IS EARTH unique in the universe? Before the early 1990s, we had no idea planets existed outside of our own solar system. The nine planets (which included Pluto at the time) orbiting the sun were the only ones known. Fast forward to 2020, and we now know of at least 4,000 confirmed planets outside of our solar system, known as exoplanets, with several thousand more potential ones awaiting confirmation. These exoplanets range in all sizes and masses, from small rocky worlds to huge gas giants. However, the search is still on for a habitable planet like Earth with temperatures that support liquid water and possible life forms.
Improved telescope technology has fueled the discovery of exoplanets. The most recent space telescopes driving the search for exoplanets include NASA’s former Kepler mission and the ongoing TESS mission. Kepler was launched in March 2009 and spent almost 10 years observing over 530,000 stars while confirming 2,662 exoplanets. It showed that planets outnumber stars in our Milky Way galaxy, an incredible fact that seemed like science fiction a few decades ago. The TESS mission was launched in April 2018 and picked up where Kepler left off. Since this past May, TESS has identified nearly 2,000 more possible exoplanets.
Based on the years of Kepler data, NASA now estimates around four billion stars in our galaxy are similar to our sun, and if only 7 percent of those stars host habitable planets (considered to be a very conservative estimate), then there could be at least 300 million Earth-like planets within the Milky Way. Those possible worlds beyond Earth are awaiting discovery.