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How are Exoplanets found?

Discovering Exoplanets

Exoplanets are planets outside our Solar System and the search for them is a very young one, since the first ever discovery of an exoplanet made in 1995, whene Michel Mayor and Didier Queloz discovered 51 Pegasi b. But how are exoplanets found? Almost every light spot we see on the night sky is, mainly, a star which might have planets orbiting around it. Other light spots are, appart from the Sun and the Moon, some planets of our own Solar System, since only Venus, Mars, Jupiter and Saturn are visible at the naked eye. So, if we mainly see other “Suns”, how can we detect planets surrounding them which must be dozens of times smaller? Here is where exoplanet detection techniques play a role. Let’s get started.

Have a look our previous post about the nearest exoplanet found!

Radial Velocity Technique

We might think that the centre of mass of a planetary system is the main star, but nothing further than reality. In fact, due to our friend Gravity (yes, you better have Gravity as a friend since is present everywhere), the main Star movement will be influenced by the planets orbting it, and thanks to this influence we will be able to detect whether there are planets around a Star or not.

“two bodies attract each other with a force which is a function of their respective masses and the distance between them”.
Newton’s 3rd Law

When a Star is influenced by the planets orbiting it, that Star will wobble back and forward, therefore a method capable of measure the velocity of a Star with respect to the Earth will allow us to deduce wheather there are planets surrounding it. Makes sense right? So this is what Radial Velocity Tehnique does, measure the radial velocity to a Star in order to determine the presence of planets around it by measuring ita velocity changes. In fact, it was by measuring Star wobble movements how 51 Pegasi b was discovered.

Astrometry

This is a method based in measuring the movement of a Star by means of precise measurements of its position in the sky. This method also allows to detect wobbles in the Star, what means that there are planets orbiting it. By the moment, only one exoplanet has been discovered with this method since it requires an extremely difficult-to-achieve precision to detect a Star holding orbiting planets, although it is also used to make tracking observations of planets that have been previously detected by other methods. European Space Agency’s GAIA mission aims to detect dozens of thousands of exoxplanets up to 1600 light years away from our Sun and has as an objective trace a map of our Galaxy.

Direct Imaging

Photographing a planet might seem complicated since the brightness of the Star it orbites will be much more powerful. However, in certain conditions, young big planets far from their host Star are possible to image. This is how also in 2004 the first directly imaged exoplanet was discovered.

Transit of Planets

Basically consists in measuring a Star brightness to detect possible exoplanets orbiting it. The basic idea is that whenever a planet transits its “Sun”, this Star will change our measured brightness, simmilar to measuring our Sun brightness and detect an airpline passing in front of it since at that moment the Sun’s brightness will decrease. Several missions have discovered many exoplanets with this technique, such as the 2006 mission CoRoT, and 2009 mission Kepler.

Gravity Microlensing

Remember Gravity, our friend, because it stills play a key role. Gravity Microlensing relays on the fact that gravity is also able to bend light, therefore whenever there is a Star passing in front of another Star, the brightness of the latter will be amplified since the former acts as a “lens” and, if the star in between hosts a planet, then the amplified light of the Star behind will increase faster rather than smoother. The first exoplanet discovery by this technique was made in 2004.

Thank you for reading!


REFERENCES

NASA. 5 Ways to Find a Planet. https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#
ESA. Exoplanet Detection Methods. https://sci.esa.int/web/exoplanets/-/60655-detection-methods
BBC. Nobel de Física: cómo es 51 Pegasi b, el primer exoplaneta descubierto en la historia por el cual Michel Mayor y Didier Queloz ganaron el premio. https://www.bbc.com/mundo/noticias-49975318