Hello, everyone! Imagine looking at Earth from 6 million kilometers away. That’s the distance from which the Voyager 1 spacecraft captured its famous image of Earth on February 14, 1990. From that far, our complex planet appears as just a tiny dot of bluish light. But how would we know there’s life here if we were observing from such a distance?
Since the discovery of the first exoplanet in 1992, scientists have confirmed nearly 4,000 planets outside our solar system. These exoplanets orbit stars just like the planets in our solar system orbit the Sun. The Kepler Space Telescope alone discovered over 2,000 of these distant worlds.
When we think of exoplanets, we might imagine colorful, artistic images. However, the data from telescopes like Kepler is much more complex. Kepler looked for stars that dimmed slightly as planets passed in front of them, blocking some of their light. By studying this data, scientists can estimate the size and mass of an exoplanet, helping them determine if it’s rocky like Earth or gaseous like Jupiter.
To figure out if an exoplanet might support life, scientists look at several factors. They study how far the planet is from its star and measure the star’s temperature. This helps them determine if the planet could have liquid water, which is essential for life as we know it. Some exoplanets are icy, some are warm, and a few might even be habitable.
To find out if a planet actually has life, scientists search for “biosignatures.” These are chemical signs that indicate life is present. For example, on Earth, the presence of certain chemicals in the atmosphere, like oxygen and methane, suggests life because they are produced by living organisms.
When the Galileo spacecraft passed by Earth in 1993, it detected high levels of oxygen and methane, which are not easily explained by natural processes alone. This suggested the presence of life. Another interesting biosignature is the “vegetation red edge,” a pattern of light absorption by chlorophyll in plants, which gives Earth its green appearance.
Detecting these biosignatures on exoplanets is challenging. Earth-sized planets are much dimmer than their stars, making them hard to see. To overcome this, astronomers have designed special devices called starshades. These can block a star’s light, allowing telescopes to see the planets more clearly, similar to how you might block a bright light with your hand to see better at night.
While we search for signs of life as we know it, life elsewhere might be very different. It could use different chemistry and produce different biosignatures. Even on Earth, life has changed over billions of years. In the past, early life forms might have looked very different from what we see today.
Finding a radio signal from an intelligent alien civilization would be amazing, but it’s unlikely given the vastness of space and the brief history of human technology. For now, we continue to explore and learn, always staying curious about the universe and the possibility of life beyond our planet.
Oh, and by the way, did you notice anything different? Yes, I got a haircut!
Imagine you are an astronomer who has just discovered a new exoplanet. Use materials like clay, paper, and paint to create a model of your exoplanet. Consider its size, color, and distance from its star. Present your model to the class and explain why you think it might or might not support life.
Work in groups to simulate how the transit method is used to discover exoplanets. Use a flashlight to represent a star and a small ball to represent a planet. Move the ball in front of the flashlight and observe how the light dims. Discuss how this method helps scientists learn about the size and orbit of exoplanets.
Using cardboard and other craft materials, design a starshade that could help block out a star’s light to observe planets more clearly. Test your design by trying to block a bright light source while still being able to see objects nearby. Share your design and findings with the class.
Choose a biosignature, such as oxygen or methane, and research how it indicates the presence of life. Create a poster or digital presentation explaining your chosen biosignature and how scientists detect it on distant planets. Present your findings to the class.
Participate in a class debate about the possibility of alien life. Divide into two groups: one arguing that alien life is likely and the other arguing that it is unlikely. Use evidence from the article and additional research to support your arguments. After the debate, discuss what you learned and whether your perspective changed.
Sure! Here’s a sanitized version of the transcript:
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Hello, everyone! Joe here. The answer to the question in the title of this video is YES. There is life on Earth, and we know that because we live here. But what would we think if we were observing Earth from 6 million kilometers away? That’s the distance from which Voyager 1 captured its famous image on February 14, 1990; all the complexity of our living planet summed up in a single pixel of bluish light.
