“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.”

In the vast expanse of the Milky Way, our beloved, radiant sun stands as a remarkable rarity. While it radiates familiarity and warmth, the majority of stars populating our galaxy are significantly smaller and cooler, boasting a mere half of the sun’s mass, or even less. Circling these diminutive stars are billions of planets, forming a bustling population within our cosmic neighborhood.

For these planets to attain habitable conditions, they must huddle closely to their modest stellar companions, eagerly seeking warmth. However, this proximity exposes them to formidable tidal forces, which can be nothing short of extreme.

In a groundbreaking analysis drawing upon the latest telescope observations, a team of esteemed astronomers from the University of Florida has made a startling revelation. It has been discovered that a staggering two-thirds of the planets orbiting these ubiquitous dwarf stars face a scorching fate, as they are unable to withstand the brutal tidal onslaught. This unfortunate outcome renders them lifeless, devoid of any potential for biological diversity. Yet, amid this cosmic ordeal, a glimmer of hope emerges.

Approximately one-third of these planets, comprising hundreds of millions of celestial bodies scattered across the galaxy, persist in a “Goldilocks” orbit. These fortunate worlds nestle at just the right distance from their parent stars, basking in a gentler gravitational embrace. This delicate equilibrium allows them to retain the precious elixir of life: liquid water. Consequently, the prospects of these planets nurturing and sustaining lifeforms become tantalizingly real.

Professor Sarah Ballard, an esteemed figure in the field of astronomy at the University of Florida, and her diligent doctoral student, Sheila Sagear, have devoted considerable time to studying these distant exoplanets. These captivating worlds, orbiting celestial bodies beyond our sun, offer a fascinating glimpse into the vast cosmic tapestry that envelops us. Their diligent research and astute analysis have culminated in the publication of their remarkable findings in the esteemed Proceedings of the National Academy of Sciences, marking a significant milestone in our collective quest for extraterrestrial knowledge.

“I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,” Sagear adds. “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.”

Sagear and Ballard conducted a study to determine the eccentricity of over 150 planets orbiting M dwarf stars, which share a similar size to Jupiter. The degree of ovalness in an orbit indicates its eccentricity. When a planet orbits in close proximity to its star, similar to the distance between Mercury and the Sun, an eccentric orbit can lead to a phenomenon known as tidal heating. This process occurs as the planet experiences stretching and deformation due to varying gravitational forces acting on its irregular orbit, generating friction and consequent heating. In extreme cases, this excessive heat could render the planet inhospitable, eliminating any possibility of sustaining liquid water.

“It’s only for these small stars that the zone of habitability is close enough for these tidal forces to be relevant,” Ballard adds.

The data utilized in this study was obtained from NASA’s esteemed Kepler telescope, renowned for its ability to capture detailed information concerning exoplanets as they traverse across their respective host stars.

To precisely gauge the exoplanets’ orbital characteristics, Ballard and Sagear placed particular emphasis on the duration it took for these planets to traverse the stellar surface. Additionally, the study incorporated recently acquired data from the Gaia telescope, an invaluable resource capable of measuring the distances to billions of stars within our galaxy.

“The distance is really the key piece of information we were missing before that allows us to do this analysis now,” Sagear adds.

Sagear and Ballard made a significant discovery indicating that stars hosting multiple planets are more inclined to possess circular orbits, which are favorable for maintaining liquid water. Conversely, stars with just one planet are prone to experiencing tidal extremes that could render their surfaces uninhabitable.

The findings from this limited sample suggest that approximately one-third of the planets examined exhibited orbits gentle enough to potentially support liquid water. Extrapolating this result, it implies that within the vast expanse of the Milky Way, there exist countless hundreds of millions of intriguing celestial bodies deserving of exploration in our quest for signs of extraterrestrial life.

Source: 10.1073/pnas.2217398120

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