Fraction of stars with planetary systems, fp, 1961 to the present

Of the Drake Equation’s seven factors, f_p, the fraction of stars with planets, is the one on which we have made the most progress. Since 1961, we have gone from ignorance to a fairly reliable determination of what this number is. Our progress has occurred in three phases. In the first, which lasted for more than thirty years after 1961, astronomers were limited to speculation and probabilities, based on the Copernican principle and their growing understanding of processes that led to the formation of our solar system. The second phase began in 1995, when the first exoplanet was discovered using the Doppler method, which kicked a new research field into high gear. Excitement mingled with confusion as hot Jupiter-like planets with highly eccentric orbits were found, planets with unprecedented properties. The next decade and a half saw a steady march of the detection limit from Jupiter mass to Neptune mass and more recently to super-Earths, exoplanets with two-to- five times Earth’s mass. The census grew from a few to over five hundred. The third phase of exoplanet discovery came with the 2009 launch of the Kepler satellite. Astronomers began to use the transit or eclipse method to detect planets smaller than Earth and, as of 2014, they have found more than 3,000 planetary candidates, about 300 of which are similar to or smaller than Earth. Both the Doppler and the transit methods provide information on potential habitability, and taken together they yield a mean exoplanet density. There is remarkable agreement in the exoplanet count derived from three different detection methods: Doppler, transits, and microlensing. With an uncertainty factor of two, there is an exoplanet for every sequence star, and most of them are small, terrestrial, and orbit red dwarfs.