The first exoplanet was discovered by Subaru Strategic Program using the IRD Infrared Spectrograph on the Subaru Telescope (IRD-SSP). This planet, Ross 508b, is a super-Earth with about four times the mass of Earth and is located near the habitable zone. Such a planet could be able to retain water on its surface and will be an important target for future observations aimed at verifying the possibility of life around low-mass stars.
Exoplanet research, which has made great strides in recent years since the discovery of a giant planet around a star similar to our sun, is now focusing on red dwarfs, which have less mass than our sun. Red dwarfs, which make up three quarters of the stars in our galaxy and exist in large numbers in the vicinity of our solar system, are excellent targets for finding exoplanets in our vicinity. The discovery of such nearby exoplanets, together with detailed observations of their atmospheres and surface layers, will allow us to discuss the presence or absence of life in environments very different from those of our solar system.
However, red dwarfs are very weak in visible light due to their low surface temperature of less than 4000 degrees. Previous searches for planets using visible light spectrometers have found only a few planets around very close red dwarfs, such as Proxima Centauri b. In particular, red dwarfs with surface temperatures below 3000 degrees (late-type red dwarfs) have not been systematically searched for for planets. The transit method, which detects changes in stellar brightness as a planet passes in front of a star, does not require as many photons as the Doppler spectroscopic method, so searching for planets around red dwarfs using the transit method has progressed in recent years. . Transiting planet searches with TESS (Transiting Exoplanet Survey Satellite) can detect terrestrial planets around relatively heavy red dwarfs (early type red dwarfs).
Although red dwarfs are important targets for the study of life in the Universe, they are difficult to observe because they are too faint in visible light. In order to solve the difficulties associated with spectroscopic observations of red dwarfs, a planetary search using a high-precision spectrograph in the infrared, where red dwarfs are relatively bright, was long overdue. For example, the luminosity of the Sun seen from 30 light-years away is five magnitudes in visible light and three magnitudes in infrared light. On the other hand, the lightest late-type red dwarfs are very faint in visible light at magnitude 19, but relatively bright in the infrared at magnitude 11.
The Astrobiology Center in Japan has successfully developed the IRD (InfraRed Doppler instrument), the world’s first high-precision infrared spectrograph for 8-meter class telescopes. The IRD mounted on the Subaru telescope can detect tiny oscillations in the speed of a star, roughly the speed of a walking person.
The transit method can only detect planets whose orbits are along the line of sight, while the Doppler method can detect planets regardless of their orientation relative to the celestial plane. It is also an important method in that it can determine the “mass” of a planet.
The IRD Subaru Strategic Program (IRD-SSP) to search for planets around late-type red dwarfs began in 2019. It is the first systematic search for planets around late-type red dwarfs and is an international project involving a hundred national and international researchers. During the first two years, scouting observations were carried out to find low-noise “stable” red dwarfs, where even small planets can be detected. Red dwarfs have high surface activity, such as flares, and this surface activity can cause changes in the star’s line-of-sight velocity even when no planet exists. Therefore, only stable red dwarfs with low surface activity are targets in the search for small Earth-like planets.
Currently, the project is in the intensive observation phase of about 50 promising late-type red dwarfs that have been carefully selected during the screening.
The first exoplanet discovered by IRD-SSP is located about 37 light-years from Earth, around a red dwarf star called Ross 508, which is one-fifth the mass of the sun. It is the first exoplanet discovered by a systematic search using an infrared spectrometer.
To confirm that the periodic oscillation of Ross 508 is indeed due to a planet, the IRD-SSP team identified several indicators of stellar activity which could produce a false positive of a planet (for example, changes in the luminosity stellar and the shape of some emission lines) and showed that the period of these indicators is markedly different from the observed planetary period. This is a more difficult task than using the Doppler method to confirm planetary candidates previously reported by the transit method, but it is an essential method for detecting non-transiting planets.
This planet, Ross 508b, has a minimum mass about four times that of Earth. Its average distance from its central star is 0.05 times the Earth-Sun distance, and it is located at the inner edge of the habitable zone. Interestingly, the planet is likely to have an elliptical orbit, in which case it would pass into the habitable zone with an orbital period of around 11 days (Figures 1 and 2).
Planets in the habitable zone could retain water on their surface and support life. Ross 508b 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, as current telescopes cannot directly image the planet due to its proximity to the central star. In the future, it will be one of the targets for searches for life by 30-meter class telescopes.
Until now, only three planets were known to orbit such very low mass stars, including Proxima Centauri b. The IRD-SSP should pursue the discovery of new planets.
“Ross 508b 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 not accurate enough, and verification by high-precision line-of-sight velocity measurements in visible light was needed.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 high accuracy even for late-type red dwarfs that are too faint to be observed with visible light,” says lead author Dr Hiroki Harakawa (NAOJ Subaru Telescope). of the discovery article.
“14 years have passed since the development of the IRD began. We have continued our development and research in the hope of finding a planet exactly like Ross 508b. This discovery was made possible thanks to the high instrumental performance of the “IRD, the Subaru Telescope’s large aperture, and the strategic observing framework that enabled intensive and frequent data acquisition. We are committed to making new discoveries,” says Professor Bun’ei Sato (Tokyo Institute of Technology ), principal researcher at the IRD-SSP.
These results emerged as Harakawa et al. “A super-Earth orbiting near the inner edge of the habitable zone around the M4.5 Ross 508 dwarf” in Publications of the Astronomical Society of Japan June 30, 2022.
Super-Earth exoplanet orbiting star discovered
Hiroki Harakawa et al, A super-Earth orbiting near the inner edge of the habitable zone around the M4.5 dwarf Ross 508, Publications of the Astronomical Society of Japan (2022). DOI: 10.1093/pasj/psac044
Provided by National Institutes of Natural Sciences
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