[Paper Review] A Search for Optical Laser Emission Using Keck HIRES

Nathaniel K. Tellis, Geoffrey W. Marcy|arXiv (Cornell University)|Apr 13, 2015

Space Science and Extraterrestrial Life2 references3 citations

TL;DR

This study searches for narrow-band optical laser emissions from 2,796 stars, including 1,368 Kepler exoplanet hosts, using high-resolution spectroscopy (λ/Δλ = 60,000) at the Keck telescope. No laser signals were detected, setting a detection threshold of ~10⁻² photons m⁻² s⁻¹ for typical Kepler stars and 1 photon m⁻² s⁻¹ for solar-type stars within 100 light-years, implying that lasers stronger than 90 W at 100 ly or 1 kW at 1,000 ly would be detectable with current methods.

ABSTRACT

We present a search for laser emission coming from point sources in the vicinity of 2796 stars, including 1368 Kepler Objects of Interest (KOIs) that host one or more exoplanets. We search for extremely narrow emission lines in the wavelength region between 3640 and 7890 Angstroms using the Keck 10-meter telescope and spectroscopy with high resolution ($λ/Δλ$ = 60,000). Laser emission lines coming from non-natural sources are distinguished from natural astrophysical sources by being monochromatic and coming from an unresolved point in space. We search for laser emission located 2-7 arcsec from the 2796 target stars. The detectability of laser emission is limited by Poisson statistics of the photons and scattered light, yielding a detection threshold flux of approximately $10^{-2}$ photons $m^{-2} s^{-1}$ for typical Kepler stars and 1 photon $m^{-2} s^{-1}$ for solar-type stars within 100 light-years. Diffraction-limited lasers having a 10-meter aperture can be detected from 100 light-years away if their power exceeds 90 W, and from 1000 light-years away (Kepler planets), if their power exceeds 1 kW (from lasers located 60-200 AU, and 2000-7000 AU from the nearby and Kepler stars, respectively). We did not find any such laser emission coming from any of the 2796 target stars. We discuss the implications for the search for extraterrestrial intelligence (SETI).

Motivation & Objective

• To search for narrow, monochromatic optical emission lines indicative of artificial laser transmissions from stars hosting exoplanets.
• To assess the detectability of extraterrestrial laser signals based on photon statistics and scattered light limitations.
• To establish detection thresholds for laser power at various distances from Earth, considering diffraction-limited beam propagation.
• To explore the feasibility of detecting laser emissions from technological constructs located tens to hundreds of AU from target stars.
• To contribute to the broader SETI effort by testing the hypothesis that advanced civilizations might use optical lasers for interstellar communication.

Proposed method

• Utilized high-resolution spectroscopy (λ/Δλ = 60,000) with the Keck HIRES instrument to analyze stellar light across 3,640–7,890 Å.
• Searched for extremely narrow emission lines (monochromatic) offset by 2–7 arcsec from target stars, where stellar light would not dominate.
• Calculated detection thresholds based on Poisson photon statistics and scattered light, yielding flux limits of ~10⁻² photons m⁻² s⁻¹ for typical Kepler stars.
• Modeled detectable laser power as a function of distance, showing 90 W lasers detectable at 100 ly and 1 kW lasers at 1,000 ly with 10-meter aperture.
• Used spatial separation (2–7 arcsec) to avoid stellar contamination, effectively probing lasers located 10–100 AU from nearby stars and 2,000–7,000 AU from Kepler stars.
• Planned follow-up searches for lasers within the 2 arcsec inner working angle in a subsequent study, where detection sensitivity would be reduced by ~100×.

Experimental results

Research questions

• RQ1 Can narrow, monochromatic optical emission lines be detected from stars hosting exoplanets, indicating artificial laser transmissions?
• RQ2 What is the minimum detectable laser power at various distances from Earth using current high-resolution spectroscopy?
• RQ3 How does the angular separation from the star (2–7 arcsec) affect the detectability of laser signals by avoiding stellar contamination?
• RQ4 What are the flux and power thresholds for detecting diffraction-limited lasers from 100 ly and 1,000 ly distances?
• RQ5 Can existing spectroscopic surveys like Sloan Digital Sky Survey be repurposed to search for laser signals, and what limitations exist?

Key findings

• No laser emission signals were detected from any of the 2,796 target stars, including 1,368 Kepler Objects of Interest.
• The detection threshold for laser flux is ~10⁻² photons m⁻² s⁻¹ for typical Kepler stars and 1 photon m⁻² s⁻¹ for solar-type stars within 100 light-years.
• Diffraction-limited lasers with power exceeding 90 W can be detected from 100 light-years away, and those exceeding 1 kW from 1,000 light-years away.
• Lasers located at 60–200 AU from nearby stars and 2,000–7,000 AU from Kepler stars are detectable with current instrumentation.
• The inner working angle of 2–3 arcsec limits detection to lasers located at least 10–100 AU from nearby stars, excluding those at 1 AU (e.g., Earth-analog planets).
• Future searches within the 2 arcsec inner working angle will require ~100× higher flux thresholds due to increased photon noise from stellar light.