‘Primordial’ Alien Planets May Be Habitable For Billions Of Years Suggests New Research


Should astronomers widen the definition of “potentially habitable” when looking at exoplanets? Yes, say the authors of a new paper published in Nature Astronomy today that argues that rocky exoplanets with atmospheres dominated by hydrogen and helium gases could host liquid water on their surface for billions of years.

“It’s exciting to be reminded that the search for other habitable worlds might need out-of-the-box thinking,” said lead author Marit Mol Lous at the University of Zürich in Switzerland.

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An exoplanet is a planet that orbits a star outside the solar system.

When assessing their potential habitability astronomers typically take into account the possibility of liquid water existing on their surfaces. “It was a vital ingredient in the emergence of life on Earth [and even] when we think of habitability in an alternative way, it is still difficult to replace liquid water as other solvents are not as suitable,” said Mol Lous. “That is why we think the presence of liquid water is so essential to the possibility of life.”

Astronomers also search for signs of oxygen and nitrogen, which dominate the atmosphere on Earth. However, the paper’s new modeling suggests that even planets with their original primordial atmospheres of hydrogen and helium (something Earth also once had) can be potentially habitable for long periods of their history.

It’s thought that large rocky exoplanets that orbit their star from afar could retain their hydrogen and helium-dominated atmospheres.

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Does this research widen what is meant by the term “habitable planet?” “We come from the other direction—our research reduced the types of planets that are considered certainly uninhabitable— but we cannot go as far as saying that the type of planet we study is habitable,” said Mol Lous.

It’s certainly possible that life could adapt to the strange environments on these planets. “We can see that on Earth life can already adapt to a very wide range of circumstances—think of pressures, chemical environments— and therefore that fact that a planet has a hydrogen and helium dominated atmosphere might not necessarily pose a problem for possible life,” said Mol Lous.

Earth also used to have an atmosphere of hydrogen and helium, but such at atmosphere is easy to lose because those two particles and very light. “If a planet is not really massive it does not have enough gravity to hold onto the lightest particles,” said Mol Lous. “And if a planet is close to the star it receives a lot of radiation which increases how quickly particles escape.”

That’s what happened on Earth, where hydrogen and helium were replaced by heavier particles of oxygen and nitrogen, but that doesn’t make it a universal process. “It could be that other planets which lose their atmosphere in the same way as Earth end up with completely different atmospheric compositions.”

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Large rocky exoplanets at some distance from their star could retain their hydrogen and helium-dominated atmospheres. However, finding hydrogen and helium-rich exoplanets of the correct mass and distance from their star, and a thick enough atmosphere (between 100 to 1,000 times thicker than the Earth’s) to host liquid water for eight billion years+ planets is not easy.

“Most exoplanets today are measured with instruments that hugely favour the detections of planets very close to their star,” said Mol Lous. “They likely lost their primordial atmosphere or they are too massive to fall into the category of planets we are looking for.”

Thankfully the instruments for observing exoplanets are getting better and the team hopes to find suitable candidates in the future.

Can the James Webb Space Telescope (JWST) help? “JWST might be capable of observing possible biomarkers from such planets, which would be very exciting,” said Mol Lous. “But we would need to be very cautious [because] if there is life on such planets it will be under very different circumstances from Earth, and thus even biomarkers might look very different.”

It’s another reminder that the search for life beyond Earth requires an open mind.

Wishing you clear skies and wide eyes.


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