From classical antiquity, through early fables such as Johannes Kepler’s Somnium (1634) or Francis Godwin’s The Man in the Moone (1638), to Martian invasions like the one described by H. G. Wells, for most of our history we have taken for granted that the worlds closest to Earth must be inhabited. There was no reason to think otherwise—that is, until science actually began finding them. For a planet to be habitable, several circumstances that are not particularly abundant have to come together. Today we know that there is no intelligent life on the Moon, Venus and Mars, but we also know that in our galaxy alone there are possibly billions of planets. With today’s science, how many of them might resemble our own world and even harbour beings with whom we could communicate?
In 1995, astronomers Michel Mayor and Didier Queloz detected the first exoplanet in a Sun-like star, 51 Pegasi b (today called Dimidium). Three years earlier, Aleksander Wolszczan and Dale Frail had discovered the first confirmed extrasolar planets, but these orbited around a pulsar, so they were not suitable for life. And although with an estimated surface temperature of nearly 1,000°C the planet discovered by Mayor and Queloz is also an uninhabitable hell, their discovery launched the hunt for possible terrestrial twins.
As of this writing in October 2021, 4,852 exoplanets have been confirmed in 3,586 star systems, 800 of which have more than one planet, but these numbers are growing steadily. The vast majority have been detected by the transit method—when a planet crosses in front of its star from our point of view, the star’s light dims slightly. This method therefore finds gas giant planets more easily and is limited to those whose transit is visible from Earth. With the data obtained, astronomers can estimate certain characteristics, such as mass, size, possible composition, proximity to the star and temperature. In this way astronomers can ascertain whether a planet is a candidate to harbour life as we know it.
Among the telescopes that have been used to detect exoplanets, the biggest hunter of other worlds has been NASA’s Kepler space telescope. Launched in 2009, its mission was planned for three and a half years, during which it would survey some 150,000 stars in a region of the sky near the constellation Cygnus (the swan). Despite certain mechanical problems that limited its functionality, the mission was able to be extended until October 2018, when the telescope finally exhausted its fuel. During its more than nine years of life, Kepler observed 530,506 stars and discovered 2,662 new worlds. In 2018, NASA’s TESS (Transiting Exoplanet Survey Satellite) took over from Kepler, and in 2019 the European Space Agency (ESA) launched CHEOPS (CHaracterising ExOPlanets Satellite) to characterise already known exoplanets.
Of all the confirmed exoplanets, only a small minority are terrestrial (rocky). Although we can never know the total number of these planets in the whole universe, let alone whether there is life on them, advances in science allow us to narrow down the estimates. One 2020 study that analysed Kepler data calculated that the Milky Way could harbour as many as six billion Earth-like planets, while another estimated the number of potentially habitable planets in our galaxy at about 300 million. But we are only just beginning to scratch the surface of the universe.