Astronomers using the NASA/ESA Hubble Space Telescope have detected water vapor in the atmosphere of K2-18b, a planet of eight Earth masses in the habitable zone of the M-type dwarf star K2-18.
This artist’s impression shows planets K2-18b and c and their host star. Image credit: NASA / ESA / Hubble / M. Kornmesser.
K2-18 is a red dwarf star located approximately 111 light-years away in the constellation Leo.
Also known as EPIC 201912552, the star hosts two massive planets: K2-18b and c.
K2-18b, discovered in 2015, has a radius of 2.2 times that of Earth and is about 8 times as massive.
The planet orbits its star every 33 days at a distance of approximately 0.15 AU and has an Earth Similarity Index of 0.73.
It receives 1.28 times the light intensity of Earth, and its equilibrium temperature is 28 degrees Fahrenheit (minus 2 degrees Celsius).
“K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition,” said co-lead author Dr. Angelos Tsiaras, a researcher in the Centre for Space Exochemistry Data at University College London.
“However, it brings us closer to answering the fundamental question: Is the Earth unique?”
Dr. Tsiaras and his colleagues used Hubble data from 2016 and 2017 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 astronomers believe that other molecules, including nitrogen and methane, may be present but they remain undetectable with current observations.
“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-lead author Dr. Ingo Waldmann, also from the Centre for Space Exochemistry Data at University College London.
“This is not only because super-Earths like K2-18b are the most common planets in our Milky Way, but also because red dwarfs are the most common stars.”
“Our discovery makes K2-18b one of the most interesting targets for future study,” added University College London’s Professor Giovanna Tinetti, co-author of the study.
“This study contributes to our understanding of habitable worlds beyond our Solar System and marks a new era in exoplanet research, crucial to ultimately placing the Earth, our only home, into the greater picture of the Cosmos,” Dr. Tsiaras said.
The discovery is reported in a paper in the journal Nature Astronomy.
A paper from a different team of astronomers will be published in the Astronomical Journal.
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Angelos Tsiaras et al. Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18b. Nature Astronomy, published online September 11, 2019; doi: 10.1038/s41550-019-0878-9
Björn Benneke et al. 2019. Water Vapor on the Habitable-Zone Exoplanet K2-18b. AJ, in press; arXiv: 1909.04642
