At a distance of 1.295 parsecs, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun's closest stellar neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days (ref. 3) and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface.

Moons orbiting extrasolar planets are the next class of object to be observed and characterized for possible habitability. Like the host-planets to their host-star, exomoons have a limiting radius at which they may be gravitationally bound, or the Hill radius. In addition, they also have a distance at which they will become tidally locked and therefore in synchronous rotation with the planet. We have examined the flux phase profile of a simulated, hypothetical moon orbiting at a distant radius around the confirmed exoplanets μ Ara b, HD 28185 b, BD +14 4559 b, and HD 73534 b. The irradiated flux on a moon at its furthest, stable distance from the planet achieves its largest flux gradient, which places a limit on the flux ranges expected for subsequent (observed) moons closer in orbit to the planet. We have also analyzed the effect of planetary eccentricity on the flux on the moon, examining planets that traverse the habitable zone either fully or partially during their orbit. Looking solely at the stellar contributions, we find that moons around planets that are totally within the habitable zone experience thermal equilibrium temperatures above the runaway greenhouse limit, requiring a small heat redistribution efficiency. In contrast, exomoons orbiting planets that only spend a fraction of their time within the habitable zone require a heat redistribution efficiency near 100% in order to achieve temperatures suitable for habitability. This means that a planet does not need to spend its entire orbit within the habitable zone in order for the exomoon to be habitable. Because the applied systems comprise giant planets around bright stars, we believe that the transit detection method is most likely to yield an exomoon discovery.

Planets orbiting within the habitable zones of M stars are prime targets for future observations, which motivates a greater understanding of how tidal locking can affect planetary habitability. In this Letter we will consider the effect of tidal locking on limit cycling between snowball and warm climate states, which has been suggested could occur for rapidly rotating planets in the outer regions of the habitable zone with low CO2 outgassing rates. Here, we use a 3D Global Climate Model that calculates silicate-weathering to show that tidally locked planets with an active carbon cycle will not experience limit cycling between warm and snowball states. Instead, they smoothly settle into “Eyeball” states with a small unglaciated substellar region. The size of this unglaciated region depends on the stellar irradiation, the CO2 outgassing rate, and the continental configuration. Furthermore, we argue that a tidally locked habitable zone planet cannot stay in a snowball state for a geologically significant time. This may be beneficial to the survival of complex life on tidally locked planets orbiting the outer edge of their stars, but might also make it less likely for complex life to arise.

ABSTRACT While most of the 6000 discovered exoplanets are highly unlike the Earth, the first rocky worlds in the habitable zone (HZ) provide intriguing targets for the search for life in the cosmos. As detections increase, it is critical to test the empirical HZ as well as its limits using known exoplanets. However, there is not yet a list of rocky worlds that observers can use to test the limits of surface habitability.We analysed data from Gaia DR3 and the NASA Exoplanet Archive (NEA) of all known exoplanets, identifying future targets to test limits of habitability through (i) orbits near the edges of the HZ, (ii) similar irradiation environments to modern Earth, and (iii) large eccentricities. We prioritize targets for transmission observations, light curve measurements, and direct imaging, identify the oldest HZ rocky worlds based on the NEA and complementary literature data, and provide theoretical limits for the empirical HZ and a 3D-HZ for each system. Our analysis shows 45 rocky worlds in the empirical HZ and 24 in a narrower 3D-HZ. For context, we compare their demographics to those of the full catalogue of exoplanets in the NEA. The resulting list of rocky exoplanet targets in the HZ will allow observers to shape and optimize search strategies with space- and ground-based telescopes – such as the James Webb Space Telescope (JWST), Extremely Large Telescope (ELT), Habitable Worlds Observatory (HWO), and Large Interferometer For Exoplanets (LIFE) – and design new observing strategies and instruments to explore these worlds, addressing the question of the limits of exoplanet surface habitability.

