Nearby Worlds May Tell Us How Life Might Look in Our Galaxy

In the constellation Aquarius, invisible to the naked eye, lies a star that might change history. Home to seven mysterious planets—each around the size of our own Earth—the TRAPPIST-1 system is regarded by some as the crown jewel of astronomy’s efforts to find life in the Milky Way. With not one, but three worlds orbiting in the so-called habitable zone, where water can flow and life can thrive, TRAPPIST-1 is one of humanity’s best and brightest opportunities to chase the discovery of a lifetime.

More than science is at stake: what we find—or don’t—on these worlds will shape science forever.

What sets TRAPPIST-1 apart is its striking commonality. At the heart of this system is a small, dim star called a red dwarf. Ranging between 8 percent and 57 percent of the mass of our own sun, red dwarfs quietly make up a remarkable 73 percent of all stars in the galaxy, and are suspected to harbor at least . Naturally, this has piqued the curiosity of those who study life in the cosmos—astrobiologists. Could alien life thrive around these small red suns?

The possibility tantalizes the philosopher, but even more so the astronomer: planets around red dwarfs are easier to find than around any other type of star. In fact, the TRAPPIST-1 system was discovered in 2016 with a telescope only two feet across. Because the star is small even by red dwarf standards, its Earth-sized planets stand out easily; when they cross, or transit, the star, they block roughly half a percent of its total light output. For comparison, the Earth only blocks 0.01 percent of our much larger sun’s light when it passes in front of it. In terms of detectability, red dwarfs seem to be the clear winner, and out of 445 red dwarf systems (I asked Jessie Christensen, the scientist who maintains the NASA Exoplanet Archive, what the latest count was), TRAPPIST-1 is one of the brightest that transits, making it a favorite target for astrobiology.

But red dwarfs have a dark side. They are not simply smaller, redder versions of our own well-behaved sun; they are turbulent, active sources of extreme radiation. While Earth experiences violent solar outbursts called coronal mass ejections (CMEs) roughly once every 25 years, a planet that orbits TRAPPIST-1 experiences them weekly. And the bigger the host star, the more powerful the CME. If a planet does not have a strong magnetic field to protect it, a CME can strip away its atmosphere until it is a barren, uninhabitable rock.

In addition, red dwarfs are born hot, and cool over time. This means that a planet may have its water inventory boiled away before it gets the chance to settle into the habitable zone, or that a planet may begin its life habitable before freezing over. Finally, red dwarf planets live very close to their star, and when two things in space orbit close together, one will eventually come to face the other—the way the same side of the moon is always facing the Earth. In the case of TRAPPIST-1’s planets, this means that one hemisphere may experience eternal daytime, and the other, eternal night: perhaps unideal conditions for life to evolve.

Nonetheless, many astrobiologists remain convinced of the adaptability and persistence of life. After all, we are but one data point here on Earth, and perhaps, in the TRAPPIST-1 system and elsewhere, hardy organisms have adapted to extreme conditions. So the question remains: Do red dwarf stars make our galaxy a lush and vibrant garden or a sparse desert?

To answer this question, scientists have enlisted NASA’s flagship space telescope, the JWST. JWST carries with it a variety of cameras and instruments known as spectrographs, and unlike the Hubble Space Telescope, it sees almost entirely in the infrared. This is particularly useful for red dwarf systems, like TRAPPIST-1, because the infrared is actually where they shine the brightest. Astronomers have a particularly clever way of using JWST to study TRAPPIST-1’s planets: they wait for the planet to cross behind the star and use the cameras to tell how much light went missing during this period. The missing light tells them about the way the planet’s surface reflects. A planet without an atmosphere reflects in a very predictable way; any deviation from this pattern indicates that an atmosphere may be present.

Do TRAPPIST-1’s planets have atmospheres? This is what astronomers set out to determine in JWST’s first observing run, called Cycle 1. One team used an infrared camera to examine the innermost planet, TRAPPIST-1b. They concluded that it does not have an atmosphere—resembling Mercury in our own solar system. Two other teams measured the next planet, TRAPPIST-1c. TRAPPIST-1c is also too hot to be in the habitable zone and has been speculated to possess thick clouds of carbon dioxide, like Venus. But the teams reported the same finding: there is no significant atmosphere on TRAPPIST-1c, either. Cycle 2, which began in July of this year, is ongoing, and will see the JWST revisit these planets.

The need for more data is clear. TRAPPIST-1’s three potentially habitable planets, e, f and g, have not yet been explored. Part of it is logistics: the farther out a planet orbits, the fewer transits it will have during an observing cycle. In addition, these outer planets are colder, and if they possess atmospheres, it will take significantly more time, and different instruments, to measure them. Nonetheless, the knowledge gained—regardless of the outcome—will be invaluable. Should TRAPPIST-1 prove to be a barren wasteland, astrobiology will be able to refocus efforts on promising sunlike stars, reallocating precious resources to where they are needed most. But if even a hint of life persists, it must be pursued with vigor: a new call to discover and study the worlds that fill our galaxy and our imaginations.

Fully surveying the TRAPPIST-1 system will take a dedicated, multiyear campaign, but it must be done. Its significance to not only science but humanity at large cannot be overstated. The possibility of life around nearly three fourths of the Milky Way’s stars is simply too big to overlook. Exploration calls, if only we are bold enough to answer.

This is an opinion and analysis article, and the views expressed by theauthor or authors are not necessarily those of Scientific American.


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