Andrew P. V. Siemion

In the search for technosignatures, it can be useful to narrow down the cosmic haystack by prioritizing time-coordinate “Schelling points” at which a beacon may be more likely to be transmitted. One such Schelling point is the antisolar point (ASP), the place and moment at which Earth would lie directly between a hypothetical transmitter in the Earth's transit zone (i.e., ecliptic) and the Sun. In this work, we describe a campaign to survey the ASP for radio technosignatures using the Allen Telescope Array, report the results of 3 initial nights of data from a pilot study in summer 2023, and discuss the future of the campaign. In the pilot study, we find no sky-localized narrowband 1–2.4 GHz technosignatures within an R.A. range of approximately 20.266–21.312 hr during the ASP, setting an upper limit flux density threshold of 115–936 Jy.

LHS 1140b, a rocky exoplanet in its host star’s habitable zone, was recently shown to have an escaping helium atmosphere. We present a search for narrowband radio technosignatures in archival Breakthrough Listen observations toward the LHS 1140 system using the 100 m Robert C. Byrd Green Bank Telescope at 1–12 GHz. We report a nondetection of candidate signals above 300 GW.

The LTT 3780 system hosts two known exoplanets—LTT 3780 b, a rocky super-Earth, and LTT 3780 c, a temperate sub-Neptune—orbiting a nearby M dwarf on opposite sides of the radius valley. LTT 3780 c has been proposed as a candidate Hycean world, making the system an important target for astrobiological investigation, particularly in light of recent JWST atmospheric observations. Although biosignature and technosignature searches both seek evidence of life beyond Earth, these approaches have historically been pursued independently. Well-characterized exoplanet systems provide an opportunity to combine these complementary search strategies. In this work, we conducted radio technosignature observations of the LTT 3780 system using both the Allen Telescope Array and the Karl G. Jansky Very Large Array (VLA). The two facilities provide complementary observational capabilities, with the ATA optimized for wide-band multibeam postprocessing analyses and the VLA enabling high-sensitivity real-time interferometric searches. Across ∼30 hr of total observing time, we searched for narrowband Doppler-drifting signals in the frequency range ∼1–10 GHz. After applying comprehensive radio-frequency interference mitigation and multibeam consistency tests, no candidate signals consistent with astrophysical or technosignature origins were identified. We place minimum detectable effective isotropic radiated power limits of 4.7 × 10 ¹² –3.6 × 10 ¹³ W across the observed bands and facilities. Although no technosignatures were detected, this work demonstrates how complementary observation and analysis strategies can be applied to exoplanets of astrobiological interest and serves as a pathfinder for future combined biosignature and technosignature investigations.

In 2025 July, the third-ever interstellar object, 3I/ATLAS, was discovered on its ingress into the solar system. Similar to the NASA Voyager missions sent in 1977, science probes by extraterrestrial life (“artifact technosignatures”) could be sent to explore other stellar systems like our own. In this campaign, we used the SETI Institute’s Allen Telescope Array to observe 3I/ATLAS from 1–9 GHz. We detected nearly 74 million narrowband hits in 7.25 hr of data using the newly developed search pipeline bliss . We then blanked hits by frequency and drift rate to mitigate radio frequency interference in our dataset, narrowing the dataset down to ∼2 million hits. These hits were further filtered by the localization code NBeamAnalysis , and the remaining 211 hits were visually inspected in the time-frequency domain. We did not find any signals worthy of additional follow-up. Accounting for the Doppler drift correction and given the nondetection, we are able to set an effective isotropic radiated power upper limit of 10–110 W on radio technosignatures from 3I/ATLAS across the frequency and drift rate ranges covered by our survey.

