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Climate Models for Red Dwarf Planets

Although it’s hard for me to believe it, there was a time nine years ago, not long after I began writing these posts, when a daily scramble for topics was fairly common. How the world has changed. These days, between the huge increase in online discussion of interstellar flight and the burgeoning exoplanet scene, the problem becomes to keep from falling too far behind. I’m already a couple of weeks out on interesting work from the University of Washington on one of my favorite topics, red dwarfs and the possibilities for life there. It’s time to catch up.

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What Aomawa Shields has been examining in her recent work is climate in the extreme, the kind of ‘snowball Earth’ event that, in several periods 600 million years ago and earlier, may have covered the planet in ice from pole to pole. Shields’ new paper in Astrobiology goes at the question of climate extremes on planets around M-dwarfs, where conditions are markedly different than around stars like the Sun. It turns out that M-dwarf planets, even when they receive the same amount of light, may be warmer than their counterparts orbiting larger, hotter stars.

The reason: While hotter stars emit more visible and ultraviolet light, cooler stars are rich in infrared and near-infrared wavelengths. Ice and snow absorb longer wavelength near-infrared light more efficiently than visible light, which tends to be strongly reflected. In the red dwarf scenario, then, the absorption of light by ices and snows helps to warm the planet, a trend that can be compounded by atmospheric greenhouse gases that also absorb near-infrared light.

Image: Proxima Centauri, the nearest star to our own, and an example of a red dwarf with frequent flare activity. New work from the University of Washington suggests that M-dwarf planets are more likely to avoid ‘snowball’ events when the planet becomes completely covered with ice. No planets have yet been discovered around Proxima Centauri, but the hunt continues. Credit: NASA/CXC/SAO.

Whereas hotter stars may have their visible and ultraviolet light reflected by ices and snows on the surface (‘ice-albedo feedback’ is the term), the M-dwarf stays warmer at the same level of incoming light. M-dwarf planets are more likely to avoid the kind of snowball states that terrestrial worlds around hotter stars fall into. From the paper (citations removed for brevity):

Because of the spectral dependence of ice albedo, the ice-albedo feedback mechanism is sensitive to the wavelength of light coming from the host star. On M-dwarf planets, a significant amount of radiation emitted by the host star is absorbed by atmospheric gases such as CO2 and water vapor, which absorb strongly in the near IR… However, a disproportionate amount of the longer-wavelength radiation that does reach the surface will be absorbed by, rather than reflected from, icy or snowy surfaces on these planets.

The effects are striking, because conditions for runaway icing don’t occur:

This will reduce the difference between ice and ocean surface albedo. Episodes of low-latitude glaciation, termed “Snowball Earth” events… may be less likely to occur on M-dwarf planets as a result of the lower-albedo ice on their surfaces, as entrance and exit into such a snowball state has been shown to be sensitive to ocean-ice albedo contrast…

Snowball states don’t preclude life, of course — Earth itself is proof of that — but Shields argues that astronomers will want to prioritize their searches for planets less vulnerable to them (see this University of Washington news release for more). That seems an unnecessary leap given that snowball events can be a catalyst for emerging life, as the paper goes on to discuss:

The Neoproterozoic Snowball Earth episodes of 750 to 635 million years ago have been linked to the emergence of multicellular life on Earth due to enhancement of the flux of dissolved phosphates into the ocean, which would have caused increased primary productivity and organic carbon burial and led to the rise of oxygen in the ocean and atmosphere… Planets less likely to experience such global-scale glaciations may therefore be dependent on alternate pathways to serve as catalysts for biological evolution. The M-dwarf planet in our simulations also exhibited more stable lower-latitude ice lines than the G- or F-dwarf planets; this may be due to the lower-albedo ice formed on its surface. A more stable low-latitude ice line on M-dwarf planets may be possible as the result of a lower albedo contrast between bare sea ice and snow-covered ice…

Shields and team used three-dimensional atmospheric circulation models to simulate planets covered by ocean, land and water ice under different conditions of incident radiation from a variety of star types. The effect of icy surfaces absorbing light works along with greenhouse gas absorption to reduce the frequency of snowball events. The effect becomes less of a factor as we move to the outer edge of the habitable zone, where greenhouse gases like atmospheric CO2 reach high levels and ice-albedo feedback plays a much smaller role.

The paper is Shields et al., “The Effect of Host Star Spectral Energy Distribution and Ice-Albedo Feedback on the Climate of Extrasolar Planets,” published online by Astrobiology July 15, 2013 (full text).

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Comments on this entry are closed.

  • Alex Tolley July 30, 2013, 10:23

    This part of the discussion seems rather relevant to the results:

    “We assumed a rotation period equal to present-day Earth (24 h) for our simulations to isolate the effect of stellar SED and ice-albedo feedback on planetary climate. Synchronous rotation, which is expected to occur on M-dwarf planets orbiting in their stars’ habitable zones (Dole, 1964; Kasting et al., 1993; Joshi et al., 1997; Edson et al., 2011), would certainly affect the results presented here. “

  • Paul Gilster July 30, 2013, 12:32

    I’m really glad you saw that, Alex, because I missed it!

  • Michael July 30, 2013, 15:26

    I am suspecting that the temperature at the surface of planets around high infrared emitting Stars will be quite low. Any carbon dioxide in the atmosphere will act as an interceptor of that radiation limiting it getting to the ground, in effect a reverse greenhouse effect. How this carbon dioxide and snow interact is up for debate.

  • Brian Hall July 30, 2013, 15:45

    Would this help with the faint young sun paradox? Does a newly formed G-type star have its peak emissions shifted toward the IR?

