Maunakea, Hawaiʻi – Astronomers have developed a new method for finding exoplanets whose portraits can be taken from Earth using large ground-based telescopes, one that has proven successful after this technique resulted in a direct image of a Jupiter-like gas giant located 132.8 light-years away in the constellation Cygnus.
The planet, called HIP 99770 b, is the first one beyond our solar system found using a powerful combination of astrometry and direct imaging.
Two Maunakea Observatories on Hawaiʻi Island – W. M. Keck Observatory and Subaru Telescope – performed the direct imaging, snapping infrared photos of the planet directly. The astrometry, which measured the position and motion of HIP 99770 b’s home star, came from the European Space Agency’s Gaia space observatory and its predecessor Hipparcos.
The study will be published in the journal Science on Friday, April 14.
“Performing both direct imaging and astrometry allows us to gain a full understanding of an exoplanet for the first time: measure its atmosphere, weigh it, and track its orbit all at once,” said Thayne Currie, an affiliated researcher at Subaru Telescope and lead author of the study. “This new approach for finding planets prefigures the way we will someday identify and characterize an Earth-twin around a nearby star.”
Detecting HIP 99770 b is tough; because the planet is faint, it can get lost in the glare of its bright host star.
“This is the kind of discovery that really could have only been done from Maunakea,” said Currie. “We are extremely grateful for the privilege of being able to study the heavens from this mountain.”
This new way of combing for nearby exoplanets is a major improvement to traditional, ground-based means; for the past 14 years, astronomers have been using so-called ‘blind’ surveys to scour the sky for stars that show potential for housing giant planets we can directly image from Earth based on the star system’s age and distance. However, this technique has a low yield. Precision astrometry on the other hand detects the movement of stars, which allows researchers to zero in on the ones tugged by the gravitational pull of an unseen companion such as a planet, then capture a picture of the star systems that are close enough to directly image.
“Our discovery really changes the way we do exoplanet science,” said Currie. “Direct imaging is very exciting but also really challenging, with many nights devoted to looking for planets around other stars from so many programs only to come up empty. By fine-tuning our technique with astrometry, we now know exactly where to look.”
HIP 99770 b serves as proof of concept, developed by an international research team led by Subaru Telescope, University of Tokyo, University of Texas-San Antonio, and the Astrobiology Center of Japan. From the Subaru Telescope and Keck Observatory data, they determined HIP 99770 b is 14-16 times the mass of Jupiter and orbits a star that is nearly twice as massive as the Sun. However, it receives nearly the same amount of sunlight as Jupiter because its host star is far more luminous than the Sun.
The team also characterized the nature of HIP 99770 b’s atmosphere, namely its temperature, gravity, clouds, and chemistry. Using Keck Observatory’s second generation Near-Infrared Camera (NIRC2) paired with the Keck II telescope adaptive optics system, they found the gas giant has a slightly higher temperature and is less cloudy than the HR 8799 planets – the very first directly imaged exoplanetary system discovered in 2008 by two Maunakea Observatories – Keck Observatory and Gemini Observatory. The planet’s atmosphere also has signs of water and carbon monoxide.
Furthermore, the HIP 99770 star system is surrounded by a disk of icy dust, similar to the Kuiper Belt in our own solar system.
“This is the first of many discoveries from our Keck and Subaru imaging program that uses astrometry to select targets. We already have additional discoveries that will be announced later this year and next year,” said Currie.
- “Subaru Telescope Images, Weighs, and Tracks Massive Benchmark Exoplanet” – Subaru Telescope Press Release, April 13, 2023
The Near-Infrared Camera, second generation (NIRC2) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.
ABOUT ADAPTIVE OPTICS
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and current systems now deliver images three to four times sharper than the Hubble Space Telescope at near-infrared wavelengths. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.