K2-18 b, a sub-Neptune exoplanet located in the habitable zone of its host star, has emerged as an important target for atmospheric characterization, and assessments of potential habitability. Motivated by recent interpretations of JWST observations suggesting a hydrogen-rich atmosphere consistent with Hycean-world scenarios, we conducted a coordinated, multiepoch search for narrowband radio technosignatures using the Karl G. Jansky Very Large Array equipped with the COSMIC backend and the MeerKAT telescope with the BLUSE backend. Our observations span frequencies from 544 MHz to 9.8 GHz and include multiple epochs that cover at least one full orbital period of the planet. In this work we outline, create, and apply a comprehensive postprocessing framework that incorporates observatory-informed radio frequency interference masking, drift-rate filtering based on the expected dynamics of the K2-18 system, multibeam spatial discrimination, primary and secondary transit filtering (when applicable), and signal-to-noise-ratio-based excision of weak and strong spurious signals. Across all bands and epochs, no signals consistent with an astrophysical or artificial origin were identified at a limit of 10 ¹² –10 ¹³ W. These nondetections allow us to place upper limits on the presence of persistent, isotropic narrowband transmitters within the K2-18 system, providing the first interferometric technosignature constraints for a Hycean-planet candidate. Our results demonstrate the efficacy of coordinated multiepoch interferometric searches and establish a methodological framework for future technosignature studies of nearby potentially habitable exoplanets.

K2-18b, a sub-Neptune exoplanet located in the habitable zone of its host star, has emerged as an important target for atmospheric characterization and assessments of potential habitability. Motivated by recent interpretations of JWST observations suggesting a hydrogen-rich atmosphere consistent with Hycean-world scenarios, we conducted a coordinated, multi-epoch search for narrowband radio technosignatures using the Karl G. Jansky Very Large Array equipped with the COSMIC backend and the MeerKAT telescope with the BLUSE backend. Our observations span frequencies from 544MHz to 9.8GHz and include multiple epochs that cover at least one full orbital period of the planet. In this work, we outline, create, and apply a comprehensive post-processing framework that incorporates observatory-informed RFI masking, drift-rate filtering based on the expected dynamics of the K2-18 system, multibeam spatial discrimination, primary and secondary transit filtering (when applicable), and SNR-based excision of weak and strong spurious signals. Across all bands and epochs, no signals consistent with an astrophysical or artificial origin were identified at a limit of 10^12 to 10^13W. These non-detections allow us to place upper limits on the presence of persistent, isotropic narrowband transmitters within the K2-18 system, providing the first interferometric technosignature constraints for a Hycean-planet candidate. Our results demonstrate the efficacy of coordinated multiepoch interferometric searches and establish a methodological framework for future technosignature studies of nearby potentially habitable exoplanets.

We report the results of one of the most sensitive pulsar surveys to date targeting the innermost region of the Galactic center (GC) using the Robert C. Byrd Green Bank Telescope at X band (8–12 GHz) using data from the Breakthrough Listen initiative. In total, we collected 9.5 hr of data covering the wider ∼ 8 ′ diameter of the GC bulge, and 11 hr on the inner 1 . ′ 4 region between 2021 May and 2023 December. We conducted a comprehensive Fourier-domain periodicity search targeting both canonical pulsars (CPs) and millisecond pulsars (MSPs), using constant and linearly changing acceleration searches to improve sensitivity to compact binaries. Assuming weak scattering, our searches reached luminosity limits of L min ≈ 0.14 mJy kpc 2 for CPs and L min ≈ 0.26 mJy kpc 2 for MSPs—sensitive enough to detect the most luminous pulsars expected in the GC. Among 5282 signal candidates, we identify an interesting 8.19 ms MSP candidate (dispersion measure (DM) of 2775 pc cm ⁻³ ), persistent in time and frequency across a 1 hr scan at a flux density of S min ≈ 0.007 mJy . We introduce a novel randomization test for evaluating candidate significance against noise fluctuations, including signal persistence via Kolmogorov–Smirnov tests and flux-versus-DM behavior. We are unable to make a definitive claim about the candidate due to a mixed degree of confidence from these tests and, more broadly, its nondetection in subsequent observations. This deepens the ongoing missing pulsar problem in the GC, reinforcing the idea that strong scattering and/or extreme orbital dynamics may obscure pulsar signals in this region.

3I/ATLAS, an interstellar object, made its closest approach to Earth, at ∼1.8 au, on 2025 December 19. On 2025 December 18, the Breakthrough Listen program conducted a technosignature search toward 3I/ATLAS using the 100 m Robert C. Byrd Green Bank Telescope at 1–12 GHz. We report a nondetection of candidate signals down to the 100 mW level.