In the Lecture Hall: The Transit Method of Exoplanet Discovery
“Astronomers make rare exoplanet discovery, and a giant leap in detecting Earth-like bodies.”
The text on my phone screen read.
I swiped through the article, trying my best to understand the sentences.
With my phone in hand, I made my way to Hall 407. It was shrouded in darkness. I stretched my hand out along the wall to my left. My fingers grazed the cool surface until I found the familiar plastic switch plate.
Three snappy clicks and a soft hum. The room filled with bright white light, revealing the rows of benches arranged in a semicircle around the long center wooden table.
I walked to the first bench of the center column and placed my backpack on one end. I leaned against the desk and opened the Wikipedia page of Exoplanets.
I was reading the section about discovery techniques when the professor walked in, his stride unhurried and assured. Behind him, the scholar almost bounded into the room with his light and quick steps.
“Hello!” I called out, putting my phone in my pocket.
“Hello.” the professor said, as he walked towards the high-backed chair kept across the table, in front of the 6 large whiteboards that hung on the wall.
“Hello there!” the scholar chimed in, with his bright voice. He dragged a wooden stool towards the left edge of the table and sat on it. “So, what are we discussing today?”
“Well, I was reading an article about the recent discovery of a new exoplanet. I, of course, couldn’t understand much of it,” I chuckled. “So here’s my question. How do we even detect planets outside our solar system? Aren’t they too small and too far away?”
“That’s a very good question,” the professor said, as he settled into the chair. Its plush leather cushioned him, giving a sense of timeless comfort.
Question — What is the transit method of exoplanet discovery?
The professor glanced at the scholar and said, “Why don’t you begin?” His voice carried a calm authority that filled the hall.
“Sure!” the scholar said and focused on me. “You are right. A planet in another star-system is too small and too far away. But, we humans are clever. We have come up with multiple methods to discover and confirm the presence of a planet around a star.
Let’s go over them one by one.”
“Why don’t we just cover one of them today?” the professor interjected. “The concepts are complex.”
“That’s true. Yes, let’s discuss just one,” the scholar thought for a few moments and continued, “Let’s talk about the transit method.”
He looked at the professor and received a nod of approval.
He pointed up and said, “Let’s consider a tube light.”
I tilted my head back and squinted against the harsh glare of the tube lights. They stretched across the room, making the ceiling hum with a quiet buzz of electricity.
“Consider an insect flying in front of it. Will you be able to see it?”
I shook my head.
“Now, let’s take a sensitive detector and measure the light’s intensity in different intervals of time.”
“How do you measure the intensity?”
“By measuring the number of photons striking the detector.”
I nodded, and he continued, “So now you have collected a table of values. One column has the intensity values, while the other has the time values. Let’s make a plot of intensity versus time. What will it look like?”
The question hung in the air as he got up from his stool and picked two markers from a box on the center table. He walked to the whiteboard behind the professor and drew two thick black lines — one vertical and one horizontal — on the board.
The professor leaned back in his chair. With a gentle push of his legs against the ground, he glided smoothly to the right. The chair’s wheels rolled quietly across the floor.
“I think,” I began slowly, “it will be a straight horizontal line.”
“Why?” the professor asked.
“If there is no major change in the tube light, the intensity will remain the same.”
He smiled.
“Exactly,” the scholar said as he drew a thick blue horizontal line, slightly above the previous one. “Now, when the insect flies in between the detector and the tube, what will happen?”
“There should be a slight dip in intensity,” I said, as I felt my confidence rising slowly.
“That’s correct,” he wiped a small section of the blue line and drew a small dent. “So, even though we can’t look at the insect directly, we can infer its presence from the changes in the light’s intensity.”
I nodded along, eager to hear his next words.
“Now,” his eyes darted towards me as he turned around, “what will it mean if the dips start to appear in regular intervals?”
“The insect is flying in front of the detector in regular intervals.”
“Good! Okay, next question. If, somehow, the insect was tied to the tube light, what kind of motion would display that pattern? Note that the motion should be easy for the insect.”
The silence stretched out as my mind sifted through different possibilities. The professor leaned forward slightly in his chair, his hands rested calmly on the table. His eyes, sharp yet warm, met mine. The scholar’s eager gaze fixed on me.
With the answer still uncertain in my mind, I replied, “The insect will be flying around the tube light. In an orbit.”
The professor’s eyes lit up with a glimmer of approval. He gave a slow affirming nod. “Exactly right,” he said.
The scholar broke a broad grin. “Yes!” he exclaimed. “This is exactly how we discover exoplanets. We monitor the intensity of a star over a period of time. If there’s a planet orbiting the star, there will be tiny periodic dips in this plot here.”
A sudden clarity washed over me. As the fog of complexity dispelled away, a genuine smile spread across my face.
“And,” the scholar continued, “we can derive multiple important parameters about the system.
For example, the depth of the dip tells us the size of the planet in comparison to the star. The periodicity of the dips informs us about the duration that the planet takes to complete an orbit.”
“That’s such a clever method to obtain all this information,” I remarked.
“Exactly,” the professor said. “This method is called the transit method, since the planet transits in front our view of the star. It is also the most widely used procedure to detect exoplanets.”
“Wow.”
“So, we can know about a planet’s size using this method. What about its mass? Is it not an important parameter?” I asked.
“It is, it is,” the professor said as he got up from his chair. “Why don’t we discuss the next method tomorrow?” his tone indicated a genuine eagerness to continue our exploration of the subject.
“Sure!” I took my backpack from the bench and swung it on my shoulder. The professor and the scholar began walking towards the door as they exchanged a few words.
I followed them out of the hall, turning off the lights on my way.
With a final wave, we parted ways.
Hardik Medhi
Stay Curious
