M-Dwarfs: A New and Wider Habitable Zone

by Paul Gilster on February 23, 2012

I want to work a new paper on red dwarf habitability in here because it fits in so well with yesterday’s discussion of the super-Earth GJ1214b. The latter orbits an M-dwarf in Ophiuchus that yields a hefty 1.4 percent transit depth, meaning scientists have a strong lightcurve to work with as they examine this potential ‘waterworld.’ In transit terms, red dwarfs, much smaller and cooler than the Sun, are compelling exoplanet hosts because any habitable worlds around them would orbit close to their star, making transits frequent.

When I first wrote about red dwarfs and habitability in my Centauri Dreams book, it was in connection with the possibilities around Proxima Centauri, but of course we can extend the discussion to M-dwarfs anywhere, this being the most common type of star in the galaxy (leaving brown dwarfs out of the equation until we have a better idea of their prevalence). Manoj Joshi and Robert Haberle had published a paper in 1997 that described their simulations for tidally locked planets orbiting red dwarf stars, findings that held open the possibility of atmospheric circulation moderating temperatures on the planet’s dark side. There seemed at least some possibility for extraterrestrial life on such a world, although the prospect remains controversial.

Image: X-ray observations of Proxima Centauri, the nearest star to the Sun, have shown that its surface is in a state of turmoil. Flares, or explosive outbursts occur almost continually. This behavior can be traced to the star’s low mass, about a tenth that of the Sun. In the cores of low mass stars, nuclear fusion reactions that convert hydrogen to helium proceed very slowly, and create a turbulent, convective motion throughout their interiors. This motion stores up magnetic energy which is often released explosively in the star’s upper atmosphere where it produces flares in X-rays and other forms of light. X-rays from Proxima Centauri are consistent with a point-like source. The extended X-ray glow is an instrumental effect. The nature of the two dots above the image is unknown – they could be background sources. Credit: NASA/CXC/SAO.

A lot of work has been done on M-dwarfs and habitability in the years since, and we also have the problem of this class of stars emitting flares of X-ray or ultraviolet radiation, making the prospects for life still uncertain. It would be helpful, then, if we could find a way to back a planet off from its host star while still allowing it to be habitable. The flare problem would be partially mitigated, and tidal lock might not be a factor. Joshi, now studying planetary atmospheric models at the University of East Anglia, has recently published a new paper with Haberle (University of Reading) arguing that the habitable zone around M-dwarfs may actually extend as much as 30 percent further out from the parent star than had been previously thought.

At issue is the reflectivity of ice and snow. M-dwarfs emit a much greater fraction of their radiation at wavelengths longer than 1 μm than the Sun does, a part of the spectrum where the reflectivity (albedo) of snow and ice is smaller than at visible light wavelengths. The upshot is that more of the long-wave radiation emitted by these stars will be absorbed by the planetary surface instead of being reflected from it, thus lowering the average albedo and keeping the planet warmer. Joshi and Haberle modeled the reflectivity of ice and snow on simulated planets around Gliese 436 and GJ 1214, finding both the snow and ice albedos to be significantly lower given these constraints.

The finding has no bearing on the inner edge of the habitable zone, as the paper notes:

The effect considered here should not move the inner edge of the habitable zone, usually considered as the locus of orbits where loss rates of water become significant to dry a planet on geological timescales (Kasting et al 1993), away from the parent M-star. This is because when a planet is at the inner edge of the habitable zone, surface temperatures should be high enough to ensure that ice cover is small. For a tidally locked planet, this implies that ice is confined to the dark side that perpetually faces away from the parent star; such ice receives no stellar radiation, which renders albedo effects unimportant.

The dependence of ice and snow albedo on wavelength is small for wavelengths shorter than 1 μm, which is where the Sun emits most of its energy, so we see little effect on our own climate. But the longer wavelengths emitted by red dwarfs could keep snow and ice-covered worlds warmer than we once thought. How various atmospheric models would affect the absorption of the star’s light is something that will need more detailed work, say the authors, but they consider their extension of the outer edge of the habitable zone to be a robust conclusion.

