China's state media reports
that astronomers using its newest
radio telescope
may have detected signals
from an extra-terrestrial civilization
for the very first time.
Or not.
In fact, probably not.
But still, it's an interesting story
because it's the first time
that its newest Five-hundred meter
Aperture Spherical Telescope was used
in a targeted
Search for Extraterrestrial Intelligence
or SETI.
The announcement was reported on WeChat
on June 14th,
but the paper that it's
based off
is still undergoing
peer review,
and that means it hasn't been accepted
for publication yet.
But even if it does get accepted,
it's by no means conclusive.
The authors aren't claiming
they've actually detected
an alien signal.
Rather, they're saying they've detected
an interesting candidate
signal that needs to be followed up.
The signal was detected
during an observation
of the Kepler-438 system.
It's home
to at least one planet
that orbits just 0.17 A.U.
from its host star.
That puts the planet
much closer
to its star than Mercury is from the Sun
However, the star is an M-type red dwarf,
so that makes it cooler
and dimmer than our Sun.
So at 0.17 astronomical
units, the planet orbits
well within the star's habitable zone.
Now, all that really means
is that the planet orbits at a distance
where the temperatures
would allow
water to flow as a liquid on the surface.
It doesn't mean that
the planet really has a liquid water,
let alone a civilization
that's trying to communicate with us.
Besides, red dwarfs
pose a lot of challenges for life
because those planets are going
to be tidally locked
with one side
getting nuked
by ultra powerful super flares.
In fact,
I talked about life around a red dwarf
in a previous video,
so feel free to check that out
if you want to learn the gory,
violent details.
But still,
Kepler-438b is a potentially
earth-like planet,
and there appears to be
an interesting signal associated with it.
Now, if any of this seems familiar, it's
because it is.
In late 2020,
there was a lot of buzz
about a possible
signal detected around the star
Proxima Centauri,
and Proxima
is another red dwarf star,
and it happens to be the closest
star to our Sun.
It hosts two known
exoplanets, one of which orbits Proxima
in its habitable zone.
In 2020, astronomers
found an interesting
signal detected by the Parkes
Radio Telescope in Australia
while it was looking at Proxima.
However, word leaked out
before the astronomers
could complete their analysis
and the internet was rife
with speculation
about the signal's origin.
The only problem was that
the signal was never heard from again.
The team published their analysis
in a pair of papers in the October 2021
issue of Nature.
The papers acknowledged
that while the signal was interesting,
it was more than likely interference
generated by nearby electronics.
Now there's a new signal of interest,
this time detected with China's
FAST telescope while observing
another red dwarf star.
And once again,
the odds are
that it's more
than likely radio interference.
But it was at least detected
with a new telescope
that was using a new detection technique.
So I thought it'd be interesting
to break it
all down in some detail
and see what's next
in the search for this particular signal.
FAST is the largest
single-dish telescope in the world.
Large radio
arrays like the Very Large Array
can certainly achieve
higher resolution than FAST can,
but FAST's giant
single-dish antenna makes the telescope
extremely sensitive.
Now, although the dish is fixed in place,
its receiver can be maneuvered
around to
listen to signals coming
from different parts of the dish,
and that allows fast to effectively point
anywhere within
a 26 degree radius of the zenith
while still using 300
meters of the dish's
total 500-meter aperture.
It's this combination of high sensitivity
and the ability
to track targets
for extended periods
that makes FAST ideal for SETI.
To that end,
an international team from China
and the University
of California,
Berkeley,
used FAST to carry out
the first targeted
SETI observations between 2020 and 2021.
During that time, FAST
looked at 33 Exoplanetary systems,
29 of which
host planets in their habitable zones.
Now traditionally, SETI assumes
that alien
signals are likely to be narrowband.
In other words,
they should only span
a very narrow range of frequencies
because those kinds of signals are easily
distinguished from those created
by natural sources.
Unfortunately, manmade
signals are narrowband as well,
and there's a lot of them everywhere.
The signals are collectively
referred
to as Radio Frequency Interference,
or RFI.
RFI is a real problem
in all of radio astronomy,
but it makes SETI
particularly difficult
because you're looking for a faint
narrowband signal
amid a cacophony of manmade signals.
It's like trying
to hear a distant whisper
in the middle of a football stadium.
And that's why the Chinese government
established a five-kilometer radio-quiet
zone around the FAST telescope.
But that doesn't mean
that there can't be some RFI
from aviation, spacecraft,
or even someone who accidentally
left their cell phone on.
And that's
why SETI uses an on-off
observing technique
to figure out
if a signal might
really be coming from a star
or if it's just background RFI.
The idea goes like this.
An incoming
signal should be confined
to a relatively small
viewing angle
that the telescope sees around the star.
This viewing angle is called
the telescope's "beam".
The signal's
origin can
therefore be tested
by simply pointing the telescope's
beam away from the star,
listening for a while,
and then returning to the star.
If the signal disappears
when the beam points away
and it reappears
when it points back toward the source,
you can at least know that the signal
is probably coming
from the direction of the star.
On the other hand, RFI
should be detectable
when the telescope is pointed
both on and off the main target.
So any persistent signals
like these are immediately
rejected as RFI.
Now the problem with this approach
is that it's inefficient.
It takes FAST
around 10 minutes
to re-point
its receiver
to a different location in the sky,
then listen,
and then another 10 minutes
to return to the star.
So that's
why FAST employs
a multiple-beam receiver to look at 19
individual beams
on the sky simultaneously.
The central beam - called Beam 1 -
is pointed directly at the target,
while the six outermost beams
serve as the off-sources.
A signal that reaches Beam 1
and any of the outer beams
is rejected as RFI.
