Super Earth Ross 508b skims a red dwarf’s habitable zone

Figure 1: Schematic diagram of the newly discovered planetary system around Ross 508. The green area represents the habitable zone (HZ) where liquid water can exist on the planet’s surface. The orbit of the planets is shown as a blue line. For more than half of its orbit, the planet is estimated to lie near the HZ (solid line) and within the HZ (dashed line) for the rest of the orbit. Credit: Center for Astrobiology

The first exoplanet was discovered by the Subaru Strategic Program using the IRD infrared spectrometer on the Subaru Telescope (IRD-SSP). This planet, Ross 508 b, is a giant Earth with a mass about four times the mass of Earth and located near the habitable zone. Such a planet might be able to retain water on its surface, and would be an important target for future observations to verify the possibility of life around low-mass stars.

Research on exoplanets, which has made significant progress in recent years since the discovery of a giant planet around a star similar to our Sun, is focused on red dwarfs, which have a lower mass than our Sun. Red dwarfs, which make up three-quarters of the stars in our galaxy and are found in large numbers near our solar system, are excellent targets for finding exoplanets in our region. The discovery of such nearby exoplanets, with detailed observations of their atmosphere and surface layers, will allow us to discuss the presence or absence of life in environments very different from those in our solar system.

However, red dwarfs are very faint in visible light due to their surface temperature being as low as 4,000 degrees. Previous searches for planets using visible-light spectrometers have discovered a few planets around very nearby red dwarfs, such as Proxima Centauri b. In particular, red dwarfs with surface temperatures below 3000 degrees (red dwarfs of the late type) have not been systematically searched for planets. The transit method, which detects changes in stellar brightness when a planet transits in front of a star, does not require as many photons as the spectroscopic Doppler method, so the search for planets around red dwarfs using the transit method has advanced in recent years. Searches for transiting planets using 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 studying life in the universe, they are difficult to notice because they are very faint in visible light. In order to solve the difficulties involved in the spectroscopic observation of red dwarfs, a planetary search using a high-resolution spectrophotometer in the infrared, where red dwarfs are relatively bright, is long overdue. For example, the brightness of the Sun visible from 30 light-years away is five intensities in visible light and three degrees in infrared light. On the other hand, the lightest late-type red dwarfs are very faint in visible light with a strength of 19 degrees, but relatively bright in infrared with a strength of 11 degrees.

The Center for Astrobiology in Japan has successfully developed the IRD (Infrared Doppler Instrument), the world’s first high-resolution infrared spectrometer for 8-meter class telescopes. The IRD installed on the Subaru telescope can detect minute oscillations in a star’s velocity, about the speed of a person walking.

The transit method can only detect planets whose orbits lie along the line of sight, while the Doppler method can detect planets regardless of their orientation with respect to the celestial plane. It is also an important method as it can determine the “mass” of the planet.

The IRD Subaru Strategic Program (IRD-SSP) began searching for planets around late-type red dwarfs in 2019. This is the first systematic search for planets around late-type red dwarfs and is an international project involving about 100 local and international researchers. During the first two years, screening observations were made to find “stable” low-noise red dwarfs, where even minor planets could be detected. Red dwarfs have high surface activity, such as flares, and this surface activity can cause changes in a star’s line-of-sight velocity even when there are no planets. Therefore, stable red dwarfs with low surface activity are the targets in the search for small Earth-like planets.

Currently, the project is in the intensive monitoring phase of about 50 promising red dwarfs of the late type that have been carefully selected through screening.

Super Earth Ross 508B is skimming the habitable zone of a red dwarf

Figure 2: The periodic variation in the line-of-sight velocity of Ross 508 observed by the IRD. It is wrapped around the orbital period of the planet Ross 508 b (10.77 days). The change in the Ross 508’s line-of-sight velocity is less than 4 meters per second, indicating that the IRD picked up a very small wobble that is slower than a person running. The red curve is best suited for observations, and its deviation from the sinusoidal curve indicates that the planet’s orbit is likely to be elliptical. Credit: Harakawa et al. 2022

The first exoplanet discovered by the 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. This is the first exoplanet discovered by a systematic search using an infrared spectrometer.

To confirm that Ross 508’s periodic oscillation is indeed due to a planet, the IRD-SSP team identified several indicators of stellar activity that could produce a planet’s false positives (for example, changes in stellar brightness and the shape of some emission lines) and showed that the period of these indicators varies clearly about the period of the observed planets. This is a more difficult task than using the Doppler method to confirm candidate planets previously reported by the transit method, but it is an essential method for detecting non-transiting planets.

This planet, Ross 508 b, has a minimum mass of about four times that of Earth. The average distance from its central star is 0.05 times the distance between Earth and the Sun, and it is located at the inner edge of the habitable zone. Interestingly, the planet likely has an elliptical orbit, in which case it will cross into the habitable zone with an orbital period of about 11 days (Figs 1 and 2).

Planets in the habitable zone can retain water on their surfaces and may harbor life. Ross 508b will be an important target for future observations to verify the habitability of planets around red dwarfs. Spectroscopic observations of molecules and atoms in the atmospheres of planets are also important, while 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 of searches for life by 30-meter class telescopes.

So far, only three planets are known to orbit such very low-mass stars, including Proxima Centauri b. The IRD-SSP is expected to continue discovering new planets.

Ross 508b is the first successful discovery of super-Earths using only near-infrared spectroscopy. Prior to this, in the discovery of low-mass planets such as super-Earths, near-infrared observations were not accurate enough, verified by high line-of-sight velocity measurements. Accuracy in visible light was essential.This study shows that only the IRD-SSP is capable of detecting planets, and clearly demonstrates the advantage of the IRD-SSP in its ability to search with such high accuracy that Dr. Hiroki Harakawa (NAOJ Subaru Telescope), lead author of the discovery paper says: late-type red dwarfs that are too faint to be observed with visible light.”

“It has been 14 years since the development of the IRD began. We have continued our development and research in the hope of finding a planet just like Ross 508b. This discovery was made possible by the efficient performance of the IRD, the Subaru telescope’s large aperture, and the strategic framework of observations that made it possible to obtain the data We are committed to making new discoveries.” says Professor Bonai Sato (Tokyo Institute of Technology), principal investigator at IRD-SSP.

These results appeared as Harakawa et al. “Super-Earth orbiting near the inner edge of the habitable zone around M4.5-dwarf Ross 508” in Publications of the Japanese Astronomical Society On June 30, 2022.

Super-Earth exoplanet discovered orbiting a nearby star

more information:
Hiroki Harakawa et al, super-Earth orbit near the inner edge of the habitable zone around M4.5 dwarf Ross 508, Publications of the Japanese Astronomical Society (2022). DOI: 10.1093/pasj/psac044

Provided by the National Institutes of Natural Sciences

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