In the last decade, we have discovered thousands of exoplanets. Now, astronomers from Germany and Northern Ireland have uncovered how many of these worlds could possibly detect us as clearly as we can detect them.
They estimated that 68 out of the known 3,667 exoplanets are located in the right position to spot at least one or more planets in the solar system. Nine of them are in an ideal place to detect Earth, but none of them are habitable. However, statistical estimates suggest that 10 undiscovered planets could both see Earth and be in the habitable zone. This analysis is reported in the Monthly Notices of the Royal Astronomical Society.
”We estimate that a randomly positioned observer would have roughly a 1 in 40 chance of observing at least one planet," co-author Katja Poppenhaeger from Queen’s University Belfast said in a statement. "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."
If we can see them, why can’t they see us? It’s all a matter of how we detect exoplanets. The extremely successful Kepler mission detects them by looking at how planets that transit in front of their stars block a small fraction of light. If they see this happening at regular intervals, this suggests a planet is orbiting that star.
For a hypothetical alien civilization to find Earth, they will need to inhabit the narrow band of the sky from where a transit is visible. Although the inclination of the orbit of the other eight planets is only a few degrees away from Earth’s, that would be enough for a telescope like Kepler to miss out on some of the other planets. The report suggests no more than three planets can be seen from any location.
Distance from the star also plays a key role. The research estimates that although the rocky planets are smaller, just being near the Sun makes us more likely to be spotted. ”Larger planets would naturally block out more light as they pass in front of their star,” explained lead author Robert Wells of Queen’s University Belfast. ”However the more important factor is actually how close the planet is to its parent star – since the terrestrial planets are much closer to the Sun than the gas giants, they’ll be more likely to be seen in transit.”
How planets transit a star is not the only way to see exoplanets, but it has been extremely successful. Maybe there are aliens out there with much better telescopes that have discovered us with other methods, but if they spotted us through the transit method, they can only live on these handful of planets.