Ideally, in the search for extraterrestrial intelligence (SETI), we would want to observe every object in the night sky at all times over all frequencies. Yet, as should be obvious, this multidimensional observing strategy is hard given the practical limitations of telescope technology. One strategy to resolve this is strategic target selection, which can narrow down the number of targets and time of observation based on SETI theory. The range of frequency of the observation is dependent on the receivers available on each telescope. Targeted searches can increase the chance of detecting extraterrestrial intelligence (ETI) by focusing on a narrow list of plausible sources to search for signals from technologically advanced extraterrestrial life, or technosignatures. However, since targeted searches rely on the assumption that SETI theory is correct. In case this is incorrect, it is also important to perform broader searches.
One of the most common techniques used to detect exoplanets, as used by TESS, is the transit method. When an exoplanet transits, it crosses between the Earth and the exoplanet's host star. The transit method looks for small dips in the brightness of the host star as the exoplanet blocks some of the star's light. This transit usually only lasts for a small fraction of the exoplanet’s orbit and works best for large exoplanets (larger exoplanets block more light resulting in a larger dip in brightness). Also, for SETI searches, during a transit any ETI signal directed away from the host star will appear brightest to an observer on Earth!
Strategically targeting TESS TOIs is important for SETI searches because of the geometrical alignment of Earth with the extraterrestrial system. Since TESS TOIs are transiting, as observed from Earth, ETI may be more likely to transmit a signal to those that can observe it transiting. This is because ETI may understand that transiting exoplanet systems are some of the most detectable and assume that those that can see their system transit would observe them for technosignatures.
Recently, as part of Breakthrough Listen (BL), Traas et al. (2021) (see the paper here and blog post here) performed a search of 28 Transiting Exoplanet Survey Satellite (TESS) Targets of Interest (TOIs), using four of the receivers (L, S, C, and X, spanning radio frequencies from 1 - 11 GHz) available at the Green Bank Telescope (GBT). Building off of Traas et al. (2021), this past summer I performed a technosignature search of targets that transit during their GBT observations. Traas et al. selected any TESS TOIs observed with all four receivers by BL. Instead, during my project we used data from TESS TOIs that were in transit during the BL GBT observation, further narrowing the targeting strategy of Traas et al. As mentioned above, by doing so, we are increasing the likelihood of detecting signals directed in the opposite direction to the planet’s host star.
In the end, we did not detect any evidence of extraterrestrial life in the systems we observed. However, we were the first to place constraints on technosignatures from TESS TOIs observed during transit. More information on our target selection, analysis, and results can be found in Franz et al. (2022), which was recently accepted for publication in The Astronomical Journal.
Main Image Credit: NASA