brown dwarf discoveries from OASIS,” El Morsy said. “These new discoveries could help clarify how planets and brown dwarfs form and how their atmospheres evolve, and provide NASA with additional targets for honing the exoplanet imaging technologies we will need to detect a habitable Earth-like planet in the future.”
The survey discoveries are presented in two newly published papers in The Astronomical Journal and The Astrophysical Journal Letters.
