How exoplanet hunters find alien worlds hundreds of light-years away and figure out if they have water
- Astronomers discovered what they believe are two alien worlds composed mostly of water.
- Telescopes have been used to discover 5,241 planets beyond our solar system so far.
Sitting in a star system about 218 light-years away in the constellation Lyra, two exoplanets orbit a dim, red dwarf star.
In the journal Nature Astronomy, astronomers announced that the two exoplanets — Kepler-138 c and Kepler-138 d — could be made up of mostly water.
But how did they figure that out, when the planets are far enough away that it would take a spaceship traveling 17,400 miles per hour more than 8 million years to reach them?
Astronomers look for tiny dips in starlight to find far away worlds
Most exoplanets are discovered through a technique called the transit method.
"That means Kepler, or another planet-hunting telescope, observed them a ton of times as they pass in front of their host star," Caroline Piaulet, a Ph.D. student at the University of Montreal's Institute for Research on Exoplanets and lead author of the study, told Insider.
Since the first worlds outside our solar system were discovered in 1992, astronomers have searched for other planets orbiting sun-like stars.
Astronomers have already captured direct evidence of 5,241 planets beyond our solar system, according to NASA's Exoplanet Archive.
To understand whether exoplanets, or planets around other stars, have the conditions to host life, researchers measure the chemical makeup of their atmospheres.
They do this by looking at graphs called a spectrum, which reveal how starlight filters through the atmosphere by dipping at very specific wavelengths that correspond to different molecules.
Astronomers typically look for the ingredients that sustain earthly life — liquid water, a continuous source of energy, carbon, and other elements — when hunting for life in distant worlds.
How they concluded Kepler-138 c and Kepler-138 d have water
But the new observations suggested the presence of another world orbiting the host star every 38 days: Kepler-138 e.
Taking into account the newly found exoplanet, researchers compared the sizes and masses of the planets in the star system and determined Kepler-138 c and Kepler-138 d were roughly the same mass and size — about 20% larger than Earth.
The team also concluded that up to half of their volumes are likely made of materials lighter than rock.
"We were surprised that they were not consistent with being rocky in composition," Piaulet said. "They had to have some lighter elements in there. Water is a good explanation because it's the most abundant of these elements."
"We often think of the Earth as the blue planet, covered in oceans," she added. "These worlds would have much more water than Earth."
On Earth, the average depth of the oceans is 2.3 miles, according to the National Oceanic and Atmospheric Administration. For Kepler-138 c and Kepler-138 d, the water atmosphere would be 1,243 miles deep, Piaulet added.
Water worlds beyond our solar system could be abundant in the universe
Worlds rich in liquid water might not be as rare as astronomers previously thought, Piaulet said.
In August 2022, another team of researchers found planet TOI-1452 b that could potentially be covered in a deep ocean. Follow-up observations with the James Webb Space Telescope will be needed to confirm the ocean's presence.
But astronomers don't have to venture into the far-off reaches of the cosmos in search of ocean worlds.
There are some of them right in our own solar neighborhood, such as Jupiter's moon Europa and Saturn's moon Enceladus, which have subsurface oceans beneath an ice layer.
Now that researchers have begun to find a bevy of water-rich worlds, "we don't know much about what type of life could be there," Piaulet said.
One limitation in scouring the cosmos for alien life is that scientists' definition of what a life-supporting planet might look like is based on what we know about life on Earth.
But by discovering and studying new worlds, astronomers can hone in on what makes a world habitable beyond a sample size of one — Earth.
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