As discussed in my book, Introduction To The Cosmos, there are hundreds of billions of galaxies, each with at least a hundred billion stars. Surrounded by the seemingly endless number of stars, humanity has long speculated about worlds beyond our own and the life they might harbor.

Opinions on whether there are planets elsewhere have varied. The Greek philosopher Epicurus  (341–270 BCE) wrote the following optimistic letter to one of his students:

“First, an infinite number of worlds exists in the universe, some of which worlds are like and some of which are unlike our own.  We conclude this because the ultimate particles are infinite in number, as was proved already.  No matter how far they move out into space, it is not possible that the number of particles has been used up in the formation of any number of worlds.  Thus there is no obstacle to the existence of an infinite number of worlds, and we conclude that there are innumerable worlds in the universe, including those, like our own, which contain living beings.”

Aristotle (384–322 BC), also a Greek philosopher, in his book On the Heavens, was more pessimistic and wrote, “…there cannot be more worlds than one.”

Opinions have varied between the two extremes since then.

A planet that is not part of our solar system is called an exoplanet and the study of exoplanets is a relatively new area of study at less than 30 years old. The first exoplanetary system discovered was in 1992 with the discovery of several planets orbiting the pulsar PSR B1257+12. Since then, more than 4,300 exoplanets have been discovered.

Like the planets of our solar system which reflect light from the Sun, exoplanets reflect the light of their host star; however, that light is in the order of billions of times fainter than their host star. Moreover, the separation of an exoplanet from its host star can be very small, making visual observations challenging.

The most common method, and the method most accessible to amateur astronomers, is the transit method. With the transit detection method, astronomers stare at an exoplanet system candidate and record the intensity of the light from the host star. As readings are taken, they produce a light curve of the star’s luminosity over time. When astronomers see a slight dip in the star’s luminosity, as shown in the following diagram, they conclude than an exoplanet must have passed in front of the star, as shown:

The transit method is so reliable that we launched a satellite in 2018 that was designed to detect exoplanets using the transit method. The satellite is called the Transiting Exoplanet Survey Satellite (TESS) and is currently in orbit around earth.

With the transit method, more than 3,000 planets have been discovered; however, there are other methods. Other methods of detection had to be developed because we approached the limit of what the transit method was capable of detecting, plus other methods provide a number of advantages, like being able to detect exoplanets that are not directly along our line of sight, as is required for the transit method.

Alternate detection methods include:

  • Radial Velocity: where astronomers measure the reflex motion of the star’s barycenter (center of mass)
  • Astrometry: repeated, high-accuracy measurements of the position and motions of a host star caused by the perturbation of an orbiting planet
  • Timing: this technique is used to detect exoplanets orbiting white dwarfs, pulsating subdwarfs, and eclipsing binaries; astronomers use radio pulsar timing to measure the periodic signature which is modulated by an orbiting planet
  • Microlensing: where astronomers use gravitational lensing in which orbital motion, using parallax, is measured during a lensing event; this method is used for larger exoplanets
  • Direct imaging: this is made possible by improvements in technology like adaptive optics and space-based imaging

I’ll discuss these methods in upcoming blog posts. I also plan to discuss how you can observe and measure a transiting exoplanet using remote telescopes.