Exoplanets are planets outside of the solar system. Since the first extrasolar planet was detected in 1992, astronomers have detected nearly 1,000 exoplanets orbiting other stars. The search for new exoplanets continues, as scientists try to determine whether there may be truly Earth-like planets elsewhere in the galaxy.
Astronomers had long suspected that planets were probably common in the universe. After all, according to NASA, there was no reason to believe that the conditions which had given rise to planets around the Sun were unique. There are several hundred billion stars in the Milky Way Galaxy alone. As a result, even if most stars do not have planets, there would still be a huge number of planets in the galaxy. The basic difficulty was finding them. At distances from several dozen to tens of thousands of light-years, detecting the dim reflection of starlight off a distant planet is simply impossible. Exoplanets therefore remained only a theory.
The solution, essentially, was to cheat. In the early 1990s, rather than try to spot exoplanets directly, several astronomers began looking for the effects those exoplanets would have on the stars they orbit. For instance, the gravitational force exerted by an orbiting planet tugs the star slightly one way and then another, making it appear to "wobble" slightly along its axis. In addition, when a planet's orbit brings it sweeping across the face of its star that points toward Earth, this causes a momentary reduction, or blinking, in the brightness of the star. Neither method is easy to actually implement. However, with sensitive telescopes and sophisticated computer programs, it is possible to survey a large number of stars looking for these subtle markers that a star is orbited by one or more planets.
The results have been impressive. In 1995, as the first results began to filter in, European researcher Ivan Zolotukhin founded the Extrasolar Planet Encyclopaedia to list all of the newly discovered exoplanets. As of July 2013, the encyclopedia has ballooned to 924 entries in 712 systems. (This means that astronomers have found nearly 150 star systems with multiple planets, like the solar system.) Astronomers working in the field are unanimous in their belief that this number will continue to grow rapidly. Recently, NASA launched the Kepler space telescope, which specializes in searching for planets and has singlehandedly identified hundreds of exoplanets. In several years, Kepler may be replaced by an even more powerful planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS). Together, Kepler and TESS will push the range in which exoplanets can be detected outward by thousands of light-years in all directions.
Exoplanet astronomers mostly share one goal: finding a planet that resembles Earth. Such a planet would be relatively small and rocky, and it would orbit its star at a distance close enough to keep water from freezing but far enough way to keep it from simply evaporating away. Unfortunately, very few such planets have been found so far. Instead, early exoplanet surveys turned up some new and quite unexpected types of planets which do not resemble those found within the solar system. For instance, most known exoplanets are so-called "hot Jupiters" - massive gas giants that orbit their stars even closer than Mercury orbits the Sun. A few are "super Earths" - rocky planets, but several times as large as Earth.
These results are disappointing, but there is still room for optimism. It might be the case that Earth-like planets are truly rare, and that most stars really are hosts mainly to super-Earths and hot Jupiters that would be inhospitable to water-based life. However, it seems much more likely that the reason large, close-in planets are found so frequently is because they are the easiest sort of planets to find. Even under perfect conditions, finding and confirming the presence of a perfectly Earth-like planet around a Sun-like star would require years of observations. The easiest planets to find are large planets which orbit close to small red dwarf stars, because those are the ones that have the strongest effects on their host stars. As a result, they're exactly the sort of exoplanet which astronomers would expect to find most frequently. In contrast, Earth-like planets are much less common. Even so, there are simply so many stars that one recent estimate pegged the number of rocky Earth-like exoplanets in the Milky Way Galaxy at an astonishing two billion.
As a result, the search continues. Over the past few years, tantalizing evidence has emerged that astronomers are closing in on Earth-sized planets. For instance, rocky planets have been found close to the habitable zone around Gliese 581, a red dwarf star just 20 light-years from Earth. Because Gliese 581 is a red dwarf, its habitable zone is much closer to the star. That makes planets easier to detect. However, it also means that stars in Gliese 581's habitable zone would experience factors unknown on Earth, like tidal locking. Even more recently, the Kepler mission has discovered a candidate planet which might truly be Earth-like: a super-Earth orbiting in the habitable zone of a yellow star 600 light-years away which is now designated as Kepler-22b.
After just twenty years, the search for exoplanets has been both surprising and rewarding. Astronomers now realize that there is a much greater variety of planets in the universe than they first expected, ranging from super-Earths to hot Jupiters. True Earth-like planets have remained stubbornly just out of sight until quite recently, but as planet-hunting technology improves, there is every reason to believe that in the coming years there will be numerous discoveries of Earth-like exoplanets.