An “exoplanet” is a planet that orbits a star other than our sun. The first exoplanet orbiting a star was not found until 1995.* According to NASA, there are now 3,572 confirmed exoplanets and another 4,500 candidates. These exoplanets are found orbiting 2,700 different stars in in a small pocket of space in the Milky Way relatively near our solar system. While finding 3,572 confirmed exoplanets is quite a feat, it pales in comparison to the estimated one trillion -3 trillion planets in our galaxy alone. There are a lot of planets to find!
The first exoplanet discovered was a gas giant (like Jupiter) called 51 Pegasi b. The early exoplanets discovered were very large planets close-in to their stars (called “hot Jupiters”) and were primarily found by detecting a wobble in the host star due to the gravitational tug from the orbiting planet. As a planet tugs on a star with its gravitational pull, it causes the star to wobble in its path across the sky. In using this wobble method, the tug is bigger when the planet is closer and larger, and so large, close-in planets are easiest to find.
In 2009 the launch of the NASA Kepler Space Telescope has ushered in a new era of exoplanet discovery. The Kepler focused on a very small patch of space of only about 150,000 stars for four years. It was looking for slight dimming of stars as planets passed in front of them. After the four years of observation it had found 2,000 confirmed exoplanets. In addition to Hot Jupiter’s, space telescopes using the transit method can find much smaller and more distant orbiting planets. Other telescopes including the Hubble Telescope have also found exoplanets mainly by using the transit method.
What is really interesting to astronomers is searching for Earth-like (rocky) exoplanets in the so-called “Goldilocks” distance – not too close and not too far from their stars to keep water on planetary surfaces from freezing or vaporizing away. As of November 2017 about 50 rocky planets in Goldilocks zones from their stars have been discovered. These rocky planets in the Goldilocks zone are the most likely to harbor extra-terrestrial life. Current estimates are there should be about 40 billion potentially inhabitable planets in our galaxy.
*Note in 1992 a planet orbiting the pulsar was discovered.