Stargazing: Astronomers estimate Earth-size planets number in the billions –

As of July 4, the NASA Exoplanet Archive ( listed 4,183 confirmed exoplanets, planets orbiting other stars, with another 2,089 candidate planets awaiting confirmation. NASA and other institutions have only studied a tiny percent of all the stars in our Milky Way. Based on the sample so far, astronomers estimate that planets outnumber stars in our galaxy. That means the Milky Way contains several hundred billion planets.

These exoplanets come in a bewildering variety. Some are Jupiter-size planets so close to their parent star that the heat from the star evaporates them. Some Earth-size planets get so hot, they rain liquid metal from their clouds. Most confirmed planets are significantly larger than Earth, but thats because larger planets are easier to discover than smaller planets. Astronomers estimate that Earth-size planets number in the billions.

Being the size of Earth doesnt mean such a planet has life on it. Many factors play into planet habitability. Distance to its parent star determines surface temperature. Too hot or too cold and habitability becomes unlikely. The type of parent star plays a crucial role. Stars smaller than our sun often produce large, dangerous stellar flares.

If Earth is a good example of what conditions necessary for a planet to support life, water is an absolute must. On our planet, where theres water, life exists, even at the bottom of the ocean, with near-freezing temperatures, in boiling hot springs or three miles underground in cracks in the rock.

Scientist Lynnae Quick, along with several other NASA scientists, looked at the likelihood of finding other life-bearing planets. This initial study contained a small sample of only 53 Earth-size planets. They specifically looked to see if the planets could support surface or subsurface oceans, as is the case with several moons in our own solar system. Of those, they calculated that 30 likely possess such bodies of water, more than half of the planets they analyzed. With a few billion Earth-size planets in our galaxy alone, that means there might be a lot of life-bearing planets out there. "Forthcoming missions will give us a chance to see whether ocean moons in our solar system could support life, said Quick.

The study didnt address the presence of intelligent aliens. There isnt enough data to decide that. But at least we have some idea of the possibilities now.

Planet Visibility Report

If youre up in the wee hours of Aug. 9, take a look at the southern sky between midnight and 1 a.m. The moon and the Red Planet, Mars, are separated by a mere three-finger widths held at arms length about 1/3 of the way up in the southeast. Jupiter and Saturn float less than a handspan apart in the southwest. Halfway between those two sits Pluto, but youll need a good-sized telescope to see it. Neptune is in the south, but, like Pluto, youll need a telescope to spot it.

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Stargazing: Astronomers estimate Earth-size planets number in the billions -

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