It has turned out that 2015 has been a banner year for the search for potentially habitable planets. It started on January 6 at a meeting of the American Astronomical Society in Seattle when Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) announced the discovery of eight planets orbiting inside the habitable zones (HZ) of their respective systems found using data from NASA’s Kepler mission (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”). At the same conference, there was also a quieter announcement of additional planet candidates found by Kepler including a couple of planets that were potential “Earth twins” – Earth-size planets in Earth-like orbits around Sun-like stars (see “Earth Twins on the Horizon?”). Just a couple weeks later, additional promising Earth twin planet candidates were identified (see “The First Look at Kepler’s Complete Primary Mission Data Set”). One of these candidates, now known as Kepler 452b, was subsequently confirmed in July to be the first (almost) Earth-size planet found orbiting (almost) inside the HZ of a Sun-like star (see ”Habitable Planet Reality Check: Kepler 452b”).

But while most of Kepler’s finds are hundreds or even thousands of light years away, there have also been on going searches for exoplanets among the nearby stars. Unfortunately, doubts continue to be cast on the existence of some potentially habitable worlds in the Sun’s neighborhood such as Kapteyn b (see “Kapteyn b: Has Another Habitable Planet ‘Disappeared’?”). But just as 2015 was wrapping up came the announcement of the discovery of three planets orbiting the nearby red dwarf known as Wolf 1061 including one that was claimed to be potentially habitable. In fact, it may just be the closet known potentially habitable planet. Before we examine this claim, first some background on our little-known neighbor.

Background

Wolf 1061, also known as GJ 628, is a type M3V red dwarf located 14.0 light years away in the constellation of Ophiuchus. It has a luminosity of only 0.79% that of the Sun which is why it has a V magnitude of just 10.1 requiring a telescope to view despite being nearby. Wolf 1061 first came to the attention of astronomers about a century ago because of its relatively high proper motion of 1.2 arc seconds per year. As a result, it was included in the catalog of high proper motion stars compiled by German astronomer Max Wolf (1863-1932) of the Heidelberg-Königstuhl State Observatory along with many other nearby red dwarf stars (see “The Real Wolf 359”). The best measurements indicate that Wolf 1061 has a surface temperature of 3393 K and it is estimated to have a mass that is 0.25 times that of the Sun. As red dwarfs go, Wolf 1061 is fairly typical of the stars in our neighborhood.

Because of its small size and relative closeness, Wolf 1061 has been considered a high priority target for various searches for extrasolar planets for some time. A recently published paper with Cassy Davison (Georgia State University) as the lead author presented the results from one of the more thorough searches for extrasolar planets in this system as part of a larger survey of nearby M-dwarf stars. Their analysis of radial velocity measurements derived from IR spectra acquired using the CSHELL cryogenic Echelle spectrograph on NASA’s 3.0-meter IRTF (Infrared Telescope Facility) located at the Mauna Kea Observatory in Hawaii indicated that there are no planets orbiting Wolf 1061 with masses greater than 1.0 to 2.5 times that of Jupiter with periods of 3 and 30 days, respectively (corresponding to orbit radii of 0.026 and 0.12 AU). The radial velocity data as well as astrometric measurements acquired using the 0.9-meter telescope at the Cerro Tololo Inter-American Observatory analyzed by Davison et al. discount the presence of brown dwarfs in orbits with periods as great as 8 years or radii as large as about 2.5 AU. Previous searches by direct imaging have also ruled out brown dwarf and small stellar companions out to 50 AU and more.

Although the results of Davison et al. eliminated the possibility of Jupiter-size planets orbiting close to Wolf 1061, they had only a limited number of radial velocity measurements acquired over a span of 3.3 years with a typical uncertainty of ±86 meters per second. Higher quality data acquired over a longer period were available from the Swiss-based team operating the HARPS (High Accuracy Radial velocity Planet Search) spectrograph attached to the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile. An early analysis by the HARPS team of 23 radial velocity measurements presented in a paper by Bonfils et al. published in 2013 had individual measurement uncertainties on the order of one meter/second. Bonfils et al. found some indications of a signal with a periodicity of about 67 days in the radial velocity measurements but the false alarm probability of 3.3% was too high to qualify as a reliable exoplanet detection.

Duncan Wright and a team of astronomers at Australia’s University of New South Wales obtained a larger set of 148 publically available HARPS spectra of Wolf 1061 acquired over 10.3 years for their new analysis. Wright et al. had more spectra at their disposal and employed a new data processing technique to extract radial velocity measurements from those spectra with a much improved signal to noise ratio and a precision better than one meter per second. Their analysis of the radial velocity measurements revealed the presence of not only the 67-day signal first hinted in the work by Bonfils et al., but two additional signals with periods of 5 and 18 days. After checking their results for the possible effects of stellar activity or spurious signals from the window function resulting from data acquired irregularly in time, it was determined that Wolf 1061 was orbited by a trio of planets. The residuals after fitting the data for the signals from these three planets was ±1.86 meters/second. The derived properties of these finds are summarized in Table 1 below including the effective stellar flux, S eff (where Earth equals 1), derived from the data in Wright et al..

Table 1: Properties of the Planets of Wolf 1061

Planet b c d Mass (Earth=1) ≥1.4 ≥4.3 ≥5.2 Orbit Period (days) 4.89 17.87 67.3 Orbit Radius (AU) 0.0355 0.0843 0.204 S eff (Earth=1) 6.2 1.10 0.19

Since the inclination, i, of the planets’ orbits with respect to the plane of the sky can not be determined from radial velocity measurements alone, only the minimum mass or M p sini of these exoplanets can be determined at this time. With minimum masses of 1.4, 4.3 and 5.2 times that of the Earth (or M E ), these planets are much smaller than the upper limits derived by the work of Davison et al. but are broadly consistent with the sizes of worlds typically found orbiting other red dwarfs (see “Occurrence of Potentially Habitable Planets around Red Dwarfs”).