The paper is Joshi and Haberle, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone,” accepted by Astrobiology (preprint).

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{ 18 comments }

andy February 23, 2012 at 15:14

One immediate question as regards the inner edge is what happens to the albedo of clouds. Changing that could result in modification of the inner edge of the HZ.

Ron S February 23, 2012 at 15:24

“The dependence of ice and snow albedo on wavelength is small for wavelengths shorter than 1 μm, which is where the Sun emits most of its energy, so we see little effect on our own climate. But the longer wavelengths emitted by red dwarfs could keep snow and ice-covered worlds warmer than we once thought. ”

Surface albedo doesn’t matter unless the radiation gets that far; that depends on the composition of the atmosphere, including oxygen and water vapor. For consideration, here is a diagram showing the opacity of Earth’s atmosphere. Note the 1 μm point.
http://en.wikipedia.org/wiki/File:Atmospheric_electromagnetic_opacity.svg

jkittle February 23, 2012 at 16:30

so if we can reach one of these worlds then is it practical to use “solar” power? we also need to take into account that the size of the planet matters. larger planets should be “habitable” at distances greater then smaller planets. Larger planets should have more geologic activity too and thus more ecological niches in geothermal areas.

jkittle February 23, 2012 at 16:38

lookign at a planet the size of neptune for example with its low surface gravity ( 1.14 g) and large surface area. perhaps in the depths of a neptune ocean we might find life. if it were close to a red dwarf star its atmosphere would protect from flares.
by the way , how do we know that Neptune does not have liquid oceans? at some depth the temperature has to be between 0 and 100 degrees c.
It turns out that microbes can live at enormous pressures…
just Say’n we may be suffereing form a lack of imagination in looking for life in our solar system or looking at planets around distant stars with too narrow a focus.

philw1776 February 23, 2012 at 16:51

A related thought. The Earth receives the sun’s higher energy photons mostly at less than 1um as stated. Instead of boiling, the Earth re radiates even more photons but less energetic in the infra red at over 1um wavelength. This increase in entropy satisfies the 2nd Law of Thermodynamics.

Not sure how all the physics works here but planets of red dwarfs receiving the lower energy longer wavelength ‘sunlight’ might possibly be less efficient re-radiating it and retain a higher portion of the heat. If so, this would do two things (1) move the inner HZ out further and (2) move the outer HZ boundary further from the star.

Extrapolating to K stars midway between our G and the Ms, perhaps further outer HZ boundaries might mean less tidal locked planets.

Scott G. February 24, 2012 at 16:38

The Proxima Centauri image caption says: “X-rays from Proxima Centauri are consistent with a point-like source. The extended X-ray glow is an instrumental effect.”

Does this mean that the telescopes we have today cannot resolve the disk of this star? The only star that I’m currently aware of where we can resolve its disk is Betelgeuse, but I was hoping that Proxima would be resolvable too, because it’s so close.

As an aside, here’s a quote from the Betelgeuse Wikipedia article: “In 1995, the Hubble Space Telescope’s Faint Object Camera captured an ultraviolet image of comparable resolution—the first conventional-telescope image (or “direct-image” in NASA terminology) of the disk of another star.”

Phil February 24, 2012 at 16:53

“…Does this mean that the telescopes we have today cannot resolve the disk of this star?…”

IIRC, quite a lot of nearby main sequence stars have had their real size measured – with high accuracy – by long baseline optical interferometry, but only a few bloated supergiants have actually been resolved visually.

