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.
The sky area in the constellation of Ophiuchus near the red dwarf star Wolf 1061. Click on image to enlarge. (CDS/Strasbourg Observatory)
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.
These color-coded plots show the fraction of planets that could have been detected orbiting Wolf 1061 as a function of orbital period and planet mass by Davison et al. Red indicates a nearly perfect detection rate. Click on image to enlarge. (Davison et al.)
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.
The ESO 3.6m Telescope equipped with HARPS was used to acquire the data used to find the new planets orbiting Wolf 1061. (ESO/H.H.Heyer)
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, Seff (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 |
| Seff (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 Mpsini 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 ME), 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.
Ground-based systems like MEarth, pictured here, should be capable of observing any transits of the newly discovered planets of Wolf 1061 starting in early 2016. (Harvard/CFA)
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 RE (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 RE and an Earth-like composition would have a mass of about 6 ME. With their currently unconstrained orbit orientation, there is less than a 3% chance that Wolf 1061b, with a minimum mass of 1.4 ME, exceeds the 6 ME 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, Seff. 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 ME 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 Seff 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 Seff 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”.
Diagram illustrating the orbits of the newly discovered planets of Wolf 1061 in relation to its habitable zone (HZ). Click on image to enlarge. (UNSW created with Universe Sandbox 2)
The situation with Wolf 1061c is not as clear cut. With an Seff 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 Seff 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.
Follow Drew Ex Machina on Facebook.
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]
The one thing I seem to be confused about is that for GJ 667 Cc with a minimum mass of ≥3.709 ± 0.682 M⊕ (Hobson et al) you said: “With this in mind there is an uncomfortably high probability that GJ 667Cc is a mini-Neptune.” but for Wolf 1061c with a much greater minimum mass of ≥4.3 (as stated in the article) you said: “it seems more likely that it is a rocky planet with some prospect of being Earth-like.” So wouldn’t Wolf 1061c be even more likely to be a Mini-Neptune?
Actually, in the review of GJ 667Cc (which can be found at http://www.drewexmachina.com/2014/09/07/habitable-planet-reality-check-gj-667c/) I was using the minimum mass of 4.1 ME given in Paul Robertson and Suvrath Mahadevan (“Disentangling Planets and Stellar Activity for Gliese 667C”, The Astrophysical Journal Letters, Vol. 793, Article ID. L24, October 1, 2014) which is essentially the same minimum mass for Wolf 1061c.
In any case, there is no conflict between the two subjective statements “there is an uncomfortably high probability that GJ 667Cc is a mini-Neptune” and, in the case of Wolf 1061, “it seems more likely that it is a rocky planet with some prospect of being Earth-like”. In both cases there is something like one chance in three or four that these worlds are mini-Neptunes. The difference in tone is the realization (by me and others) over the last year or so of the uncertainties in the current mass-radius relationship which is still being refined by astronomers.
What’s the metallicity of the star? I remember mention of a paper that argued that the super-earths around high-metallicity systems tend to pick up and retain hydrogen atmospheres if they’re above about 1.2 Earth radii and 2 times Earth mass.
I have read the same thing recently about the correlation between a stars metallicity and the occurrence rate of mini-Neptunes. I did some digging and the [Fe/H] for Wolf 1061 is –0.02 ± 0.17 dex – basically it has a Sun-like metallicity to within the measurement errors.
Thanks for a well rounded and comprehensive review as ever ,Drew. Peer reviewed standard as normal. Planetary characteristics apart Wolf 1061 is an M3 dwarf so right on tne boundary being a fully convective star. Ordinarily such a property might lead to an unpredictable ,unanchored intrinsic magnetic field but this star appears to have a slow rotation rate of around 100 days ( Samus et al) . Apart from reducing any Dynamo like effects it also implies the star is relatively old and as with most such M dwarfs should have “calmed down” considerably in terms of its innate activity , conferring more life friendly characteristics on the star at least . Given the large size of any planets and their capacity for potentially thick atmospheres these might prove to have greater resistance in the face of any battering received previously when Wolf was somewhat more fierce . It also raises the interesting if unlikely possibility that should “c’ or even ‘d’ could indeed have started as “mini Neptunes ” but subsequently become the recently theorised, somewhat mystical “HECs”. Habitable evaporated cores .
The surface gravity is over 2 gee for the Hab-Zone planet, so it’s a bit on the nasty side for large terrestrial bipeds to visit.
Great article, on the planets that orbit so close to a red dwarf, would they be locked to each others orbital time since b and c pass only 5 million miles apart and c and d – 11 million. The tidal lock of the red dwarf may not be strong enough since the mass of these planets may cause rotatation resonance like Venus and Earth or a chaotic rotation like Pluto,s smaller moons. Baldwin comment on evaporated cores makes sense but has anyone studied what would be left, I can just imagine miles deep of organic goo!
Hi Drew, I’m Duncan Wright, the author of the recent Wolf1061 paper. I usually keep an eye on your site and like the work you do here, and I think you’ve done another good job in your summary of the Wolf1061 system.
I was gathering some notes to put up on my own site (when it gets back up), the kind of broader perspective on habitability that is left out of papers and often can’t be covered well in interviews, but your work here makes my notes look like a rough outline. I think it’s easiest for me if I just point people to this analysis!
