The search for extraterrestrial intelligence (SETI) will likely be sped up thanks to new results that narrow down how alien radio signals would drift in frequency as a result of the Doppler shift caused by their home planet's orbit around its star.
A Doppler shift is the lengthening or shortening of the frequency of a signal caused by the motion of the transmitter. If the transmitter is moving away from us, the wavelength becomes stretched and the frequency decreases; if it's moving towards us, the wavelength shortens and and the frequency increases. This results in the signal appearing to "drift" across a range of frequencies as the transmitter moves. (Think of how the sound of a police or ambulance siren changes as it approaches and then passes you.)
Both the orbital motion and the daily rotation of an exoplanet, plus Earth's own orbital motion and daily rotation, contribute to the frequency drift of any signal that may be transmitted from the exoplanet and received here on Earth. Radio astronomers know that Earth's orbital motion causes a drift rate of 0.019 nanoHertz (nHz) and that Earth's spinning on its axis creates an additional 0.1 nHz drift. These shifts can be accounted for when analyzing signals. However, while astronomers do not always know how fast exoplanets are spinning — the exception is tidally locked planets, which have a day that is the same length as their year — they can measure an exoplanet's orbital period and derive a maximum frequency drift from this figure.
The drift rate is dependent upon the orbital characteristics of an exoplanet — the inclination of its orbit with respect to us, how far from circular its orbit is and how much it precesses, or wobbles. Machine-learning algorithms that are able to sift through data, looking for signals that display a drift rate, require a maximum value for the drift rate so that they can limit their search. SETI searches usually assume a small value for the frequency drift, less than 10 nHz, but previous calculations based on actual measurements of the most extreme exoplanet orbits known placed an upper limit on the drift rate of plus or minus 200 nHz.
Using plus or minus 200 nHz as a maximum drift rate requires increased computational resources, slowing down the speed with which data from SETI searches is analyzed.
Now, by modeling about 5,300 real exoplanets, a team led by graduate student Megan Li of the University of California, Los Angeles was able to refine and reduce the maximum value for the drift rate caused by the orbital motion of exoplanets to plus or minus 53 nHz.
This means that, for 99% of planetary systems, the frequency of a signal detected from a distant exoplanet would be expected to drift in frequency at a maximum rate of plus or minus 53 nHz. This new result is more accurate because it measures the drift rate at all points in an exoplanet's orbit, not just at those points that maximize the drift rate. And being a lower value than plus or minus 200 nHz, it will reduce the amount of computational resources required and speed up the search. There's even scope to reduce it much further, study team members said.
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"The 53 nHz value is for all the planets that we currently know, but it contains some biases that are making the value higher, because transiting exoplanets have a higher drift rate than non-transiting exoplanets, and bigger exoplanets have bigger drift rates than smaller exoplanets," Li told Space.com in a telephone interview. (Transiting exoplanets cross their host stars' faces from our perspective on Earth.)
The current catalog of known exoplanets is not entirely representative of the wider population of exoplanets out there. Current detection methods still favor larger planets closer to their stars, because they are the easiest to find. So, to try and avoid any biases, Li's team also measured the maximum drift rate of over 5,000 simulated planets that we might expect to be more representative of the true population of exoplanets in terms of their orbital characteristics, with smaller planet sizes, longer orbital periods and a more uniform spread of orbital inclinations. The imagined planets were placed into 20 groups, each consisting of 5,286 worlds, split into 10 groups with nearly circular orbits and 10 groups with increasingly non-circular (known as eccentric) orbits. From these, Li's team was able to derive much lower drift rates — plus or minus 0.27 nHz for the low-eccentricity orbits and plus or minus 0.44 nHz for the high-eccentricity orbits.
These values are far lower than the calculated drift rate of plus or minus 53 nHz.
"The biases are what's making the 53 nHz value so large," said Li. "We think the real drift rates would in most cases be much closer to those lower values of 0.44 and 0.27 nHz."
As a wider range of alien worlds are discovered in the future by upcoming missions such as the European Space Agency's PLATO (Planetary Transits and Oscillations of stars), the maximum drift rate calculated from real exoplanetary data should begin to better reflect the values seen in the simulated results. This will have the effect of making the analysis of potential SETI signals even more efficient.
Drift rates are one way to tell whether a signal has come from deep space, although it is not a foolproof method. Radio frequency interference (RFI) from transmitters on Earth — cell phones, airport radars and so on — have a drift rate of zero because they are on Earth with our receivers. Satellites in low Earth orbit usually have negligible drift rates, but some, such as SpaceX's Starlink megaconstellation and the U.S. government's GPS network, do exhibit some frequency drift in their radio signals.
"We can try to figure out what the drift rate of those satellites would be so that if they do appear as RFI — which they often do — we can throw those out," said Li.
With SETI searches now targeting up to a million stars, being able to analyze data quickly is important to avoid a logjam and to spot any potential alien signals before they switch off. Calculation of the drift rate may seem a more technical challenge, but it's vital for speeding up that search, team members said. For example, the new findings will improve computing costs and search times on Breakthrough Listen's SETI project on the MeerKAT radio telescope array in South Africa by three orders of magnitude. In the end, if E.T. is really out there, we can now find them faster.
The findings are published in the November 2023 issue of The Astronomical Journal.