P

Michael February 24, 2012 at 19:48

There are places where the U.V and X-ray intensity would be a lot lower and that is on the terminator (tidally locked) of the planet as the atmospheric opitcal depth is a lot thicker,
Also worlds that are near to their stars and are so called tidally locked may not be so, because ice could buildup on the dark side, this would alter the centre of gravity of the planet and it could cause a small torque turning the planet slowly round, as it turns the ice melts and is re-deposits on the dark side again so keeping the effect going – The lower the mass of the planet and right land to water ratio should help this effect -however this effect may be over whelmed by tidal drag – just a thought

torque_xtr February 25, 2012 at 3:50

It is said that the inner edge of HZ is defined by the distance at wich water loss becomes significant – Venus had earthlike water amounts, but it is too close to Sun, so all water have photodissociated and then hydrogen atoms have escaped to space because of their light mass and high temperature around exobase, and oxygen combined with surface rocks. But how is this consistent with characteristics of planets like Kepler 11-f and especially Kepler-26b, which both have escape velosities lower that Earth’s and Venus’s but are hotter than Mercury and apparently are light neptunes? It’s possibly no other way to explain their very low densities than to assume they have large amounts of H2O or/and H2/He in their compositions, even the “oxygen gas giant” produced by complete H2O breakdown in small neptune would be denser. So we have extraordinarily high deuterium content in Venusian atmosphere, which strongly suggests H2O escaped to space, together with Venus Express-measurements of escape rate, and these low density hot sub-neptunes in orbit around sun-like stars. Contradiction?

spaceman February 25, 2012 at 16:28

Hi,

Like many natural phenomenon, I am guessing there is an approximately bell-shaped distribution of planetary habitability with respect to stellar spectral type. Perhaps most habitable planets reside around spectral type late G to early K with the ‘tail ends of habitability distribution’ reaching the early M dwarf spectral type and the late F spectral type.

The study mentioned here seems to increase the probability of our finding relatively hospitable planets around M type stars due to the possibility of a wider habitable zone for some types of planets around these long-lasting red stars. A wider habitable zone around red dwarfs would mean that some of the habitability impediments associated with planets needing to be so close to the parent star may be somewhat ameliorated. Fascinating work. This may be unrealistic, but I’d sure love to explore one of these systems at some point.

Nick February 26, 2012 at 0:31

torque_xtr, very interesting comments. I suspect the contradiction may be resolved by the fact that, even though the hot light Neptunes have a similar surface gravity to Venus, they have a much higher escape velocity, which may be sufficient to prevent the hydrogen from escaping.

It is utterly wonderful that the diversity of the kinds of planets is nowhere close to being limited to the diversity of the planets in our own solar system. I suspect before this is all over we will discover many more new kinds of planets that are simply not represented in our own solar system.

andy February 26, 2012 at 17:25

@torque_xtr:
Regarding the Kepler-11 planets, there are large error bars on the masses. The situation with regards to atmospheric escape could be helped if the masses are at the high-end of the allowable range. Furthermore there is the issue of initial composition to take into account – if the Kepler-11 planets formed beyond the ice-line from primarily volatile-rich material and migrated into their current location, while the terrestrial planets in our system essentially formed in situ, the Kepler-11 planets would have a far larger reservoir of material from which to generate their atmospheres.

Eniac February 26, 2012 at 22:56

@Andy:

The situation with regards to atmospheric escape could be helped if the masses are at the high-end of the allowable range.

Right. Furthermore, in this situation, where planets are near the limit of observability, the actual mass of the observed planets will be strongly biased away from that limit, i.e. is much more likely to be at the high end of the allowable mass range than at the low end. This is because the posterior condition of having observed the planet increases its probability of being observable dramatically. Sounds like a tautology, but has this very real bias effect.

torque_xtr February 27, 2012 at 3:22

@Andy
Even in case of largest mass and smallest radius, we have sqrt(2GM/R) around 15 km/s for Kepler-11f, 17.5 km/s for Kepler-11e, 19 km/s for Kepler-11d and 10 km/s for Kepler 26b, (10.5 km/s, 15 km/s, 14 km/s and 8 km/s for the most likely values), which is around ~1.5 times higher than for Venus. The estimated effective temperatures and the mean molecular velosities for hydrogen atoms are, in turn, 2, 2.5, 3 and 4 times higher that for Venus, so the water/hydrogen escape proplem is significant even with the assumptions of the most suitable parameters. (Of course, it is zero’th approximation which doesn’t take into account that the exobase temperatures in H2/He atmospheres may be lower than in CO2 or N2/O2-ones, and/or the influence of planetary magnetic fields)

