- 4236 -
EXOPLANETS - home of extraterrestrials? 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.
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4232 - EXOPLANETS
- home of extraterrestrials?
-
- Exoplanet orbits
could be key to finding ET. Here's why.
The type of orbit an exoplanet occupies could result in astronomers
missing out on possible radio transmissions coming from the plane
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- 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.)
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- 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.
-
- 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.
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- 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 they were 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.
-
- 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.
-
- 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.
Transiting exoplanets cross their host stars' faces from our perspective
on Earth.
-
- Current detection
methods still favor larger planets closer to their stars, because they are the
easiest to find. Astronomers 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 they were 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. 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.
-
- 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 is vital for speeding up that search. 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. If E.T. is really out there, we can now find them faster.
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-
November 22, 2023
EXOPLANETS - home
of extraterrestrials? 4236
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