Could life evolve in the degenerate era of the universe?

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.}

posted about 8 years ago

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