A stable-frequency transmitter with relative radial acceleration to a receiver will show a change in received frequency over time, known as a ‘drift rate.’ For a transmission from an exoplanet, astronomers must account for multiple components of drift rate: the exoplanet’s orbit and rotation, the Earth’s orbit and rotation, and other contributions. Understanding the drift rate distribution produced by exoplanets relative to Earth, can help scientists constrain the range of drift rates to check in a Search for Extraterrestrial Intelligence (SETI) project to detect radio technosignatures, and help them decide validity of signals-of-interest, as they can compare drifting signals with expected drift rates from the target star. In a new study, University of California, Los Angeles astronomer Megan Grace Li and colleagues modeled the drift rate distribution for over 5,300 confirmed exoplanets, using parameters from the NASA Exoplanet Archive.
Li et al. used the known population of exoplanets and extrapolated to the much larger, unknown population of exoplanets to set better thresholds for planetary effects on signals from extraterrestrial intelligences. Image credit: Breakthrough Listen Initiative.
“Our work gives deeper insight into what extraterrestrially transmitted signals may look like if they come from exoplanets, informing not only the parameter space of technosignature searches but also possible interpretations of detected signals,” Li said.
In the study, Li and co-authors focused on exoplanets from the NASA Exoplanet Archive (NEA).
They calculated the orbital drift rate distributions for over 5,300 known exoplanets, in the process, creating a tool with which researchers can quickly calculate expected drift rates from any exoplanetary system.
They found that 99% of the total drift rate distribution fell within 53 nHz.
In a previous paper, the researchers discovered that exoplanetary systems showed drift rates up to 200 nHz in the most extreme cases and recommended this as a threshold.
The new work builds upon this foundation by considering not only the maximum drift rates from extreme systems but also the average or most likely drift rates from all known systems.
“These results imply that, in many cases, the drift rate will be so low that we can prioritize other parameters (such as covering more frequencies or analyzing datasets faster) without worrying that we will miss true signals,” saod Dr. Sofia Sheikh, a researcher at the SETI Institute.
The authors then simulated ‘de-biased’ populations of exoplanets that might better represent exoplanetary characteristics in any random sample of the galaxy instead of just the exoplanets that are the most obvious.
For example, known planets tend to have ‘edge-on’ orbits because these systems are easiest to detect using the two most common planet-finding techniques, the transit method and the radial velocity method.
However, edge-on orbits also have much higher drift rates than planets that are ‘inclined,’ or angled, randomly compared to the observer’s line-of-sight.
The astronomers simulated a de-biased population of exoplanets, going beyond the common edge-on orbit case in the NEA and correcting for other observational biases (such as a bias in the NEA for exoplanets that are particularly close to their stars).
They found that a drift rate of merely 0.44 nHz for any random star would be sufficient to capture 99% of hypothetical signals from any orbiting exoplanets.
Searching twice as many drift rates — for example, up to 2 nHz instead of 1 nHz — takes twice as many computations for low drift rates.
This new research, which drops the recommended limits by a factor of 4 (for stars with known planets) or over 400 (for stars without known planets), will significantly reduce unnecessary calculations and allow future SETI scientists to fine-tune the drift rate parameters in their searches to better match the particular systems they’re observing.
These new, narrower ranges of maximum drift rates represent a significant efficiency gain in the quest to detect potential radio signals from technologically capable extraterrestrial life.
“Our new thresholds built to encompass most of the drift rates produced by stable-frequency transmitters on exoplanets can improve the computing costs and times of future searches, such as the one Breakthrough Listen intends to conduct on MeerKAT,” the researchers said.
Their paper was published in the Astronomical Journal.
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Megan G. Li et al. 2023. Developing a Drift Rate Distribution for Technosignature Searches of Exoplanets. AJ 166, 182; doi: 10.3847/1538-3881/acf83d
