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Wolf 1061 Unlikely to Host Habitable Worlds

A key way to learn more about a given exoplanet is to home in on the properties of its star. So argue Stephen Kane (San Francisco State University) and colleagues in a new paper slated for the Astrophysical Journal. The star in question is Wolf 1061 (V2306 Ophiuchi), an M-class red dwarf some 13.8 light years away in the constellation Ophiuchus. In December of 2015, Australian astronomers announced the discovery of three planets around the star.

Drawn out of data from the HARPS spectrograph at La Silla, the planets are all super-Earths, their radial velocity data supplemented with eight years of photometry from the All Sky Automated Survey. All three seem likely to be rocky planets, but firming this up would take transits, which the discovery team at the University of New South Wales estimated might occur, with a likelihood of about 14 percent for the inner world, dropping to 3% for the outer.

Kane and team investigate the transit question in light of the fact that that two recent papers have produced sharply different orbital periods for the outermost planet here, though both find planet c near or within the habitable zone, which itself depends on the star’s luminosity and effective temperature. Both the recent papers see reasonably high transit possibilities, but Kane’s work rules out transits of the two inner worlds, leaving open a possibility for the outer.

The researchers have used observations from the Center for High Angular Resolution Astronomy (CHARA) interferometric array at Mount Wilson Observatory near Los Angeles. From this emerges a precise stellar radius measurement (0.3207±0.0088 R), from which the team has calculated the star’s effective temperature and luminosity. The photometry data reveal a stellar rotation period of 89.3±1.8 days. The work has useful implications for upcoming space-based exoplanet studies. As the paper notes:

The assessment of host star properties is a critical component of exoplanetary studies, at least for the realm of indirect detections through which exoplanet discoveries thus far have predominantly occurred. This situation will remain true for the coming years during which the transit method will primarily be used from space missions such as the Transiting Exoplanet Survey Satellite (TESS), the CHaracterising ExOPlanet Satellite (CHEOPS), and the PLAnetary Transits and Oscillations of stars (PLATO) mission. Of particular interest are the radius and effective temperature of the stars since the radius impacts the interpretation of observed transit events and the combination of radius and temperature is used to calculate the extent of the HZ.

And indeed, it is through this painstaking analysis of stellar properties that the researchers have been able to calculate habitable zone boundaries for Wolf 1061 of 0.09–0.23 AU. Have a look at the paper’s Figure 8, which shows the Wolf 1061 system as diagrammed from above.

Screenshot from 2017-01-26 09-00-26

Image: Figure 8 from the paper shows the orbits of the planets overlaid on the habitable zone. The scale here is 1.0 AU to the side, with the ‘conservative’ habitable zone shown as light gray, and the more optimistic extension shown in dark gray. Credit: Kane et al.

Notice the outer planet (d), which passes briefly through the habitable zone at closest approach to the star before widening further out in its eccentric orbit. Indeed, only 6 percent of the orbital period takes place within the habitable zone. Planet c spends 61 percent of its orbit within the habitable zone, but only within the optimistic assumptions for the HZ. Taking into account the recent orbital solutions for this system, both inner worlds are problematic:

…planet c is quite similar to the case of Kepler-69 c, which was proposed to be a strong super-Venus candidate by Kane et al. (2013). Indeed, both of the inner two planets, terrestrial in nature according to the results of both Wright et al. (2016) and Astudillo-Defru et al. (2016b), lie within the Venus Zone of the host star (Kane et al. 2014) and are thus possible runaway greenhouse candidates.

Measuring stellar parameters unlocks the boundaries of the habitable zone, allowing the researchers to study the planetary orbits and weigh the chances for liquid water on the surface. The results hardly favor the Wolf 1061 system as a promising candidate for life.

We find that, although the eccentric solution for planet c allows it to enter the optimistic HZ, the two inner planets are consistent with possible super-Venus planets (Kane et al. 2013, 2014). Long-term stability analysis shows that the system is stable in the current configuration, and that the eccentricity of the two inner planets frequently reduces to zero, at which times the orbit of planet c is entirely interior to the optimistic HZ. We thus conclude that the system is unlikely to host planets with surface liquid water.

