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A Search for Technosignatures from 14 Planetary Systems in the Kepler Field with the Green Bank Telescope at 1.15–1.73 Ghz

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A Search for Technosignatures from 14 Planetary Systems in the Kepler Field with the Green Bank Telescope at 1.15–1.73 Ghz The Astronomical Journal, 155:209 (9pp), 2018 May https://doi.org/10.3847/1538-3881/aabb03 © 2018. The American Astronomical Society. All rights reserved. A Search for Technosignatures from 14 Planetary Systems in the Kepler Field with the Green Bank Telescope at 1.15–1.73 GHz Jean-Luc Margot1,2 , Adam H. Greenberg2 , Pavlo Pinchuk2 , Akshay Shinde3, Yashaswi Alladi3, Srinivas Prasad MN4, M. Oliver Bowman1, Callum Fisher2, Szilard Gyalay2, Willow McKibbin2, Brittany Miles2, Donald Nguyen2, Conor Power4, Namrata Ramani5, Rashmi Raviprasad2, Jesse Santana2, and Ryan S. Lynch6,7 1 Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA; [email protected] 2 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA 3 Department of Computer Science, University of California, Los Angeles, CA 90095, USA 4 Department of Electrical Engineering, University of California, Los Angeles, CA 90095, USA 5 Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, USA 6 Green Bank Observatory, P.O. Box 2, Green Bank, WV 24494, USA 7 Center for Gravitational Waves and Cosmology, Department of Physics and Astronomy, West Virginia University, White Hall, Box 6315, Morgantown, WV 26506, USA Received 2018 January 29; revised 2018 March 29; accepted 2018 March 30; published 2018 April 25 Abstract Analysis of Kepler mission data suggests that the Milky Way includes billions of Earth-sized planets in the habitable zone of their host stars. Current technology enables the detection of technosignatures emitted from a large fraction of the Galaxy. We describe a search for technosignatures that is sensitive to Arecibo-class transmitters located within ∼420 ly of Earth and transmitters that are 1000 times more effective than Arecibo within ∼13000 ly of Earth. Our observations focused on 14 planetary systems in the Kepler field and used the L-band receiver (1.15–1.73 GHz) of the 100m diameter Green Bank Telescope. Each source was observed for a total integration time of 5 minutes. We obtained power spectra at a frequency resolution of 3Hz and examined narrowband signals − with Doppler drift rates between±9Hzs 1.Weflagged any detection with a signal-to-noise ratio in excess of 10 as a candidate signal and identified approximately 850,000 candidates. Most (99%) of these candidate signals were automatically classified as human-generated radio-frequency interference (RFI). A large fraction (>99%) of the remaining candidate signals were also flagged as anthropogenic RFI because they have frequencies that overlap those used by global navigation satellite systems, satellite downlinks, or other interferers detected in heavily polluted regions of the spectrum. All 19 remaining candidate signals were scrutinized and none were attributable to an extraterrestrial source. Key words: astrobiology – extraterrestrial intelligence – planetary systems – planets and satellites: general – techniques: spectroscopic 1. Introduction emission spans about 550 Hz and corresponds to OH (1612 MHz) maser emission(Cohen et al. 1987). A mono- Analysis of Kepler mission data suggests that the Milky Way ± includes billions of Earth-sized planets in the habitable zone chromatic signal that shifts by 10 Hz as a function of time (HZ) of their host stars (e.g., Borucki 2016). The possibility according to a complex sequence in a manner similar to that ( that intelligent and communicative life forms developed on one used in transmitting the 1974 Arecibo message The Staff at the ) or more of these worlds behooves us to conduct a search for National Astronomy & Ionosphere Center 1975 is another extraterrestrial intelligence. Here, we describe an L-band radio technosignature. This work focuses on detecting signals that survey of 14 planetary systems selected from the Kepler are narrow in the frequency domain, and is sensitive to both of mission field. Our analysis methods are generally similar to these examples. Our data are also amenable to searching for those used by Siemion et al. (2013), but our observations signals that are narrow in the time domain (e.g., pulses). sample a different slice of the search volume. In addition, our Our search is not predicated on the assumption of deliberate analysis examines signals of lower signal-to-noise ratio (S/N; transmissions aimed at Earth. Earthlings, for instance, use high- 6 10 versus 25) and larger range of Doppler drift rates power (∼10 W) transmissions to study asteroids that may pose (±9Hzs−1 versus ±2Hzs−1) than recent Breakthrough Listen an impact hazard(e.g., Naidu et al. 2016). These transmissions results (Enriquez et al. 2017). use monochromatic, binary phase-coded, or chirp signals, all of We define a “technosignature” as any measurable property or which would be recognized