A rocky planet transiting a nearby low-mass star (1511.03550v1)

Abstract: M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun -- are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.

• The paper reveals GJ 1132b as a rocky exoplanet discovered using precise transit photometry and radial velocity measurements.
• It employs data from the MEarth-South telescope and HARPS spectrograph to determine a 1.2 Earth-radius and 1.62 Earth-mass, indicating an Earth-like composition.
• The planet’s proximity and moderate temperature offer a rare opportunity for detailed atmospheric studies with instruments like JWST.

Discovery and Characterization of the Rocky Exoplanet GJ 1132b

The paper "A rocky planet transiting a nearby low-mass star" presents the discovery and initial characterization of an exoplanet, GJ 1132b, orbiting a nearby M-dwarf star, GJ 1132. The discovery is significant due to the planet's proximity, size, and the possibility of its atmosphere being amenable to further paper, despite it not being within the habitable zone.

Observational Techniques and Findings

Utilizing the MEarth-South telescope array, the research team monitored the brightness of M-dwarf stars within 33 parsecs of Earth, leading to the identification of a transiting event in the star GJ 1132's light curve. The transit indicates a planet with a radius approximately 1.2 times that of Earth. The subsequent radial velocity measurements obtained through the HARPS spectrograph provide a mass estimate of 1.62 Earth masses, with a density suggesting an Earth-like rocky composition.

GJ 1132b receives approximately 19 times more stellar radiation than Earth, placing it above the classical habitable zone. Although the planet is too hot for life as we know it, its temperature is moderate enough to retain a substantial atmosphere, potentially suitable for spectroscopic atmospheric analysis. Measurements estimate the planet's equilibrium temperature to lie between 409 K and 579 K, depending on the assumed albedo.

Implications for Planetary Atmosphere Studies

The nearby location (12 parsecs) and small size of the host star, GJ 1132, enhance the ability to paper the atmospheric properties of GJ 1132b. The planet presents a rare opportunity as a transiting rocky exoplanet close enough to enable the observation of atmospheric absorption features with existing and future telescopes. Theoretical models suggest that the planet's density aligns with compositions similar to those of Earth or Venus, primarily indicating a mix of rock and iron with negligible volatile layers like hydrogen or helium.

Given its non-habitable yet moderate temperature, GJ 1132b is particularly intriguing as its atmospheric escape processes can be contrasted against denser, hotter rocky exoplanets known for rapid atmospheric loss. Such studies could illuminate the evolutionary pathways of terrestrial planets under varying stellar irradiance conditions.

Future Observational Prospects

Future missions, especially those with capabilities for infrared observations, like the James Webb Space Telescope (JWST), offer the potential to not only observe the transmission spectra of GJ 1132b’s atmosphere but also to detect thermal emissions directly from the planet. Such data would yield insights into the composition and pressure structure of the atmosphere, contributing to the broader understanding of atmospheric retention and loss processes in exoplanets orbiting M-dwarfs.

The paper of GJ 1132b exemplifies the confluence of radial velocity and photometric transit methodologies yielding vital information about exoplanetary characteristics. It underscores the value of these techniques in the context of small stellar hosts, thereby impacting the strategies employed in ongoing and future searches for exoplanets and their atmospheres. Continued observations will refine mass estimates and help discern any potential atmospheric components, further informing models for the atmospheric evolution of terrestrial planets under non-habitable conditions.

GJ 1132b serves as a target for atmospheric characterization, with implications for understanding rocky exoplanets around low-mass stars—a topic of considerable interest due to the frequency of such stars and their associated planets in the galaxy.