Ever since astronomers first began searching out planets around other stars, they've found thousands of "planet candidates" and confirmed more than a thousand as actual worlds. There could be billions of worlds out there. The tools of the search are ground-based telescopes, the Kepler Telescope, Hubble Space Telescope, and others. The idea is to look for planets by watching for slight dips in the light of a star as the planet passes in its orbit between us and the star. This is called the "transit method" because it requires that a planet "transit" the face of the star. Another way to find planets is to look for tiny shifts in the star's motion that are caused by a planet's orbit. Detecting planets directly is very difficult because stars are quite bright and planets can get lost in the glare.
Finding other Worlds
The first exoplanet (a world circling other stars) was discovered in 1995. Since then, the rate of discovery grew as astronomers launched spacecraft to look for distant worlds.
One fascinating world that they've found is called Kepler-452b. It circles a star similar to the Sun (a G2 star type) that lies about 1,400 light-years from us in the direction of the constellation Cygnus. It was found by the Kepler telescope, along with 11 more planet candidates orbiting in the habitable zones of their stars. To determine the planet's properties, astronomers conducted observations at ground-based observatories. Their data confirmed the planetary nature of Kepler-452b, refined the size and brightness of its host star, and pinned down the size of the planet and its orbit
Kepler-452b was the first near-Earth-sized world found, and it orbits its star in the so-called "habitable zone". That's a region around a star where liquid water could exist on the surface of a planet. It is the smallest planet ever found in a habitable zone. Others have been larger worlds, so the fact that this one is closer to our own planet's size means astronomers are close to finding Earth twins (in terms of size).
The discovery does NOT tell whether or not there IS water on the planet, or what the planet is made of (that is, whether it is a rocky body or a gas/ice giant). That information will come from further observations. Yet, this system does have some interesting similarities to Earth. Its orbit is 385 days, while ours is 365.25 days. Kepler-452b lies just five percent farther away from its star than Earth does from the Sun.
Kepler-452, the parent star of the system is 1.5 billion years older than the Sun (which is 4.5 billion years old). It's also a bit brighter than the Sun but has the same temperature. All these similarities help give astronomers a comparison point between this planetary system and our own Sun and planets as they seek to understand the formation and history of planetary systems. Ultimately, they want to know how many habitable worlds are "out there".
About the Kepler Mission
The Kepler space telescope (named for astronomer Johannes Kepler) was launched in 2009 on a mission to spy out planets around stars in a region of the sky near the constellation Cygnus. It performed well until 2013 when NASA announced that failed flywheels (that keep the telescope pointed accurately) were failing. After some research and help from the scientific community, mission controllers devised a way to keep using the telescope, and its mission is now called K2 "Second Light". It continues to search out planetary candidates, which are then re-observed to help astronomers determine the masses, orbits, and other characteristics of the possible worlds. Once Kepler's planet "candidates" are studied in detail, they are confirmed as actual planets and added to the growing list of such "exoplanets".