Gliese 667 Cc Gliese 667 Cc is an extrasolar planet 22.18 light years away. It orbits around the red dwarf star Gliese 667 C, one of the Gliese 667 triple star system. The system is in the constellation of Scorpius. It is the most Earth-like planet in 50 light years. Its ESI (Earth Similarity Index) is 0.85. It is present in the hot inner edge of the habitable zone than the Earth. It absorbs 90% of light from it's star, as the Earth does but it is tidally locked.[1] Gliese 667 Cc was first found by the European Southern Observatory on 21 November 2011. They used the wobble method (also known as radial velocity). They watched how the star "wobbled" as Gliese 667 Cc circled around its star.[2] On 2 February 2012 researchers at the University of Göttingen and the Carnegie Institution for Science confirmed the planet's existence.[3] Physical characteristics Gliese 667 Cc circles its star every 28 days,[1] and is 0.125 AU away from its star. It fits within the habitable zone. Its mass is 3.8 times that of the Earth.[1] Its size is probably about 50% larger than Earth. Tidal locking Because of Gliese 667 Cc's distance from its star, it is predicted to be tidally locked to its star. Gliese 667 Cc (also known as GJ 667 Cc, HR 6426 Cc, or HD 156384 Cc) is an exoplanet orbiting within the habitable zone of the red dwarf star Gliese 667 C, which is a member of the Gliese 667 triple-star system, approximately 23.62 light-years (7.24 parsecs; 223.5 trillion kilometres) away in the constellation of Scorpius. The exoplanet was found by using the radial velocity method, from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star. Gliese 667 Cc is sometimes considered as the first confirmed potentially habitable exoplanet. The planet orbits a red dwarf (M-type) star named Gliese 667 C, orbited by two planets. The star is part of a trinary star system, with Gliese 667 A and B both being more massive than the smaller companion. Gliese 667 C has a mass of 0.31 M☉ and a radius of 0.42 R☉. It has a temperature of 3,700 K, but its age is poorly constrained, estimates place it greater than two billion years old. In comparison, the Sun is 4.6 billion years old and has a surface temperature of 5,778 K. This star is radiating only 1.4% of the Sun's luminosity from its outer atmosphere. It is known to have a system of two planets: claims have been made for up to seven, but these may be in error due to failure to account for correlated noise in the radial velocity data. Since red dwarfs emit little ultraviolet light, the planets likely receive minimal amounts of ultraviolet radiation. Gliese 667 Cc is the second confirmed planet out from Gliese 667 C, orbiting towards the inner edge of the habitable zone. From its surface, the star would have an angular diameter of 1.24 degrees and would appear to be 2.3 times the visual diameter of the Sun as it appears from the surface of the Earth. Gliese 667 C would have a visual area 5.4 times greater than that of the Sun but would still only occupy 0.003 percent of Gliese 667 Cc's sky sphere or 0.006 percent of the visible sky when directly overhead. The apparent magnitude of the star is 10.25, giving it an absolute magnitude of about 11.03. It is too dim to be seen from Earth with the naked eye, and even smaller telescopes cannot resolve it against the brighter light from Gliese 667 A and B. Based on black body temperature calculation, Gliese 667 Cc should absorb a similar, but slightly higher, amount of overall electromagnetic radiation than Earth, making it a little warmer (277.4 K [4.3 °C; 39.6 °F]) and consequently placing it slightly closer to the "hot" inner edge of the habitable zone than Earth (254.3 K [−18.8 °C; −1.9 °F]). According to the Planetary Habitability Laboratory (PHL), Gliese 667 Cc is (as of July 2018) the fourth-most Earth-like exoplanet located in the conservative habitable zone of its parent star. Its host star is a red dwarf, with about a third as much mass as the Sun. As a result, stars like Gliese 667 C may live up to 100–150 billion years, 10–15 times longer than the Sun's lifespan. This, however, does not equate to a longer period of favorable conditions for life. A 2017 paper employed bayesian inference to show that if Earth is assumed to be typical of a habitable planet, then there must be some constraint that prohibits habitability and the evolution of life on planets that orbit stars of less than 0.65 M☉. Given that Gliese 667 Cc orbits a star of mass 0.31 M☉, its chances of habitability may be considerably smaller than estimates based purely on how Earth-like the planet is. Furthermore, the planet is likely tidally locked, with one side of its hemisphere permanently facing towards the star, and the opposite side being dark and cold. However, between these two intense areas, there could be a sliver of habitability—called the terminator line, where the temperatures may be suitable (about 273 K [0 °C; 32 °F]) for liquid water to exist. Additionally, a much larger portion of the planet may be habitable if it supports a thick enough atmosphere to transfer heat to the side facing away from the star. However, in a 2013 paper, it was revealed that Gliese 667 Cc is subject to tidal heating 300 times that of Earth. This in part is due to its small eccentric orbit around the host star. Further simulations of the interaction between Gliese 667Cc and Gliese 667Cb show that it would be subjected to intense tidal energies. The energy that it is subjected to would warm the planet up by 1.6 Kelvin every one hundred thousand years causing the partial or complete melting the mantle and covering the surface in lava. Because of this, the chances of habitability are very likely to be lower than originally estimated. KELT-4Ab – another exoplanet in a triple-star system LTT 1445 – a triple M-dwarf system with three planets orbiting LTT 1445A List of potentially habitable exoplanets