as technosignatures by alien effect that provides scientific evidence of past or present civilizations. In most such observations, less than a millionth of technology, by analogy with “biosignatures”, which provide the energy is absorbed and scattered by the asteroid, and the evidence of past or present life. The detection of a remainder propagates beyond the asteroid at the speed of light. technosignature such as an extraterrestrial signal with a time– Our search is agnostic about whether radio transmissions were frequency structure that cannot be produced by natural sources intended for detection by a distant civilization (e.g., a beacon) would provide compelling evidence of the existence of another or not (e.g., a radar or inter-planet telecommunication system). civilization. A signal that is narrow (<10 Hz) in the frequency Sections 2–5 describe the observations, analysis, discussion, domain is a technosignature because natural sources do not and conclusions, respectively. emit such narrowband signals. The narrowest reported natural 1 The Astronomical Journal, 155:209 (9pp), 2018 May Margot et al. Table 1 from 1.1 to 1.9 GHz. The signal was channelized into 256 Target Host Stars Listed in Order of Observations channels of 3.125MHz bandwidth each. The raw voltages of Host Star Distance (ly) HZ Category the in-phase and quadrature channels were digitized with 8-bit quantization. GUPPI’s baseband recording mode enables Kepler-399 NA L +42 reduction of the data storage requirements by a factor of four Kepler-186 561-33 Cat. 1 with minimal signal degradation with an optimal four-level Kepler-452 NA Cat. 2 (two-bit) sampler(Kogan 1998). In this mode, the quantization Kepler-141 NA L thresholds are set to −0.981599σ,0,+0.981599σ, where σ is Kepler-283 NA Cat. 1 L the root-mean-square (rms) of the voltage and the quantized Kepler-22 620 ± ± Kepler-296 737+91 Cat. 1 levels are set to 1 and 3.335875. The quantization -59 fi Kepler-407 NA L ef ciency, which is the ratio of signal power that is observed Kepler-174 NA Cat. 2 with the four-level sampler to the power that would be obtained Kepler-62 1200 Cat. 1 with no quantization loss, is ηQ=0.8825 (Kogan 1998). Eight Kepler-439 2260+215 L computers handled the transfer of our data to eight disk arrays -124 − − +65 L 1 1 Kepler-438 473-75 at an aggregate rate of 800 MB s or 2.88 TB hr . +52 Kepler-440 851-150 Cat. 2 Calibration procedures at the beginning of our observing +62 Kepler-442 1115-72 Cat. 1 window consisted of recording a monochromatic tone at 1501MHz (center frequency +1 MHz), performing a peak and Note.Distances in Parsecs are from the NASA exoplanet archive. Habitable focus procedure on a bright radio source near the Kepler field, fi ( Zone categories 1 and 2 refer to small (Rp<2RE) planets in the conservative and observing a bright pulsar near the Kepler eld PSR B2021 and optimistic habitable zones described by Kane et al. (2016), respectively. In +51; Manchester et al. 2005). multi-planet systems, only the lowest category is listed. 3. Analysis 2. Observations 3.1. Validations We selected 14 exoplanet host stars (Table 1) from the We verified the validity of our data-processing pipeline by Kepler catalog. A majority of these stars host small HZ planets analyzing the monochromatic tone data and recovering the < ’ ( with radii Rp 2RE, where RE is Earth s radius Kane signal at the expected frequency. We also folded the pulsar data et al. 2016). Although such planets may be advantageous for at the known pulsar period and recovered the characteristic the development of extraterrestrial life forms, advanced pulse profile. civilizations may be capable of thriving in a variety of environments, and we do not restrict our search to small HZ 3.2. Data Selection planets. Indeed, because planets and HZ planets are common among most stars in the Galaxy(e.g., Petigura et al. 2013; We unpacked the data to 4-byte floating point values, Batalha 2014), there is no compelling reason to search the computed Fourier transforms of the complex samples with the Kepler field(e.g., Siemion et al. 2013) as opposed to other FFTW routine (Frigo & Johnson 2005), and calculated the fields. There is, however, a possible increased probability of signal power (Stokes I) at each frequency bin. Signals with detecting technosignatures when observing planetary systems frequencies outside the range of the L-band receiver edge-on. (1150–1730 MHz) were discarded. We used the GBT’s notch Our observing sequence was inspired by a solution to the filter to mitigate interference from a nearby aircraft surveillance traveling salesperson problem, which minimized the time spent radar system. Signals with frequencies within the 3 dB cutoff repositioning the telescope. Each target was observed twice in range of the notch filter (1200–1341.2 MHz) were also the following 4-scan sequence: target 1, target 2, target 1, target discarded. 2. The integration time for each scan was τ=150 s, yielding a total integration time of 5 minutes per target.
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