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A few days ago, there was a particular space news story that was on everyone’s lips: a seventeen-year-old intern at NASA had discovered an exoplanet. On the third day of his internship!

The teenager, Wolf Cukier, enjoyed internet fame as well as local fame that was widespread. After all, how often does one discover a planet on the third day of one’s internship? People were fascinated by his story and were inspired by his passion. Cukier himself was very happy with his discovery, and hopes to continue his pursuit of astronomy, and wishes to research further in the future.

But many of you might be wondering, “How did he discover an exoplanet?” After all, in the grand scheme of things, planets are small balls of rock or gas floating through a boundless expanse, with none of the explosive, energetic fusion of stars, or the constant radio-wave pulses of pulsars, or the distinctive gravitational footprint of black holes. Indeed, looking at even relatively close-by planets like Jupiter need precise timing, as well as the ability to discern the gas giant from the hundreds of bright points that dot the night sky. A planet seems very hard to find, especially if it is hundreds or even thousands of light-years away.

And that, precisely, is what we are going to discuss today.

An exoplanet is any planet that is not in our solar system. In other words, if a planet is found that is not Mercury, Venus, Mars, Jupiter, Saturn, Uranus, or Neptune, it is considered to be an exoplanet.

Some exoplanets are understandably more exciting than others. While most planets are either too close to their stars and are barren desert wastelands, and other planets are too far away and look like Elsa’s castle, some planets are found in that critical “Goldilock’s Zone”. Named after the famous fairytale, the Goldilock’s Zone is the area around any star in which the temperature can enable water to exist in the liquid state.

The first exoplanets were found relatively recently, in 1992, by astronomers Aleksander Wolszczan and Dale Frail. They observed the planets orbiting around the pulsar PSR 1257+12, and the discovery caused a considerable uproar throughout the scientific community. It was the first confirmed sighting of a planet outside the solar system, and it gave credibly once more to theories that there could be life out there.

After the first two were discovered, well, the sky was the limit. The number of exoplanets found jumped from another orbiting a main-sequence star in 1995 to over 715 by 2014. But how were they found in the first place?

The main method of finding exoplanets is known as the Transit Method. About 97% of the planets that have been discovered to this day were discovered by this method.

In order to explain this method, let me create an analogy. Imagine that you are falling asleep to the TV. If you are like some of my friends, you need the TV to be on constantly until you finally nod off. So, imagine that a boring program is playing, and you are getting comfortable listening to the soothing voice of the narrator talking about rocks, or (I hope not) outer space.

Then, suddenly, your reverie is disrupted. The sound dips in volume, and your zen is disturbed. You sit up to find your mother blocking the speaker and glaring at you. “You should sleep without the TV!” she says.

But how did you know that your mother was there in the first place?

“Ah-ha!” you say. “I head the volume reduce because my mother blocked the speakers.”

The same analogy can be applied to other things as well. Whenever there is a projector and someone puts their hand in front of it, one sees the light being blocked and recognize that something is doing the blocking. Whenever there is a stream of water cascading on one’s scalp, a sudden break means that something is blocking the water. 

The discovery of exoplanets works in roughly the same way.

 Image result for exoplanet transit method detection

 Credit: The Planetary Society

As is summarized succinctly in the diagram, the transit method essentially involves watching and continuously monitoring the brightness of the star. When a planet passes in front of the star, the brightness will suddenly dip down.

The same thing is also observed with eclipses. If you did not know that a total solar eclipse was about to happen, and your world went dark in the middle of the day, you might be confused at first, but would then realize that something is blocking the sun. In this case, that something is the moon.

Scientists constantly observe hundreds of thousands of stars, keeping an eye on their brightness. Whenever they notice a routine dip in the brightness of the stars, they know that something is passing in front of the star. The first conclusion is always that there might be an exoplanet, and the brains of the scientific world scramble to point their telescopes there, hoping to catch a glimpse of the famed little rock (or a big ball of gas).

Something that I find fascinating about the transit method is that the nature of the procedure is highly conducive for spectroscopy. Essentially, as scientists are using the simple fact that the planet is passing in front of the star as evidence that there is a planet in the first place, the scientists can then observe the light that is passing through the atmosphere of the planet to determine its composition.

One of the most important elements that a planet needs to have in its atmosphere in order to be a viable candidate to support life is oxygen. Another is nitrogen. Another is carbon.

As I discussed in the “Light” blog post, each element has what is basically its own fingerprint. When light is shined through each element, the unique electron configuration will result in some colors being exemplified. Here are some important elements and their spectra:

 Credit: Lumen Learning

 

As a result, by looking at the light through a big spectrometer, scientists can break it down to its component colors and can tell which element that the atmosphere has in it. As the spectrum of each element is already recorded, scientists simply have to compare their recorded spectrum to existing spectra in order to determine the identity of the element.

In this way, if a world is a sulfur wasteland, scientists won’t have to waste time, money, and energy sending a probe their only to have it return disappointing results. Observing exoplanets using spectroscopy to determine their atmospheric composition is the astronomy counterpart of a company doing background checks on a prospective employee. In both cases, if something fishy turns up, there’s no hope!

More recently, some scientists have been preemptively using single optic technology and have been observing potential star systems with the hope of finding a new world. The transit method, though, works much better and is much easier to read and analyze.

So far, we have discussed the technicality of what is observed to detect exoplanets. But how are these things detected in the first place?

The pioneer of the exoplanet discovery process was the Kepler Space Telescope. In classic NASA fashion, the mission was predicted to last about 3.5 years. It actually lasted 9.5!

Kepler was revolutionary in its simplicity. It contained an energy source, a shield from radiation, a receiver, a transmitter, and a photometer. Compared to the fine-tuned optics and the aligned instruments of the James Webb and Hubble Space Telescopes, the Kepler was more robust and more monochromatic.

Kepler’s only instrument was the photometer, and it had only one job: constantly monitor the light of stars to notice discrepancies or dips. And even though the system was so simple, it enjoyed tremendous success; over its nine-and-a-half years, Kepler was able to successfully detect a stunning 2,600 exoplanets. 

But Kepler still had a big limitation. The nature and simplicity of the spacecraft meant that it could only view one swath of the Milky Way at a time. Admittedly, it was a large swath, but there still wasn’t–pardon the pun–universal coverage of the galaxy.

Then, in April 2018, another satellite called the Transiting Exoplanet Survey Satellite (TESS) was launched. TESS was created with the intention of remedying the shortcomings of Kepler. Essentially, with its novel, highly elliptical orbit and extreme wide-field cameras, TESS is able to map an astounding 85% of the sky, one hemisphere at the time. Here is an example of how it works:

Image result for tess field of view

Credit: TESS|NASA

Cukier was able to discover the exoplanet that was named TOI 1338 b using TESS data. When he was reviewing some data sheets, he noticed a few dips in the brightness of the binary star system that the satellite was observing, and when checking it with his mentor, lo and behold! An exoplanet!

The prevalence and success rate of this method shows how far we have come as a species, as well as how far we have to go. And lucky for us, the sky is the limit.

 …

As we progress further into the future, the survival of the human race will become more and more vital upon our leaving the Earth. As we as a species continue to cause the detriment of the planet, it will become more and more critical that we find a way off this planet. And the study of exoplanets can facilitate just that. 

Of course, the sights are not so dim yet. If we work hard, we can content ourselves with observing these pale, distant, beautiful worlds from a distance. But who knows? One day, they might be our new home.

Clear skies!