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Our universe is incredibly young, relative to its total life span. How young? Well, it contains stars. Giant balls of readily fusing hydrogen and helium that give off a pleasant glow, suitable for igniting the formation of life forms on planets orbiting at optimal distances with healthy chemical compositions.

About 100 trillion years from now, the universe will look very different. All hydrogen burning stars will have exhausted their fuel, and all pockets of free floating gasses large enough to form into new stars will have done so, and then those stars will have all run out of fuel. The only remaining stars in this future universe will be degenerate ones: white dwarfs, neutron stars, and black holes. The universe will have entered what's known as the Degenerate Era.

Assuming that there are chemically ideal planets for it to evolve on (these will stick around for a few hundred trillion more years after the stars have all burned away), could life evolve in such a universe? How would life in the Degenerate Era differ from life in the current universe?

What is life?

When considering the question of the evolution of life, we must first answer an important question: what is life? While there are many working definitions of life, here's a simple one that may work for the purpose of this question. In order to be considered "life", something must meet these four criteria:

  1. Reproduce - Life must be able to reproduce in a manner which passes on the traits of the parent organism(s).
  2. Grow - Life must consume matter and use this matter to grow in size.
  3. React to stimuli - Life must be capable of reacting to external stimuli.
  4. Metabolize - Life must be able to utilize chemicals and energy to change its structure and physical state.
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  • $\begingroup$ A crucial question for such an era is what is your definition of life? When you're only considering situations like those on Earth, we can be loose with the definition, but when you start pushing into the extreme enviornments, we need a rather wide definition of life. $\endgroup$ – Cort Ammon Mar 11 '16 at 22:08
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    $\begingroup$ Evolve is unlikely , but any race that exists for such a long time should be able to harness the technology to create more Suns and repeat the cycle $\endgroup$ – Freedo Mar 12 '16 at 0:50
  • $\begingroup$ The problem I foresee is that a planet can't be "chemically ideal" if it's frozen under 300 miles of ice and sources of heat/energy are going to very difficult to come by in this setting. $\endgroup$ – ApproachingDarknessFish Mar 12 '16 at 2:30
  • $\begingroup$ What about moons of rogue planets that could have subsurface oceans because of tidal heating like Europa? $\endgroup$ – Kasie Ream Nov 18 '17 at 19:13
  • $\begingroup$ I recommend Manifold Time by Stephen Baxter. Without giving too much away, the book centers around this kind of question. $\endgroup$ – Cort Ammon Nov 20 '17 at 0:35
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There are several possibilities - actually, quite a few - for the development of life in the Degenerate Era. Some have potential; some don't.

  1. The planet is a rogue planet. It has been proposed that rogue planets could retain heat and support life via geothermal energy from radioactive decay (see also Stevenson (1999)). However, it seems unlikely that a planet's core could remain "hot" for any time period of an order greater than billions of years (see here and here, as well as here and here, noting the disagreement between the latter two links).

    You would have to have a planet form in the degenerate era for this to be possible. I find this unlikely simply because most planets would likely have formed long before this time. These planets would then be generating minimal internal heat via radioactive decay, if any.

  2. The planet gains energy from tidal heating. This answer mentions it; indeed, it has been proposed as a potential mechanism for life on Europa, an the reason that its hypothetical subsurface ocean could exist. However, the tidal forces would have to be significant, meaning that the planet would have to be very close to the body it orbits, which could be dangerous. This might, though, be your best option.

    The heat produced by tidal heating is computed by $$q=\frac{63}{38}\frac{\rho n^5r^4e^2}{\mu Q}$$ where the important orbital components here are $r$ (distance to primary), $e$ (eccentricity) and $n$ (mean motion). The equation makes it seem that $q\propto r^4$; however, given that $n\propto a^{-3/2}$, where $a$ is the semi-major axis, the heat generated drops off for orbits with greater semi-major axes, instead of increasing.

  3. New stars form from collisions. The Wikipedia article containing a section on the Degenerate Era notes that collisions between bodies (e.g. white dwarfs, brown dwarfs, etc.) can produce Type Ia supernovae, carbon stars, or even red dwarfs, if two brown dwarfs collide correctly. However, this does not mean that the star will be suitable for life. These cases make it unlikely that there will be a planet orbiting the resulting object. The odds of capturing a rogue planet are quite slim. Getting the planet into an orbit that could bring it inside the star's habitable zone is even harder.

