Habitable Zone Of Red Dwarfs May Be Larger Than Once Thought

Stretching the spectrum: a hypothetical red dwarf planetary system (Research.gov)

Given that 80% of the stars in the Universe are M-type ‘red dwarfs’, research into the habitability of planets in these stars’ orbits has received relatively little attention in the past as they were generally considered unsuitable for hosting habitable planets due to their low mass and temperatures, as well as the propensity for planets in their orbit to be ‘tidally locked’. However, this trend has shown signs of reversal over the past few years, and habitability assessments have generally returned favourable reviews of M-star planets. The issue of tidal locking, where one hemisphere of a planet constantly faces the star, doesn’t seem to be resolved yet, but more research is being carried out and a definitive assessment may be forthcoming soon.

A paper published in Astrobiology this month has bolstered the habitability assessment of red dwarf systems even further. Manoj Joshi, now at the University of East Anglia, and Robert Haberle at the NASA Ames Research Center, have considered the effect that the longer wavelength spectra of M-stars may have on the ice-albedo feedback operating on planets within their habitable zones. Albedo describes the fractional reflectivity of a given surface, from 0 (nothing reflected, a hypothetical ‘black-body’ ) to 1 (all light reflected). On Earth, the albedo of ice is ~0.5 (50% of light reflected), whilst snow has an albedo of ~0.8.

The ice-albedo feedback is a fundamentally important abiotic feedback mechanism that has a powerful control over the planetary climate: it describes the ability of ice and snow to reflect light away from the surface, thereby cooling it further and causing more ice/snow to form, which continues to exacerbate the effect in what is termed a ‘positive’ or destabilising feedback loop. More ice, more light reflected away, cooler temperatures, more ice and so on.

The ice-albedo feedback is thought to have been at least partially responsible for the ‘Snowball’ or ‘Slushball’ Earth events that occurred in the late Proterozoic eon, approximately 600 million years ago, which saw the Earth frozen from pole to pole, with possible refugia at the equator. This interpretation is still rather contentious within the geosciences, but most researchers agree that the Earth experienced a period of extreme glaciation around this time, but its full extent, and how the Earth emerged from this deep-freeze, is still not fully understood.

The amount of incident light, as well as atmospheric greenhouse effects, exhibit a strong control on the ability of the ice-albedo feedback to enter a ‘runaway’ state by preventing temperatures from falling below a critical level of ice cover. Accordingly, this mechanism is often considered a controlling factor on the outer boundary of the habitable zone because of its very powerful ability to destabilise the planetary environment into an irreversible state of complete glaciation.

Joshi and Haberle constructed a simple model to test how the the ice-albedo feedback would operate on planets within the habitable zones of M-stars when considering the longer wavelength, lower energy emissions of these stars. Red dwarfs, as their name suggests, emit much of their radiation in the red and near-infrared portion of the electromagnetic spectrum. Observations from the red dwarfs Gliese 436 and GJ 1214 mentioned by the authors show that they emit much of their radiation at wavelengths greater than 0.7 μm, and significantly more in the 3 to 10 μm region than would be expected from a ‘black-body’ hypothesised M-type of a similar temperature. The albedo of ice and snow begins to decrease at wavelengths greater than 1 μm, and therefore the albedo of snow and ice covered surfaces on planets in the orbit of red dwarfs would be proportionally lower than that of the same surface on Earth (or any other planet in orbit around a G- or K-type star), meaning they reflect less radiation away from the surface, and that the ice-albedo feedback mechanism is weakened. For example, the authors show that snow or ice covered surfaces on planets orbiting GJ1214 may have albedos of 0.43 and 0.23 respectively, representing a significant decrease in the amount of incident light reflected from the surface and a dampening of the ice-albedo feedback mechanism.

Because of the diminished effect of the ice-albedo feedback mechanism around red dwarfs, the authors propose that their habitable zone may be 10-30% further from the star than was previously considered. This finding has a significant impact on the search for habitable exoplanets and for astrobiology, and, as is often the case with good science, has been drawn from a relatively simple experiment – in this case, by analysing the reflectivity of frozen or snowy surfaces under the observed radiative regime of red dwarfs. It seems that the tide really is turning in terms of our understanding of the habitability of planets in the orbits of red dwarfs, and that these numerous and ubiquitous stars should receive renewed research and observational attention.


Click here for the Astrobiology article (requires subscription).


Manoj M. Joshi and Robert M. Haberle (2012). Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone Astrobiology, 12 (1) DOI: http://arxiv.org/abs/1110.4525

Gliese 667Cc: A new ‘Super-Earth’ basking in the light of three Suns

Astronomers announce with excitement the latest exoplanet found to be orbiting within the habitable zone of its star. In addition, the newly discovered Gliese 667Cc is a member of a very unique orbital system. Its parent star, the red dwarf Gliese 667C itself orbits a binary system of two K-type stars, Gliese 667A & B at an enormous distance roughly equivalent to 6 times that between the Sun and the dwarf planet Pluto. Accordingly, the distant binary system, whilst bound gravitationally, has no affect over the planetary environment of Gliese 667Cc, nicknamed ‘Vulcan’ by astronomers after the triple-star system home to Star Trek‘s Spock. I’m not much of a sci-fi fan, despite my interest in all things exoplanet, so I’ll stick to an shortened ‘Cc‘ for brevity.