Regarding the formation, as it was said in some article on arxiv and could be speculated a little further – in case of Keler-11 and other tight-packed sub-neptune systems, which resemble scaled up versions of Jupiter satellite system, it may be possible that the circumstellar disc was very dense and massive, and the planets had formed before the primary ignited fusion and it’s radiation blew away all volatiles. The other findings of compact light neptune planetary systems may indicate this is a common scenario, contrary to our Solar System. (if there’s no “observational bias” by the Kepler team, which announced these results first :-) In our case, the disk was less dense, and the Sun achieved main sequence before the material could coalesce into something massive, so all volatiles were blown beyond snow line (where mixed with pre-existing material), and the terrestrial planets had to be born from rocky leftovers with mass comparable to that of Kepler-11′s inner planets rocky content.

In all cases, Kepler-11 is older than the Sun, so all the planets were there for billions of years and yet retained their gaseous envelope – it’s very unlikely that all 5 planets are now simultaneously in the late stages of atmospheric loss…

Ronald February 27, 2012 at 9:17

From the paper: “The effect of such spectral dependence can move the habitable zones of planets orbiting M-stars outward by 10-30% in terms of distance from the star and increase the chance of finding habitable planets orbiting M-stars.”

Well, though of course a 10-30% increase in HZ is not insignificant, it is not spectacular either, also given the disadvantages of M dwarfs.

I tend to agree with spaceman, that probably “there is an approximately bell-shaped distribution of planetary habitability with respect to stellar spectral type. Perhaps most habitable planets reside around spectral type late G to early K with the ‘tail ends of habitability distribution’ reaching the early M dwarf spectral type and the late F spectral type.”
Or something like that. I would rather put the optimum somewhere around mid-G (G5/6) orso.

philw1776 February 27, 2012 at 14:33

As always it depends on what’s meant by habitable. Why only include late F stars? Sure the early Fs start puffing up by the time they’re our Earth’s age and earlier but was not the Earth habitable billions of years ago? Life thought so.

Were we to have magic unicorn starships capable of quick interstellar flight, Fs and maybe some As with their big wide HZs might contain lots of ocean planets where we could seed the oceans with O2 producing plants and the land with grasses, trees and shrubs. No mosquitos please.

The fact that the parent star would heat things up too much after “only” a couple tens of million years could be left as an engineering exercise for later generations.

amphiox February 27, 2012 at 19:00

The fact that the parent star would heat things up too much after “only” a couple tens of million years could be left as an engineering exercise for later generations.

Habitable planets around the big stars might prove to be excellent places for advanced civilizations to colonize (assuming, and it is a big assumption, that sufficiently advanced civilizations would find themselves needing or wanting to colonize planets at all), but would not be the best places to look for indigenously evolved life. (At least not as a first target. I would imagine that once we get good and comfortable at looking at late F and smaller stars, and bag a few identifications, we’ll surely start branching out to look at the bigger ones too. All things in good time, it would seem….)

Ronald February 29, 2012 at 7:29

Slightly off-topic, but: the latest Kepler data are just in!

http://arxiv.org/abs/1202.5852
PDF is also available.

Quote from summary:

“The largest fractional increases are seen for the smallest planet candidates (197% for candidates smaller than 2Re compared to 52% for candidates larger than 2Re) and those at longer orbital periods (123% for candidates outside of 50 day orbits versus 85% for candidates inside of 50 day orbits).
(…)
The fraction of all host stars with multiple candidates has grown from 17% to 20%, and the paucity of short-period giant planets in multiple systems is still evident. The progression toward smaller planets at longer orbital periods with each new catalog release suggests that Earth-size planets in the Habitable Zone are forthcoming if, indeed, such planets are abundant.”

The present data are from the period May 2009 – September 2010, which means that soon there shoud be even a lot more data available.

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