Gotta find them all!!

Knowing about all the planets in our own solar system made us wonder if there are other planetary systems out there as well. And there are!! But how did we detect these extrasolar planets?

There are four ways to detect extrasolar planets. The simplest way, but not always the easiest way, is direct observation. That’s exactly what it sounds like: using telescopes to look for planets and observe the spectra of those planets. Only a few planets have been detected using direct observation, since these faraway planets may not be able to be seen by using the telescopes we have now.

We can also use the astrometric method, which involves observing a star and seeing whether it changes position over time. Recall that a planet and its star are both involved in the orbit around their center of mass (even though it doesn’t look like that because the star is usually much more massive than the planet so the center of mass is actually inside of the planet!). From this, we can conclude that if a star wobbles, there must be a force acting on it, which is the force from a PLANET orbiting! This method is only really useful for discovering large planets that are far from their star.

Another method is to use spectra and see if there is Doppler shifts in the star’s spectra. We can use that to figure out if there are planets orbiting the star. This method is great for finding large planets with smaller orbits.

Our final, and my favorite method, is the transit method. This method can only be used if a planet is edge-on when viewed from Earth, and is awesome for finding small planets!! We observe whether the star gets dimmer in regular time intervals (this can show that a planet is moving in front of the planet, thus blocking out some light) and then conclude the existence of that planet with follow-up observations using other methods. The star’s brightness can be graphed, and seeing recurring curves shows that it’s the same planet moving in front of the star.

As a planet travels in front of its star, we observe the star get a little dimmer. (image credit: Roen Kelly)

Today, over 5,000 exoplanets have been discovered, and so many potential exoplanets are still being observed! Learning about other planetary systems is really cool, because we can see similarities between our solar system and other ones, as well as learn about new properties.