Science & Technology

Scientists detect two water worlds 218 light years away

Study of the twin planets shows exoplanets are more diverse; new planet in same system discovered  

By Rohini Krishnamurthy
Published: Friday 16 December 2022
Cross-section of the Earth (left) and the exoplanet Kepler-138 d (right). Image: Benoit Gougeon, Université de Montréal_

Two alien planets about 218 light years away from Earth have found a twin in the ocean worlds of Europa and Enceladus — moons orbiting Jupiter and Saturn, according to a new study.

The exoplanets, Kepler-138 c and Kepler-138 d, are likely water worlds — a feature that scientists have theorised for a long time, the study published in the journal Nature Astronomy noted.

They are also larger-scale versions of Enceladus (Saturn’s moon) and Europa (Jupiter’s moon), the findings predicted.

These twin planets of the same size and mass are more massive than Earth but lighter than ice giants Uranus and Neptune.   

But they are different from the planets in our solar system, which is chiefly composed of rocky planets like Earth and gas giants like Jupiter.

“We have now shown that these two planets, Kepler-138 c and d, are quite different in nature and that a large fraction of their entire volume is likely composed of water,” Björn Benneke from the University of Montreal and the study’s author said in a statement.

These exoplanets, along with Kepler-138 b, orbit Kepler-138, a red dwarf star. The star is smaller and cooler than the Sun, a yellow dwarf star.

Benneke and his colleagues used NASA’s Hubble and the retired Spitzer space telescopes to observe the planetary system.

The volume of the two alien worlds was three times that of Earth and mass twice as big, they calculated.

Further, their observations showed that the Kepler- 138 c and d are made up of ingredients lighter than rock (rocky planets like Earth) but heavier than hydrogen or helium (gas-giant planets like Jupiter). 

This signals the presence of water: Up to half of the mass of the twin worlds should be water, the researchers estimated.

Until now, scientists expected that worlds slightly larger than Earth would likely have rocky features.

Further, the density of the twin exoplanets was lower than Earth but comparable to Enceladus and Europa.

“The secure identification of an object with the density of the icy moons of the solar system but that are significantly larger and more massive, clearly demonstrates the great diversity of exoplanets,” Jose-Manuel Almenara of Grenoble Alpes University in France explained. 

But unlike Enceladus or Europa, these worlds are not ocean worlds. “The temperature in Kepler-138c's and Kepler-138d’s atmospheres is likely above the boiling point of water,” Caroline Piaulet from the University of Montreal, said in a statement.

She expects to find a thick, dense steam-filled atmosphere on these planets. This would allow liquid water at high pressure to exist, she added.

Further, the researchers also discovered a fourth planet in the Kepler planetary system: Kepler-138 e.

The new planet takes 38 days to complete an orbit. It is in the habitable zone, meaning it is located in an orbit that receives just the right amount of heat from its star to allow water to exist in a liquid form.

The researchers are yet to study its nature and size. The researchers hope to address this knowledge gap and find more water worlds in the future.

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