On the other hand,
if a signal is detected in Beam 1,
but not in any of those outermost beams,
well then it becomes a candidate
for follow-up.
This technique is called
Multi-Beam Coincidence Matching or MBCM.
The advantage is that fast
can make both on and off
source observations
simultaneously
without ever having to move its receiver.
So let's suppose
there is a signal that shows up
only in the central beam,
but doesn't show up anywhere else.
How then, can we know it's
potentially extraterrestrial in nature?
Well,
FAST is mounted on Earth
and Earth is rotating.
And that means sometimes the telescope
is rotating
toward the source, while other times
it's rotating away.
And that's going to introduce
a change in frequency
due to the Doppler effect.
So over the course of an observation,
we should expect the signal
to drift in frequency,
maybe going from a higher frequency
to a lower
or possibly even from a lower
to a higher frequency.
Either way, we get
what's called a "waterfall" plot
because it kind of
looks like the profile of a waterfall.
Likewise,
any signals that don't drift at all
are therefore rejected as RFI
because they're probably just coming
from a stationary ground source.
In the 2020 survey,
all observations
looked for a narrowband signal drifting
between 1.05 and 1.45 Gigahertz.
And that puts it
within the range of frequencies
that have very little natural
background noise.
It's also close
to the classic "waterhole"
range of frequencies
that might be chosen
to get the attention
of water-drinking civilizations
like ours.
Unsurprisingly,
the team found a lot of signals
within this range
- over 1.3 million of them.
To narrow them down
further,
any signal
that only illuminated the central beam
but none of the outer
most beams were promoted to "events",
and that brought the total down
to a little over 2,000 potential events.
But even then, many of these events
were still rejected as RFI
because of the presence of known aircraft
or spacecraft at the time.
And these can also produce
waterfall signals
because they're moving
relative to the telescope.
Still,
there was one signal
that could not really be
ruled out this way.
It was the signal recorded
while observing Kepler-438.
It was detected at 1.14 Gigahertz,
and its width
was on the order of just 1 Hertz or so.
Its drift
rate was consistent
with that of a transmitter
moving with an exoplanet.
And even more intriguing,
it was the only event
that was picked up in
just the central beam.
It also persisted
for the entire 20 minute observation,
with just a small variation
in the drift rate.
The team couldn't
find any records of a satellite
or deep space probe
that was entering the central beam
during the observation either.
Does this mean that fast really detected
the signal coming from Kepler-438?
Well, no,
the team doesn't seem to think so.
It turns out there's one bit of evidence
that suggests the signal
may, in fact, be coming from some ground
based source
in including within the instruments
onboard FAST itself.
This bit of evidence has to do
with the signal's polarization,
and polarization
is essentially
the angle of the incoming wave.
Sometimes the wave is approaching
us horizontally,
sometimes vertically,
and usually it's approaching us
from some angle in between.
Well FAST's
multiple beam
detector can measure polarization.
So the team processed the signals
against two polarization angles
that are exactly 90 degrees
from each other.
And these two angles were chosen
because FAST's instruments
tend to generate signals of their
own at one of these angles.
The chances of an alien signal
having exactly the same polarization
as FAST's instruments isn't exactly zero,
but it should be fairly unlikely.
However, the Kepler-438 signal does
in fact
appear to be
fairly strong
in one of those two polarizations.
If the signal appeared
to have roughly
the same intensity at either
angle, well,
that would certainly have strengthened
the signal's candidacy.
But the fact that it lines up
with one of FAST's
own polarization
angles
suggests that it's probably
some kind of RFI that's
either associated
with one of FAST's instruments
or some other electronics
that are nearby.
Now, in order to really know for sure,
we'd have to confirm this
result with a different telescope.
Ideally,
that telescope would be built on the far
side of the Moon,
because then the Moon itself
would block RFI
from both the Sun and Earth,
and that would make the Moon's far side
the most radio-quiet place
in the entire solar system.
But such a telescope is a long ways off,
and that's
why additional observations
will have to be made from the ground
for now.
Now, certainly FAST is going to take
another look at Kepler-438,
but it's also hoped
that additional telescopes
like Parkes in Australia
could follow up with Kepler-438 as well.
An independent detection
with another telescope would
at the very
least rule out
RFI from the area immediate
surrounding FAST.
A simultaneous detection between FAST
and another telescope
would probably rule out RFI
coming from Earth.
But even then, there's still a chance
that the RFI signal
could be manmade signals
coming from deep space probes
or anything else
we have flying around up there.
Now, if all
this sounds like hard, tedious work
with a lot of dead ends,
well, that's because it is.
But the cool thing about the story
is that it demonstrates
how Multi-Beam Coincidence
Mapping can be a viable technique,
not just for the FAST telescope,
but for any telescope
as long as their outermost
beams are far enough
away from their central beam.
For now, though,
nobody seriously is saying that
this is likely
a signal
from an extraterrestrial civilization.
Not even the authors of the paper
are saying that.
It's probably a new form of RFI
that we just need
to learn about
in the vicinity of the FAST telescope.
Now, isolating and identifying possible
sources of RFI
is definitely not sexy, but it is crucial
for continuing the search
for alien signals.
And SETI in the United States
is actually a privately funded venture,
so if you'd like to help them out,
consider making a one-time donation
or even a recurring donation.
And no, they're not asking me
to say this.
I just think
that it's something
that everybody should donate
to at least once.
And speaking of support, a huge
thanks to these wonderful people
right here
for helping
to keep Launch Pad Astronomy going.
And I'd like to welcome Daniel Durkin,
Matt Langford,
Christian Frinken, Mathias
Skagerholm,
and Lisa Terry
as my newest Patreon supporters.
And if you'd like to join me
on this journey
through this incredible universe of ours,
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Until next time.,
stay curious, my friend.