According to a new theoretical study published on Tuesday in Monthly Notices of the Royal Astronomical Society, only 8 percent of the potentially habitable planets that will ever form in the universe existed when our Solar System was born 4.6 billion years ago.
Using an assessment of data, collected by the U.S. space agency NASA's Hubble Space Telescope and the prolific planet-hunting Kepler space observatory, researchers created a "family album" of galaxies, which outlines the history of star formation.
"Our main motivation was understanding the Earth's place in the context of the rest of the universe," said Peter Behroozi, study author of the Space Telescope Science Institute. "Compared to all the planets that will ever form in the universe, the Earth is actually quite early."
The study shows that the universe was making stars at a fast rate 10 billion years ago, but the fraction of the universe's hydrogen and helium gas that was involved was very low. Today, star birth is happening at a much slower rate than long ago, but there is so much leftover gas available that the universe will keep cooking up stars and planets for a very long time to come.
According to NASA, Earth-like worlds are defined as rock planets orbiting within their host star's habitable zone, the region in which water might exist in liquid form. Kepler's planet survey indicates that Earth-sized planets are ubiquitous in our galaxy.
Scientists believe that there should be currently about 1 billion Earth-sized worlds in the Milky Way, and a good portion of them are likely rocky. That estimate skyrockets when including the other 100 billion galaxies in the observable universe.
The universe's evolving galaxies are going to keep making stars and planets. The researchers say that future Earths are more likely to appear inside giant galaxy clusters and also in dwarf galaxies. Our own Milky Way has used up a great deal of its gas, making the prospect of future star births less likely.
This leaves plenty of opportunity for untold more Earth-sized planets in the habitable zone to arise in the future. The last star isn't expected to turn off the lights for another 100 trillion years. That's plenty of time for literally anything to happen on the planet landscape, NASA said.
"A big advantage to our civilization arising early in the evolution of the universe is our being able to use powerful telescopes like Hubble to trace our lineage from the big bang through the early evolution of galaxies. The observational evidence for the big bang and cosmic evolution, encoded in light and other electromagnetic radiation, will be all but erased away 1 trillion years from now due to the runaway expansion of space. Any far-future civilizations that might arise will be largely clueless as to how or if the universe began and evolved," the researchers concluded.
Xinhua - china.org.cn
22/10/15
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