The paper is Kane et al., “Characterization of the Wolf 1061 Planetary System,” accepted for publication at the Astrophysical Journal (preprint).


Comments on this entry are closed.

  • Andrew LePage January 26, 2017, 12:20

    This new result isn’t too surprising. My original assessment about the potential habitability of Wolf 1061c published a year ago (which was based on the original assumption at the time of these planets discovery that their orbits were circular) was not all that promising.


    I’m still waiting to see some preprints of some additional recent papers on the Wolf 1061 (in addition to this one by Kane et al) before I do a detailed reassessment of the potential habitability of Wolf 1061c – but it isn’t looking too promising for a variety of reasons.

  • susan purdy January 26, 2017, 12:35

    where is the Earth located in the ‘goldilocks’ zone of Our Sun…. compared to other exoplanets in other solar systems….
    in the middle
    closer to our sun
    or further away

    thank you

    • Ashley Baldwin January 26, 2017, 16:43

      A quick ( but crude ) calculation of a stellar habitable zone ,
      HBZ ( in AU) = 0.95 x square root (stellar bolimetric ( all emitted wavelengths ) luminosity (ratio to Sun) ) – 1.4 bolimetric luminosity (

      The “Earth equivalent insolation distance” , EEID , a closely related figure is represented by the square root of the bolimetric luminosity .

      So approximately 0.95-1.4 for the current Sun and obviously 1 AU for the EEID . That puts Earth towards the current inner edge ( which other work suggests could be as far out as 0.99 AU ) and 1.35 AU Mars towards the outer. ( bearing in mind the Sun has been up to 25 % less luminous in its early life thus the values in the equation would be different with the HBZ moving outwards as the Sun evolves and brightens ) .

      This makes allowance for CO2 greenhouse effect with “runaway” occurring interior to the inner edge and the outer edge representing the maximum possible greenhouse effect before CO2 condenses out of the atmosphere.

      In the past when the Sun was dimmer other ( more potent ) greenhouse gases like methane probably played a big part in permitting habitability and also the formation of a “reducing atmosphere” which allowed the build up of the critical prebiotic molecules that would rapidly have been oxidised by Earth’s current oxygen rich iteration.

    • FrankH January 26, 2017, 17:08

      The Earth at 1AU from the Sun is close to the inner edge (0.95AU) of the Sun’s habitable zone:


  • Bruce Mayfield January 26, 2017, 13:18

    But still the Wolf 1061 system has some things going for it too. From its Wikipedia article: “Wolf 1061 is very stable and most likely does not have any significant activity such as sunspots or flares.” Its the 36th closest star system.

    This system could be usefull at some point in the distant future.

  • andy January 26, 2017, 14:26

    Note that there has apparently been a substantial revision of the outer planet’s properties since the discovery paper for this system: according to the Kane et al. paper, the new orbital period is ~3 times longer and the eccentricity is substantially greater (from what I can tell, the original orbital solution would have put planet d in the HZ). The paper referenced for the new orbital solution is not yet published.

  • RobFlores January 26, 2017, 15:39

    Another planetary distribution that makes me suspect a horrible thing.
    that I’ve mentioned before. There maybe a bias against smaller terrestrial type planets forming/final position in the prime locations within HZs of stars.

    You may recall there are few sub 2.5 RE within the HZ for all Kepler
    discoveries. (Yes, this maybe bias in the data, and perhaps Earth sized terrestrial are really favored in the prime HZ areas, and cannot spot them) but the count of larger terrestrial worlds lying within HZ is scant indeed, and quite a lot of them are in the HZ in name only as they scrape the edges as this one does. We need better data. A Robust successor to Kepler. not Just the JW telescope but a dedicated planet searcher.

    I hope I am wrong because it makes the Dream of a Twin Earth that much
    further out there in light years.

