Exoplanets: what they are and how to find them

*This text was written by a columnist from TechWorld; learn more at the end.

Have you heard the term exoplanets and wondered what could it be? The prefix “exo” is not from exotic  —that doesn’t stop them from being super exotic planets—but from “outside”. Out of where? From our Solar System. Exoplanets are nothing but planets, like in our neighborhood, but orbiting other stars!

Artistic representation of different planetary systems (out of scale)Source:  Wikipedia

Within our Solar System, according to the official definition of planet 2006, we only have 8 planets. Yes, the same definition that removed Pluto from office. Summing up the current definition of the International Astronomical Union, a planet must:

  1. orbit a star
  2. Have hydrostatic balance (be a sphere)
  3. To be the dominant celestial body in its orbit

Today we don’t consider Pluto a planet because it doesn’t meet item number 3. We know that Pluto has a companion, Charon, almost his size. The united system is considered a pair of dwarf planets.

Dual dwarf planet system: Pluto and CharonDual dwarf planet system: Pluto and CharonSource:  APOD

But, going outside the Solar System, today we already know more than 4,500 exoplanets! In addition to these, there are more than 7,000 candidates to be confirmed — or not. The first confirmation of the detection of an exoplanet was made on October 6, 1995 by researchers at the University of Geneva, Michel Mayor and Didier Queloz. Both shared the 2019 Nobel Prize in Physics for the discovery! The detection of the exoplanet around the 51-Pegasi star used the radial velocity method. This means that the planet’s presence alters the star’s motion. This alteration, following the laws of physics, informs us of the presence of this planet, as well as its mass and distance to the star.

In addition to this method, there are now other ways to detect exoplanets. Some of the best known, in addition to radial velocity, are direct imaging and planetary transit.

Direct imaging, as the name suggests, is when we observe the exoplanet directly. For this to happen, it is necessary to block the star’s light, as it obscures the planet’s presence. A classic example is the HR 8799 system, below. In the center, we can see an empty sphere, which is where the covered star is. In addition, you can observe the movement of 4 bright points, which are 4 planets orbiting the star in real time!

HR 8799 planetary system, with 4 exoplanetsHR 8799 planetary system, with 4 exoplanetsSource:  Wikipedia

The second method is planetary transit. This phenomenon occurs when the planet passes in front of a star, blocking part of its light. Transit also takes place in the Solar System with Mercury and Venus, which are in more internal positions. This technique was the first responsible for mass detection of exoplanets with the Kepler Space Telescope. With constant monitoring of starlight, it was possible to detect cyclical variations in brightness, representing the transit of exoplanets!

Different snapshots of the transit of Mercury. The black dots in the upper left corner of the Sun are the planet Mercury at different times of transit.Different snapshots of the transit of Mercury. The black dots in the upper left corner of the Sun are the planet Mercury at different times of transit.Source:  APOD

With the transit method it is necessary to detect the variation in the brightness of stars more than once. This is to confirm the presence of the exoplanet. The first detection generates an exoplanet candidate. The second tells us its orbital period. That is, how long it takes for the exoplanet to complete its orbit. With the orbital period, astronomers can predict the third transit. If this is confirmed, we have confirmation of exoplanet detection!

With over 4,500 confirmed exoplanets and 7,000 candidates, it is inevitable to ask yourself: are we alone in the Universe?

Camila de Sá Freitas, columnist of TechWorld, holds a bachelor’s and master’s degree in astronomy. She is currently a PhD student at the European Southern Observatory (Germany). Self-styled Galaxy Examiner, he investigates evolutionary scenarios for galaxies and possible changes in star-making. It is present on social networks as @astronomacamila

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