A third of the most common planets in the galaxy are probably in the habitable zone


June 06, 2023

(Nanowerk News) In a new analysis based on recent telescope data, University of Florida astronomers have found that a third of planets around the galaxy’s most common stars may be in goldilocks orbits that are close enough, and gentle enough, to hold liquid water. – and possibly save life.

The remaining two-thirds of the planets around this small, ubiquitous star were likely roasted by gravitational tides, sterilizing them.

UF astronomy professor Sarah Ballard and doctoral student Sheila Sagear published their findings in Proceedings of the National Academy of Sciences (“Orbital eccentricity distribution of planets orbiting M dwarfs”). Ballard and Sagear have long studied exoplanets, worlds orbiting stars other than the sun. Illustration of exoplanet Many exoplanets orbiting ordinary, small stars like this could harbor liquid water and potentially life. (NASA/JPL-Caltech)

“I think these results are very important for the next decade of exoplanet research, as eyes turn to this stellar population,” said Sagear. “These stars are excellent targets for searching for small planets in orbits where liquid water is possible and therefore possibly habitable.”

Our familiar and warm yellow sun is relatively rare in the Milky Way. By far, the most common stars are much smaller and cooler, with only half the mass of our sun at most. Billions of planets orbit this common dwarf star in our galaxy.

Scientists think that water in liquid form is necessary for life to evolve on other planets, as it did on Earth. Because these dwarf stars are cooler, any planet would have to be crowded very close to its star to get warm enough to accommodate liquid water. However, this close orbit makes the planet vulnerable to extreme tidal forces caused by the star’s gravitational effect on the planet.

Sagear and Ballard measured the eccentricity — how oval the orbits are — from a sample of more than 150 planets around this dwarf star, which is about the size of Jupiter. If a planet orbits close enough to its star, about the distance Mercury orbits the sun, its eccentric orbit can cause it to undergo a process known as tidal heating. As the planet stretches and deforms by changing gravitational forces in its irregular orbit, friction heats it up. Ultimately, this could roast the planet, eliminating all possibility of liquid water.

“Only for small stars is the habitability zone close enough for these tidal forces to be relevant,” Ballard said.

The data comes from NASA’s Kepler telescope, which captures information about exoplanets as they move in front of their host stars. To measure planetary orbits, Ballard and Sagear focused mainly on how long it takes for planets to move across the star’s surface. Their study also relied on new data from the Gaia telescope, which measures the distances to billions of stars in the galaxy.

“Distance is really an important piece of information that we missed before that allows us to do this analysis now,” said Sagear.

Sagear and Ballard found that stars with lots of planets are the most likely to have the kind of circular orbits that allow them to hold water in liquid form. Stars with only one planet are the most likely to see extreme tides that would sterilize their surface.

Since a third of the planets in this small sample have orbits soft enough to accommodate liquid water, it is likely that the Milky Way has hundreds of millions of promising targets for investigating signs of life outside our solar system.


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