    • Andrew LePage January 26, 2017, 18:53

      > Yes, this maybe bias in the data

      There is absolutely positively no “maybes” abut it – there *ARE* very strong selection biases in the data concerning the occurrence rates of planets orbiting red dwarf and other star. ANY attempts to draw conclusions from just one system or even the ensemble of all known stars of a particular type without taking the absolutely necessary step of accounting for those biases and the detection limitations in a rigorous statistical manner will produce incorrect results and leave an erroneous impression. Here is a link to a review article on the occurrence rate of potentially habitable planets orbiting red dwarfs which take these biases very carefully into account:


      And here is another review about the occurrence rates of Earth-size planets orbiting more Sun-like stars (again taking selection biases and the limitations of the detection method into account in the derivation of the statistics):


      In short, there does not seem to be any shortage of Earth-size planets – they are just tough to detect using methods we have available today (e.g. precision RV measurements, transit method, etc.)

    • Bruce Mayfield January 26, 2017, 20:37

      I think this remains just an ease of detection bias issue. Smaller things tend to out-number larger in nature, such as with stars. Why wouldn’t this be the case for planets also?

      If earth sized planets really are rarer than super-earths it would be a strange thing.

      • RobFlores January 27, 2017, 13:08

        So just taking into account the super terrestrials that can be detected by Kepler Smaller than mini-Neptunes. Is any one studying these planets as a group. It would be nice to know if there is random chance for them
        to reside in the HZ of their host star. Or is there a bias For or Against
        the probability of a kepler detected Super Terrestrial being inside the Habitability Zone.

  • hiro January 26, 2017, 16:33

    Moon(s) of the outer planet d (if any exists) is interesting, I wonder whether that region is too hot for exomoon keeping water on its surface.

  • Ashley Baldwin January 26, 2017, 17:28

    I think there is a really important take home message from this paper and that is just how dependent the two main exoplanet discovery techniques ( RV spectrometry and Transit photometry ) are on stellar properties not least photospheric activity. To know the planet you first need to know the star . Exoplanet radius can only be calculated by Transit photometry . Wolf 1061, a fairly typical neighbouring mid M dwarf , epitomises this with even its close proximity not allowing easy characterisation .

  • CZ January 27, 2017, 4:11

    Imagine a tidally locked exoplanet way inside the habitable zone with a boiling atmosphere on the facing side and liquid water on the dark side with plant life fueled by powerful aurora . Need better next gen tech to resolve exoplanets. Current tech sucks

    • Ashley Baldwin January 28, 2017, 15:07

      Don’t think it’s as bad as that. Exoplanet science is really a still new field , one that is evolving fast though incrementally . RV spectroscopy has come on a long way since the first ( and easiest ) hot Jupiters were discovered 20 years ago . Orbiting chromospherically stable sun like stars .

      Spectrographs like ESPRESSO now have the resolution to image Earth like planets in habitable zones of sunlike stars , but have reached a new baseline of background chromospheric “noise ” . The next step will be able to overcome this , most especially for M dwarfs who by their very nature are active and thus much more difficult .

      One of the other changes from the Wright paper seen in the Kane study is that Wolf 1061 is not considered as quiescent despite its leisurely 89 day orbit and by proxy an older > 5 Gyr age . With a mass around 0.25 Msun it is also around the “transition to complete convection ” zone of the main sequence with all the magnetic field uncertainty ( and related chromospheric activation ) that brings with it . See Houdebine et al arxiv yesterday -“Rotational Activation Correlation “.

  • Herp McDerp January 27, 2017, 5:11

    That eccentric orbit for planet d — if it exists — has me wondering: Could the poor characterization and presumed high eccentricity of d be due to the gravitational perturbations of two planets in near-circular orbits and roughly 1:2 period ratio? That’s what has happened with several other systems that initially were thought to include planets on eccentric orbits. If that were the case it would be good news, since other, smaller planets would be more likely to occupy stable orbits in the space between planets c and d (and outside the habitable zone, between d and hypothetical planet e as well).

  • Brian A Schmidt January 27, 2017, 16:14

    Six percent of the time in the HZ is still something, and that’s not even considering potential effects of greenhouse gases stronger than anticipated.

    Plenty of time for micro-organisms to get busy.

    It would be interesting to look for similar habitats on earth and see what survives there. High latitudes would be better comparisons than high altitudes because the daily cycle at altitude would be worse for life than longer, seasonal cycles.

  • Andrew Palfreyman January 28, 2017, 15:27

    This truly is an exciting time for exoplanet discovery – new telescopes arriving and the promise of atmospheric analysis just around the corner. I count myself very lucky to still be around to witness it all.