Could it Answer the Question: “Are We Alone?” –Profile of Exoplanet K2-18b 

NASA Exoplanets


In the 1980’S Carl Sagan observed that the human species had reached the shore of the cosmic ocean. A mere two decades ago, astronomers debated the existence of planets beyond our solar system. Today, astronomers estimate that an average of one to two planets orbit every star in the Milky Way –That’s an estimated 400 billion planets.  NASA recently confirmed that the number of discovered exoplanets just ticked past the 5,000 mark, representing a 30-year journey of discovery led by NASA space telescopes. In the absence of evidence, defying intuition and common sense, scientists continue to doubt he existence of life, either microbial or advanced, beyond our pale blue dot. Our scientific imagination, Einstein was fond of noting, is a preview of coming attractions. 

K2-18b, which is eight times the mass of Earth, is the only exoplanet known to have both water and temperatures that could support life. The 2018 discovery is the first successful atmospheric detection for an exoplanet orbiting in its star’s ‘habitable zone’, at a distance where water can exist in liquid form.

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“Finding water in a potentially habitable world other than Earth is incredibly exciting,” said first author Angelos Tsiaras with the University of London College Centre for Space Exochemistry Data (UCL CSED). “K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?”

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Found in Hubble Archive Data

The team used archive data from 2016 and 2017 captured by the NASA Hubble Space Telescope and developed open-source algorithms to analyze the starlight filtered through K2-18b’s atmosphere. The results revealed the molecular signature of water vapor, also indicating the presence of hydrogen and helium in the planet’s atmosphere.




The authors believe that other molecules including nitrogen and methane may be present but, with current observations, they remain undetectable. Further studies are required to estimate cloud coverage and the percentage of atmospheric water present.

Orbits a Red-Dwarf Star

The planet orbits a cool red-dwarf star that is about 110 light years from Earth in the Leo constellation. Given the high level of activity of its red dwarf star, K2-18b may be more hostile than Earth and is likely to be exposed to more radiation.

K2-18b was discovered in 2015 and is one of hundreds of super-Earths — planets with a mass between Earth and Neptune — found via the transit method by NASA’s Kepler spacecraft. NASA’s TESS mission is expected to detect hundreds more transiting super-Earths in the coming years.

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The artist’s impression at top of the page shows K2-18b, its host star, and an accompanying planet in this system (ESA/Hubble, M. Kornmesser). K2-18b is now the only super-Earth exoplanet known to host both water and temperatures that could support life.

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“With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets,” said co-author Ingo Waldmann (UCL CSED). “This is not only because super-Earths like K2-18b are the most common planets in our Galaxy, but also because red dwarfs—stars smaller than our Sun—are the most common stars.”

Exoplanet Discovered Three-Times the Mass of Jupiter

The next generation of space telescopes, including the NASA/ESA/CSA James Webb Space Telescope and ESA’s ARIEL mission, will be able to characterize atmospheres in more detail as they will carry more advanced instruments. ARIEL is expected to launch in 2028, and will observe 1,000 planets in detail to get a truly representative picture of what they are like.

“Our discovery makes K2-18 b one of the most interesting targets for future study,” said Professor Giovanna Tinetti (UCL CSED), co-author and Principal Investigator for ARIEL. “Over 4,000 exoplanets have been detected but we don’t know much about their composition and nature. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution.”

“This study contributes to our understanding of habitable worlds beyond our Solar System and marks a new era in exoplanet research, crucial to ultimately place the Earth, our only home, into the greater picture of the Cosmos,” said Tsiaras.

A Preview of Coming Attractions

“Finding other planets that might have the capability to support life is one of the holy grails of astronomy research,” said Dr. Colin Vincent, Head of STFC’s Astronomy Division. “This result based on data from the Hubble Space Telescope gives an exciting taste of what may be possible in the next few years as a number of new telescopes and space missions supported by STFC and the UK Space Agency come online.”

Image credit top of page: NASA

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via NASA and University College London

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