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Upper limit on the fraction of alien civilizations that develop communication technology

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Fewer than 1 in 200 intelligent species develop communication technology, and silicon-based life in nitrogen lakes could tighten the limit to about 1 in 1,300.

desk verdict The silicon/nitrogen habitable zone distances are new and the algebra is clean, but the claimed factor-of-6 tightening of the ξ_biotec bound is an unquantified assumption dressed as a result. read the letter →

arxiv 1908.01335 v1 pith:WYXBAHOC submitted 2019-08-04 physics.pop-ph

Luis A. Anchordoqui , Susanna M. Weber This is my paper · ORCID
classification physics.pop-ph
keywords FermiparadoxDrakeequationhabitablezonesilicon-basedlifeliquidnitrogenexoplanetoccurrencerategamma-raybursthazardscommunicationtechnologyfraction
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper asks how rare technological civilizations really are. Using the fact that we have detected no artificial signals from the Milky Way, it sets a statistical upper limit on the average fraction of living intelligent species that develop communication technology: less than $5\times10^{-3}$, about one in two hundred, at 95% confidence. It then asks whether allowing life to be based on silicon rather than carbon, living in liquid nitrogen lakes instead of water, changes the answer. The paper finds that if planets with surface nitrogen lakes are as common as planets with subsurface water oceans, the limit tightens by roughly a factor of six, to about $7.5\times10^{-4}$. The significance is that the calculation turns a silence in the sky into a concrete bound on one of the most uncertain factors in the Drake equation.

What carries the argument

The argument runs on the Drake-equation split $N = \langle \zeta_{\rm astro}\rangle \langle \xi_{\rm biotec}\rangle L_\tau$, where $\langle \zeta_{\rm astro}\rangle$ is the astrophysical production rate of habitable planets and $\langle \xi_{\rm biotec}\rangle$ is the biological-technological factor being bounded. The upper limit comes from a small-sample Poisson confidence interval: with zero observed signals, $N<3.09$ at 95% CL. The extension to non-carbon life uses a radiative-balance temperature formula, $T_p = [F_{r_p}(1-\alpha_p)/(4\sigma)]^{1/4}$, to define a nitrogen habitable zone where the planetary surface temperature lies between the freezing and boiling points of liquid nitrogen (63.15 to 77.36 K), and takes the albedo of such worlds from Triton ($\alpha \approx 0.6$). That zone is what converts the existence of silicon biochemistry in a nitrogen solvent into an extra population of habitable planets and thus into a lower value of $\langle \zeta_{\rm astro}\rangle$, which tightens the limit on $\langle \xi_{\rm biotec}\rangle$.

What would settle it

A census of exoplanets in the liquid-nitrogen temperature annuli—for ultra-cool dwarfs, roughly 28 to 42 million km from the star—that finds few or no planets there would show $\eta$ is far below 1 and erase the factor-of-six tightening; alternatively, detection of a single confirmed artificial signal would invalidate the $N=0$ premise on which the entire upper-limit argument rests.

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Extended reading notes

Core claim

The central discovery is a revised upper limit on $\langle \xi_{\rm biotec}\rangle$, the product of the fraction of suitable planets where life appears, the fraction of living species that develop intelligence, and the fraction of intelligent species that develop communication technology. From the non-observation of artificial signals, modeled as zero events in a Poisson process, the 95% confidence upper limit is $N<3.09$, which translates to $\langle \xi_{\rm biotec}\rangle < 5\times10^{-3}$ for fiducial astrophysical parameters. Extending habitability to non-carbon, silicon-based life in liquid nitrogen, the paper computes nitrogen habitable zones for Sun-like stars (about 1.24 to 1.85 billion km) and for ultra-cool dwarfs such as TRAPPIST-1 (about 28 to 42 million km), and argues that, as with subsurface ocean worlds, the frequency of planets hosting such life may be $\eta \approx 1$. Taking $\eta=1$ tightens the limit to $\langle \xi_{\rm biotec}\rangle < 7.5\times10^{-4}$, a factor of about six more restrictive.

Load-bearing premise

For the headline factor-of-six tightening, the load-bearing premise is that planets with surface liquid nitrogen are about as common as planets with subsurface water oceans ($\eta \approx 1$); the paper asserts this frequency rather than deriving it, and if the true value is closer to the conservative $\eta = 0.15$, the tightening disappears.

