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Narrowing down the search for habitable exoplanets

Jenny Winder, News Writer
Dec 10, 2014, 16:20 UTC

Sen—The search for life in the Universe just got easier, with new research from Cornell University’s Institute for Pale Blue Dots that offers estimates for where and when habitable infant Earths are most likely to be found. 

The distribution of liquid water and its change during the formation of planetary systems is important in understanding how planets become habitable.

Most known exoplanets orbit stars roughly similar to the Sun. It turns out that the Habitable Zone, where water can be liquid on the surface of a planet and where signals of life in the atmosphere can be detected with telescopes, is located further away from the young stars that these worlds orbit than previously thought.

“The search for new, habitable worlds is one of the most exciting things human beings are doing today and finding infant Earths will add another fascinating piece to the puzzle of how ‘Pale Blue Dots’ work,” says Lisa Kaltenegger, associate professor of astronomy in Cornell’s College of Arts and Sciences.

“This increased distance from their stars means these infant planets should be able to be seen early on by the next generation of ground-based telescopes,” says research associate Ramses M. Ramirez. “They are easier to spot when the Habitable Zone is farther out, so we can catch them when their star is really young.”

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The increased distance of the Habitable Zone from pre-main sequence stars makes it easier to spot infant earths. Image credit: Astrophysical Journal Letters

Since the pre-main sequence period for the coolest stars is up to 2.5 billion years, it’s possible that life could begin on a planet during its sun’s early phase and that life could then move to the planet’s subsurface (or underwater) as the star’s luminosity decreases.

The paper by Kaltenegger and Ramirez, to be be published in the January 1, 2015, issue of Astrophysical Journal Letters, offers estimates for where infant Earths, capable of hosting life, are most likely to be found. 

As reference points, they also assess the maximum water loss for rocky planets that are at equivalent distances to Venus, Earth and Mars from our Sun.

The research predicts that planets around cooler stars need to initially accrete more water than Earth did or, alternatively, have additional water delivered later during  pre-main sequence phase to remain habitable. The findings are also consistent with recent claims that Venus lost its water during accretion.

The researchers also found that during the early period of a solar system’s development, planets that end up being in the Habitable Zone later on, when the star is older, initially can lose the equivalent of several hundred oceans of water or more if they orbit the coolest stars. However, even if a runaway greenhouse effect is triggered, a planet could still become habitable if water is later delivered to the planet, after the runaway phase ends.

“Our own planet gained additional water after this early runaway phase from a late, heavy bombardment of water-rich asteroids,” says Ramirez. “Planets at a distance corresponding to modern Earth or Venus orbiting these cool stars could be similarly replenished later on.”