Scientists report exoplanet K2-155d could hold liquid water

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Wednesday, March 14, 2018

Data from NASA's Kepler spacecraft, the Equipo de Seguimiento de Planetas Rocosos Intepretando sus Transitos, and the Nordic Optical Telescope in Spain and Suburu Telescope in Hawaii indicate that one of the three roughly Earth-sized planets around K2-155 may be the right distance from its sun to host liquid water, within what scientists call the habitable zone, and climate simulations support this conclusion. The findings were published in The Astrological Journal last month, but the researchers, led by Teruyuki Hirano of Tokyo Institute of Technology's Department of Earth and Planetary Sciences, began to discuss them with the public on Monday.

Artist's conception of a red dwarf star with a planet and moon.
Image: NASA/ESA/G. Bacon (STScI).

"It's important to note that the number of planets around red dwarfs is much smaller than the number around solar-type stars," Hirano said in a press release. "Red dwarf systems, especially coolest red dwarfs, are just beginning to be investigated, so they are very exciting targets for future exoplanet research." According to the research team, one of the key findings of this work is that planets around red dwarf stars may be more similar to Earth-type planets than had previously been believed.

Much of the interest in exoplanets capable of hosting liquid water is water's role as a prerequisite for life as we understand it. To paraphrase a description given in The Atlantic of 2011 conference on the subject, on a practical level, solving the riddle of what makes a planet habitable is solving the riddle of whether it would have water, not so cold that it would remain frozen year-round nor so hot or inhospitable that it would evaporate. The planet must also have a solid surface where water can gather, which rules out gas giants similar to our Jupiter. It also cannot be so small that any protective gaseous atmosphere would float away, which scientists believe happened on our Mars. Hirano conceded that the climate modeling systems used to predict K2-155d's friendliness to water are limited: "In our simulations, the atmosphere and the composition of the planet were assumed to be Earth-like, and there's no guarantee that this is the case." Further study, he says, is necessary. He went on to express enthusiasm for NASA's upcoming Transiting Exoplanet Survey Satellite (TESS), due to launch next month. "TESS is expected to find many candidate planets around bright stars closer to Earth," he said.

The planet, named K2-155d because it is the fourth star circling its sun, is one of many exoplanets to have been found around red dwarf stars—this report alone describes 15—but its star is brighter than the others and did not exhibit any solar flares during the study's 80-day observation period, which may make it gentler on living organisms than stars with fierier tempers.

K2-155d is located about 200 light-years from Earth. According to the research findings, it is tidally locked, meaning that it does not spin on its axis to create day and night. Rather, the same side always faces the sun and the other side the stars. Its years are roughly 40 days long.

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