From W.M. Keck Observatory: “A Rocky Planet Around One of Our Galaxy’s Oldest Stars”

W.M. Keck Observatory, two ten meter telescopes operated by Caltech and the University of California, Maunakea Hawaii USA, altitude 4,207 m (13,802 ft). Credit: Caltech.

UCO Keck Laser Guide Star Adaptive Optics on two 10 meter Keck Observatory telescopes, Maunakea Hawaii USA, altitude 4,207 m (13,802 ft). Credit:

Mauna Kea Observatory, Hawaii USA, altitude 4,213 m (13,822 ft).

From W.M. Keck Observatory

January 11, 2021

Mari-Ela Chock, Communications Officer
W. M. Keck Observatory
(808) 554-0567

University of Hawaiʻi Astronomers Using W. M. Keck Observatory Discover Ancient Magma World Orbiting a Chemically Unusual Star

Artist’s rendition of toi-561, one of the oldest, most metal-poor planetary systems discovered yet in the milky way galaxy. this 10 billion-year-old system has a hot, rocky exoplanet (center) that’s one and a half times the size of earth as well as two gas planets (to the left of the rocky planet) that are about twice as large as earth.
Credit: W. M. Keck Observatory/Adam Makarenko.

“They should have sent a poet,” says Ellie Arroway in the film Contact as, suspended in outer space, she gazes upon a spiral galaxy. Almost all of the planets discovered to date (including the solar system planets) are confined to the plane of the Milky Way, unable to glimpse such a sweeping vista of our galaxy.

However, astronomers at the University of Hawaiʻi Institute for Astronomy (IfA) using the W. M. Keck Observatory on Maunakea have discovered a rocky planet with a different kind of view.

The planet orbits the star TESS Object of Interest (TOI) 561, named for the ongoing NASA TESS planet hunting mission.

NASA/MIT TESS replaced Kepler in search for exoplanets.

TOI-561 belongs to a rare population of stars called the galactic thick disk. Thick disk stars are chemically distinct, with fewer trace heavy elements (and especially less iron) than typical stars of the Milky Way, suggesting they formed early, approximately 10 billion years ago. They also have wandering motions that can lift them out of the galactic plane, providing an epic view of our own spiral galaxy.

“The rocky planet orbiting TOI-561 is one of the oldest rocky planets yet discovered. Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago,” says Dr. Lauren Weiss, Beatrice Watson Parrent Postdoctoral Fellow at UH IfA and leader of the team that discovered the TOI-561 planetary system.

The result was announced at a press conference today, January 11, 2021, at the January 2021 meeting of the American Astronomical Society and is published in The Astronomical Journal.

The rocky planet orbiting TOI-561 transits its star, meaning that the planet passes in front of its star as seen from Earth, blocking a fraction of the starlight. The planet is small, with a radius only one and a half times that of Earth. As a result, the reduction of light it causes is miniscule, just 0.025% of the star’s brightness.

Astronomers at UH IfA noticed this change in intensity and used Keck Observatory’s High Resolution Echelle Spectrometer (HIRES) [below] to confirm the presence of the planet. By measuring the wobble of the star induced by the planet’s gravity, they were able to infer that the planet has three times the mass of Earth. Combining this mass with the radius determined from the transits, the team concluded that the planet is most likely rocky, perhaps with less iron than Earth.

TOI-561 has at least two other planets transiting the star, both of which have about twice Earth’s radius and are too large and low-mass to be rocky.

The origin of stars in the galactic thick disk is unclear, with some studies suggesting that they formed in a distinct, old galaxy that our younger and more massive Milky Way galaxy later cannibalized. Alternatively, they could be some of the first stars that formed within the Milky Way.

“I wonder what view of the night sky would have been accessible from the rocky planet during its history,” says Weiss.

Illustration showing the structural components of the Milky Way galaxy. The star TOI-561 is located in the thick disk (marked in red-orange), which contains a rare, older population of stars. While nearly all known planets are found within the thin disk (marked in orange), the newly discovered rock-and-lava exoplanet orbiting TOI-561 is one of the first confirmed rocky planets orbiting a galactic thick disk star. Credit: Kaley Brauer/MIT.

Although the past 10 billion years of the planet’s history are murky, it likely does not host life now. The planet orbits its star twice every Earth day, so close to its host star that the estimated average surface temperature is over 3100 degrees Fahrenheit (or about 1726 Celsius) — far too toasty for life as we know it. However, this rocky, magma planet is perhaps a harbinger of a population of rocky worlds yet to be discovered around our galaxy’s oldest stars.

The observations and analysis in this study were carried out as part of the TESS-Keck survey, a multi-institutional effort to characterize the properties of planets discovered by the NASA TESS mission.

See the full article here .

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The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometer and world-leading laser guide star adaptive optics systems. Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Today Keck Observatory is supported by both public funding sources and private philanthropy. As a 501(c)3, the organization is managed by the California Association for Research in Astronomy (CARA), whose Board of Directors includes representatives from the California Institute of Technology and the University of California, with liaisons to the board from NASA and the Keck Foundation.

Keck UCal


Keck 1

HIRES – The largest and most mechanically complex of the Keck’s main instruments, the High Resolution Echelle Spectrometer breaks up incoming starlight into its component colors to measure the precise intensity of each of thousands of color channels. Its spectral capabilities have resulted in many breakthrough discoveries, such as the detection of planets outside our solar system and direct evidence for a model of the Big Bang theory.