There are some reasons why I don't like the odds of life arising on a planet orbiting one of the degenerate objects you listed in your question1.

  • White dwarfs. These are actually the most benign of the three, insofar as there's not too much deadly radiation or extreme tidal forces hanging around, for the most part. The obvious problem is that white dwarfs may evolve to become black dwarfs, and thus emit virtually no light. It has been estimated that black dwarfs will form on a timescale on the order of ~1015 years, possibly near the end of the Degenerate Era.

    The bigger problem is that the circumstellar habitable zone will be extremely tiny. Agol (2011) estimates that it extends from ~0.005 AU to ~0.02 AU. You would need planets to be extremely close to the star. Putting aside the risk of tidal forces making the planets uninhabitable, this begs another question: How did the planets get so close to begin with? A Sun-like star would have expanded far, far beyond this during its AGB phase, meaning that (as Agol notes) the planets would have to form later on or migrate inwards. This may not be likely, but it is certainly possible:

    Formation mechanisms must be modeled to help motivate future surveys. For example, gravitational interactions of a planet and star with a third companion body may be responsible for creating hot Jupiters (Fabrycky & Tremaine 2007), which is also promising for moving distant planets around white dwarfs to $2a_R\approx0.01\text{ AU}$, the tidal circularization radius (Ford & Rasio 2006). It is also possible that tidal disruption of a planet or a companion star will result in the formation of a disk which may cool and form planets (Guillochon et al. 2010), out of which a second generation of planets might form (Menou et al. 2001; Perets 2010; Hansen et al. 2009).

    That said, planets might not survive this long. Adams & Laughlin (1997) show that, assuming a number density of objects that is similar to the number density of stars in the galaxy today, a planet with a nearly circular orbit of radius $R$ would be disrupted on a timescale of $$\tau=1.3\times10^{15}\text{ years }\left(\frac{R}{1\text{ AU}}\right)^{-2}$$ Other similar problems arise when we consider that planetary systems with more than one planet are generally chaotic, as matscienceman mentioned. This could either help or hurt the chances of habitability, by making the planet better or worse for life through collisions or ejections. Additionally, planetary orbits will decay over time. This timescale depends on both the initial orbital radius of the planet and the mass of its parent star.

  • Neutron stars. You again have the issue of habitability. I wrote a lot about the issues specific to planets orbiting pulsars here. There are a couple main problems, neglecting the radiation (which would likely be emitted elsewhere, such as from the poles of the neutron star, and so be [mostly] harmless). The applicable one here is that the planets might have been captured, and would likely be orbiting far away from the neutron star - too far for them to get any benefits from tidal heating or light. However, this is not necessarily the case (see Veras et al. (2011), an article that cites it, and an answer that cites that).

    Other problems:

    • The neutron star could have an accretion disk, which could produce more radiation and in general make life unpleasant.
    • Neutron stars don't emit a lot of radiation conducive to life.
  • Black holes. Insert all of the issues that come with neutron stars, and then add more. By now, Worldbuilding Stack Exchange has beaten to death the idea of life on a planet orbiting a black hole. Issues include:

    • Tidal forces[1]
    • Radiation from an accretion disk (told you!)[1]
    • A fairly large magnetic field[1]
    • Minimal tidal heating at safe distances[2]
    • Plasma (mainly around supermassive black holes)[3]

    I could go on. As Serban Tanasa wrote in the first of those three answers, a planet orbiting a black hole would most likely be "a radiation swept molten rock horror."

Even though the Degenerate Era will be an interesting place, it seems unlikely that life will find a planet enjoying stable enough conditions to be habitable. That said, as DJMethaneMan said, there are most likely enough planets in the universe for all of these cases to successfully yield habitable conditions somewhere.


