As of 4 October 2024, there are 5,765 confirmed exoplanets in 4,304 planetary systems, with 965 systems having more than one planet. Most of these were discovered by the Kepler space telescope. [ There are an additional 1,982 potential exoplanets from Kepler's first mission yet to be confirmed, as well as 975 from its "Second Light" mission and 4,630 from the Transiting Exoplanet Survey Satellite (TESS) mission.] Here is a list of known exoplanets.

Overwhelmingly the most probable types of planetary systems are "similar" and "mixed."  The "ordered" and "anti-ordered" arrangements are extremely rare.  We can see planetary systems in the process of forming, the Orion nebula is particularly full of them, and the relatively random nature of the overall process makes these outcomes easy to understand. The nearest other planetary system to our own is the system of Alpha Centauri. It is a triple star system, and apparently the two sunlike stars in the system do not have planets; only the dimmest star, Proxima is known to have at least two planets, both rocky. Proxima is also the nearest star, 4.2 light years away. The average distance from our sun to a star with planets is about 750 parsecs or roughly 2400 light years!  About 85% of all stars are part of binary systems, but there are four known quintuple star systems!  Better not to try to imagine what orbits of planets might be like in a system with 5 suns!

Speculation about the probabilities of life originating on other worlds besides our earth seems to be utterly pointless and fruitless because we know of only one example of life forming and evolving on a planet, namely the earth itself. Just using the past history of earth as a guide, we can see how relentlessly hostile nature is to the existence of life. In the known history of life on earth, there have been around 6 major "extinction events," in which all life on earth came close to dying completely out!  Any one of these catastrophes could easily have wiped out every living thing on earth, leaving the earth barren of life for the remainder of its existence, until it is destroyed as the sun evolves into a red giant. To the best of our current knowledge, life seems to have spread out on earth during or just after the so-called Late Heavy Bombardment, about 3.7 billion years ago. Did the bombardment itself seed life?  This might have been either the worst possible time or the best possible time for the chemical processes that produced self-replicating molecules and the earliest molecular forms of life....  In any case, even if the basic chemistry on which life is based can be assembled with high probability, it can certainly be destroyed with approximately equal high probability. Only time will tell if, on other worlds, life has survived against all odds and has even produced multicellular organisms which can manipulate their environment extensively.

Just on the basis of our knowledge of the fundamental forces of nature, interstellar travel seems almost impossibly difficult. It is very hard to imagine how an alien civilization could ever achieve a state of constraint-free technology, where even limited interstellar travel would become a priority. The exploration would have to be by robots, and these robot probes would not be able to get information back to the parent planet for hundreds or even thousands of years after initial launch.

The real quest is well underway--- Spacecraft currently on their way to destinations around the Solar System: