# Close the door on your way out - Life lit by a blue dwarf star

I'm seeking a hard science setting for a piece of xenofiction with a decidedly non-sciencey feel. That said, there is no magic or magic technology.

The idea is as follows:

1. A red dwarf star has a rocky, icy planet well outside the habitable zone.
2. After ~6 trillion years the red dwarf begins a transition into its blue dwarf stage for a further 400 billion years
3. The massively increased stellar luminosity of the blue dwarf stage melts the icy planet turning it into a habitable planet, on which a civilisation develops. Since the planet is not too close to the star, it is not tidally locked, but is now in the habitable zone of a star with similar luminosity to the sun (according to article linked above).
4. A timeline of 400 billion years is allowed for the creation of a habitable planet and for evolution.

This civilisation would be very alone in the universe at the end of the stellar phase of the universes evolution, with perhaps only a few other blue dwarf stars visible in the night sky, if any. All the other stars have turned into black dwarfs.

Obviously the timescales are immense so my question is this:
Is the notion of a planetary thawing of a truly ancient rocky ice planet some 6 trillion years after its formation is within the realms of plausibility.

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

• So the source states that the luminosity increase will be from ~2200 K to ~5800 K, for a 0.1 M$_{\odot}$ red dwarf. Given that the inner and outer radii of the habitable zone are proportional to $\sqrt{L}$, and for a blackbody $L$ is proportional to $T^4$, then the radii are proportional to $T^2$, and so multiplying the temperature by ~2.5 will extend the inner and outer radii outward by a factor of about ~6. I wouldn't characterize this change as massive, so the planet can't be too far "well outside" the initial habitable zone. – HDE 226868 Feb 13 '16 at 20:03
• @HDE226868: Nice back-of-the-envelope calculation. I think a orbit radius factor of 6 can easily mean a planet is "well outside" the habitable zone... Were the Earth six times as far away from the Sun as it is now (so further than Jupiter) it would be a dead frozen lump. – fgysin reinstate Monica Feb 15 '16 at 11:26

Yes, it is plausible as the timescales of which a red dwarf is a blue dwarf are quite big, to the point where if your icy planet is distant enough it will thaw and potentially develop life. What matters is the placement of your planet and its size, as if your icy planet is too small it won't retain a atmosphere and if it's too far the temperature increase will only make the planet only go from Triton cold to Europa cold. There are exoplanets with substantial amounts of water whose origin comes from a icy planet that migrated inwards. They are worlds with world spanning oceans and very thick atmospheres of vapor. Your world, being a world that thawed will get its atmosphere from when its icy surface slowly melts away. Since water vapor is lighter than nitrogen, your world is likely going to be bigger than earth to retain its atmosphere.

The biggest concern for your world I see comes from geology, as with such a ancient planet the geological processes within the world likely have died down after trillions of years. This may actually inhibit life because of the role geological activity on earth played in the origin of life, and may play on other worlds like Europa if there is hydrothermal vents on Europa. You likely could circumvent this issue by having life actually form extremely early on in the red dwarf's life when your world is geologically active, but be frozen or evolutionarily static for trillions of years due to having no incentive to evolve. (This does happen on earth in certain instances, such as with the Coelacanth who were thought extinct yet still are around, in a much simpler, far less biodiverse ecosystem, life not advancing beyond simple organisms like bacteriums for trillions of years may actually be possible). Your life is likely to be radiation resistant, because being around a red dwarf exposure to higher radiation levels is a given. This is good news for your life as a ancient world trillions of years in age is going to be geologically dead no matter what the size of your planet. One possible way to circumvent this is by having your world part of a binary planet system (and therefore gets geological activity from tidal stress, a process that is seen around pluto).

A civilization may well develop, they have the time and evolution being the fickle and semi-chaotic thing it is on a world with rocky landmasses mixed with water has the potential for intelligent life to develop. However, if they are a space faring society they'll be going a very different tech path than on earth to get there because they'd lack access to fire due to a water vapor atmosphere and without geological activity they'd also lack access to any form of volcanic metallurgy. You should consider inventing creatures that can circumvent this issue somehow, perhaps through intense body heat. They may take far longer to develop a space faring civilization than per say, a world like earth might, but since they have hundreds of billions of years they have more than enough time to figure things out.

• Great answer - thanks! And thanks for the links. Presumably by varying the initial frozen composition I could end up with something that isn't dominated by water vapour? – rumguff Aug 23 '15 at 18:44
• Could you circumvent the "geologically dead" by a Thea like impact? – Sidney Feb 15 '16 at 22:03

Yes, it can happen.

For this to be possible, you first have to put the planet far enough away that it can become totally icy. Then, after the temperature rises as the red dwarf transitions into a blue dwarf, the habitable zone must encompass the planet's orbit, melting the ice and making it a better place for life. We can easily calculate whether or not the final habitable zone can ever extend far enough for this to be possible.

