The possibility of liquid water on the surface of a third of the planets orbiting red dwarfs | Sorae Portal site to space

University of Florida doctoral student Sheila Sager and University of Florida astronomer Sarah Ballard have discovered an exoplanet orbiting a red dwarf (M-type star), a common type of star with a diameter and mass smaller than the Sun. A third of them may have liquid water on their surface.

These exoplanets could be the best targets for the search for extraterrestrial life, and it is estimated that there are hundreds of millions of target planets in the Milky Way alone. I think the results of this will be very important for the study of exoplanets in the next 10 years.”

Humans have discovered more than 5,400 exoplanets so far, many of which have been found around red dwarfs that are supposed to be rocky like Earth. Depending on conditions such as circulation in the habitable zone (a region where liquid water can exist stably on the surface of a celestial body), there is a possibility that life could exist on these exoplanets.

The surface temperature of a red dwarf star is less than 4,000 degrees Celsius, and the habitable zone extends to a place not far from the main star, so the planets orbiting it are not affected by the tidal force caused by the gravity of the red dwarf star. Main star. Receive strongly. A celestial body that receives a tidal force is deformed slightly, but if a planet’s orbit is deformed into an ellipse rather than a perfect circle, the degree of deformation will change cyclically each time it orbits. The planet generates heat due to friction.

In this way, the phenomenon in which the interior of a celestial body is heated by tidal forces is called “tidal heating”. Jupiter’s moon Io is known to have very active volcanism which is heated by tidal heating due to the gravitational attraction of Jupiter and other moons.

Tidal heating may also occur on exoplanets as well, and some have suggested volcanic activity, but the strength of the heating is an issue from a habitability perspective. If extreme tidal heating occurs, the planet could become too hot for liquid water to exist on its surface.

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Related: Could there be volcanic activity? Earth-sized exoplanet discovered 86 light-years away (May 20, 2023)

Sagir and Ballard used data from NASA’s Kepler space telescope and the European Space Agency’s Gaia space telescope to study 101. We investigated the orbital eccentricity (orbital eccentricity) that affects the strength of tidal heating of a total of 163 exoplanets orbiting red dwarfs.

The eccentricity is a numerical value indicating the shape of an orbit: 0 for a complete circle, greater than 0 and less than 1 for an ellipse, 1 for a parabola, and greater than 1 for a hyperbola. For example, the Moon’s orbit is elliptical with an eccentricity of 0.0549, so there is a difference of about 40,000 km between when it approaches and when it moves away from Earth. The full moon that appears as it approaches Earth is called a super moon.

The analysis showed that two-thirds of the studied planets have eccentricity and cannot hold liquid water on their surfaces due to heating caused by strong tidal forces. On the other hand, the remaining third of the planets are not very hot, which indicates that they may be able to hold liquid water on their surfaces and possibly even support life. They also found that in planetary systems in which multiple planets have been detected, the eccentricity of the orbit is small and tends to be close to a perfect circle, while in a planet orbiting its parent star alone, the eccentricity tends to be large.

In addition, it is believed that not only the distance from the main star and the eccentricity of the orbit, but also the activity of the red dwarf affects the habitability around the red dwarf. Recently, as a result of an analysis of the activity of the red dwarf star “Proxima Centauri” about 4.2 light-years away, the habitability of the exoplanets “Proxima Centauri b” orbiting around its habitable zone may be low. The search results have been published.

Also, in the past, research results have been announced that the activity of red dwarfs promotes the formation of an ozone layer in the planet’s atmosphere, and that the effects on the planets are limited depending on the latitude at which the flares occur on the surface of the red dwarfs. more.

communication
・ Isn’t the exoplanet Proxima Centauri b “habitable”? You may experience strong stellar winds (January 2, 2023)
Ozone formation could be enhanced on planets exposed to red dwarf flares (December 2, 2022)
·good news? Red dwarf flares may have limited effect on exoplanets (August 10, 2021)

Sagear and Ballard’s results indicate that even among the exoplanets orbiting the habitable zones of red dwarfs, where strong tidal heating can occur, there are many planets that could be targets for the search for extraterrestrial life. It shows potential and impact on future research will be noted. Their research results were published in the Proceedings of the National Academy of Sciences (PNAS) on May 30, 2023.

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• Image credit: NASA/JPL-Caltech, NASA’s Goddard Space Flight Center/Chris Smith (KRBwyle)
• University of Florida – A third of the most common planets in the galaxy could be in the habitable zone
• INAF media A third of the outer planets are in the habitable zone
• Sagir and Ballard – orbital eccentric distribution of planets orbiting M dwarfs (PNAS, arXiv)

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