Editorial extensions

If this is right

  • If the limit holds, at most a few tenths of a percent of living intelligent species in the galaxy ever reach communication technology, so the absence of detected signals is consistent with life being common but technological civilization being rare.
  • Including nitrogen-lake silicon life with $\eta \approx 1$ tightens the 95% upper bound to $\langle \xi_{\rm biotec}\rangle < 7.5\times10^{-4}$.
  • The calculation makes the Fermi paradox quantitative: given current estimates of star formation and habitable-planet production, our silence implies that the average path from life to radio technology is very inefficient.
  • The same statistical method can be applied to future null results: as the communicative lifetime $L_\tau$ or the astrophysical rate estimates improve, the upper limit on $\langle \xi_{\rm biotec}\rangle$ scales linearly with the assumed values.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The factor-of-six tightening is the weakest link: it assumes $\eta \approx 1$ for nitrogen lakes purely from the existence of a temperature zone, without an occurrence-rate measurement; if future exoplanet surveys find few planets in those annuli, the tightening evaporates.
  • Extending the argument to other cryogenic solvents, such as liquid methane on Titan-like worlds, would require the same $\eta$ question, but the paper's radiative-balance machinery could be applied directly to produce testable habitable-zone annuli for each solvent.
  • A direct testable extension is a census of planets in the nitrogen habitable zone around ultra-cool dwarfs (28 to 42 million km for TRAPPIST-1-like stars); that census would settle whether the factor-of-six is real.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper re-examines the SETI-based upper limit on the average fraction of living intelligent species that develop communication technology, ⟨ξ_biotec⟩, by extending the analysis to non-carbon, silicon-based life in liquid-nitrogen solvents. Using a blackbody equilibrium temperature argument, the authors derive circumstellar zones where liquid nitrogen could exist: 1.24–1.85 billion km for a Sun-like star and 28–42 million km for TRAPPIST-1. They then combine the null result for artificial signals (N<3.09 at 95% CL) with estimates of star formation, habitable-planet frequency η, and gamma-ray-burst survival to obtain the baseline limit ⟨ξ_biotec⟩<5×10^{-3} at 95% CL. The paper claims that if silicon/nitrogen biospheres are included and one takes η∼1, the limit tightens by up to a factor of 6.

Significance. If the claimed tightening were properly supported, the paper would provide a concrete, quantitative illustration of how broadening the definition of habitability sharpens Fermi-paradox constraints, and it would usefully connect astrochemistry speculations about silicon life to observational SETI limits. The paper's blackbody algebra and the propagation of the null detection into a 95% CL upper limit are transparent and internally consistent, and the presentation gives the reader a clear chain from assumptions to Eq. (17). However, the significance of the central new result depends entirely on an unsupported occurrence-rate assumption for silicon/nitrogen planets, so the current manuscript does not yet deliver the factor-of-6 tightening it advertises.