Your initial goal is to place the planet "well outside the [initial] habitable zone". That's fine. We can find the initial inner and outer boundaries of the habitable zone using the formulae found here. The outer radius of the initial habitable zone is $$r_o=\sqrt{\frac{L_*}{0.53}}\text{ AU}$$ Let's take the example of the 0.1-M$_\odot$ red dwarf used in the blog post. Its initial (i.e. pre-blue dwarf) luminosity is ~1/2400 L$_\odot$, or ~1.595$\times$1023 watts. Plugging this in, we get $$r_o\approx2.80\times10^{-2}\text{ AU}$$ Now, this is for a very dim red dwarf (I would think about an M5V class or an M6V class dwarf), so it's not surprising that this is so small.1

You say you want to have an ice planet. We can estimate the inner radius at which it can form by calculating the distance of the initial frost line. If we use the model attributed to Hayashi (1981) by Ida & Lin (2005), then $$r_f=2.7\left(\frac{L_*}{L_{\odot}}\right)^{\frac{1}{2}}\text{ AU}\approx5.50\times10^{-2}\text{ AU}$$ Now, it is clear that $L_*$ will change with time even before the transition to the blue dwarf stage. Kennedy & Kenyon (2008) model these changes on various stars and compare them to earlier results, including those of Ida & Lin.2 The differences are drastic, in some cases. See their Figure 1:

For lack of more information, though, I'm going to have to stick to the original estimate.

Your article states that the initial surface temperature of the star is ~2230 K, prior to entering the blue dwarf phase. It then rises to ~5810 K, a change of a factor of about 2.5 The luminosity of a blackbody is proportional to its temperature to the fourth power (see the Stefan-Boltzmann law), so the inner and outer radii of the habitable zone are proportional to the temperature squared. This means that the final outer edge will be ~6 times as far out as it originally was - way beyond the initial frost line, about three times as far out.

This seems pretty good, but what about the inner edge of the final habitable zone. Where will that be? According to the same site that provided the formula for the outer edge, the inner edge will be $$r_i=\sqrt{\frac{L_*}{1.1}}\text{ AU}\approx0.70r_0$$ This is about twice as far out as the frost line originally was.

So, yes, from at least these estimates, the scenario is quite plausible.

1 Some results state that the habitable zone should be even smaller, although others disagree and say that it should be bigger.
2 Even more drastic changes are shown in Martin & Livio (2012).

You've got a problem here:

The radioactive materials that keep the planet's core hot will be virtually gone by the time your planet warms. With no molten core you will have no plate tectonics and thus there will be no vulcanism or uplift. The result is your planet erodes.

By the time you're trying to make it habitable you'll have a world of constant depth oceans.

• Will that prevent life from evolving? – k-l Feb 14 '16 at 3:28
• You'll have a world totally without land and a surface totally without stone. How will technology develop? – Loren Pechtel Feb 14 '16 at 4:21
• Perhaps underwater, or floating on the surface. – k-l Feb 14 '16 at 4:23
• @KiranLinsuain But out of what? The only materials to construct things from are bone and shell. There would be no fire. – Loren Pechtel Feb 14 '16 at 6:13
• I don't think that technology absolutely needs fire to be developed. We have to keep in mind that life on a planet like that one would be completely different. Knowing how split and complex are the Earth living being, and even what they do, I suppose it's completely possible to have advanced technology without fire. – Eithne Feb 15 '16 at 13:49

As noted by others, such an old planet is possible, but has its problems.

The "dead" geology of the planet is the primary problem, and needs some kind of external energy injection to either maintain geologic activity for such a lengthy time or to jump start the processes once again (planetary viagra?).

• Jump starting the process would likely require a Theia-type impact. This would require a lengthy period of time for the crust and atmosphere to settle into a more life-friendly environment, but this impact could happen just prior to the conversion to blue dwarf, surreptitiously aligning the renewed geologic cycle with the increase of stellar luminosity . To maintain an active core, a significant Moon is likely required, as little radioactivity will be available to keep it going. A potential problem to your scenario is that the planet will not be "warming" into a habitable planet, but cooling.
• Maintaining geologic activity for such a lengthy period of time would require your planet to have a significant orbital partner to keep the core active through tidal forces. This could be a significant moon, a double-planet, or perhaps your "planet" is a significantly sized moon of gas giant. This option keeps the crust of your planet renewed, and may even result in local hot spots where simple life could develop and survive, allowing it to rapidly bloom when the stellar luminosity warms the entire surface.

I think the deal breaker is that after trillions of years your planet would be colder than liquid nitrogen all the way through to the core. The heat from the star would have to be very hot to warm up the surface of the planet to a comfortable level, but step out of the shade and you are likely to get a nasty burn. At night the ground would freeze and surface temperatures would plummet. Even with a thick atmosphere to help buffer the heat from the star, as soon as night falls the air would start to condense and freeze out of the sky. A few billion years increased heat from the star would not be enough to warm up the interior of the planet enough to buffer the thermal gradients on the surface. Basically, I think the the habitable zone will be too narrow in a scenario like this.

... Maybe if the planet rotated very fast it might be enough to smooth out the temperature changes.

Given the difficulties of keeping the core of any world warm on the timescale of stellar evolution with radioisotopes are pretty well insurmountable so instead of a planet orbiting a red dwarf I would suggest a world near the barycentre of a system like Luyten 726-8 a binary red dwarf. This would have a number of advantages, such a world is illuminated by two blue dwarfs thus having a better heat budget but more importantly the gravitational flexion it will experience under the influence of two such stars will keep the world warm at it's core and allow tectonics etc... to proceed under the ice, keeping life ticking over ready for the big thaw.