Our search for life on another planet needs to look for a light source that did not come from a sun.
Humans produce light unlike the sun—
I am not allowed to insert images but at night our freeway car lights emit a unique light frequency different from the suns.
If you can find a light source with this wavelength, you have found what we have been looking for----
Agree/Disagree?
Not wrong in theory, but very, very, very hard to do in practice. The amount of light put out by even a planetary population’s worth of artifical lighting would be swamped by many orders of magnitude by the light of the star itself. We can barely see very large planets a very long distance from their star.
And although the amount of energy a civilization utilizes would be expected to keep growing as the civilization advances, I see no reason to expect the amount of light they require to increase (except in proportion to population).
Isn’t the “light” from a sun different than from light from a cars light?
Yeah, it’s a tad brighter.
Yes, but is about scales. The electric lights from a populated planet might make a star’s light look 0.0000001% different. That would be non-trivial to detect.
Think of it this way: you are staring at the sun when the ISS happens to pass in front of it. The ISS has a window. There may be control panels with lights visible from outside that window. You are looking to notice a control panel light in the ISS while it is in front of the sun.
The main ways we detect exoplanets are by either noticing the decrease in sunlight caused by the planet getting between us and its star or by the gravitational wobbles it causes on the star’s movement through space. There are several variations on these, but the bottom line is that we don’t actually see the planet itself because “a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out.” We’re probably several decades away from seeing a planet by its own (reflected) light, and much longer away from seeing artificial lights that inhabitants generate.
Yes and no.
The stars are black body radiators. That has a quite specific spectrum that depends upon the temperature of the surface of the individual star. In addition the makeup of the star in terms of the traces of different elements in the star (along with the hydrogen and helium that make up the bulk of the star) add very specific adsorption lines to the spectrum of each star. The combination allows us to classify stars usefully and gain some knowledge about the nature of a star (how big, how old, intrinsic brightness and the like.)
Car headlights, and other artificial light sources are a mix of technologies now. Especially with LED lights in use now. Headlights used to be tungsten lights, which are black body radiators, again with different temperatures that determine the colour (ie whether the light is yellowish, roughly white, or even blueish). LED and gas discharge lamps are different again. They are a mix of emission lines depending upon the exact design. They look white to us, but they are tuned to the human eye, and would not look white to anything else.
It is reasonably easy to tell all these light sources apart, if they are of similar brightness. If you were in orbit above the earth you can easily see the night lighting.
Even if you were only as far away as the moon, you would have little hope of making out the nightime illumination on the earth. It is just so feeble. We mostly cannot actually see exoplanets at all. We detect them by their effects on their sun. Slight wobble of position, or slight dimming in brightness as the exoplanet travels in front of it.
One of the favourite questions to pose is whether we could detect any clear sign of life on the Earth from even without our solar system. Could a being on Mars, Saturn, Jupiter determine whether there was intelligent life, on Earth? The answer probably not. Life yes, but beyond that, very hard to impossible to detect anything against the background.
Purely a WAG, but I would venture to guess that the ability to detect artificial lights on any known exoplanet that is potentially habitable is zero, with any conceivable technology. Such light would be fantastically weak relative to the light of the star as well as in absolute terms.
A case in point is the exoplanet Trappist-1e, 40 light-years away, which is of special interest because it’s a rocky earth-sized planet in the habitable zone of its parent star. Everything we think we know about it has been inferred indirectly. Among the important things we don’t know, which remains for future space-based telescopes to discover, is the planet’s albedo (reflectivity) or anything about its atmosphere (or if it even has any). We have no idea of the planet’s albedo because we cannot even detect how bright the planet is when it’s in full daylight. So even with incredibly advanced technology, we’re not likely to be able to detect artificial lighting 40 light-years away!
The other thing is that stars emit radiation across a broad spectrum, and since any intelligent aliens will have evolved visual organs that are sensitive to the stronger parts of that spectrum, they will produce artificial lights that emit in a similar spectrum. It may be different, but not much different.
So, much, much easier to try to detect alien life the way we’re doing it, primarily by (a) looking for biosignatures in the exoplanets (the presence of an atmosphere, the presence of water, the presence of oxygen) and (b) looking for radio transmissions that aren’t from natural sources.
Interesting side note: Because the parent star of Trappist-1e is so small and cool, it’s an incredibly long-lived star. It’s already 7 billion years old, almost twice as old as our solar system, which means that if life evolved on that planet, it could be incredibly old and incredibly advanced by now. The star itself, unlike our sun which is about halfway through its ~8 billion year life expectancy, will likely burn for 12 trillion years, making it one of the last stars in the universe to go out.
Back in the 1970s, we called our first landers “Mars1”, “Mars 2” etc., so the Martians could not possibly have known where they came from.
Conversely, if an alien space probe lands on Earth, it will probably just be called “Earth 1”, and we won’t have a clue where it came from.
Wouldn’t the Martians just assume it comes from the planet in their solar system where the inhabitants refer to the fourth planet by the same name as their ancient god of war?
How would they know that? Marsipedia?
It would have come from the silent planet.
Say someone at Alpha Centauri A wanted to communicate with us. They could shine a laser at us. Alpha Centauri A has a brightness of around 2.5e-8 W/m^2. A green laser with an aperture of 1 km would have a beam divergence of around 3.5e-10 radians, which gives a spot size of 1.45e7 m at Earth’s distance and an area of 1.6e14 m^2.
To achieve the same brightness, then, would require an input power of 4e6 W. That’s only 4 megawatts; not too bad. And in fact nowhere close to that would be needed, since the laser would have a narrow spectral line that would be easily detectable. You could probably get away with a factor of 1000 less, which makes the laser very modest indeed.
But that is just the closest bright star. Go 1000 light years out and you need almost 100,000 times the power. That’s still pretty modest, in the grand scheme of things–even a gigawatt laser is not outside the bounds of Earth technology, let alone alien.
All of this however is assuming deliberate communication. If the light is just some side effect of something else, spread equally in all directions, it becomes far harder to detect (probably impossible at less than Dyson sphere scales).
How accurately would they have to know Earth’s orbit to aim the laser so that it would hit us four years from when they turned it on? Also, if we wanted to do the same thing, do we have the technology to point a laser that accurately? I doubt it…
If you know a planet even exists at all, you’re going to know its orbit very well.
We would notice one of these.
Why would their radio transmissions be any more detectable than their headlights and bonfires? (Maybe we could see their forest fires?)
One point I have not seen brought up is that, detectable or not, the light the OP is talking about would be generated by intelligent life. I would imagine that is very much rarer that just life so you’d have to be lucky to point your detector at the right planet.