Earth Detecting Earth: At what distance could Earth's constellation of technosignatures be detected with present-day technology? (2502.02614v1)

Abstract: The field of the Search for Extraterrestrial Intelligence (SETI) searches for ``technosignatures'' that could provide the first detection of life beyond Earth through the technology that an extraterrestrial intelligence (ETI) may have created. Any given SETI survey, if no technosignatures are detected, should set upper limits based on the kinds of technosignatures it should have been able to detect; the sensitivity of many SETI searches requires that their target sources (e.g., Dyson spheres or Kardashev II/III level radio transmitters) emit with power far exceeding the kinds of technology humans have developed. In this paper, we instead turn our gaze Earthward, minimizing the axis of extrapolation by only considering transmission and detection methods commensurate with an Earth-2024 level. We evaluate the maximum distance of detectability for various present-day Earth technosignatures -- radio transmissions, atmospheric technosignatures, optical and infrared signatures, and objects in space or on planetary surfaces -- using only present-day Earth instruments, providing one of the first fully cross-wavelength comparisons of the growing toolbox of SETI techniques. In this framework, we find that Earth's space-detectable signatures span 13 orders of magnitude in detectability, with intermittent, celestially-targeted radio transmission (i.e., planetary radar) beating out its nearest non-radio competitor by a factor of $10^3$ in detection distance. This work highlights the growing range of ways that exoplanet technosignatures may be expressed, the growing complexity and visibility of the human impact upon our planet, and the continued importance of the radio frequencies in SETI.

• The paper demonstrates that Earth's radio transmissions, especially targeted radar signals, can be detected up to 12,000 light-years, making radio SETI the most robust method.
• The paper quantifies atmospheric technosignatures by showing that nitrogen dioxide signals are detectable up to 5.71 light-years, aligning with biosignature search strategies.
• The paper evaluates optical and radar technosignatures, revealing that city lights and reflective objects like Echo satellites have extremely limited detectability ranges.

Earth Detecting Earth: Analyzing Technosignature Detectability with Earth-level Technology

The investigation presented in the paper "Earth Detecting Earth: At what distance could Earth's constellation of technosignatures be detected with present-day technology?" by Sheikh et al. provides a comprehensive evaluation of the detectability of Earth's technosignatures at various distances, using current Earth-based instrumentation. This analysis serves as a valuable cross-disciplinary endeavor in the field of Search for Extraterrestrial Intelligence (SETI), emphasizing a minimal-extrapolation approach by focusing on what could be termed "Earth-level" technosignatures ($\iota = 1$).

Study Overview

The authors detail the detectability of different classes of technosignatures, categorized broadly under radio transmissions, atmospheric technosignatures, optical and infrared emissions, and objects in space or on planetary surfaces. The fundamental goal of the paper is to leverage existing Earth-based technologies to understand the detectability distance for each technosignature type.

Key Findings and Numerical Results

1. Radio Transmissions: Radio transmissions remain the most detectable technosignature of Earth by a large margin. The paper finds that intermittent, celestially-targeted radio transmission, such as planetary radar signals, can be detected at distances up to 12,000 light-years. This constitutes the most favorable scenario for detecting Earth-level technosignatures with present-day radio technology. By comparison, more persistent transmissions like those from NASA's Deep Space Network are detectable up to 65 light-years.
2. Atmospheric Technosignatures: Technosignatures from atmospheric constituents, particularly nitrogen dioxide (NO$_2$), can be detected up to 5.71 light-years. This finding is notable, given the overlap with ongoing biosignature search strategies, affirming the relevance of atmospheric studies for detecting extraterrestrial technosignatures.
3. Optical and Infrared Emissions:
   • City Lights: Detectability of city lights is extremely limited, with an estimated range of merely 0.036 light-years (36,000 AU), significantly constrained by Earth's low urbanization fraction.
   • Targeted Lasers: Lasers, particularly those used in deep space optical communication, provide a detectability up to 1.8 parsecs given present-day technology.
4. Objects in Space or on Planetary Surfaces: The detectability range is largely limited by the effective radar cross-section and system capabilities. For example, using radar, reflective objects like historical Echo satellites can be detected up to 0.145 AU.

Implications for SETI

This analysis underscores the propensity of radio wavelengths to offer the greatest distances for the detection of technosignatures, which continues to validate radio SETI as a primary search strategy. Moreover, the paper is a reminder of the potential contribution atmospheric observations may play, not only in the field of biosignatures but also technosignatures.

The paper's exploration of the variability and time derivatives of atmospheric technosignatures could guide future directions in detecting rapid atmospheric changes signaling advanced technological activities, including those resulting from anthropogenic climate impacts.

Future Developments

The proposed framework encourages a holistic approach to technosignature detection, integrating across various types and scales. Future developments in observational technologies, particularly those enhancing sensitivity and resolution, stand to improve the detectability limits. Additionally, expanding the ichnoscale concept beyond current Earth technology, to speculate on higher levels of technological capacity, can further enrich strategic SETI endeavors.

In conclusion, "Earth Detecting Earth" provides a structured and insightful framework reaffirming the complexity and multi-wavelength nature of detecting technosignatures, underscoring the pivotal role of both current and future advancements in astrophysical instrumentation in the search for extraterrestrial intelligence.