Nearby Star Hosting a Potentially Habitable Super-Earth
Red dwarf stars are the most common stars in the Universe. With low masses and low luminosities, red dwarf stars are excellent targets to search for potentially habitable rocky planets. Due to their diminutive nature, the habitable zones around red dwarf stars are much closer-in. As a result, a planet in the habitable zone of a red dwarf star will exert a more detectable gravitational tug on its host star than if the planet were in the habitable zone of a more luminous star. Additionally, the low-mass nature of a red dwarf star only makes such a signal more detectable.
Wolf 1061 is a relatively nearby red dwarf star located only 14 light years away. It has 25 percent the Sun’s mass, 0.787 percent the Sun’s luminosity, and its estimated surface temperature is 3393 K. Using archival radial velocity data from the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory’s 3.6 m telescope in La Silla in Chile, Wright et al. (2015) present the detection of three planets around Wolf 1061. The three planets were discovered due to the gravitational tug they exert on their host star, which causes their host star to exhibit a detectable wobbly motion.
The three planets are identified as Wolf 1061b, Wolf 1061c and Wolf 1061d. Wolf 1061b has at least 1.36 times the mass of Earth and an orbital period of 4.888 days (0.0355 AU); Wolf 1061c has at least 4.25 times the mass of Earth and an orbital period of 17.867 days (0.0843 AU); and Wolf 1061d has at least 5.21 times the mass of Earth and an orbital period of 67.274 days (0.2039 AU). The values in parentheses indicate the distance of each planet from Wolf 1061 in astronomical units (AU), whereby one AU is the average Earth-Sun distance. All three planets are small enough to potentially be rocky worlds.
The conservative habitable zone around Wolf 1061 is predicted to lie between 0.092 to 0.18 AU, while the optimistic habitable zone around Wolf 1061 is predicted to lie between 0.073 to 0.19 AU. The habitable zone around a star is where temperatures are just right for liquid water, and possibly even life, to exist on the surface of a planet. Wolf 1061b (inner planet) is well inside the inner boundary of the optimistic habitable zone, making it too hot to be habitable; Wolf 1061c (middle planet) is well inside the optimistic habitable zone and just outside the inner boundary of the conservative habitable zone, making it potentially habitable; and Wolf 1061d (outer planet) is outside the outer boundary of the optimistic habitable zone, making it a little too cold for habitability.
From the mass-radius relation for planets, the radii of the three planets are estimated to be 1.44 (inner planet), 1.64 (middle planet) and 2.04 (outer planet) times the radius of Earth. The middle planet, Wolf 1061c, is of particular interest because it is likely a potentially habitable rocky planet and it joins a small but growing group of potentially habitable rocky planets orbiting nearby stars cooler than the Sun. At present, Wolf 1061c is one of the closest known potentially habitable rocky planets.
Wright et al. (2015), “Three planets orbiting Wolf 1061”, arXiv:1512.05154 [astro-ph.EP]