Earth, as a habitable planet, was an early arrival in the time span of the universe, meaning most of the potentially life-bearing planets the universe may eventually create have yet to be born, a theoretical study suggests.

Our solar system, and our planet, came into existence around 4.6 billion years ago, when just 8 percent of potentially life-supporting planets scientists estimate the universe could ever form had evolved, NASA researchers say.

That means another 92 percent of such planets are waiting to be born, they explain in a study appearing this week in the Monthly Notices of the Royal Astronomical Society.

Those numbers come out of analysis of data from the Hubble Space Telescope and NASA's Kepler planet-hunting space observatory, the scientists say.

"Our main motivation was understanding the Earth's place in the context of the rest of the universe," says Peter Behroozi of the Space Telescope Science Institute (STScI) in Baltimore, Md. "Compared to all the planets that will ever form in the universe, the Earth is actually quite early."

By peering far into space and far back in the universe's history, Hubble and Kepler have provided a chronicle of star formation in the universe, which was occurring at a rapid rate some 10 billion years ago but which only used up a small portion of the available hydrogen and helium gases.

There's enough gas still left for the universe to continue making stars—and Earth-like planets—for a long time, the researchers say.

"There is enough remaining material [after the Big Bang] to produce even more planets in the future, in the Milky Way and beyond," says STScI co-researcher Molly Peeples.

Kepler has found that Earth-like planets in their stars' habitable zone—a distance that would allow liquid water to exist on a planet's surface—are everywhere in our Milky Way galaxy.

Around 1 billion such worlds may exist in our galaxy at the present, a figure likely to apply to the other 100 billion or so galaxies in the observable universe, the researchers say.

More billions of Earth-sized planets are likely to be born between now and when the last existing star in the universe fizzles out 100 trillion years from now, they predict.

Many of those will arise in huge galaxy clusters and dwarf galaxies, where the available gases have not all been consumed and will be forming stars, solar systems and planets for a long time to come, they say.

Because our Earth evolved at a fairly early point in the life of the universe, we have the advantage of being able to utilize powerful instruments like Hubble and Kepler to study the universe from almost the point of the Big Bang up through the early formation of galaxies.

That cosmic history, preserved for now in light and other wavelengths in the electromagnetic spectrum, will have faded away for the most part a trillion years from now.

That means civilizations that may arise in the far future will be essentially clueless about the beginnings of the universe, the researchers point out.

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