Possible Photometric Signatures of Moderately Advanced Civilizations: The Clarke Exobelt

Brief

Socas-Navarro (2018) models the transit light-curve signature of artificial geostationary/geosynchronous satellite belts around exoplanets, parameterized by face-on opacity χ_o, belt width, and inclination. Earth's current Clarke belt yields χ_o ≈ 3×10^-13 using 1,738 publicly tracked satellites (UCS database, release 9-1-17), far below the ~10^-4 detection floor, but exponential growth in satellite density extrapolates to detectability around year 2200. Numerical simulations show a 10 m telescope could resolve a CEB at χ_o = 5×10^-5 around Proxima b in J-band, and χ_o = 5×10^-4 produces observable signatures for TRAPPIST-1 planets d–g. CEBs are distinguishable from natural ring systems by a characteristic light-curve morphology: the first pre-ingress dip is always concave and the second always convex, the inverse of the ring pattern, with a sharp derivative discontinuity at dip endpoints that rings lack.

Metadata

Category

Search

Venue

The Astrophysical Journal

Type

Peer-reviewed

Year

2018

Authors

Hector Socas-Navarro

DOI

10.3847/1538-4357/aaae66

arXiv

1802.07723

Access

Open access

Length

2.5 M

Programs

Breakthrough Listen, Allen Telescope Array, SERENDIP, Kepler, TESS, Ĝ survey

Instruments

Kepler photometer, 10 m aperture telescope (modeled), JWST, GMT, E-ELT, TMT

Data sources

Union of Concerned Scientists satellite database (release 9-1-17), Kepler spectral response filter, Mauna Kea J-band filter definition

Tags

SETI, technosignature, transit photometry, exoplanet, Clarke exobelt, megastructure-search

Key points

• Earth's Clarke belt face-on opacity is χ_o ≈ 3×10^-13, computed from ~580 geostationary/geosynchronous satellites (one-third of 1,738 in the UCS database) assuming 1 m typical object radius, roughly nine orders of magnitude below the ~10^-4 detection threshold.p.4
• Satellite belt opacity χ_o has grown exponentially over the 15 years prior to publication; extrapolating that trend places Earth at the observable threshold (~10^-4) around year 2200, though the paper explicitly cautions this is not a prediction.p.4
• A synthetic Earth-Sun light curve at 10 light-years with χ_o = 10^-3 produces CEB transit signatures clearly above the noise at the ppm level, comparable to Kepler's stated ~10 ppm photometric precision.p.5
• Proxima b (4.22 ly) simulated at χ_o = 5×10^-5, γ = 20°, i = 80° with a 10 m telescope in near-infrared J-band yields a detectable CEB signature, below the 10^-4 threshold.p.6
• TRAPPIST-1 planets d–g each show observable CEB signatures at χ_o = 5×10^-4 (γ = 15°, i = 80°); reducing to χ_o = 10^-4 leaves all detectable except the innermost, TRAPPIST-1 d.p.6
• Geostationary orbit radius r_C scales as the cube root of planet mass, so a 37% spread in Earth-like planet densities translates to only ~11% uncertainty in r_C, making CEB altitude a robust, geometry-independent prediction even without radial velocity mass measurements.p.5
• CEBs produce a first transit dip that is always concave and a second that is always convex; ring systems switch from convex to concave in both dips, and lack the sharp derivative discontinuity at dip endpoints that CEBs exhibit.p.8
• As of 2018, no confirmed stable ring system had been identified among 3,700+ known exoplanets; the sole candidate (1SWASP J140747.93-394542.6) is interpreted as a transient protosatellite disk rather than a stable ring.p.7

Verbatim

• “In some cases, a Clarke exobelt with a fractional face-on opacity of ∼ 10 − 4 would be easily observable with existing instrumentation.”

  p.1

• “If this trend is extrapolated into the future, we would reach the "observable" threshold ( ∼ 10 − 4 ) around the year 2200.”

  p.4

Most interesting

• Earth's satellite belt is currently ~9 orders of magnitude below the opacity needed for detection from a neighboring star, yet the paper demonstrates the concept is observationally tractable for a more densely populated exoplanet belt using hardware that already exists.
• 97% of active geosynchronous satellites fall within the ±15° inclination protected zone, and Earth's belt has an altitude spread of only ~150 m, a few parts per million of the belt radius, closely matching the paper's idealized thin-belt geometry.
• Because r_C scales as (M·T²)^(1/3), tidally locked planets, where rotation period equals orbital period, allow trivial CEB radius predictions; the paper notes that half of Kepler planet candidates and the vast majority of TESS targets around M- and K-type stars are expected to be tidally locked within 1 Gyr.
• A space elevator, also conceived by Arthur C. Clarke, would 'tremendously facilitate access to geostationary orbit' and would likely accelerate the rate of χ_o growth, potentially compressing the timeline to detectability for a technological civilization.
• All light-curve results in the paper scale linearly with χ_o in the χ_o << 1 regime, meaning every simulated curve can be rescaled to any arbitrary belt opacity by simple multiplication, the formalism is analytically transparent.
• The Ĝ survey, the most extensive systematic search for Dyson sphere signatures, searched for megastructures requiring engineering capabilities orders of magnitude beyond the Clarke exobelt concept; the CEB explicitly targets civilizations at Earth's current level of development, not exotic post-Kardashev Type II societies.