When you look up at the stars at night, it's easy to feel we're alone in the universe, but calculations based on the work done by NASA suggests that there are one hundred billion planets in the Milky Way galaxy with one in ten, rocky planets like Earth.
These tiny worlds, known as exoplanets, are several light-years away and were only imagined or part of science fiction for centuries. Until 1992, when the first planet orbiting a dead-star was discovered it was like trying to find an ant walking on this very road, from London. Three years later an exoplanet similar to the gas giant was detected orbiting its star closer than Mercury circles our sun.
Researchers can calculate a planet’s surface temperature from the world’s orbital period and its star’s temperature and analyse the planet's atmosphere for telltale signs of life, such as oxygen and methane gases. The tally of confirmed alien worlds grows weekly, but the hunt for an Earthlike planet continues. Who knows, if scientists found one with the right size, temperature, and water, they may also find life!
Finding exoplanets involves two main methods.
The most successful is called the Doppler wobble, with most newfound worlds, discovered this way. Astronomers look for a “wobble” in a star's light spectrum. The more massive a planet the greater its effect on the host star.
The second way to find far off worlds is using the transit method. This looks for periodic dips in a star's brightness as an orbiting planet passes in front of its star. By measuring the amount and frequency of a star’s dimming, astronomers can estimate the orbits and masses of its planets.
As of 1 August 2020, there are 4,301 confirmed exoplanets in 3,176 systems, with 703 systems having more than one planet.