Keck High-Resolution Echelle Spectrometer (HIRES), at the Keck I telescope.

LRIS – The Low Resolution Imaging Spectrograph is a faint-light instrument capable of taking spectra and images of the most distant known objects in the universe. The instrument is equipped with a red arm and a blue arm to explore stellar populations of distant galaxies, active galactic nuclei, galactic clusters, and quasars.

UCO Keck LRIS on Keck 1.

VISIBLE BAND (0.3-1.0 Micron)

MOSFIRE – The Multi-Object Spectrograph for Infrared Exploration gathers thousands of spectra from objects spanning a variety of distances, environments and physical conditions. What makes this huge, vacuum-cryogenic instrument unique is its ability to select up to 46 individual objects in the field of view and then record the infrared spectrum of all 46 objects simultaneously. When a new field is selected, a robotic mechanism inside the vacuum chamber reconfigures the distribution of tiny slits in the focal plane in under six minutes. Eight years in the making with First Light in 2012, MOSFIRE’s early performance results range from the discovery of ultra-cool, nearby substellar mass objects, to the detection of oxygen in young galaxies only 2 billion years after the Big Bang.

Keck/MOSFIRE on Keck 1, Mauna Kea, Hawaii, USA.

OSIRIS – The OH-Suppressing Infrared Imaging Spectrograph is a near-infrared spectrograph for use with the Keck I adaptive optics system. OSIRIS takes spectra in a small field of view to provide a series of images at different wavelengths. The instrument allows astronomers to ignore wavelengths where the Earth’s atmosphere shines brightly due to emission from OH (hydroxl) molecules, thus allowing the detection of objects 10 times fainter than previously available.

Keck OSIRIS on Keck 1

Keck 2

DEIMOS – The Deep Extragalactic Imaging Multi-Object Spectrograph is the most advanced optical spectrograph in the world, capable of gathering spectra from 130 galaxies or more in a single exposure. In ‘Mega Mask’ mode, DEIMOS can take spectra of more than 1,200 objects at once, using a special narrow-band filter.

Keck/DEIMOS on Keck 2.

NIRSPEC – The Near Infrared Spectrometer studies very high redshift radio galaxies, the motions and types of stars located near the Galactic Center, the nature of brown dwarfs, the nuclear regions of dusty starburst galaxies, active galactic nuclei, interstellar chemistry, stellar physics, and solar-system science.

NIRSPEC on Keck 2.

ESI – The Echellette Spectrograph and Imager captures high-resolution spectra of very faint galaxies and quasars ranging from the blue to the infrared in a single exposure. It is a multimode instrument that allows users to switch among three modes during a night. It has produced some of the best non-AO images at the Observatory.

KECK Echellette Spectrograph and Imager (ESI) on Keck II.

KCWI – The Keck Cosmic Web Imager is designed to provide visible band, integral field spectroscopy with moderate to high spectral resolution, various fields of view and image resolution formats and excellent sky-subtraction. The astronomical seeing and large aperture of the telescope enables studies of the connection between galaxies and the gas in their dark matter halos, stellar relics, star clusters and lensed galaxies.

Keck Cosmic Web Imager on Keck 2 schematic.

Keck Cosmic Web Imager on Keck 2.

NEAR-INFRARED (1-5 Micron)

ADAPTIVE OPTICS – Adaptive optics senses and compensates for the atmospheric distortions of incoming starlight up to 1,000 times per second. This results in an improvement in image quality on fairly bright astronomical targets by a factor 10 to 20.

LASER GUIDE STAR ADAPTIVE OPTICS [pictured above] – The Keck Laser Guide Star expands the range of available targets for study with both the Keck I and Keck II adaptive optics systems. They use sodium lasers to excite sodium atoms that naturally exist in the atmosphere 90 km (55 miles) above the Earth’s surface. The laser creates an “artificial star” that allows the Keck adaptive optics system to observe 70-80 percent of the targets in the sky, compared to the 1 percent accessible without the laser.

NIRC-2/AO – The second generation Near Infrared Camera works with the Keck Adaptive Optics system to produce the highest-resolution ground-based images and spectroscopy in the 1-5 micron range. Typical programs include mapping surface features on solar system bodies, searching for planets around other stars, and analyzing the morphology of remote galaxies.

Keck NIRC2 Camera on Keck 2.


Keck Near-Infrared Echellette Spectrometer on Keck 2.

Future Instrumentation

KCRM – The Keck Cosmic Reionization Mapper will complete the Keck Cosmic Web Imager (KCWI), the world’s most capable spectroscopic imager. The design for KCWI includes two separate channels to detect light in the blue and the red portions of the visible wavelength spectrum. KCWI-Blue was commissioned and started routine science observations in September 2017. The red channel of KCWI is KCRM; a powerful addition that will open a window for new discoveries at high redshifts.

KCRM – Keck Cosmic Reionization Mapper KCRM on Keck 2.

KPF – The Keck Planet Finder (KPF) will be the most advanced spectrometer of its kind in the world. The instrument is a fiber-fed high-resolution, two-channel cross-dispersed echelle spectrometer for the visible wavelengths and is designed for the Keck II telescope. KPF allows precise measurements of the mass-density relationship in Earth-like exoplanets, which will help astronomers identify planets around other stars that are capable of supporting life.

KPF Keck Planet Finder on Keck 2