New consideration doubles the theoretical number of habitable worlds in Milky Way

A hypothetical water world with the red dwarf Gleise581 glowering on the horizon.

A hypothetical water world with the red dwarf Gleise 581 glowering on the horizon.

Look at a big spiral galaxy like the Milky Way and its blazing with the blue-white light of hot, young stars. But the vast majority of stars in our galaxy are the smaller, redder types. Of the estimated 400 billion suns in our galaxy, astronomers believe at least half of them are red dwarfs. One problem with red dwarfs is they tend to be active, they can flare and dim dramatically over short periods of time compared to our reliable, stable yellow sun. Because they are so much dimmer, planets in the Goldilocks zone where water might exist as a liquid on the surface would need to huddle much closer to the red stars. Not only would those planets would be pummeled by stellar outburst, they’d become tidally locked, always the same side facing the sun, similar to the way our moon always presents the same face to earth. All things being equal, a planet that takes a few weeks to rotate instead of a day would have a less powerful magnetic field to ward off the solar wind. Both Mars and Venus have mostly lost whatever primeval magnetic field they once had and that probably played a role in vaporizing whatever oceans they may have started out with.

But there’s a newer consideration, one which could help protect the planets of red dwarfs, now being modeled and tested. This increases the number of potentially habitable exo-planets to 60 billion! How? Think of a cold Venus:

Link — The team’s three-dimensional global calculations determined, for the first time, the effect of water clouds on the inner edge of the habitable zone. The simulations are similar to the global climate simulations that scientists use to predict Earth’s climate. These required several months of processing, running mostly on a cluster of 216 networked computers at UChicago. Previous attempts to simulate the inner edge of exoplanet habitable zones were one-dimensional. They mostly neglected clouds, focusing instead on charting how temperature decreases with altitude.

“There’s no way you can do clouds properly in one dimension,” Cowan says. “But in a three-dimensional model, you’re actually simulating the way air moves and the way moisture moves through the entire atmosphere of the planet.”These new simulations show that if there is any surface water on the planet, water clouds result. The simulations further show that cloud behavior has a significant cooling effect on the inner portion of the habitable zone, enabling planets to sustain water on their surfaces much closer to their sun.

Astronomers observing with the James Webb Telescope will be able to test the validity of these findings by measuring the temperature of the planet at different points in its orbit. If a tidally locked exoplanet lacks significant cloud cover, astronomers will measure the highest temperatures when the dayside of the exoplanet is facing the telescope, which occurs when the planet is on the far side of its star. Once the planet comes back around to show its dark side to the telescope, temperatures would reach their lowest point.

But if highly reflective clouds dominate the dayside of the exoplanet, they will block a lot of infrared radiation from the surface, says Yang, a postdoctoral scientist in geophysical sciences. In that situation “you would measure the coldest temperatures when the planet is on the opposite side, and you would measure the warmest temperatures when you are looking at the night side, because there you are actually looking at the surface rather than these high clouds,” Yang says.


  1. M can help you with that. says

    I love that this model actually comes with a set of observations that would let us know whether it actually applies. Too many of the exciting possibilities of extrasolar habitable planets rely on plausible but not-yet-testable possibilities; I’m not an astronomer, but from what I recall of the state of the technology it shouldn’t be too long before we could potentially get a temperature (or at least albedo!) measurement of a planet orbiting a (relatively close to us) red dwarf…

    …and if red dwarf stars have a usable habitable zone, they offer a nice long window for life to develop and/or thrive; much longer than stars like our sun (most of Earth’s life-suitable time has already passed!), and far longer than the big dramatic giants and the like.

  2. trollofreason says

    Erf. If I may?
    The “Goldilock zone” hypothesis has always troubled me. Which is alright, since recent advances in astromony, not to mention even more recent discovery of exoplanets themselves, and hydrology as old as the steam age suggests the G-zone might be a crock of steaming shit. Pardon my language.
    Liquid water is one of the fundamental building blocks of life as we know it, true, but water can be rendered into a liquid, organic-compatible solvent suitable for complex chemical interactions under a stupidly varied number of circumstances. Circumstances, by the way, only marginally related to a planet’s distance from its star and more to do with the properties of the planet itself in relation to pressure, temperature, and impurities.
    I know it’s a small thing, but at the same time it’s not since the entire point of this article is to back up what I just said: There is no such thing as the Goldilocks zone, and life might not be as special or as rare as we think.

  3. Randomfactor says

    Mars and Venus have mostly lost whatever primeval magnetic field they once had

    Mars’ day is almost identical to ours, and no moon to speak of to slow it down so presumably it’s been that way for a while.. Is the effect more pronounced because it’s smaller? Less iron in the core?

  4. StevoR : Free West Papua, free Tibet, let the Chagossians return! says

    @ ^ Randomfactor : Mars woes stem mainly from its low mass as I understand it.

    Its small mass meant it had less internal heat which meant Mars couldn’t drive plate tectonics, cooled off too quickly, lost its magnetic field and thus the solar wind could erode its atmosphere away and irradiate its oil with UV and thus it became the rusty,salty frozen thin aired hostile place it is today.

    If we terraform Mars one idea that may pose problems is how to significantly increase its mass enabling it to have a gravity capable of holding more atmosphere for longer otherwise in time – well, potentially over a very long time – the newly blue red planet will end up back at square one.

    Good thought about red dwarf worlds here but I do wonder how they’ll cope with the extreme flares almost all red dwarfs are known for emitting :

    “Imagine M-dwarf (red dwarf – ed) bathing on the beach and having your star suddenly – with no warning at all become ten times brighter.”

    – James B. Kaler, page 31, ”The Faintest Stars” article in ‘Astronomy’ magazine August 1991.

    Could pose a bit of a challenge but, who knows, hopefully life will find a way to cope.

  5. StevoR : Free West Papua, free Tibet, let the Chagossians return! says

    One of my favourite astronomer – writers Ken Croswell offers a few more thoughts on red dwarf planets hospitality for life :

    Bad news in the above link, good news here in the below link :

    and also more good news here :

    Although I guess there’s still really insufficient evidence to draw any firm conclusions either way as yet.

    If only we could send out starships now and learn from say the nearest ten such stars. Sigh.

Leave a Reply