Planet Hunt - The Search for Exoplanets

An artist's rendering of the pulsar Lich and its three discovered planets, two of which were the first ever confirmed exoplanets. Image Credit: NASA/JPL-Caltech

For as long as humans have been telling stories, we’ve dreamed of worlds beyond our own. Today those worlds are no longer mere stories. We live in a golden age of exoplanet discovery.

What is an Exoplanet?

An exoplanet is any planet that exists beyond our Solar System, and since the 1990s, we’ve confirmed over 6,000 of them. It’s safe to assume that planets would exist around other stars. After all, there’s nothing special about the Sun. It’s a very average star. But we didn’t have actual proof that exoplanets existed until 1992, when we discovered two of them around a pulsar called PSR B1257+12, also known as Lich. Current evidence suggests that there is, on average, at least one planet around every star.

The first two exoplanets discovered, named Poltergeist and Phobetor, were detected based on the way their gravity affected the timing of Lich’s pulses. A pulsar is a type of star that rotates at extreme speeds, making it seem to pulse like a lighthouse. These pulses have extremely regular timing, so when we noticed the timing of Lich’s pulses was slightly off, we knew it had to be because of planets orbiting it. This makes the name Poltergeist perfect for the exoplanet, as poltergeists are ghosts said to cause objects to move.

How To Find an Exoplanet

Since the discovery of Poltergeist and Phobetor, we’ve refined our techniques and gotten even better at detecting these far-off planets. But how do you hunt for exoplanets? You can’t just point a telescope and visually inspect a star to look for any planets, as stars are billions of times brighter than planets. Imagine standing across a football field from a spotlight shining directly in your eyes. In front of that spotlight, someone lights a match. Would you be able to see the light from the match? Similarly, the extremely bright and massive stars will drown out nearly any chance at directly seeing a planet orbiting it. Instead, we usually rely on indirect methods of detection, such as pulsar timing.

Pulsars are relatively rare and we want to find planets around regular stars, so different methods are required. One similar technique is often called the wobble method. As the planets in our Solar System orbit around the Sun, the Sun is also tugged by their gravity. Because of this, the Sun’s barycenter, its center of rotation, is not actually at its center. That means that if you watched the Sun carefully enough over time, it would appear to wobble. If we see other stars wobble, we can deduce it’s because of at least one planet orbiting it. By measuring the magnitude and frequency of the wobble, we can determine how many planets are causing it and how big those planets are.

There’s also the transit method. By carefully watching a star and measuring how much light it puts out, we can detect brief dips in light caused by an object passing in front of it. If this decrease in light output happens periodically, then we can confirm it’s caused by a planet orbiting the star, rather than interstellar debris coincidentally passing by. This is the method favored by the Kepler space telescope.

In addition to those primary methods, there are many others, including direct imaging. While a direct image of an exoplanet is extremely difficult to get, it is possible, assuming that the planet is far away from its host star and is newly formed. A newly formed planet will have heat left over from its violent formation, making it easier to detect.

A direct image of an exoplanet orbiting another star. The exoplanet, PDS 70c, is the dot on the right of the center. The ring is the protoplanetary disc around the star that the planet formed from. Image Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al. (CCA 4.0)

Type of Exoplanets

One of the most common types of planets we find is the hot Jupiter. A hot Jupiter is a large gas giant planet, similar to Jupiter, that orbits closer to its parent star than Earth does to the Sun. They orbit so close that they can often complete an entire orbit in just a few days. While we find these sorts of planets more often than others, that doesn’t mean that they’re the most common type of planet in the galaxy. It’s rather that they’re far easier to find than other types of planets. Being both so large and so close to their star means they have a greater impact on the star, making it easier for them to be found through most of the methods we employ. In reality, a hot Jupiter is likely to be a relatively rare type of planet, as gas giants usually form further away from a star, while smaller rocky planets form closer. The hot Jupiters we see likely formed further away but then migrated closer due to outside influences.

Another type of exoplanet we find a lot of are the so-called super-Earths. While Earth is the largest rocky planet in our Solar System, in others we find terrestrial planets many times the size of Earth. Once again, the reason these seem so common is that their relative size and proximity to a star makes them easier to spot than the true Earth-like planets. The term super-Earth refers purely to the size of the planet and is not meant to imply that it is in any way habitable. If anything, a super-Earth would be harder to live on for Earth-like life, due to extreme surface gravity.

Exo-Earth

Every once in a while, we do find a planet that could potentially be habitable; one that’s similar in size and composition to Earth, while also being within its star’s “Goldilocks zone,” a region at just the right distance from a star so as not to be too hot or too cold to support liquid water on the surface. By using tools such as the James Webb Space Telescope’s spectrograph, we can even get an idea at a distance what kind of elements make up an exoplanet atmosphere. Currently, the exoplanets Kepler-62f, Kepler-186f, and Kepler-442b are considered to be the most likely candidates for habitable exoplanets. They are located at 1000, 490, and 1120 light-years away, respectively.

The hunt for exoplanets could be an important next step for humanity. Perhaps we’ll find a second home out there. Or maybe we’ll detect a planet with the first definitive signs of extraterrestrial life. As we refine our existing methods and create new ones, we’ll continue to discover more and more worlds beyond our own. Who knows what wonders they might hold.