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Exoplanets - What are they and how do we find them?

Updated: Jul 18, 2023

Exoplanets stands for extrasolar planets, as in outside of the Solar system. These planets are becoming increasingly common discoveries since the first was found back in 1992 where not one, but two exoplanets were discovered around a pulsar, a particularly interesting form of neutron star. The two exoplanets were three and four times the mass of the Earth. Since that first discovery, there has been a burst of discoveries of 5,338 exoplanets that are currently confirmed with more than 9,000 candidates on the way at this time of writing.

These planets are incredible and can have truly crazy characteristics. Such amazing planets include:

  • Kepler-16b - a planet that orbits two stars. This planet orbits far out from two stars and is in a stable orbit. This planet is a real-life Tatooine though the planet may not be desert-like, it definitely would have two stars in the sky.

  • 51 Pegasi b - a plant that is half the mass of Jupiter, but orbits so close and fast to its parent star that it orbits completely every four days!

  • HD 189773b - This planet has incredible winds which whip around the planet at 8,700 km/h, these winds push glass sideways which can be thought of as glass gales blasting around. The blue color comes from reflection on silicate which is made from the 1300 °C temperature producing grains of glass from heating.

  • HR 5183b - the whiplash planet, this planet has a very elliptical orbit where it goes all the way towards the edge of it’s solar system and then comes back towards its host star and speeds around close to the star and is slingshot back out again.

  • TrES-2b - The darkest planet discovered to date, this planet is so dark it reflects very, very little light and appears darker than a piece of coal. It reflects less than 1% of light that lands upon it.

So exoplanets exist and we have discovered many of them, but how do we find them? There are many ways to detect an exoplanet and they are particularly interesting, so let’s dig into what these methods are. Below these are shown with some help from the European Space Agency (ESA).

Transit Photometry - planet eclipse

The idea of transit photometry is pretty simple. You get a telescope that looks at a star and its output for a long time, then if there is a planet that goes between us on Earth and the star, there will be a small drop in the light output. It would not be big, especially if the planet is smaller, but with advances in technology it would be detectable.

What can we tell from this method the size of the planet and their orbital periods, well sometimes it will be possible to tell the atmospheric composition as some of the light from the parent star will pass through the planets atmosphere. This means that scientists are able to tell what elements are in the atmosphere by how it impacts the light coming from the star when the planet is in front of the star.

Radial Velocity - star shaking

This method means that the planet is orbiting around the star and needs enough gravity to pull the star back and forward a little. To wobble it. If it can do that, then it will cause blue-shifting and red-shifting of the light that comes from it. In this way, we can tell how much gravitational influence the planet has on the star.

What we can tell from this method is the planet's mass and to some degree it’s orbit, if it is more circular or more elliptical. Image Credit: Las Cumbres Observatory

Transit timing variation - double planet wiggles

In this method, astronomers are able to determine when there is more than one planet in a system since the multiple planets will interact to change the wobble that we detect from the star. Basically, the biggest planet will cause a drop in the light coming from the star by being between the star and the telescope and then the second planet will cause a wobble in that drop in light, because that second planet causes a change in the timing of the orbit of the first and usually bigger planet.

This method is particularly useful for finding planets around stars that give off periodic emissions such as pulsars or finding multiple planets in a given system that impact timing. So finding many exoplanets all at once!

Microlensing - gravity and light

When a star is directly behind an exoplanet and in the right position, micro-lensing might occur, this is where the light from the star is bent around the planet due to the gravity of the planet. The planet acts as focussing lens for the light making it temporary brighter for Earth to see and telescopes and detectors pick this up.

This method is particularly useful for finding the most distant exoplanets and this method can find planets thousands of light years away. This method can tell about mass of the planet and distance from the star itself.

Image Credit: NASA / ESA / K. Sahu / STScI

Astrometry - sky wobbles

With this method, precise measurements are taken of the stars location in the sky. If there is an unknown planet, then it could cause a wobble and different movements around the sky. If the star is measured precisely, then this movement can be detected with very careful observation over a long period of time.

This method is quite sensitive and gives an accurate estimate for the mass of the planet rather than just a minimum of possibility.

Direct imaging - take a picture

This is the easiest method to understand and it is simply a matter of taking a snapshot of a planet which often looks bright in the infrared range. This is because the planet reflects the light of the star off its atmosphere and if it is bright enough, then an image can be taken. This method is quite distance limited and does not work on distant exoplanets, but only those close enough to be resolved.

Image credit: NASA/JPL-Caltech/Palomar Observatory

The above methods are the interesting ways we can find exciting and new exoplanets and lead to discoveries about the fascinating nature of the universe through planetary exploration.

The more we know about different planets and systems, then the more we appreciate our special little solar system with our eight fantastic planets and Earth sitting in the perfect place in the Solar system.

Finally for those interested in a bit of light reading about space and all its wonderful parts and puzzles, please consider picking up my ebook in an easy to access pdf format (link here). Thanks for considering it.

Thanks for reading.

Cheers and stay curious

Oliver - The Teaching Astrophysicist

(Note: This blog post was NOT generated by AI and is conceived, typed and uploaded by a real person.)

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