In 1977, Meatloaf sang “Two Out of Three Ain’t Bad,” but we doubt he calculated the galaxy’s life-supporting planets. Because new data imply that two out of three are negative, unless you want uninhabitable baked worlds with boiling oceans.
Scientists are thrilled about the remaining one third because it still contains hundreds of millions of planets that might be in a “just right” goldilocks orbit around a M (red) dwarf star, allowing them to retain liquid water and perhaps sustain life.
“This result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,” said University of Florida professor Sheila Sagear. “These stars are excellent targets to look for small planets in orbits where water might be liquid and the planet might be habitable.”
Life on exoplanets is debated. Some scientists believe 300 million planets might arrive in the habitable zone, including more Earth-like planets than previously imagined.
There might be life on hundreds of millions of planets.
However, solar flares from active M dwarf stars may kill life on a planet’s surface, and numerous unknown atmospheric conditions might render its promised moderate orbit a moot issue.
Sagear and researcher Sarah Ballard used NASA’s Kepler space telescope data from 2009 to 2019 and ESA’s Gaia telescope data to investigate 163 M-dwarf exoplanet orbit eccentricities. They measured orbits by measuring how long the planets took to travel across their star using Kepler photography. Gaia data measured billions of stars’ distances.
Sagear added, “The distance is really the key piece of information we were missing before that allows us to do this analysis now.
Planets’ orbits around stars are shaped by eccentricity. Earth’s orbit, like others, isn’t round, but its distance from the Sun means it doesn’t affect weather.
If a planet circles closer to its star, an eccentric path generates substantial variations in gravitational forces, causing friction and tidal heating. It makes these planets inhabitable for life.
“These tidal forces are only relevant for these small stars,” Ballard added.
A third of Kepler’s 2,600 planets had moderate orbits that kept them in the habitable zone, allowing closer examination for lifeforms.
M dwarf stars—70% of known stars—are half the size of the Sun and cooler. They are “just right” for life.
M dwarf stars with several planets were more likely to have these more circular, benign orbits, whereas single planets had more orbital extremes that made their surfaces uninhabitable.
“Since one-third of the planets in this small sample had gentle enough orbits to potentially host liquid water, that likely means that the Milky Way has hundreds of millions of promising targets to probe for signs of life outside our Solar System,” the researchers said.