For thirty years astronomers have been in hot pursuit of planets outside our solar system. The search for “exoplanets” markedly accelerated since 2006. Today nearly fifty ground-based search projects plus ten space missions have been commissioned. Most of them are still ongoing, surveying a relatively small portion of the galaxy (from this we know that there are many more planets than stars in the Milky Way).
The results have been spectacular. NASA’s website reports 4,422 confirmed exoplanets, 7,445 candidates in 3,328 planetary systems. Of these close to 1500 are Neptune like, 1400 are gas giants, over 1300 are Super Earths, 165 are Terrestrial, and the number is rising. NASA estimates that in a decade, they will number in the tens of thousands as more and more robotic telescopes enter the race.
Combining a variety of discovery methods, scientists can determine their diameter and masses, and surface temperature; they can estimate the level of light they receive from the star they orbit, the composition of their atmosphere (if they have one), and guess the color of their sky.
This provokes a fascinating thought: If we can detect such astonishing details of other worlds “out there,” observers from these worlds can gather the same information about ours. If they did, we would be aliens and possible targets of their curiosity, a possibility that has been on the minds of astronomers for years.