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Astronomers have identified exoplanets from which potential life forms are likely to be able to observe a transit of one of our solar system's planets:
"The detection of thousands of extrasolar planets by the transit method naturally raises the question of whether potential extrasolar observers could detect the transits of the Solar System planets," they wrote in a paper published [open, DOI: 10.1093/mnras/stx2077] [DX] last month in the Monthly Notices of the Royal Astronomical Society.
[...] The transit method only works if a planet is aligned in a way that it crosses the star. In the Solar System, the terrestrial planets – Mercury, Mars, Earth and Venus – are more likely to be spotted in this way than the gas and ice giants – Jupiter, Saturn, Uranus and Neptune. Up to three planets in various combinations can be seen at any one time, the researchers found. The next step is to find which boundaries are located in the best positions to observe more than one of the terrestrial planets crossing the Sun, and count up the number of exoplanets inside these "transit zones."
Katja Poppenhaeger, co‑author of the study and assistant professor at Queen's University Belfast, estimated that "a randomly positioned observer would have roughly a 1 in 40 chance of observing at least one planet. The probability of detecting at least two planets would be about ten times lower, and to detect three would be a further ten times smaller than this." A full sweep shows there are currently 68 known exoplanets that are in a good spot to catch a planet transiting the Sun. From this list, nine of them are temperate and have sizes similar to Earth, but none are considered to be habitable. That doesn't mean the chances of aliens potentially spying on Earth are completely zero. The researchers estimate that there are ten other unconfirmed exoplanets that have more favorable conditions of sustaining life, and are within the transit zones.
(Score: 2, Interesting) by pTamok on Tuesday September 12 2017, @08:35AM (1 child)
Hmm. I wonder if it is technologically feasible to build a sufficiently powerful laser to mess with the transit luminosity profile enough to send a (possibly slow) message? That is, deliberately do something that looks something like KIC 8462852 [wikipedia.org] to an observer on an exoplanet looking in Earth's direction.
An observer on an exoplanet watching a transit of Earth would see a predictable change in the sun's luminosity. If you fire up a sufficiently large light source on earth during the transit, pointing directly at the exoplanet, the luminosity profile the observer sees would change. By modulating the light source intensity, you could probably transmit quite a few bits of information. If you wanted to communicate, the round-trip-delay would be non-negligible.
While I have a lot of blank envelope-backs, I don't have the smarts to work out what power of light source might be required.
There's a few tricks you could do to optimise things:
1) Use a laser. It's directional, so you minimise how much power you throw away in useless directions.
2) Use a laser - it's pretty monochromatic, so you could generate output of about the order of the level of solar luminosity for only a small part of the solar spectrum. This assumes observers on an exoplanet would monitor the spectrum and see the abnormality.
3) Turn it on and off in an easily identifiable pattern. This reduces the overall power requirement and/or allows higher peak power, and makes the signal more obviously artificial.
4) Build the laser to operate at a wavelength that corresponds to a relatively low-radiance emission wavelength for the sun - i.e. your background noise is lower, so you get more signal. I think somewhere near Hydrogen Lyman-alpha might be right, but I'm not sure.
Pointing lasers at celestial objects:
XKCD What-if "Laser Pointer" [xkcd.com]
Discussion about the above on the XKCD forums. [xkcd.com]
(Score: 2, Interesting) by pTamok on Tuesday September 12 2017, @12:01PM
Hmm.
Equatorial radius of Sun: 695,700 km -> Area of solar disc: 4,371,000 km2 (4.371x1012 m,sup>2)
Equatorial radius of Earth: 6,378 km -> Area of Earth's disk: 40,074 km2 (40.074x109 m,sup>2)
Percentage of total solar disc covered by earth during full transit (i.e. not at beginning or end): 0.9168 %
If an observer can detect the sun dimming by 1%, then assuming that if we can output light at 1% of the sun's output, it will be detectable during the transit at a blip in luminosity. That's rather a lot. Especially as the laser aperture is probably not the size of the earth's disc. If the laser aperture is 1 square meter, then it would need to be 40.074x109 times brighter than the sun (possibly 'only' at a particular frequency) to be detectable. Full spectrum full sunlight is about 1366 W/m2, so for a 1 m2 laser aperture, we would need need 1366 x 40.074x109 watts, or 5.474x1013 watts of power. That's 5.474 Terawatts. I don't think that is easily achievable continuously for any reasonable. However - if we 'only' need to achieve that for a small subset of the spectrum - say 0.1 %, that reduces the power requirement to 'only' 5.4 Gigawatts and we use a group of 1 metre aperture lasers - say 100 - then each laser 'only' needs 54 Megawatts of power.
By comparison, the lasers used at the USA's Argonne National Laboratory to investigate Inertial Confinement Fusion deliver 1.9 Megajoules in just over a nanosecond - about 500 Terawatts. Hmm. I doubt that observers would be set up to see nanosecond blinks in luminosity during a transit, but I'm beginning to think it might actually be do-able.