by Hudson

exoplanets

An exoplanet is quite simply any planet outside of our solar system. Most of these are found orbiting a star similar to how the Earth orbits the Sun. However, some planets, known as rogue planets, are not gravitationally bound to any stars, and instead exist outside of any solar systems. To date, we have observed the existence of over five thousand exoplanets, and that's just a small fraction of the estimated billions that are out there!


Exoplanets are discovered through a couple different, surprisingly complex, methods: direct imaging, the transit method, and Doppler shifts.

direct imaging

You might guess that we can just see exoplanets, like how we can use a telescope to look out at the stars. And while it is possible, less than a hundred exoplanets have been found from this method. The process involves examining infrared light coming from a star, or rather just next to a star. Since the brightness of stars often blocks out any hope of seeing planets next to them, astronomers use a coronagraph, which blocks out the direct light from the star to reveal the faint shine of the planets orbiting it. This method actually proves to be the most valuable when it comes to measuring the composition of the exoplanet’s atmosphere, though opportunities to use this method are rare since more often than not stars are just too bright to see any planets orbiting them.

Direct Imaging CGI Simulation Created by NASA. Source credit : WFIRST coronagraph: Imaging Giant Exoplanets Around Nearby Stars - NASA Science

The Transit method:

The much more common way that we look for and study exoplanets is by looking at the brightness of the stars the planets orbit. When a celestial body of notable size, in this case an exoplanet, passes in front of a star, the star gets measurably dimmer. This change in brightness is usually quite small though, normally on the order of 1/10000th of the star’s standard brightness. From this dip in the light, scientists can determine the size, orbit distance and period, and atmospheric composition (by studying the effects of the planet’s atmosphere on the dimming of the star’s light).

Star brightness varying as the orbiting planet passes in front of it (Image credit: Graph Created by NASA).

This method works best for larger planets that are close to their star, as this causes the greatest change in apparent brightness. These planets are often referred to as Hot Jupiters, which comes from them being large gas giants like Jupiter, and close to the star (“hot”). For smaller planets that are farther away from the star it can be a greater challenge to get precise measurements. Additionally, we are only able to see dimming if the orbital plane for the planets in the system around a star is aligned with our point of view. As in there could be many more exoplanets out there that we cant see because they simply never cross in front of the star they orbit from our perspective.

The Doppler Shift Method:

The final method involves looking at how a star moves due to the existence of a planet orbiting it. All objects with mass exert a gravitational pull, thus just as the star pulls the planets, the planets also pull the star. This means that the star actually orbits around the center of mass of the solar system it lies in!

This orbit is usually quite small however, and not measurable directly. Instead, we can see this by looking at the spectra of light coming from the star, and how it is affected by the star’s movement through the Doppler effect. The Doppler effect describes how when the source of an electromagnetic wave, such as light from a star, moves towards an observer the light shifts more towards the blue spectrum (a blueshift), and when the source is moving away the light shifts towards the red spectrum (a redshift). So, as the star is pulled by the planet its light periodically gets redshifted and blueshifted, which tells us that there indeed is a planet orbiting that star! Unfortunately, this method suffers from similar limitations to the transit method, meaning that hot Jupiters are still the easiest to spot, but the method remains a powerful tool in discovering exoplanets regardless.

Doppler Shifts impacting the observed spectra of a star with an orbiting exoplanet (doppler shift)
(image credit: Created by the James Webb Team at NASA, ESA, and Leah Hustak.)

Studying exoplanets helps us understand the different ways in which planets form and evolve, and can even help us understand our own planet’s past. We can also learn a lot about what conditions might be right for other lifeforms to exist on other planets. Currently, the existence of life outside Earth remains a mystery, but studying exoplanets is one of the foundational pillars to knowing who else might be out there.

Going forward, NASA’s Nancy Grace Roman Space Telescope will be the next big project to help advance exoplanet study, it aims to launch by May 2027 and will shine new light on dark energy and dark matter, but will also look for the existence and composition of unseen exoplanets. The world of exoplanet study still has a lot of room for growth, and there is much that we still do not know and much to learn!

Sources