Tidal locking has several implications: 1. No Seasons The lack of axial tilt on a tidally locked planet means there are no seasons. This stability could prevent the variation in temperatures needed to support diverse ecosystems. 2. Extreme Temperature Differences One side of the planet would be perpetually hot, with direct exposure to the star’s radiation, while the opposite side would remain cold in constant darkness. However, a narrow region between the two sides, known as the terminator line, might have milder conditions. This thin band could theoretically support habitable conditions, although the atmosphere’s ability to circulate and moderate temperatures would be critical. There might be life on Gliese 581 c. We sent a message to that planet, it will take approximately 6 years to arrive. Assuming we receive a response, it could take an additional 6 years, meaning we would get a reply in about 12 years from the time the message was sent.[3] References - ↑ https://www.space.com/36196-gliese-581c.html | Gliese 581c: Super-Earth Exoplanet | March 24, 2017 - ↑ "New 'super-Earth' found in space". BBC News. 25 April 2007. Retrieved 25 April 2007. - ↑ Hurley, Steve (2014-09-27). Gliese 581c (; Gl 581c or GJ 581c) is an exoplanet orbiting within the Gliese 581 system. It is the second planet discovered in the system and the third in order from the star. With a mass about 6.8 times that of the Earth, it is classified as a super-Earth (a category of planets with masses greater than Earth's up to ten Earth masses). At the time of its discovery in 2007, Gliese 581c gained interest from astronomers because it was reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and, by extension, potentially capable of supporting extremophile forms of Earth-like life. However, further research cast doubt upon the planet's habitability. Based on newer models of the habitable zone, the planet is likely too hot to be potentially habitable. In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.55 light-years (194 trillion kilometres; 121 trillion miles) in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give its exact location in the Milky Way. "rock giants" mostly of silicate; "cannonball" planets of solid iron; "gas dwarfs" mostly of helium and hydrogen; carbon-rich "diamond worlds"; purely hot "ice VII worlds"; purely "carbon monoxide worlds". If the planet transits the star as seen from the direction of the Earth, the radius should be measurable, albeit with some uncertainty. Unfortunately, measurements made with the Canadian-built MOST space telescope indicate that transits do not occur. The new research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold onto their large atmospheres. Rather than evolving to a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope. Gliese 581c has an orbital period ("year") of 13 Earth days and its orbital radius is only about 7% that of the Earth, about 11 million km, while the Earth is 150 million km from the Sun. Since the host star is smaller and colder than the Sun—and thus less luminous—this distance places the planet on the "warm" edge of the habitable zone around the star according to Udry's team. Note that in astrophysics, the "habitable zone" is defined as the range of distances from the star at which a planet could support liquid water on its surface: it should not be taken to mean that the planet's environment would be suitable for humans, a situation which requires a more restrictive range of parameters. In any case, based on newer models of the habitable zone, the planet is likely too hot to be potentially habitable. A typical radius for an M0 star of Gliese 581's age and metallicity is 0.00128 AU, against the Sun's 0.00465 AU. This proximity means that the primary star should appear 3.75 times wider and 14 times larger in area for an observer on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.