A Third of Planets Orbiting Red Dwarf Stars Could be in the Habitable Zone

A recent study published in the Proceedings of the National Academy of Sciences, a pair of researchers from the University of Florida (UF) examine orbital eccentricities for exoplanets orbiting red dwarf (M dwarf) stars and determined that one-third of them—which encompass hundreds of millions throughout the Milky Way—could exist within their star’s habitable zone (HZ), which is that approximate distance from their star where liquid water can exist on the surface. The researchers determined the remaining two-thirds of exoplanets orbiting red dwarfs are too hot for liquid water to exist on their surfaces due to tidal extremes, resulting in a sterilization of the planetary surface.

Artist’s illustration of a young red dwarf star with three exoplanets orbiting around it. (Credit: NASA/JPL-Caltech)

“I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,” said Sheila Sagear, who is a PhD student at UF and lead author of the study. “These stars are excellent targets to look for small planets in an orbit where it’s conceivable that water might be liquid and therefore the planet might be habitable.”

For the study, Sagear and her advisor, Dr. Sarah Ballard, analyzed the orbital eccentricities of 163 exoplanets orbiting red dwarf stars across 101 systems using data from NASA’s Kepler mission. For context, red dwarf stars are approximately the size of Jupiter, so they’re much smaller than our own Sun. This smaller size means red dwarfs give off far less energy and heat than our Sun, meaning the HZ exists much closer to the star, resulting in shorter orbital periods for planets that orbit within the HZ.

Illustration depicting habitable zones for various types of stars displaying too hot (red), too cold (blue), and just right (green) for liquid water to exist on a planetary surface. Since red dwarfs are cooler than our own Sun, their habitable zone is closer to the star. (Credit: NASA/Kepler Mission/Dana Berry)

A planetary body’s orbital eccentricity refers to the shape of its orbit. While Earth’s orbit is almost perfectly circular, astronomers have discovered planetary bodies both within and beyond our solar system to exhibit more eccentric, or oval-shaped orbits. Eccentric orbits can result in massive fluctuations within the interiors of planetary bodies, regardless of their size. One such example within our solar system is Jupiter’s moon, Io, whose eccentric orbit results in it being the most volcanically active body in our solar system.

Throughout its orbit, Io is constantly stretched and compressed from the gravitational interactions with Jupiter since its distance changes, becoming closer at times and farther away from Jupiter at other times. Over great amounts of geologic time, the interior of Io heats up from the friction being produced within its interior, which leads to heat, and the volcanic activity we observe to this very day. This process is known as tidal heating, which is what this most recent study explores with exoplanets.

In the end, Sagear and Dr. Ballard discovered that red dwarf stars possessing multiple exoplanets held the highest promise of exhibiting more circular orbits much like the Earth, meaning they could house liquid water on their surfaces. In contrast, the researchers discovered that red dwarfs boasting only one exoplanet were more likely to exhibit an orbit with a higher eccentricity, resulting in it experiencing tidal extremes, much like Jupiter’s Io, and less likely to house liquid water on its surface.

While the study found that only one-third of exoplanets in the 163-sample size could possibly house liquid water on their surfaces, this also means that there are potentially hundreds of millions of these worlds throughout just the Milky Way Galaxy.

Launched in 2009, NASA’s Kepler mission has been instrumental in expanding our understanding of exoplanets and the likelihood of their habitability. During its 9-year mission that ended in 2018 after its fuel was expended, Kepler confirmed the existence of almost 2,800 exoplanets with almost 2,000 still potentially awaiting confirmation, known as exoplanet candidates. While this most recent study encompassed a small slice of those confirmed exoplanets, the data from Kepler will undoubtedly keep scientists busy for the next several years.

Artist rendition of the Kepler telescope searching for exoplanets. (Credit: NASA/Kepler mission/Wendy Stenzel)

What new discoveries will scientists make about M dwarfs, their exoplanets, and their characteristics? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

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