major comments (3)
  1. [III and Eq. (17)] The claimed factor-of-6 tightening of the upper limit in Eq. (17) follows entirely from substituting η=1 into the denominator, but Section III provides no estimate of the frequency of planets in the silicon/nitrogen habitable zones it computes. The section identifies only radial ranges (1.24–1.85 billion km for the Sun, 28–42 million km for TRAPPIST-1) and explicitly notes that Saturn, the only solar-system body in the Sun's silicon zone, is a gas giant; for TRAPPIST-1 all seven known planets orbit at about 0.062 AU, well inside the quoted silicon zone at 0.187–0.281 AU. The sentence "As for subsurface ocean worlds, we may take η∼1" is an assertion rather than a derivation, and the analogy to subsurface ocean worlds is not transferable because surface nitrogen lakes require both a narrow equilibrium-temperature window and a rocky, nitrogen-bearing planet. If η for the silicon zone were instead comparable to the conservative water-HZ value η⊕∼0.15, the factor of 6 would disappear. This missing support is load-bearing for the paper's central new claim and must be supplied or the conclusion must be explicitly reframed as conditional on an unmeasured parameter.
  2. [III, Eq. (13)] The silicon habitable zone is derived from Eq. (13), which assumes a blackbody planet with a single albedo α≃0.6 and neglects greenhouse warming. The paper itself states in Sec. II that greenhouse effects change surface temperatures dramatically (e.g., about 40 K on Earth and 760 K on Venus), so applying the same formula to a hypothetical nitrogen-rich atmosphere without a greenhouse correction is not justified; the resulting zone boundaries could shift by an amount comparable to the 14 K width of the liquid-nitrogen range. The authors should either incorporate a greenhouse model or explicitly state this as a large systematic uncertainty and test how sensitive the claimed zone (and hence η) is to it.
  3. [IV, Eqs. (14)–(17)] The central claim that the upper limit can be up to a factor of 6 more restrictive is not robust to reasonable parameter variations. Equation (17) scales as 1/η with the text allowing 0.15<η<1; taking η=0.5 rather than η=1 reduces the tightening from a factor of about 6.7 to about 3.3, and taking η=0.15 removes it entirely. Because Section III offers no independent estimate of η for silicon/nitrogen planets, the advertised restriction is effectively the input assumption rescaled into the output. The authors should state the result as an explicit function of the silicon-zone η or provide a physical occurrence-rate calculation.
minor comments (5)
  1. [III] The word "wether" should be "whether," and the sentence ending "as solubi." is incomplete, likely missing a word such as "solubility."
  2. [II and Table I] Table I has "planer" where "planet" is meant, and the quoted range for η⊕ in Sec. II is typeset in a way that makes the asymmetric uncertainties hard to read.
  3. [IV] Before Eq. (16), the text writes "⟨ξbiotect⟩" where "⟨ξbiotec⟩" is intended.
  4. [IV] The text attributes the N<3.09 95% CL bound to exact binomial probabilities, but 3.09 is the Feldman–Cousins or Poisson upper limit for zero observed events; the statistical prescription should be stated consistently.
  5. [III] The star is referred to as "TRAPPIST-1A"; the standard designation is TRAPPIST-1, with the planets denoted by lowercase letters.

Circularity Check

1 steps flagged · score 6.0 of 10

The claimed factor-of-6 tightening is the assumed η=1 rescaled through Eq. (17); the silicon-zone calculation supplies no occurrence rate.

  1. other [Section III (last paragraph), Section IV Eq. (17), and Section V conclusion]
    "This seems to indicate the frequency of planets hosting any form of life must be extended. As for subsurface ocean worlds, we may take η∼ 1 for intelligent, conscious and technologically sophisticated species. ... ⟨ξbiotec⟩< 5× 10−3 (3 yr−1 / Γ⋆)(0.15/η)(0.044/ζdot) (17) ... The upper limit can be up to a factor of 6 more restrictive if we assume there may be non-carbon based living organisms."

    The factor-of-6 is the direct algebraic consequence of setting η=1 in Eq. (17): 5×10^-3 × 0.15/1 ≈ 7.5×10^-4, i.e., a factor ~6.7 tighter. Section III derives only equilibrium-temperature radial ranges for liquid nitrogen (1.24–1.85 billion km for the Sun; 28–42 million km for TRAPPIST-1). It does not estimate how many planets occupy those ranges: its own solar-system check finds Saturn within the Sun's silicon zone but unsuitable (gas giant) and Uranus outside it. No independent value of η for silicon/nitrogen planets is derived or cited; the paper simply imports the subsurface-ocean-world assumption. Hence the claimed new upper limit is the assumed input η=1 rescaled by the paper's own formula, not a result of the silicon-habitability analysis.

full rationale

The paper's base limit ⟨ξbiotec⟩ < 5×10^-3 at 95% CL follows from the N=0 observation through Eq. (17), with astrophysical inputs (Γ⋆, η, ζdot) and Lτ; this chain is internally consistent and the GRB-survival factor is taken from the authors' prior work [5], but that self-citation is an external input rather than a circular redefinition. The one genuinely circular move is the silicon-life extension: the 'up to a factor of 6 more restrictive' result in the conclusion is nothing more than Eq. (17) evaluated at an assumed η=1. Section III's radiative calculation establishes only that a liquid-nitrogen temperature zone exists at certain radii; it never establishes the occurrence rate η for planets in that zone, and the text's own solar-system example (Saturn in zone but unsuitable; Uranus out) shows zone existence alone does not imply η≈1. The tightening is therefore an assumption rescaled into the output by construction, warranting a partial-circularity score of 6. The remaining components—star formation rate, habitable-zone occurrence rates, GRB survival probability—are ordinary inputs, not circular.