Potential Habitability

The first step in assessing the potential habitability of the planets orbiting Wolf 1061 is to determine what sort of worlds they are: are they rocky planets like the Earth or are they volatile-rich mini-Neptunes with little prospects of being habitable in an Earth-like sense. Unfortunately, the only information currently available about these new planets is their minimum mass. The actual mass and the radius are needed to get a handle on their bulk compositions. Given their tight orbits, direct imaging of these new planets will require a ten-meter class, space-based telescope with an advanced starshade – a piece of hardware that will likely not be available for decades. There is the possibility that one or more of these planets have their orbits align by random chance to produce observable transits. Calculations suggest that there are 14%, 7% and 3% probabilities that Wolf 1061b, c and d produce transits, respectively. In addition, it is expected that the change in brightness would be large enough to be detected using Earth-based instruments like those used by MEarth and MINERVA. Wolf 1061 will be in a position to be monitored for transits starting in early 2016.

In lieu of this vital information, statistical arguments can be made about the probability these new planets have a rocky composition. A recently published analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Leslie Rogers (a Hubble Fellow at Caltech) strongly suggests that planets transition from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 R E (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). While rocky planets larger than this are possible, they become more uncommon with increasing radius. A planet with a radius of 1.6 R E and an Earth-like composition would have a mass of about 6 M E . With their currently unconstrained orbit orientation, there is less than a 3% chance that Wolf 1061b, with a minimum mass of 1.4 M E , exceeds the 6 M E threshold and it is most likely a rocky planet. The probability that Wolf 1061c and d exceed this threshold is higher at about 30% and 50%, respectively. It seems more likely (but not certain) that Wolf 1061c is rocky, although it is a toss up whether or not Wolf 1061d is a mini-Neptune.

Another major factor involved in assessing a planet’s potential habitability is its effective stellar flux, S eff . Looking at the latest models for the conservative limits of the HZ from Kopparapu et al., the HZ of a red dwarf like Wolf 1061 for a 5 M E planet would have effective stellar flux values ranging from about 1.00 for the inner edge (corresponding to the runaway greenhouse limit) out to about 0.24 (corresponding to the maximum greenhouse limit). Wolf 1061b with an S eff of 6.2 exceeds this limit by a large margin and is quite likely a larger and hotter version of Venus. At the other extreme, Wolf 1061d with an S eff of 0.19 is beyond the outer edge of the HZ. While the presence of additional greenhouse gases like methane or even hydrogen might make Wolf 1061d marginally habitable, the high probability that Wolf 1061d is a mini-Neptune combined with questionable prospects of being habitable in the conventional sense make this world a poor candidate for being “potentially habitable”.

The situation with Wolf 1061c is not as clear cut. With an S eff of 1.10, this world seems to orbit just beyond the inner edge of the conservatively defined HZ. But given the current uncertainties in the precise position of the inner edge of the HZ as well as the uncertainties in the properties of Wolf 1061c, it just might still reside inside the HZ. Given its close proximity to its red dwarf sun, it is expected that Wolf 1061c is a synchronous rotator. Models of the limits of the HZ for synchronous rotators developed by Yang et al. suggest that the S eff of the inner edge of the HZ of Wolf 1061 may actually be about 1.6 times that of the Earth. If this proves to be the case, a synchronously rotating Wolf 1061c would be comfortably inside the HZ. All things considered (and ignoring for the moment the other, still-unresolved issues with the habitability of worlds orbiting red dwarfs), Wolf 1061c is a fair candidate for being potentially habitable.

Conclusion

Contrary to the hype associated with too many recent exoplanet discoveries, the claim that Wolf 1061c is a potentially habitable planet appears to have some merit, keeping in mind how little is currently known about this world at this time. While it has some chance of being a mini-Neptune, it seems more likely that it is a rocky planet with some prospect of being Earth-like. If Wolf 1061c beats the odds and is observed to produce transits, it should prove possible not only to get a better handle on its bulk properties, but also to probe its atmosphere, especially given the relatively high apparent brightness of its sun. Habitable or not, studies of Wolf 1061c and its siblings will shed much light on the properties of planets in or near the HZ of red dwarf stars. No matter how the situation turns out, the three planets found orbiting Wolf 1061 are just a taste of things to come as astronomers continue their search of the nearby stars and beyond for extrasolar planets.

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Related Video

Here is a short video from UNSW illustrating the three new planets found orbiting Wolf 1061.

Related Reading

“The Real Wolf 359”, Drew Ex Machina, November 17, 2015 [Post]

“Occurrence of Potentially Habitable Planets around Red Dwarfs”, Drew Ex Machina, January 12, 2015 [Post]

General References

X. Bonfils et al., “The HARPS search for southern extra-solar planets. XXXI. The M-dwarf sample”, Astronomy & Astrophysics, Vol. 549, ID A109, January 2013

Cassy L. Davison et al., “A 3D Search for Companions to 12 Nearby M Dwarfs”, The Astronomical Journal, Vol. 149, No. 3, Article ID 106, March 2015

R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

D.J. Wright et al., “Three planets orbiting Wolf 1061”, arXiv 1512.05154 (submitted to The Astrophysical Journal Letters), December 16, 2015 [Preprint]

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014

“Discovery: Nearby star hosts closest alien planet in the ‘habitable zone’”, UNSW Press Release, December 16, 2015 [Press Release]