The Gliese 667C system revolves around a M1.5V red dwarf, a small star only 31% as massive as the Sun and much less luminous, located 22 light-years away from Earth in the constellation Scorpius. The habitable zone extends from 0.11 AU out to 0.23 AU, well within the orbit of Mercury if superimposed onto the Solar System.  Cc has a minimum mass equivalent to roughly 4.5 Earths and orbits at 0.12 AU, straddling the inner edge of the habitable zone. Accompanying Cc in orbit is Gliese 667Cb, a large (5.7 Earth masses) planet nestled at 0.05 AU, and possibly another planet of equal mass, dubbed Gliese 667Cd, at 0.24 AU.

Gliese 667 Cc performed very well in a habitability assessment undertaken by the Habitable Exoplanet Catalog (HEC), ranking as the planet with the greatest habitability potential of all discovered exoplanets to date:

Habitability assessment of Gliese 667Cc by the Habitable Exoplanet Catalog (information and graphics by HEC, 2012)

Figures in red are subject to large uncertainty, and will only be refined with more detailed observation. A quick refresher of the HEC metrics in the context of Cc: ESI is the ‘Earth Similarity Index’  and consists of several planetary characteristics, namely radius, density, escape velocity, and surface temperature that are used to determine the relative similarity of the planet to Earth on a scale from 0 (completely dissimilar) to 1 (identical). An ESI 0f 0.82 represents an ‘Earth-like’ world, but the large mass (5.2 as the mean expected mass) of Cc has negatively affected this value.

SPH is the Standard Primary Habitability, a measure (from 0 to 1), calculated from surface temperature and humidity, of the ability of the planet to support terrestrial primary producers. In the case of SPH, Cc outranks even the Earth! Its position half-way between the very centre of habitable zone and its inner edge, represented here by the metric HZD, means that it is extremely favourable to supporting a ecosystem of primary producers similar to those on Earth. However, as a red dwarf, Gliese 667C emits much of its radiation in the red, near-infrared (NIR) and infrared (IR) portion of the electromagnetic spectrum. Red dwarfs like Gliese 667C are also known to be more variable and prone to flaring.  The affect of this shift in wavelength would have very negative repercussions for Earth-based photosynthetic mechanisms which utilise visible light, but the possibility of photosystems evolved to exploit lower-energy NIR/IR radiation is hypothetically possible.

Other values to note are the comfortable planetary temperature of 29 °C, large mass and somewhat more suppressive gravity. A year on Cc lasts 28 days. Unfortunately, it isn’t possible to determine whether Cc is a rocky, watery or gas planet without an accurate measurement of its size, a parameter still unavailable at this stage. The effects of a possible atmosphere cannot be accounted for just yet but a thick greenhouse of water vapour, carbon dioxide or methane would elevate the planetary temperature beyond that considered habitable.

Lack of public interest

So it seems that Cc  is the new champion of the habitable planet competition being held by scientists on Earth, and the evidence seems to back up their claims. Why then the lack of public interest? Outside of popular science websites and publications, news of this new planetary utopia is hard to find. Contrast the scarcity of coverage with the hype surrounding Kepler 22b two months ago, and I fear the predictions I made in these posts may have come to fruition. The wider public is bored; they’ve heard it all before and become desensitised our disinterested. Kepler 22b is habitable, so is Gliese 581d and now so is Gliese 667Cc. It’s disappointing, but inevitable, that the furore of excitement surround these planet discoveries wasn’t sustainable. The thing is, we still haven’t stumbled across the perfect Earth analogue, a replica of our watery, rocky globe. Yet. We will do, and when this day comes and the discovery is announced, I fear the room may be empty save for a few dedicated science correspondents that realise the very real implication of finding a planet like this.

Update (08/02)

It seems that in my haste to bemoan the lack of mainstream press coverage of Cc, I neglected to detect the underlying politics of the announcement. The main reason that Kepler 22b attracted so much more attention is that Cc was not announced by NASA. The NASA PR machine is an effective beast. Also, the discovery of Gliese 667Cc was first announced last November by a European team of astronomers led by Xavier Bonfils from Université Joseph Fourier in Grenoble, France. However, it’s confirmation came yesterday from an international team lead by two American astronomers, Guillem Anglada-Escudé and Paul Butler from the Carnegie Institute for Science. Cc‘s discoverer is therefore under debate.

The coverage of Gliese 667Cc also seems to suffer from a somewhat of a geographical disconnect. Daniel Fischer, who runs the excellent ‘The Cosmic Mirror‘ site, notes that the coverage of Cc has been extensive in his native Germany because of Anglada-Escudé’s link with the University of Göttingen. Parodies and further analysis can be found here and here, respectively (in German – thanks Google Translate!).

It seems that the story of Gliese 667Cc is far from over.