NASA’s Kepler satellite mission has been in the news lately with reports of a potentially earth-like planet (Kepler 452b) orbiting a sunlike star about 1400 light years away, well within its habitable zone. Kepler is a technological tour de force. It detects exoplanets by measuring the minute variation in a star’s light caused by a planet crossing in front of the star, partially eclipsing it, as seen from the line of sight of star-planet-earth. There are a variety of methods for detecting exoplanets, but this particular technique, the transit method, appears to be the most sensitive and successful as far as I know.
The transit method does have one drawback. If the orbital path of the exoplanet lies either above or below the star’s disk, we can’t see the eclipse. IOW, unless the orbital plane of the planet is almost perfectly aligned (edge on) with us, the planet will pass above or below the star and we will not be in the shadow. This has nothing to do with our technology, or lack of it. Neither is it dependent on our distance to the system. It is strictly an inevitable result of the geometry. It limits our primitive tech just as much as it would provide an obstacle to any advanced extraterrestrial observer using the transit method to try and find other planets. Even under otherwise ideal conditions, the percentage of planets which can be detected with this method is very small (assuming planetary orbits are randomly oriented in space), and is constrained solely by the size of the stellar disk, and the orbital radius of the planet. The bigger a star is, and the closer the planet is to it, the wider an angle on the celestial sphere the system will subtend to a distant observer, i.e., the more visible it will be to distant observers.
This table, abstracted from the NASA Kepler web page, summarizes the visibility of our solar system using the transit method. For example, for systems identical to Sun/Earth,
only 0.47% of the potential “Earths” would be detectable, and then only from a band 1.65 degrees wide about its orbital plane. That’s about three full moons.
Orbital radius (AU), % visibility, visibility angle (degrees)
Mercury 0.39 1.19 6.33
Venus 0.72 0.65 2.16
Earth 1.00 0.47 1.65
Mars 1.52 0.31 1.71
Jupiter 5.20 0.089 0.39
Saturn 9.5 0.049 0.87
Uranus 19.2 0.024 1.09
Neptune 30.1 0.015 0.72
If the transit method is indeed the most effective one for searching for exoplanets, it is reasonable to expect that astronomically capable ET civilizations with SETI ambitions would be using this technique to look for potential neighbors. Furthermore, for these hypothetical ETs to detect our system (using transits) their home star would have to be located within several degrees of our ecliptic, our own Sun’s yearly path through the celestial sphere. The ecliptic is the projection of Earth’s orbital plane (as well as all the major planets in our system) onto the celestial sphere.
Speculating further, if it is reasonable to assume that ET is willing to devote additional resources to researching known planetary systems, and perhaps even initiating communication with them by radio or laser, then a band several degrees wide about our own ecliptic might be a productive place to concentrate our own passive SETI observations. That’s where the races who have used transit observations to locate us will be already concentrated.
For those of you interested in following up on this, the link below considers in some mathematical detail the geometry involved in the Kepler observational program
http://kepler.nasa.gov/Science/about/characteristicsOfTransits/