As of 1 October 2026, there are
6,375 confirmed exoplanets
in 4,772 planetary systems, with 1,064 systems having more than
one observed planet. Here is a list
of known exoplanets. Most detection methods involve
looking for planetary transits across the central star, which
requires that from our viewpoint we are looking closely parallel
to the system's orbital plane. This is a rare
circumstance, so the fact that we still see so many exoplanets
can only mean that nearly every star has planets.
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. It is a flare star, so the planets are
radiation-blasted bare rock. Proxima is also the nearest
star, 4.2 light years away. The average distance from our sun to
a star with known 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.... There is some evidence that life on earth may have originated independently two separate times. 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 some few and widely separated 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 radioed information back to the parent
planet for hundreds or even thousands of years after initial
launch. It would be almost infinitely easier to construct
gigantic telescopes in planetary orbit, which could actually
image planets in distant systems. There is no imaginable
incentive to justify physical travel from one system to another,
even if it were practical, which it does not seem to be.
The real quest is well underway---
Spacecraft currently on their way to destinations around the
Solar System:
