The search for extraterrestrial intelligence (SETI) is likely to be accelerated thanks to new results mapping how alien radio signals would deviate in frequency due to 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. As the transmitter moves away from us, the wavelength is stretched and the frequency decreases; as it moves toward us, its wavelength shortens and its frequency increases. This results in the signal appearing to “drift” over 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 diurnal rotation of a exoplanetplus Earth’s own orbital motion and daily rotation, contribute to the frequency drift of any signal that can be sent and received from the exoplanet. Soil. Radio astronomers know that the Earth’s orbital motion causes a drift rate of 0.019 nanoHertz (nHz) and that the Earth’s rotation on its axis causes an additional drift of 0.1 nHz. These shifts can be taken into account when analyzing signals. However, while astronomers don’t always know how fast exoplanets rotate (the exception to this is tidally locked planets, which have a day as long as their year), they can measure an exoplanet’s orbital period and infer a maximum frequency drift from this figure. .
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The drift rate depends on an exoplanet’s orbital characteristics: the inclination of its orbit relative to us, how far its orbit is from circular, and how much it precesses or wobbles. Machine learning algorithms that can search data, looking for signals that represent a drift rate, need a maximum value for the drift rate so they can limit their search. SETI searches typically assume a small value for the frequency drift, less than 10 nHz, but previous calculations based on actual measurements of the most extreme known exoplanet orbits placed an upper limit on the drift rate of plus or minus 200 nHz.
Using plus or minus 200 nHz as the maximum drift rate requires more computing power, slowing down the speed at which SETI query data is analyzed.
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 is 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 those points that maximize the drift rate. And because it is a lower value than plus or minus 200 nHz, it will reduce the amount of computing resources required and speed up the search. There is even room to reduce it much further, members of the research team said.
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“The value of 53 nHz applies to all planets we currently know of, but contains some biases that make the value higher, because transiting exoplanets have higher drift rates than non-transiting exoplanets, and larger exoplanets have larger drift rates than smaller ones. exoplanets,” Li told Space.com in a telephone interview. (Transiting exoplanets cross the faces of their host stars from our perspective on Earth.)
The current catalog of known exoplanets is not entirely representative of the broader population of exoplanets out there. Current detection methods still favor larger planets closer to them stars, because they are the easiest to find. To avoid any biases, Li’s team therefore also measured the maximum drift speed of more than 5,000 simulated planets, which we would expect to be more representative of the real population of exoplanets in terms of their orbital characteristics, with smaller planet sizes, longer orbital orbital speeds. periods and a more uniform distribution 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 less circular (known as eccentric) orbits. From this, 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 much lower than the calculated drift rate of plus or minus 53 nHz.
“It’s the biases that make the 53 nHz value so large,” says Li. “We think that in most cases the true drift rates would 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 stellar oscillations), the maximum drift velocity calculated from real exoplanetary data should better reflect the values from the simulated results. This will make the analysis of potential SETI signals even more efficient.
Drift speeds are one way to determine if a signal is coming 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 – has a zero drift rate because they are on Earth with our receivers. Satellites in low Earth orbit usually have negligible drift speeds, but some, like those of SpaceX Starlink mega-constellation and that of the US government GPS network, exhibit some frequency drift in their radio signals.
“We can try to find out what the drift speed of those satellites would be so that if they show up as RFI – which they often do – we can throw them out,” Li said.
With SETI searches focusing on up to a million stars, being able to quickly analyze data is important to avoid a logjam and spot any alien signals before they’re turned off. Calculating drift speed may seem like a more technical challenge, but it’s critical to speeding up that search, team members said. For example, the new findings will improve computational costs and search times on Breakthrough Listen SETI project on the MeerKAT radio telescope array in South Africa with three orders of size. Ultimately, if ET really exists, we can find them faster now.
The findings are published in the November 2023 issue The astronomical magazine.