  • Doug M. July 31, 2013, 4:30

    @Brian, the answer is no. The temperature of stars stays pretty fixed throughout their life on the main sequence (though their luminosity will change). So the young Sun was fainter, but had just about the same peak emission.

    Doug M.

  • Doug M. July 31, 2013, 4:40

    @Michael, no. You don’t get a significant “reverse greenhouse effect” until you reach the very coolest and reddest stars, the late Ms. And even then it’s modest.

    Why? Because the way the greenhouse works is, carbon dioxide admits peak solar radiation — visible light, near infrared, and near UV — which is then absorbed by the Earth and re-emitted as thermal radiation. CO2 does not admit the thermal radiation; it blocks it, acting like a blanket. Right?

    Well, the thermal radiation has a much longer wavelength than the peak solar. It’s mostly down in the middle infrared, 4–100 μm. The CO2 “blanket” doesn’t start to kick in until around 10 μm. So, you’re not going to see a reverse greenhouse unless you have a star so cool that it’s emitting a significant percentage of its energy at or below 10 μm.

    Doug M.

  • Doug M. July 31, 2013, 4:48

    Life around an M star would run into other problems. For example, the absorption of light by liquid water has a local minimum around 400 nm. That’s the blue-violet bit of the visible light spectrum. Blue light passes through water more easily than red. That’s why the light underwater is blue-ish, and gets bluer as you go deeper.

    Well and good. But — the curve on either side of the local minimum is quite steep. Water absorbs near UV pretty well, and harder UV very well indeed: if you could see in the 200nm spectrum, the oceans would look black. Similarly, water absorbs near IR very well, and if you go much below a micrometer wavelength it absorbs so well that it’s pretty much dark in those wavelengths below the first few centimeters of water depth.

    So, life evolving in liquid water would be a lot more constrained around an M dwarf.

    Doug M.

  • Ronald July 31, 2013, 5:16

    Slightly off-topic but related, pertaining to the HZ issue:
    the HZ may be even more shifted toward the outside, with the runaway greenhouse effect being triggered more easily than previously thought.
    Hence, our own planet may be even closer the the inner edge of the HZ and closer in time to the runaway greenhouse stage;
    Low simulated radiation limit for runaway greenhouse climates, by Goldblatt et al.
    http://www.sciencedaily.com/releases/2013/07/130730163146.htm

    I could not download the original article PDF for free.
    The Science Daily article mentions ‘a billion and a half years or so’ before earth moves intot the runaway greenhouse stage, but that seems way, way overly optimistic, since recent previous estimates (the recent Kasting update of the HZ) already reduced that significantly, the inner edge of our HZ now being at 0.97 to 0.99 AU. That implies that we would almost certainly leave the HZ within about 0.5 gy and possibly within 0.3 gy.

    This new model would reduce that even further.

  • Ronald July 31, 2013, 5:24

    Brian: I would rather think that a young G-type star has its peak emissions shifted toward the lower wave lengths, also lots of UV.

  • Doug M. July 31, 2013, 5:55

    …if you really want to get deep into the weeds on greenhouse stuff, water vapor has two absorption peaks in the near IR around 1400 and 1900 nm. So, a planet with a lot of water vapor in the atmosphere in orbit around a red dwarf might see some reverse greenhouse, depending on things like whether there was a high altitude water trap and how well the stratosphere mixed with the troposphere. You’d probably need a good 3D model to work out the details, though.

    Doug M.

    Doug M.

  • Michael July 31, 2013, 16:02

    @Doug,

    Percentage wise there is a lot more absorption of energy at the four main Co2 absorption wavelengths@ 4000K versa 5500K ~double at the same radiated energy. Now looking at water vapour there is significant absorption of radiated energy due to the multitude of absorption bands (7 to 8). Each will process will have a noticable effect, but together they will absorb a lot of incoming radiated energy more than with a Sun like we have and therefore will have an effect on the surface temperature. How the ground and water respond to more thermal radiation is open to debate.

  • Doug M. July 31, 2013, 18:18

    @ Ronald, that doesn’t seem to be how it works. Once a star has settled down on the main sequence, its emission spectrum doesn’t change much.

    @ Michael, at 4,000K the output in the CO2 absorption bands is much higher than at 5,000K — but it’s still just a small fraction of the star’s total output. You have to get cooler still before the effect becomes significant. I can do the math, but I’d like someone to give me a cookie first.

    Also, a hypothetical “reverse greenhouse” is going to be dynamically rather different than the greenhouse we’re familiar with. Our CO2 greenhouse absorbs energy that would have been lost to space. The reverse greenhouse absorbs energy that would have been absorbed by the planet anyway… just, at the top of the atmosphere instead of down at the surface. Does that end up cooling the planet? Maybe, but it’s going to be a while before we have a good strong model.

    Doug M.

  • Michael August 1, 2013, 2:51

    @Doug,

    With 1.5 to 2 microns @ 4000 K for Co2 represents 11.89% of the emission energy, this significant and is ~ double what our suns spectrum will be absorbed per same energy emitted! Water vapour has an even greater absorption total and whats more is that the peak 4000 K emission (0.724 micron) lands at the foot of an absorption band and there is a lot more of them going to longer wavelengths. As for cookies I give them away every time is data sent over the net.

    Mick,

  • Ronald August 2, 2013, 4:10

    Doug, you are absolutely right about that and I stand corrected. I read that stars enter the main sequence stage already within 50 million years.
    However, I also read that sunlike stars do emit large doses of UV (tens of times to a few hundred times present solar amount) in their early youth. How early that is and for how long (tens or hundreds of millions of years?) I do not know.