1 Let's assume that the planet wasn't captured. I've already mentioned the difficulties that arise with that scenario.

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    $\begingroup$ Nice list. In addition, the chaotic nature of asteroids and comets, even planets, when disrupted by the gravity of white dwarf from outside their solar system, could cause major collisions. This could melt the planet, in the same manner as the collision that produced our moon melted the Earth. Combine these with tidal forces, and the Europa type path towards life could result. $\endgroup$ – matscienceman Mar 13 '16 at 9:35
  • $\begingroup$ @matscienceman Great point; I hadn't considered that. I edited some of that in. Thank you. $\endgroup$ – HDE 226868 Mar 13 '16 at 13:51
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    $\begingroup$ I would say that even with all of the reasons it could not realistcally happen, the sheer number of stars and planets will mean that somewhere in the universe it will happen. $\endgroup$ – JDSweetBeat Mar 14 '16 at 16:23
  • $\begingroup$ @DJMethaneMan Right, of course. Edited. $\endgroup$ – HDE 226868 Mar 14 '16 at 21:16
  • $\begingroup$ @matscienceman At these timescales, everything that could have collided had already collided a long time ago, also everything that was subject to tidal forces is already tidally locked, if the orbiting bodies are not already ejected away through random stellar encounters. Of course, such an encounter would stir tidally locked planet and give it maybe some billions of years worth of energy, but such events will become increasingly rare. $\endgroup$ – Radovan Garabík Mar 15 '16 at 18:28
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There is one little explored possibility for abundant life - the stellar remnants themselves. On their way towards black dwarfs, stars will inevitably pass through favourable temperature range, where chemistry is possible. First at upper atmosphere layers, then as the star cools down the "habitable zone" moves deeper and deeper, increasing in pressure, heated by still hot core which brings (via convection) needed heavier elements and chemical energy.

White dwarfs are better candidates than red dwarfs, since the latter contain mostly hydrogen and helium, while the former stars burned through interesting elements, such as carbon and oxygen. Since there is still much hydrogen present, life will probably be based around $CH_4$ and $H_2$, not $CO_2$. And it's dark, so life depends on chemical energy. Gravity is crushing and there is no definite solid surface (think of Superjupiters as fluffy light and immaterial gas balls in comparison), thus life would be probably floating (Jovian-like). With cooling down, it will move towards extreme pressures and exotic chemistry.

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The degenerated era of the universe is the era in which no new stars will form and the present ones will burn out one by one and leaving only black hole behind (ok, that is simplicistic)

Probably if a star like our Sun will form a couple of billion years before entering in the degenerated era, it can sustain life (more or less) as we know.
Since our sun is not a star that has a very long life (about 15/20 billion years, compared with the trillion years of a red dwarfs), once the sun-like stars burn out, life as we know (or comparable with life as we know) will easily cease to exist, eventually leaving space to some more extreme forms of life, comparable to our extremophiles.

At the end of the degenerated era, once the universe will enter in the black hole era, but probably much earlier, all life will probably cease to exist, if we exclude the possibility of some very extreme extremophiles that will adapt to live near a black hole findind some way to use the Hawking radiation to survive, but this is really very unlikely.

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    $\begingroup$ Wouldn't the very existence of such a sun-like star mean we have not quite reached the degenerate era? $\endgroup$ – JDSweetBeat Mar 14 '16 at 16:26
  • $\begingroup$ Theoretically yes, but I suppose that the Universe era are not exact dates, so if such a star will form as one of the last stars, you are in a situation between the stelliferous era and the degenerated era: no new stars will form but you still have a yellow star. If you get as starting point of the degenerated era the instant the last star will born, then you will have a (maybe dying) yellow star in the degenerated era. I think I stretched a little the boundaries ;-) $\endgroup$ – Gianluca Mar 14 '16 at 18:58
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Life depends on energy to exist. One possibility is a rogue planet, hospitable to life, captured by a white or red dwarf star, in a tight orbit.

Then, let's hope that the star remains luminous for time enough for life to evolve (or re-evolve, if it had life before).

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Why wouldn't it? As long as the temperature and such is nice enough (which should be possible with a proper orbit), there's a lot more time for it to evolve - your typical degenerate-era star is going to remain in a fairly stable state for several trillion years (unlike our Sun, which only has a lifespan of less than ten billion years).

It's the "temperature and such" thing that can be a problem (and perhaps anything in a sufficiently close orbit will receive too much hard radiation). I'm not enough of an astrophysicist to speculate further.

That said, I agree with Freedo's comment - some civilization that evolves before the degenerate era will likely try to work for continuation of life into the future. A lovely setting based on that concept had been discussed a few years ago on AlternateHistory.com, named "Ten quintillion A.D." (search for it if you happen to be a member there).

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