2023-04-24 18:15:48
Could an extraterrestrial civilization endowed with technology similar to ours have detected or be about to detect the Earth by the transit method?
An alien civilization with technology similar to ours would be unable to detect Earth by direct imaging. That same civilization would have a very, very difficult time discovering our world with the radial velocity method. But let’s think about the most fruitful procedure to date, that of transits or eclipses. Would it serve to find our planet from the depths of space?
Eclipses interestelares
The photometric, or transit, method only works when, by chance, the orbital plane of the planet coincides with the line of sight. In that case the planet passes its star at each turn, a transit takes place, and this causes a very faint eclipse, a drop in apparent brightness that might or might not be detected.
Extrasolar planetary systems have randomly oriented orbital planes, and therefore for a planet to transit its star on every turn as viewed from the Solar System would seem a highly unlikely coincidence.
Charbonneau et al, Astrophysical Journal, 529:L45-L48 (2000)
And so it is. Let’s think about Earth. As with all planets, its orbit is contained in a fixed, unchanging plane. That means that, as our world circles the Sun, there is only a very small region of the cosmos from which the planet could be seen, silhouetted as a tiny black dot against the solar disk: it is a narrow band of sky at a side and the other of our orbital plane.
With the statistics against
The width of the band depends on the geometry of each case, but it grows for large planets and/or very close to their stars. Dimidius, the first exoplanet discovered, is so large and so close to its star, Helvetios (or 51 Pegasi), that its eclipses can be seen from a band of sky more than six degrees wide on either side of the orbital plane. , twelve degrees in all, twenty-four times the apparent size of the full Moon. This means that Dimidian eclipses passing in front of Helvetios can be observed from more than 10% of the entire sky. The Sun is outside that band, that is, eclipses are not detected from Earth in the 51 Pegasi planetary system, which was found by the radial velocity method.
Things get worse for smaller planets that are further from their star. The observation band for eclipses caused by Jupiter covers just over a tenth of a degree, not even 0.1% of the entire sky! The Earth, due to its greater proximity to the Sun, has it somewhat better: it is possible to see our eclipses from a band of sky half a degree in total width and centered on the plane of the Earth’s orbit. This width coincides with the size of the full Moon seen from Earth.
This band does not cover even 0.5% of the celestial sphere. For any extraterrestrial civilization, the chances of the Earth transiting before the Sun from their point of view are reduced to less than one in two hundred!
The Earth transits before the Sun only for one of each two hundred civilizations of our environment in the Galaxy. But it would still be necessary to take into account what intensity and what periodicity these eclipses have.
Parts per million
A planet that is large in comparison to its star, such as Dimidius, would hide more than 42,000 parts per million (ppm) of the light from the central body. Current ground technology in space observatories captures fadings in starlight as small as 200 ppm. This, in astronomical jargon, is expressed as “0.2 millimagnitudes.”
Jupiter, with its 10,000 ppm eclipses, would be an easy target even for non-professional alien instrumentation. Saturn (7,500 ppm) is also comfortably within the range of our technology, while Uranus and Neptune are a little closer to the limit, with photometric signals of 1,200 ppm.
The bad news, or the good news if we prefer not to be detected by the aliens, is that eclipses caused by the Earth hover at only 84 ppm. If current terrestrial technology marks the frontier, our planet is absolutely undetectable throughout the cosmos by this method.
To the previous difficulty is added that of the period. A signal that is repeated every four days, like Dimidio’s, is more likely to be picked up. But data must be accumulated over several orbital periods to confirm the planet’s existence, so it would take at least two years for Earth and well over twenty for Jupiter.
In addition, in those very long time intervals, in which continuous observation would have to be maintained, the transits occupy a very short fraction. For the aliens, the Earth would cross the solar disk in less than 13 hours and Jupiter in a day and a quarter.
The terrestrial planets detected up to now by the transit method have been found around stars much smaller than the Sun, which greatly increases the depth of the eclipses until bringing it within the range of sensitivity of current instruments.
¡Plato!
But advances in space photometry have been announced that should make it possible to capture the photometric signal due to the eclipse caused by planets similar to Earth when they transit before stars similar to the Sun. These improvements are being implemented in a space mission of the European Space Agency that, in case if successful, it will be a huge breakthrough in the search for worlds like Earth: the satellite Plato.
ESA
The dish should fly in the year 2026 and aims for a photometric precision of 34 ppm. This puts at your fingertips a not inconsiderable sample of stars like the Sun around which worlds similar to Earth could be found. If alien technology follows our steps, it could take one in two hundred civilizations more or less five years to capture the photometric signal of our transits before the Sun using an instrument like Plato.
Now, if they detect us, would they know if our world is habitable?
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