When Meatloaf sang “Two Out of Three Ain’t Bad” in 1977, we’re all but certain he was not doing the math on the potential of planets in the galaxy having the right conditions to harbor life. Because new findings suggest that two out of three are actually bad, unless you’re in search of uninhabitable baked planets with boiling oceans.
However, it’s in the remaining one third that now has scientists excited, as this fraction still accounts for hundreds of millions of planets that could be in a "just right" goldilocks orbit around a M (red) dwarf star that would enable it to retain liquid water and potentially support life.
“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, a professor at the University of Florida. “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.”
There’s been much conjecture surrounding the ability of exoplanets to support life. Some scientists have suggested as many as 300 million planets could land in the habitable zone, and that Earth-like planets could be more prevalent than we’d previously thought. Yet there are conflicting factors, such as solar flares from the active M dwarf stars shooting life-zapping radiation onto a planet’s surface, as well as many unknown factors about atmospheric makeup that could make its promising gentle orbit a moot point.
For this new study, Sagear and researcher Sarah Ballard analyzed the orbit eccentricities of 163 M-dwarf exoplanets, using data from NASA’s Kepler space telescope captured between 2009 and 2019, and ESA’s Gaia telescope. Through the Kepler imaging, they were able to measure orbits by assessing how long the planets took to pass across the face of their star. New intel from Gaia provided the means to measure the distance to billions of stars across the galaxy.
“The distance is really the key piece of information we were missing before that allows us to do this analysis now,” Sagear said.
Orbit eccentricities relate to the shape of a planet’s path around their star. Most orbits aren’t circular, including Earth’s, but our planet has enough distance between it and the Sun that the shape of the journey doesn’t have the same kind of effect on weather.
In comparison, if a planet orbits closer to its star, an eccentric path causes drastic changes in gravitational forces, resulting in friction that heats the surface, a phenomenon known as tidal heating. As such, it renders these planets inhabitable for life as we know it.
“It’s only for these small stars that the zone of habitability is close enough for these tidal forces to be relevant,” said Ballard.
A third of the 2,600 planets identified by Kepler had gentle enough orbits to keep them in the habitable zone, opening the door to exploring them more closely for evidence of lifeforms.
While M dwarf stars, around 70% of known stars, are half the size of the Sun, or less, they’re also not as hot. So they have the "just right" potential to sustain life.
The researchers also found that M dwarf stars with multiple planets were more likely to have these more circular, gentle orbits, while single planets were subjected to more orbital extremes making their surfaces inhospitable.
“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 noted in their paper.
The study was published in the journal PNAS.
Source: University of Florida
