When the Solar System was born about 4.6 billion years ago only 8% of the potentially habitable terrestrial exoplanets that will ever form in the Universe existed. This theoretical conclusion is based on an assessment of star-birth data gathered by the NASA/ESA Hubble Space Telescope and planet surveys carried out by NASA’s Kepler/K2 mission.
An artist’s impression of innumerable potentially habitable terrestrial exoplanets that have yet to be born over the next trillion years in the Universe. Image credit: NASA / ESA / G. Bacon, STScI.
The results were published online yesterday in the Monthly Notices of the Royal Astronomical Society.
“Our main motivation was understanding the Earth’s place in the context of the rest of the Universe,” said co-author Dr Peter Behroozi, 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 Hubble data show that the Universe was making stars at a fast rate 10 billion years ago, but the fraction of the Universe’s as 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.
Kepler’s surveys indicate that potentially habitable terrestrial planets are ubiquitous in our Milky Way Galaxy.
Astronomers predict that there should be one billion Earth-sized planets in the Galaxy at present, a good portion of them presumed to be terrestrial. That estimate skyrockets when you include the other 100 billion galaxies in the observable Universe.
“We expect there to be 1020 Earth-like and 1020 giant planets. Our own Galaxy is expected to host 109 and 1010 Earth-like and giant planets, respectively,” said Dr Behroozi and his colleague, Molly Peeples, also of the Space Telescope Science Institute.
“The last star isn’t expected to burn out until 100 trillion years from now. That’s plenty of time for literally anything to happen on the planet landscape,” the astronomers said.
“Future Earths are more likely to appear inside giant galaxy clusters and also in dwarf galaxies, which have yet to use up all their gas for building stars and accompanying planetary systems.”
“By contrast, the Milky Way Galaxy has used up much more of the gas available for future star formation.”
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Peter Behroozi & Molly S. Peeples. 2015. On the history and future of cosmic planet formation. MNRAS 454 (2): 1811-1817; doi: 10.1093/mnras/stv1817
