When you say “Subaru” to most people, they think of the Japanese car company with the funny commercials featuring dog families promoting the vehicles loved by both human families and outdoors types. Others may think of the Pleiades star cluster M45 or the "Seven Sisters" whose Japanese name is the source of both the Subaru company name and its iconic six star logo with the seventh being invisible. However, those truly in the know will think of the Subaru Telescope, which is located at the Mauna Kea Observatory on Hawaii and is the primary telescope for those searching for Planet Nine (for now – we’re looking at you, James Webb Space Telescope). While the Subaru Telescope hasn’t found the elusive and still unproven Planet Nine yet, this week it was announced that astronomers have used it to find something real – a super-Earth exoplanet with about four times the mass of the Earth that is located near the habitable zone of its star system. Could we someday soon see a family of space dogs in their Subaru spaceship watching their Subaru telescope-generated GPS system heading for Ross 508b?
"Ross 508 b is the first successful detection of a super-Earth using only near-infrared spectroscopy. Prior to this, in the detection of low-mass planets such as super-Earths, near-infrared observations alone were not accurate enough, and verification by high-precision line-of-sight velocity measurements in visible light was necessary. This study shows that IRD-SSP alone is capable of detecting planets, and clearly demonstrates the advantage of IRD-SSP in its ability to search with a high precision even for late-type red dwarfs that are too faint to be observed with visible light."
Dr. Hiroki Harakawa (NAOJ Subaru Telescope), the lead author of the discovery paper published this week in the journal Publications of the Astronomical Society of Japan (PASJ), explains in the press release how the Subaru telescope was crucial in the discovery of Ross 508 b – a “super-Earth” exoplanet orbiting Ross 508, a 13th magnitude low mass red dwarf star 37 light years away. NASA is a strong supporter of searching red dwarfs for potentially habitable exoplanets since three-quarters of the stars in the Milky Way galaxy are red dwarfs. Dr. Harakawa says his team has now proven that near-infrared spectroscopy can detect exoplanets orbiting red dwarfs that are too faint to see because of their low surface temperature. Proxima Centauri b, another exoplanet circling a red dwarf, was detected using visible light spectrometer – it helped that Proxima Centauri is the closest star to our Sun. The Subaru telescope doesn’t need light to detect red dwarfs – it just needs a tiny bit of motion for a special instrument.
“The Astrobiology Center in Japan has successfully developed IRD (InfraRed Doppler instrument), the world's first high-precision infrared spectrograph for 8-meter class telescopes. IRD mounted on the Subaru Telescope can detect minute wobbles in the velocity of a star, about the speed of a person walking.”
Development on the InfraRed Doppler began 14 years ago and has finally paid off, says Professor Bun'ei Sato (Tokyo Institute of Technology), the principal investigator of IRD-SSP and a co-author of the study. The large aperture of the Subaru Telescope gave the IRD the data it needed to determine that Ross 508b has a minimum mass of about four times that of the Earth; however, its average distance from Ross 508 is only 0.05 times the distance from the Earth to the Sun (5 million km or 3.1 million miles – for comparison, Mercury is 60 million km or 3.1 million miles from our Sun), which gives it a blazing orbit time of only 10.77 days. (An illustration of the star, planet and habitable zone can be seen here.) At that distance, it’s a good thing this is a cool red dwarf. This also means that Ross 508’s habitable zone is also very close to the star, and the study reveals that Ross 508 b’s elliptical orbit crosses into the inner edge of its star’s habitable zone, making it a good candidate for retaining water and providing a livable environment for life forms. However, that elliptical orbit means it also moves out of the habitable “Goldilocks” zone, so Ross 508 b may not be ideal for life forms similar to Earth. Fortunately, the Subaru telescope is strong enough to deliver some more good news, as the study reveals:
“We have explored the possibility that the planet has a high eccentricity and its host is accompanied by an additional unconfirmed companion on a wide orbit. Our discovery demonstrates that the near-infrared RV search can play a crucial role in finding a low-mass planet around cool M dwarfs like Ross 508.”
A second planet orbiting Ross 508 is suspected but unconfirmed, and its orbit may or may not be in or passing through the Goldilocks zone like Ross 508 b, but the possibility is there. After the presence of water, another key to life support is a breathable atmosphere, and the study gives the disappointing news that the Subaru telescope and the InfraRed Doppler instrument, as good as they are, can’t detect them on Ross 508 b. However, help is on the way.
“Planets in the habitable zone could retain water on their surfaces and may harbor life. Ross 508 b will be an important target for future observations to verify the possibility of habitability on planets around red dwarfs. Spectroscopic observations of molecules and atoms in the planetary atmosphere are also important, while the current telescopes cannot directly image the planet due to its closeness to the central star. In the future, it will be one of the targets of life searches by 30-meter class telescopes.”
Just a stone’s throw away from the Subaru telescope at the Mauna Kea Observatory, the aptly named Thirty Meter Telescope (TMT) is under construction and when completed will be the largest visible-light telescope on Mauna Kea. Unfortunately, its construction has been halted a number of times by demonstrations over road blockages by heavy trucks and protests against the usage of conservation lands by those supporting Native Hawaiian cultural practices and religious rights. The protests and subsequent shutdowns and restarts in construction have occurred regularly since 2014 and as of June 20, 2022, no further construction has been announced or initiated, but instrument design, mirror casting & polishing, and other critical operations are finished or still being worked on. It is possible that the TMT will be completed soon and signs of life could be found on Ross 508 b.
The search for habitable planets and signs of intelligent life continue to be exciting and controversial.