The search for worlds beyond our own is one of humankind’s greatest quests. Scientists have found thousands of exoplanets orbiting other stars in the Milky Way, but are still ironing out the details of what factors truly make a planet habitable. But thanks to researchers at Cornell University, their search may become a little easier. A team at the Institute for Pale Blue Dots has zeroed in on the range of habitable orbits for very young Earth-like planets, giving astronomers a better target to aim at when searching for rocky worlds that contain liquid water and could support the evolution of life.
The Habitable Zone (HZ) of a star is its so-called “Goldilocks region,” the not-too-hot, not-too-cold belt within which liquid water could exist on orbiting rocky planets. Isolating planets in the HZ is the primary objective for scientists hoping to find evidence of life. Until now, astronomers have mainly been searching for worlds that lie in the HZ of stars that are in the prime of their lives: those that are on the Main Sequence, the cosmic growth chart for stellar evolution. According to the group at Cornell, however, scientists should also be looking at cooler, younger stars that have not yet reached such maturity.
As shown in the figure above, cool stars in classes F, G, K, and M are more luminous in their pre-Main Sequence stage than they are once they mature. Planets that circle around such bright stars tend to have more distant orbits than those that accompany dimmer stars, making transits more visible and providing a larger HZ for astronomers to probe. In addition, the researchers found that fledgling planets can spend up to 2.5 billion years in the HZ of a young M-class star, a period of time that would allow ample time for life to flourish.
But just because liquid water could exist on a planet doesn’t mean that it does. A rocky planet must first acquire water, and then retain it long enough for life to develop. The Cornell group found that a watery world could lose its aqueous environment to a runaway greenhouse effect if if forms too close to a cool parent star, even if the planet was on course to eventually stray into the star’s HZ. These seemingly habitable planets would have to receive a second supply of water later on in order to truly support life. “Our own planet gained additional water after this early runaway phase from a late, heavy bombardment of water-rich asteroids,” offered Ramses Ramirez, one author of the study. “Planets at a distance corresponding to modern Earth or Venus orbiting these cool stars could be similarly replenished later on.”
Estimations for the HZs of cool, young stars and probable amounts of water loss for exoplanets orbiting at various distances are provided in a preprint of the paper, available here. The research will be published in the January 1, 2015, issue of The Astrophysical Journal.
I continued to be astounded at the incredible optimism that we will soon discover signs of life on other worlds. This article certainly narrows the search in a very logical manner, to concentrate on planets in the HZ. As Vanessa said:
“But just because liquid water COULD exist on a planet doesn’t mean that it DOES.”
And as a corollary to this very true principle, “Just because life COULD exist on a planet doesn’t mean that it DOES, or ever did, or ever will.” Soooooo many things can go wrong that would prevent life that would otherwise be able to exist on a planet within the HZ of its star. We are gradually proving that unpopular fact as we search the moon, Venus, Mars (all within Sol’s HZ), and eventually various other interesting moons of other solar planets (where the effects of the gravity of the moons’ mother planets warms the moons enough to allow liquid water to exist, at least below the surface). The more we come to understand what can go WRONG on planets in the HZ, the more we can extrapolate what our chances are of finding life on planets/moons in the HZ of other suns; in effect, the more realistic and less optimistic we become. It would appear that we should expect to have to search millions of exo-planets in the HZ before we MIGHT find signs of life, because of all the unfortunate factors that can go wrong. I say search away! But please be realistic. Don’t feed us with overblown and exaggerated claims that we are on the verge of finding that illusive inhabited planet/moon. I submit that we are in for a VERY LONG HAUL, as hard as that is to swallow. By all means “haul away”.