Spots, Rocks, and Goldilocks: How to Find an Earth-Like Exoplanet

Did you know that every star in the night sky is a sun, like our sun, and that each one likely has planets orbiting with it? Imagine places like Pandora from the movie Avatar, with floating mountains, glow-in-the-dark forests, and people as tall as trees—these might exist!

Rebecca from Reno
Our Solar System

By the time I was eight years old, I dreamed of becoming an astronomer—a scientist that studies planets and stars. I had all nine planets in our universe memorized—yes, there used to be nine—and I knew their names, their order of appearance, and their sizes. I even chose a favorite planet, Saturn, because I thought its rings were neat. Now, as an adult, I advocate for outer space exploration, and one of my jobs is to educate the public about the benefits and joys of the cosmos.

Exoplanets are planets outside our solar system

Today I want you to know that there are many planets in space that are just like Earth or better, and I am going to teach you the basics for how to identify an Earth-like exoplanet, which is a planet that resides outside of our solar system. For this, you will want to remember three things: spots, rocks, and Goldilocks. I’ll explain.

Spots travelling across stars may be planets/ Image NASA

What I mean by spots is that you need a fancy outer space camera that looks for exoplanets, like NASA’s Transiting Exoplanet Survey Satellite, or TESS. This camera fixates on stars in our galaxy and waits to see if any spots transit in front of the star, or sun. If a planet is orbiting a host star, it will create a dip in luminosity between that sun and the camera, forming an eclipse shadow as it passes by. If this spot keeps coming back, reappearing in the same line of travel, we consider it a potential exoplanet candidate. From this information, we can also determine the size of the planet and the duration of its year.

Rebecca From Reno
TESS: Transiting Exoplanet Survey Satellite / Image NASA

Once we find spots, we use special calculations to determine if the planet is made from gas, ice, or rocks. If it is rocky, we look at the third requirement for a possible Earth-like planet: The Goldilocks zone. If you remember the story of Goldilocks, she was a little girl who got lost in the forest and chanced upon the house of the Three Bears. The bears had gone to market and Goldilocks went inside looking for food. She saw the table had three bowls: Papa Bear’s food was much too hot, Mama Bear’s food was much too cold, but Baby Bear’s food was “just right,” and Goldilocks ate it all before taking a nap.

The moral of the story is that Earth is in this “Goldilocks Zone,” also known as the Habitable Zone—we are not too close to our sun that the surface water would boil and evaporate, and not too far from the sun that our surface water would freeze and we would die from temperatures four times colder than in Antarctica, which is the coldest place on our planet. Earth is in the sweet spot of our solar system, where organic life exists because temperatures are just right

Earth is in the “Goldilocks Zone”

And now you know that there are more than nine planets in our universe! In fact, scientists estimate that for every 200 stars you see, one of them is host to a rocky, Earth-like exoplanet in the Goldilocks Zone. Why should you care about this? Maybe you don’t even like outer space. I understand. However, in the words of one of the wisest men of our century: to confine your attention to terrestrial matters would be to limit your human spirit—Dr. Stephen Hawking. If there are 10 billion planets like Earth in your night sky, then space truly is the biggest thing your mind can imagine.

 To find out more on exoplanets, visit

Rebecca Schembri
Rebecca Schembri is a Harvard University graduate student concentrating in space diplomacy. She is from Reno, Nevada, USA.

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