This is an
artist's impression of innumerable Earth-like planets that have
yet to be born
over the next trillion years in the evolving universe.
Credits: NASA,
ESA, and G. Bacon (STScI)
(October 20, 2015) Earth
came early to the party in the evolving universe. According to a new
theoretical study, when our solar system was born 4.6 billion years ago only
eight percent of the potentially habitable planets that will ever form in the
universe existed. And, the party won't be over when the sun burns out in
another 6 billion years. The bulk of those planets - 92 percent - have yet to
be born.
This conclusion is based on an assessment of data collected
by NASA's Hubble Space Telescope and the prolific planet-hunting Kepler space
observatory.
"Our main motivation was understanding the Earth's
place in the context of the rest of the universe," said study author Peter
Behroozi of the Space Telescope Science Institute (STScI) in Baltimore,
Maryland, "Compared to all the planets that will ever form in the
universe, the Earth is actually quite early."
Looking far away and far back in time, Hubble has given
astronomers a "family album" of galaxy observations that chronicle
the universe's star formation history as galaxies grew. The data show 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.
"There is enough remaining material [after the big
bang] to produce even more planets in the future, in the Milky Way and
beyond," added co-investigator Molly Peeples of STScI.
Kepler's planet survey indicates that Earth-sized planets in
a star's habitable zone, the perfect distance that could allow water to pool on
the surface, are ubiquitous in our galaxy. Based on the survey, scientists
predict that there should be 1 billion Earth-sized worlds in the Milky Way
galaxy at present, a good portion of them presumed to be rocky. That estimate
skyrockets when you include the other 100 billion galaxies in the observable
universe.