Exoplanets

Episode #5 of the course “Strangest Things in Space”

An exoplanet is what we call planets that orbit a star other than the sun. The earth, for example, would be an exoplanet to aliens from another planet. Since the first exoplanets were found in the 1990s, scientists have discovered that exoplanets are actually very common. In all likelihood, just about every star in the universe has at least one planet orbiting it. The Exoplanet Orbit Database lists around 3,700 exoplanets in 2,792 systems currently proven to exist.

For a solar body to classify as an exoplanet, it has to meet certain criteria. Most importantly, as you might expect, it has to be a planet. This isn’t as black and white as it sounds, however, since outside our solar system, there is no formal definition of a planet. Within our solar system, the IAU provides the most widely-accepted definition of a planet—an object that orbits the Sun in a spheroidal shape and is the dominant gravitational object in its orbit.

We can look at two examples to illustrate that definition. Asteroids are not planets because, while they orbit the sun, they are sharing similar orbits with other objects that are more massive than them. Thus, they are not the dominant gravitational object. This is similar to the case of Pluto, a solar body that at one point in time was thought to be a planet but has since been declassified because it shares part of its orbit with the much larger Neptune.

Identifying exoplanets is done through five primary methods: the Doppler technique, direct imaging, light curves, transit timing variations, and microlensing. One of the most common methods monitors light blockage from the stars. Telescopes in space look at thousands of stars at the same time. They’re able to track “blips” when the sun’s light is less than normal. If the blips are in repeatable patterns, it is safe to infer that the blip of less light is an orbiting planet passing between the star and the telescope.

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