Anyway, good job and keep it up.
also @Ashley – I agree, it is a very calm and stable star both spectroscopically and photometrically, so likely pretty old.
and @Adam – without knowing the radius we can’t be sure (I’m still hopeful it’s transiting – then the science potential will be incredible!) – but a reasonable guess is a surface gravity of 1.6 – 2.2g for Wolf1061c (an Earth mean-density is the minimum surface gravity value here). But for a synchronously rotating (aka tidally locked) planet there are other reasons to suspect it might be unpleasant to visit!
@Mike – I agree, there are some very poorly known numbers in the tidal locking calculations, but it does depend so strongly on the orbital radius (to the power of 6 IIRC) that tidal locking does seem likely. However, as you mention, what situations cause rotation-orbit resonances (e.g. Mercury) I don’t think is well understood, so who knows…
Duncan, thanks for your comment and your supportive words! Feel free to link this page as a synopsis of your work.
Very interesting and welcome reply from the author. My take away from this , and I was reading several review of M dwarfs recently given the imminent arrival of ESPRESSO at the VLT . The fear is that despite its RV sensitivity approaching the levels that could spot an Earth mass planet around Sun like or smaller stars , that baseline Spectoscopic noise would prevent this, certainly in smaller stars . I was a bit surprised and somewhat relieved to see that as with other classes of main sequence stars , even M dwarfs have quieter members with greater age as ever helping further. Photometrically calm too.Heaven sent . Pray for a transit or twenty ! It is good to know that as far as M dwarf systems go , that there is such an interesting one nearby. Not as close as Proxima but it will be interesting see if Gaia has the sensitivity at this relatively close distance to complete a detailed “full range” classification of Wolf’s planets . With thick atmospheres , you never know , Leconte’s work looking at their effect at mitigating tidal synchronisation might yet help make them somewhat less hostile .
Just read an article on Astrobiology magazine on “habitable evaporated core” (HEC) and was under the impression that these would only be water worlds. Now if you look at Uranus and Neptune a large percentage of there ice mantel is methane, so what would happened to it? What I’m wondering is there a pressure and tempature that both water and methane can both be liquids, like a salt water ocean above and maybe a salt methane ocean below it on a HEC. Have not been able to find much on organic liquids and gases in a high pressure atmospheres, it would be interesting to see how the different chemical compounds would effect the climate through out the habitable zones around red dwarfs, especially since methane absorbs the red to infrared part of the spectrum. The only other blue planets in our system…
HECs are along shot to be fair , but with our limited knowledge I was trying to show that all bets are still on. The evidence for HECs is as robust as the EUV eviscerating that M dwarf stars are meant to take in the “pre main sequence” stage. A better call is Gaia. Though an astrometry precision figure of 24 micro arc second accuracy is quoted in the literature that is across the entire stellar sample size . That figure improves dramatically with brighter stars and nearer, smaller stars too. Up to the point where too much light will saturate its CCDs. In terms of brightness its at its best around magnitude 10 . Smaller stars are more subject to lateral attraction by orbiting planets , bigger planets and also those around an AU out ( at odds with Doppler spectroscopy and transit photometry where close is best ) . Close proximity to Earth helps too ( esp within thirty light years ) . Wolf 1061 is magnitude 10.10 , 0.25 Mass Sun and just four parsecs away , Proxima, magnitude 11 ,0.123 Mass Sun and just over one parsec away. So we can easily expect single figure astrometric precision for those two stars which should do better much than the gas giants quoted in the media , Super Earths certainly and may be even larger terrestrial planets if suitably placed orbitally , especially if Gaia has an extended secondary mission . It then visits its stars more often and increasing its precision even further. Here’s to hoping. Astrometry gives inclination which combined with Doppler spectroscopy will give exact mass. Get really lucky and some of Wolfs planets will transit too which brings TESS and JWST into play . So we get bulk density as well as transit spectrographic characterisation too and every reason to be pleased with as much detailed information on any planet known to man bar Earth itself. Optimism plus plus at the end but you never know !
New funding in 2016 budget;
Wide-Field Infrared Survey Telescope (WFIRST): The Omnibus includes $90 million for WFIRST-AFTA and instructions that it should be moved into Phase-A in January. This is 5.4 times the NASA request for WFIRST!
https://thespacereporter.com/2016/01/former-spy-satellites-will-search-dark-energy-exoplanets/
http://astronomicaltelescopes.spiedigitallibrary.org/issue.aspx?journalid=166&issueid=934639
The signs of an earlier launch than 2025 look good. The issue is to get its coronagraph as potent as possible ( with good progress being made I’m told despite its internal architecture and difficult entry pupil) and also have it adapted for use with a star shade for a later date. Only costs a few tens of millions , with the potency of any future external occulter mission dependent on where its light enters the telescope’s optical train in order to ensure as high a throughput as possible ( the actual light that reaches the spectrograph and imager , 68% max for a star shade compared to just 37 % for the for the coronagraph . For a very dim target like an exoplanet every extra a photon of light makes a difference !).
It’s likely to be a long life telescope given its robust design and potential for service even if at L2 so a secondary star shade mission could happen late next decade with a bit of luck. Even a five year mission would only cost a bit more than $500 million which when compared to $2.5 billion WFIRST and $8.5 billion JWST , or even $1.2 billion Gaia is an absolute snip given its critical science return too.
Does anyone know how long “1 year” would be on Wolf 1061c?
I’m writing a novel and would really appreciate the help.
Thank you
It is listed above in Table 1 under “Orbit Period” (which is the length of a “year” for this world) as 17.87 days (Earth days, of course).