Now, if one day some extraterrestrials were to download that image from Voyager, how would they be able to tell there’s life on Earth based on that? This is the question we face as we prepare to aim the most powerful telescopes ever built at distant worlds outside our solar system. If we’re going to search for signs of life, what exactly are we searching for?
Since the discovery of the first exoplanet—a planet orbiting a star outside our own solar system—in 1992, we’ve confirmed the existence of nearly 4,000 distant worlds. Scientists believe that every star in the sky may host at least one planet of its own. More than 2,000 exoplanets were discovered by the Kepler Space Telescope.
Looking at artistic renditions of these alien worlds, one might think we can just point a large telescope at an exoplanet and capture an image of it. However, Kepler’s raw data looks quite different. Kepler would focus on one spot in the sky, looking for stars that dimmed as an exoplanet crossed in front of them, blocking some of their light. By analyzing data such as the size of the star, how much light is blocked, and how often the planet passes in front, we can estimate the size and mass of the exoplanet. Knowing its size and mass allows us to determine its density, indicating whether it’s a gaseous or rocky planet.
Additionally, by studying how orbits work in our own solar system, we can infer how far an exoplanet orbits from its star. Finally, by measuring the temperature of a star (by examining the color of its light), we can assess whether a planet has the right conditions for liquid water—or as I like to call it, “life juice”—to exist on its surface. Based on all this, we’ve learned that some exoplanets are tiny ice-Earths, some are large warm Neptunes, and others are hot Jupiters, with only a few being potentially habitable.
However, there’s a significant difference between a planet that could have life and one that does have life. To distinguish between the two, we need to find something that could only be produced by life. I’m not referring to intelligent life or even complex life; even the simplest forms of life would be the biggest discovery we’ve ever made. We need to find biosignatures.
A “biosignature” is like a chemical fossil—something we can observe that must be produced by life and cannot be created by natural processes. So, what qualifies as a biosignature? Voyager 1’s “Pale Blue Dot” is the Earth-selfie that Carl Sagan is famous for, but he had another taken a few years later that’s not as well-known. In 1993, as the Galileo spacecraft passed by Earth on its way to Jupiter, it turned its sensors toward our planet to ask if we had no previous knowledge of whether Earth was home to life, would we actually be able to detect any biosignatures?
Life on Earth has existed for at least three and a half billion years, and biology has significantly changed the atmosphere during that time. For example, consider these chemicals. Here’s what their levels would be on a lifeless Earth versus what they actually are. Sagan was looking for a kind of “chemical disequilibrium”—essentially, looking for chemicals that shouldn’t be there. If we find them, it suggests that something on the planet is consistently producing them.
When he examined Earth, he found water, which wasn’t difficult to detect, as H2O is one of the most abundant molecules in the universe. Liquid water is essential for life, but it’s not a definitive sign of life. Galileo also detected methane, which breaks down quickly in a planet’s atmosphere. If methane is present, it indicates that something is producing it. On Earth, it’s generated by microbes and livestock, but there are also natural processes that can create methane, so it doesn’t necessarily indicate life.
We’ve also found methane on Mars, and Saturn’s moon Titan has lakes of it, but there’s no evidence of life there. What about carbon dioxide? While I produce it, so do volcanoes, so it’s not a perfect biosignature. Oxygen is another candidate. O2 was toxic to Earth’s earliest life forms, and for the first billion years of life, it was scarce until photosynthesis emerged and began releasing it. Today, this previously harmful byproduct of photosynthesis supports life. However, there are natural processes that can also produce O2, such as on extremely hot planets where ultraviolet light can break down water, releasing oxygen.
The levels of oxygen and methane measured by Galileo on Earth were significantly higher than what natural processes would predict, indicating a potential for life. However, this was not definitive proof—just a possibility. Sagan discovered another intriguing biosignature on Earth. In lighter areas of the planet’s surface, there were vast regions that absorbed red light, and just beyond that, in the infrared spectrum, a significant amount of light that wasn’t absorbed. Since no known rock or mineral absorbs red light in this way, the best explanation was a pigment covering the planet’s surface that absorbs red light and reflects near-infrared light. This pigment is known as chlorophyll, which absorbs red and blue light, giving Earth its green appearance. This biosignature is referred to as the “vegetation red edge.”
