A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars

Abstract

We searched high resolution spectra of 5600 nearby stars for emission lines that are both inconsistent with a natural origin and unresolved spatially, as would be expected from extraterrestrial optical lasers.The spectra were obtained with the Keck 10-meter telescope, including light coming from within 0.5 arcsec of the star, corresponding typically to within a few to tens of au of the star, and covering nearly the entire visible wavelength range from 3640 to 7890 Å. We establish detection thresholds by injecting synthetic laser emission lines into our spectra and blindly analyzing them for detections. We compute flux density detection thresholds for all wavelengths and spectral types sampled. Our detection thresholds for the power of the lasers themselves range from 3 kW to 13 MW, independent of distance to the star but dependent on the competing "glare" of the spectral energy distribution of the star and on the wavelength of the laser light, launched from a benchmark, diffraction-limited 10-meter class telescope. We found no such laser emission coming from the planetary region around any of the 5600 stars. As they contain roughly 2000 lukewarm, Earth-size planets, we rule out models of the Milky Way in which over 0.1% of warm, Earth-size planets harbor technological civilizations that, intentionally or not, are beaming optical lasers toward us. A next generation spectroscopic laser search will be done by the Breakthrough Listen initiative, targeting more stars, especially stellar types overlooked here including spectral types O, B, A, early F, late M, and brown dwarfs, and astrophysical exotica.

Model of the Search for Extraterrestrial Intelligence with Coronagraphic Imaging

“…This was roughly independent of dis-tance, since they varied the exposure times for stellar spectra in order to yield a nearly consistent number of photons per pixel, and therefore they achieved similar detection limits relative to the stellar continuum for any laser emission line. Our detection limit results for GPI and WFIRST are below the Tellis & Marcy (2015 limits, and comparable to some commercially available and higher powered lasers. The geometry in Figure 4 can also be reversed to establish the minimum power required for an Earth-based laser to outshine the Sun.…”

“…However, their inner working angle (2000 AU) is far from the habitable zone of a Sun-like star. Tellis & Marcy (2017) additionally established detection limits using unresolved Keck HIRES spectroscopy within the same wavelength channel (i.e. mixed with the stellar spectrum) ranging from 3kW to 13 MW.…”

“…Our results assume a case where ETI would transmit a laser of wavelength λ centered in GPI's H-band (1.55 µm) or in WFIRST's V-band (575 nm). These lasers would be transmitted from a diffraction limited telescope with a diameter D (assumed to be 10 m, as is standard in the OIRSETI calculations; e.g., Tellis & Marcy 2017;Wright et al 2014b).…”

“…Any hypothetical laser will have much lower total flux than a planet-hosting star, so some method is needed to distinguish the laser signal from the star. This can be done temporally, searching for photon arrival time coincidences in a pulsed laser (Wright et al 2014a(Wright et al , 2004Howard et al 2004), or spectroscopically, using high spectral resolution to distinguish a narrow-band signal of a pulsed or continuous wave (CV) laser from the stellar continuum (Tellis & Marcy 2015Reines & Marcy 2002).…”

The Breakthrough Listen Search for Intelligent Life: Searching Boyajian's Star for Laser Line Emission

“…These spectra will be valuable in the search, still in progress, for periodic variations in spectral features. Spectroscopic optical SETI programs take advantage of the prospective concentration of laser energy flux into a narrow beam and the tiny range of wavelengths to increase the ratio of laser flux to stellar flux Tellis & Marcy (2015, making them a potentially effective way to communicate across interplanetary and interstellar distances.…”