What Is an Exoplanet and Why Do Astronomers Have Interest in Them?
An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. To date, scientists have found exoplanets in a wide variety of sizes. Some are much larger than Jupiter, while others are much smaller than Earth.
Starting in 1988, and as of 1 December 2016, there have been 3,544 exoplanets in 2,659 planetary systems and 597 multiple planetary systems confirmed. HARPS (since 2004) has discovered about a hundred exoplanets while the Kepler space telescope (since 2009) has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which about 11% may be false positives.
On average, there is at least one planet per star, with a percentage having multiple planets. About 1 in 5 Sun-like stars have an “Earth-sized” planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, one can hypothesize that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.
Astronomers, however, tend to be a curious bunch that’s not satisfied with staying within the confines of just our solar system. Instead, they know that there are billions of other stars like our Sun spread across the universe.
With so many other stars out there, they began searching the neighborhoods around those stars for planets like Earth. Could there be an Earth-like planet orbiting some other star with all the perfect conditions to support life? Maybe!
Scientists have learned that other solar systems share similarities with our own solar system. For example, they’ve learned that exoplanets can orbit their stars at different distances. Some exoplanets are so close to their stars that they have surface temperatures that could melt iron.
Other exoplanets might orbit at just the right distance that they fall in their star’s habitable zone, which is the zone in which an exoplanet would enjoy temperatures that would permit liquid water to exist at its surface. The existence of liquid water at the surface is one of the key factors scientists believe would permit life to form.
Unfortunately, exoplanets are not easy to see from Earth. Exoplanets tend to be relatively small and dim compared to the bright glare from the stars they orbit, which makes them hard to spot. Clever scientists have developed an indirect method of discovering exoplanets that they call the “transit method.”
Knowing that exoplanets orbit their stars just like Earth orbits the Sun, scientists reasoned that exoplanets would pass between their stars and Earth on a regular basis. By measuring the brightness of stars over long periods of time, scientists can identify periodic decreases in the brightness of stars that correspond to exoplanets orbiting them.
Although astronomers have long believed that exoplanets existed, the first exoplanets weren’t confirmed until the 1990s. Since that time, over 3,000 exoplanets have been identified. Over 1,000 exoplanets have been identified by one device alone: NASA’s Kepler space telescope.
The Kepler space telescope reached orbit in 2009 and searched for exoplanets using the transit method for four years. Even though its original mission has ended, the Kepler space telescope collected a tremendous amount of data, and scientists are still making new discoveries from that data even today.