Since the first exoplanet was discovered in 1995, well over 3,500 planets orbiting stars other than our own have been detected. This explosion in exoplanet discovery has largely happened in the last decade due to drastically improved methods of observation. Today, the main instrument in the exoplanet hunter's toolbox is transit photometry, which detects exoplanets by measuring the decrease in a star's brightness as a planet passes in front of it.
Now, a team of scientists from Queen's University Belfast and the Max Planck Institute for Solar System Research want to know if the same methods could be used by aliens to observe Earth. Based on their initial research, it seems at least nine known exoplanets have a good view of Earth—although none of these are capable of sustaining life as we know it. Still, the researchers estimate that there are ten other planets that are ideally situated to observe Earth and habitable.
To understand how an alien on one of these exoplanets might see Earth, the researchers first identified the areas in the sky in which the transit zones—where a planet passes in front of the Sun—of Mercury, Venus, Earth and Mars could be seen. The researchers only focused on the four innermost planets of our solar system because these are the most likely to be observed by an ET using transit photometry.
"Larger planets would naturally block out more light as they pass in front of their star," said Robert Wells, a graduate student at Queen's University Belfast and the paper's lead author. "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."
To determine which exoplanets would have the best chance of observing our solar system, the researchers determined which parts of the sky would be able to see more than one planet's transit in front of the Sun. As Wells and his colleagues discovered, at most three of the four terrestrial planets could be observed in transit from any point outside of our solar system.
Statistically speaking, this means that a randomly placed alien outside the solar system has a 1 in 40 chance of observing a single terrestrial planet in our solar system. "The probability of detecting two planets would be about ten times lower, and to detect three would be a further ten times smaller than this," said Katja Poppenhaeger, an astrophysicist at Queen's University Belfast.
Of the 3,500 known exoplanets, the team calculated that only 68 are situated such that they could observe at least one planet in our solar system. Of these, nine are ideally situated to observe Earth, but none of these nine planets are habitable.
All hope is not lost for cosmic voyeurism, however. The team also estimated that based on the current distribution of exoplanets, there may be dozens of yet-to-be-discovered planets in the habitable zones of their star that can also see Earth.
The team hopes to confirm this based on data from NASA's K2 mission, which is hunting for exoplanet transits in certain areas of the sky. Each K2 campaign, or the time the orbital telescope spends observing a certain region of the sky, lasts for around 83 days. The researchers expect K2 to discover around a dozen exoplanets that would be able to see planetary transits in our solar system during each campaign.
With any luck, one of those exoplanets might be gazing back at us.