Press Releases

DATE2023.04.14 #Press Releases

Subaru Images, Weighs, and Tracks Massive Benchmark Exoplanet

April 14, 2023

“We are now in a new era for imaging other worlds,” says Thayne Currie, lead author of the ground-breaking paper published in Science.

Direct imaging is a method that will someday reveal an Earth-like exoplanet around a nearby star. In the past 14 years, large ground-based telescopes equipped with adaptive optics (AO) to sharpen starlight have taken key steps towards this goal, revealing the first direct images of Jupiter-like gas giant exoplanets. These discoveries draw from so-called blind surveys: targets are selected based on system properties like age and distance but are otherwise unbiased. Unfortunately, the low yields of these blind surveys show that exoplanets we can image with current telescopes are rare.

Direct imaging searches focused on stars showing dynamical evidence for a planet may greatly increase the rate of imaging discoveries. Precision astrometry -- measuring the position and motion of stars on the sky -- could identify which stars are being pulled by the gravitational influence of an unseen companion and thus may host planets we can image.

An international research team led by the University of Tokyo, Subaru Telescope, the University of Texas-San Antonio, and the Astrobiology Center of Japan report the world’s first joint direct imaging and astrometric discovery of an exoplanet, using Subaru Telescope’s extreme adaptive optics system (SCExAO) coupled with its near-infrared spectrograph combined with astrometry from European Space Agency’s Gaia mission and its predecessor, Hipparcos. The planet was imaged around the nearby bright star HIP 99770, located in the constellation Cygnus.

“Once we knew which star to look at, Subaru’s extreme adaptive optics system was able to sharpen starlight so well that our infrared instruments could see the faint planet hinted at by Gaia and Hipparcos” notes Olivier Guyon, the Principal Investigator of SCExAO.

Figure 1. Infrared image of HIP 99770 taken by the Subaru Telescope. The bright main star at the position marked with * is hidden. The dashed ellipse shows the size of Jupiter’s orbit around the Sun to scale. The arrow points to the HIP 99770 b extrasolar planet.  (Credit:  T. Currie/Subaru Telescope, UTSA)

Figure 2. A movie showing the orbital motion of HIP 99770 b, made by combining the Subaru Telescope’s images taken from 2020-2021. (Credit:  T. Currie/Subaru Telescope, UTSA)


The planet – HIP 99770 b – is about 100,000 times fainter than the star it orbits. Its CHARIS spectrum, combined with follow-up imaging from the W.M. Keck Observatory, reveals an atmosphere shaped by water and carbon monoxide, with a temperature about 10 times hotter than Jupiter’s. Its atmosphere resembles an older and slightly less cloudy counterpart to the atmospheres of the first imaged planets, HR 8799 bcd.

By jointly analyzing data from the Subaru Telescope, Keck, Gaia and Hipparcos, the team was able to directly measure the planet’s mass and constrain its orbit. HIP 99770 b is about 15 +/- 1 times the mass of Jupiter in our own Solar System, and orbits a star that is nearly twice as massive as the Sun. The planet’s orbit is three times larger than Jupiter’s around the Sun or just over half of Neptune’s distance from the Sun. However, it receives nearly the same amount of light as Jupiter because it’s host star is far more luminous than the Sun

“Combining direct imaging from Subaru and Keck with precision astrometry tells us far more about planets like HIP 99770 b than was previously possible,” says Currie.

The combined approach will also allow us to find an Earth-like planet around a nearby star with upcoming ground-based observatories like the Thirty Meter Telescope or space-based ones like the Habitable Worlds Observatory. Such a planet will be much closer to its star than any planet imaged to date and so will spend a large amount of time either in front or behind that star, making it impossible to see.

“The indirect detection method will help us to guess when a rocky, terrestrial planet could be imaged. Once we know when to look, we hope to learn whether this planet has an atmosphere compatible with life as we know it on Earth,” says Motohide Tamura, Professor of the University of Tokyo.

The Subaru Telescope and the W. M. Keck observatory are located at the summit of Maunakea in Hawai`i, an inactive volcano known for its unsurpassed qualities as an astronomy site and its deep personal and cultural significance to many Native Hawaiians.

These results appeared as Currie et al. “Direct Imaging and Astrometric Detection of a Gas Giant Planet Orbiting an Accelerating Star” in Science on April 14, 2023.

Project Assistant Professor Jungmi KWON and Professor Motohide TAMURA (Department of Astronomy, School of Science, The University of Tokyo) contributed to this research.


Publication details

Journal  Science
 Direct Imaging and Astrometric Detection of a Gas Giant Planet Orbiting an Accelerating Star
 Currie et al.