Super-Earth Ross 508b overflies the red dwarf’s habitable zone

Super-Earth Ross 508b overflies the red dwarf's habitable zone

Figure 1: Schematic representation of the newly discovered planetary system around Ross 508. The green region represents the habitable zone (HZ) where liquid water can exist on the planet’s surface. The planetary orbit is shown as a blue line. The planet is estimated to be closer than the HZ (solid line) for more than half of its orbit and inside 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 Infrared Spectrograph IRD on the Subaru Telescope (IRD-SSP). This planet, Ross 508b, is a super-Earth about four times Earth’s mass and is located near the habitable zone. Such a planet may retain water on its surface and will be an important target for future observations to test 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 Sun-like star, is now focusing on red dwarfs, which are less massive 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 neighborhood. The discovery of such nearby exoplanets, with detailed observations of their atmospheres and surface layers, will allow us to discuss the presence or absence of life in environments vastly different from those of our solar system.

However, due to their low surface temperature of less than 4,000 degrees, red dwarfs are very faint in visible light. Previous planet searches using visible-light spectrometers have found few planets around very nearby red dwarfs like Proxima Centauri b. In particular, red dwarfs with surface temperatures below 3,000 degrees (late-type red dwarfs) have not been systematically searched 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 spectroscopic Doppler, so the search for planets around red dwarfs using the transit method has made progress in recent years. transiting planet searches using TESS (Transiting Exoplanet Survey Satellite). 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 observe because they are too faint in visible light. To solve the difficulties in spectroscopic observations of red dwarfs, a planet search with a high-precision spectrograph has long been awaited in the infrared, where red dwarfs are relatively bright. For example, at a distance of 30 light-years, the Sun is five orders of magnitude in visible light and three orders of magnitude in infrared. On the other hand, the lightest late-type red dwarfs are very faint in the visible at 19 magnitudes, but relatively bright in the infrared at 11 magnitudes.

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. The IRD mounted on the Subaru telescope can detect minute variations in a star’s speed, about the speed of a person walking.

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 with respect to the celestial plane. It is also an important method as it can determine the “mass” of a planet.

The IRD Subaru Strategic Program (IRD-SSP) to search for planets around late-type red dwarfs started in 2019. This is the first systematic search for planets around late-type red dwarfs and an international project involving about 100 local and international researchers . During the first two years, screening observations were carried out to find “stable” red dwarfs with low noise, where even small planets can be detected. Red dwarfs have high surface activity, such as B. Flares, and this surface activity can lead to changes in the star’s line-of-sight velocity even when no planets exist. 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 phase of intensive monitoring of about 50 promising late-type red dwarfs, carefully selected through the screening.

Super-Earth Ross 508b overflies the red dwarf's habitable zone

Figure 2: Periodic variation of the line-of-sight velocity of the star Ross 508 observed by the IRD. It wraps around the orbital period of the planet Ross 508b (10.77 days). Ross 508’s line-of-sight velocity change is less than 4 meters per second, indicating that IRD detected a very small wobble slower than a person running. The red curve fits the observations best, and its deviation from a sinusoid indicates that the planet’s orbit is most likely elliptical. Photo 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 through a systematic search using an infrared spectrometer.

To confirm that Ross 508’s periodic wobble is indeed due to a planet, the IRD-SSP team identified several indicators of stellar activity that could result in a planet being falsely positive (e.g., changes in stellar brightness and form of some emissions). lines) and showed that the period of these indicators differs significantly from the observed planetary period. This is a more difficult task than using the Doppler method to confirm planet candidates previously reported by the transit procedurebut it is an essential method for detecting non-transiting planets.

This planet, Ross 508b, has a minimum mass of only about four times that of Earth. Its average distance from its host star is 0.05 times the Earth-Sun distance, and it is located on the star’s inner edge habitable zone. Interestingly, the planet likely has an elliptical orbit, in which case it would enter the habitable zone with an orbital period of about 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 test the possibility of habitability on planets around red dwarfs. Spectroscopic observations of molecules and atoms in the planet’s atmosphere are also important, while current telescopes cannot directly image the planet due to its proximity to the host star. In the future, it will be one of the life-searching targets through 30-meter-class telescopes.

So far, only three planets were known that 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 a super-Earth using only near-infrared spectroscopy. Previously, for the detection of low-mass planets such as super-Earths, near-infrared observations alone were not required to be accurate enough to be verified by high-precision line-of-sight velocity measurements in visible light. This study shows that IRD-SSP alone is capable of detect planetsand clearly demonstrates the advantage of IRD-SSP in its ability to search even late type with high accuracy Red Dwarfs who are too weak to be observed visible light” says Dr. Hiroki Harakawa (NAOJ Subaru Telescope), the lead author of the discovery paper.

“14 years have passed since the development of the IRD began. We continued our development and research in hopes of finding a planet just like Ross 508b. This discovery was made possible by the high instrumental performance of the IRD, the large aperture of the Subaru telescope, and the strategic observing framework that enabled intensive and frequent data collection. We are striving to make new discoveries,” says Professor Bun’ei Sato (Tokyo Institute of Technology), the principal investigator of the IRD-SSP.

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


Super-Earth Exoplanet Orbiting Nearby Star Discovered


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

Citation: Super-Earth Ross 508b skims habitable zone of red dwarf (2022, August 1), retrieved August 2, 2022 from https://phys.org/news/2022-08-super-earth-ross-508b-skims -habitable.html

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