Assumptions & free parameters 4 free parameters · 4 assumptions · 1 invented entities

The central claim rests on the unmeasured frequency η=1 for silicon/nitrogen life and on the blackbody approximation for the nitrogen habitable zone; both are assumptions rather than derived results.

free parameters (4)
  • eta (frequency of planets hosting silicon/nitrogen life) = 1 (upper extreme; range 0.15-1)
    The factor-of-6 tightening of the upper limit comes from taking η=1 in Eq. (17), but the paper never estimates this frequency from the silicon HZ calculation; it is an input, not a measured or derived quantity.
  • Albedo of silicon-HZ planet = 0.6 (Triton-like)
    Adopted in Sec. III to compute the nitrogen HZ boundaries via Eq. (13); the boundaries change with albedo, but the factor-of-6 limit itself does not depend on this value.
  • Minimum communication lifetime L_tau = 0.3 Myr
    Chosen so that c L_tau exceeds Galactic scales; this sets the absolute scale of the upper limit but not the factor of 6.
  • GRB critical survival parameter p_zeta = 0.044 (lower bound)
    Used to set the fiducial lowest astrophysical production rate; from the authors' prior work [5].
assumptions (4)
  • domain assumption Life can exist with a non-carbon, silicon-based biochemistry in a liquid nitrogen solvent.
    Invoked in Sec. III with citations [15-18]; the paper does not derive or test this, and it is essential for the η=1 assumption that drives the factor-of-6 claim.
  • domain assumption The habitable zone for silicon life is set by the effective temperature range 63.15-77.36 K with a Triton-like albedo and no greenhouse correction.
    Used in Sec. III via Eq. (13); the paper does not correct for greenhouse effects or atmospheric pressure, though it notes these for the water HZ.
  • domain assumption Non-detection of artificial signals implies N < 3.09 at 95% CL via a Binomial zero-event interval, as if the entire Galaxy had been monitored.
    Sec. IV [49] applies Feldman-Cousins to N=0, but the paper does not account for incomplete sky/time coverage or signal strength; this sets the absolute bound.
  • domain assumption The Drake factorization N = <zeta_astro> <xi_biotec> L_tau with a single averaged <xi_biotec> is valid.
    Eq. (2) from [4]; the averaging over diverse origins is acknowledged as a crude approximation in Sec. I.
invented entities (1)
  • Silicon-based organisms with liquid nitrogen as solvent
    purpose: To extend the habitable zone and justify η~1, which lowers the upper limit on <xi_biotec> by a factor of 6.
    The paper cites theoretical feasibility (e.g., [18]) but provides no direct observational or experimental evidence; the claim rests on plausibility arguments.

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Pith. "Pith review of Upper limit on the fraction of alien civilizations that develop communication technology." pith.science (2026). https://pith.science/paper/WYXBAHOC

@misc{pith2026190801335,
  author       = {Pith},
  title        = {Pith review of: Upper limit on the fraction of alien civilizations that develop communication technology},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/WYXBAHOC}},
  note         = {Machine review of arXiv:1908.01335}
}
abstract

We re-examine the likelihood for alien civilizations to develop communication technology on the basis of the general assumption that life elsewhere could have a non-carbon chemical foundation. We particularized the discussion to a complex silicon-based biochemistry in a nitrogen solvent, and elaborate on the environment in which such a chemistry is feasible, and if so, on what scales. More concretely, we determine the region outside the habitable zone where such organisms can grow and flourish and after that we study how our findings impact the recently derived upper limit on the fraction of living intelligent species that develop communication technology $\langle \xi_{\rm biotec} \rangle$. We also compare this new restriction on $\langle \xi_{\rm biotec} \rangle$ with that resulting from the extension of the habitable zone to accommodate subsurface exolife, originating in planets with subsurface (water) oceans.

Discussion (0). Continue with ORCID to comment.

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