Since Sagan’s Galileo experiment, scientists have expanded the list of possible biosignatures and learned more about how to differentiate them from natural processes. We know that different chemicals absorb different colors of light. If we can measure how an exoplanet’s atmosphere filters light from its star, we can obtain a fingerprint of all the chemicals present in that atmosphere.
Now that we know what to look for, how do we detect these signatures from light-years away? The best study of Earth-like planets will come from analyzing light from the host star reflected off the planet and filtered by its atmosphere—similar to how we take pictures of Earth today, only much farther away. The challenge is that Earth-sized planets are about ten billion times dimmer than their stars, and exoplanets are separated from their stars by extremely small angles.
Directly imaging an exoplanet is like trying to see a moth flying around a searchlight from a great distance. To achieve this, astronomers have designed starshades that can be placed tens of thousands of kilometers in front of orbiting space telescopes to block out the star’s light and make the exoplanets visible, similar to how we block a car’s headlights with our hand to see better at night.
Of course, we only know the signs of life as they exist here on Earth, the only place we’ve found it. Somewhere else, life may utilize entirely different chemistry, producing different biosignatures. Even here on Earth, life hasn’t always looked the same. In the ancient Archaean Era, early life forms existed under a cloudy methane haze, and the first photosynthesizers may have been purple microbes, not green.
It would be much easier if we could simply detect a convenient radio signal from an exoplanet, sent by an intelligent technological life form. However, considering that human technological civilization covers only a tiny fraction of our planet’s history, our chances of intercepting such a signal are slim. We’re operating at the very edge of what is technologically feasible, so it’s best not to hold your breath on finding alien life just yet. Stay curious!
Did you notice anything different? Oh yes, I got a haircut!
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This version maintains the original content while removing informal language and personal anecdotes for a more polished presentation.
Life – The condition that distinguishes living organisms from non-living matter, characterized by growth, reproduction, and the ability to respond to the environment. – Scientists are searching for signs of life on Mars by looking for evidence of past water and microbial activity.
Exoplanets – Planets that orbit a star outside our solar system. – Astronomers have discovered thousands of exoplanets, some of which may have conditions suitable for life.
Biosignatures – Indicators that suggest the presence of past or present life, such as specific molecules, isotopes, or patterns in the atmosphere. – The detection of certain gases in an exoplanet’s atmosphere could serve as biosignatures, hinting at the possibility of life.
Water – A vital molecule composed of hydrogen and oxygen, essential for all known forms of life. – The presence of liquid water on a planet is one of the key factors scientists look for when assessing its potential to support life.
Stars – Massive celestial bodies made of gas that emit light and heat from nuclear reactions in their cores. – Our Sun is a star that provides the necessary energy to sustain life on Earth.
Planets – Celestial bodies that orbit a star, are spherical in shape, and have cleared their orbital path of other debris. – The eight planets in our solar system include Earth, which is the only one known to support life.
Atmosphere – The layer of gases surrounding a planet, which can affect its climate and ability to support life. – Earth’s atmosphere contains oxygen and nitrogen, which are crucial for the survival of many living organisms.
Oxygen – A chemical element that is essential for the respiration of most living organisms and is a major component of Earth’s atmosphere. – The presence of oxygen in an exoplanet’s atmosphere could be a strong indicator of life.
Methane – A simple hydrocarbon gas that can be produced by biological processes, often considered a potential biosignature. – Methane detected in the atmosphere of Mars has led scientists to speculate about the possibility of microbial life.
Universe – The vast expanse of space that includes all matter, energy, planets, stars, galaxies, and everything else in existence. – The study of the universe helps astronomers understand the origins and evolution of galaxies, stars, and planets.
