The Universe is passing through the stelliferous era --its peak of star formation--but appears to be still peaking in its formation of planets, according to Dimitar Sasselov, Professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative. There are more stars in the Universe than there are grains of sand on Earth and there are an equal number of planets.
There are 200 billion stars in the Milky Way and 90% are small enough and old enough to have planets in orbit. And only 10% of these stars were formed with enough heavy elements to have Earth-like planets with 2% of these --or 100 million super-Earths and Earths-- will orbit within their star's habitable zone.
Differing from Sasselov, an important study by an international team of astronomers has established that the rate of formation of new stars in the Universe is now only 1/30th of its peak and that this decline is only set to continue.
The accepted model for the evolution of the Universe predicts that stars began to form about 13.4 billion years ago, or around three hundred million years after the Big Bang. Many of these first stars are thought to have been monsters by today's standards, and were probably hundreds of times more massive than our Sun. Such beasts aged very quickly, exhausted their fuel, and exploded as supernovae within a million years or so. Lower mass stars in contrast have much longer lives and last for billions of years. Much of the dust and gas from stellar explosions was (and is still) recycled to form newer and newer generations of stars.
Our Sun, for example, is thought to be a third generation star, and has a very typical mass by today's standards. But regardless of their mass and properties, stars are key ingredients of galaxies like our own Milky Way. Unveiling the history of star formation across cosmic time is fundamental to understanding how galaxies form and evolve. Enlarge This diagram indicates the changing ‘GDP’ of the Universe over time.
The results, reported by a team led by David Sobral of the University of Leiden in the Netherlands, are published in the journal Monthly Notices of the Royal Astronomical Society. Their findings indicate that, measured by mass, the production rate of stars has dropped by 97% since its peak 11 billion years ago.
In the new study, scientists used the UK Infrared Telescope (UKIRT), the Very Large Telescope (VLT) and the Subaru telescope to carry out the most complete survey ever made of star-forming galaxies at different distances, with around ten times the data of any previous effort. With the range of distances, the time taken for the light to reach us means that we see identically selected galaxies at different periods in the history of the universe, so we can really understand how conditions change over time.
By looking at the light from clouds of gas and dust in these galaxies where stars are forming, the team are able to assess the rate at which stars are being born. They find that the production of stars in the universe as a whole has been continuously declining over the last 11 billion years, being 30 times lower today than at its likely peak, 11 billion years ago.
The diagram above shows how the total mass of stars in the Universe should have changed over the last 11 billion years based on the new observations (lines) and how it actually did (symbols; different measurements by other teams). This provides an excellent agreement between both and strengthens the prediction of the new results that no more than a further 5% of stars will come into existence, even if we wait forever.
"You might say that the universe has been suffering from a long, serious "crisis": cosmic GDP output is now only 3% of what it used to be at the peak in star production!,: said Sobral." If the measured decline continues, then no more than 5% more stars will form over the remaining history of the cosmos, even if we wait forever. The research suggests that we live in a universe dominated by old stars. Half of these were born in the 'boom' that took place between 11 and 9 billion years ago and it took more than five times as long to produce the rest. The future may seem rather dark, but we're actually quite lucky to be living in a healthy, star-forming galaxy which is going to be a strong contributor to the new stars that will form.
"Moreover, while these measurements provide a sharp picture of the decline of star-formation in the Universe, they also provide ideal samples to unveil an even more fundamental mystery which is yet to be solved: why?"
According to Harvard's Sasselov, we may soon discover that intelligent life, indeed, may be in it's "very young" stage in the observable Universe. Its 200 billion galaxies show a clear potential to continue on as we see them today for hundreds of billions of years, if not much longer. Because planets and life are so young in our Universe, says Harvard's Dimitar Sasselov, perhaps "the human species are not late comers to the party. We may be among the early ones."
That may explain why we see no evidence of "them" and may go a long way to explaining the famous Fermi Paradox, which asks if there's advanced intelligent life in the Universe, where are they? Why haven't we discovered any evidence of their existence.
The story of the Universe according to Sasselov in is new study, The Life of Super-Earths, looks like this: generations of stars made enough iron and oxygen, silicon and carbon, and all the other elements from the original hydrogen and helium about 13 billion years ago to be able to form the Earth we live on and the planets the Kepler Mission is discovering today.
Stable environments in galaxies that were enriched enough to have planets only became available some nine billion years ago and rocky Earth-like planets and larger super-Earths, only some 7 to 8 billion years ago. And Life had to wait until that time if not later to begin its emergence throughout the Universe. Between 7 and 9 billion years ago, enough heavy elements were available for the complex chemistry needed for life to emerge were in place along with the terrestrial planets with stable environments necessary for chemical concentration.
Enrico Fermi argued that given the old age of the Universe and given the large number of stars and planetary systems and the incredibly short timescale it took humans to develop technology that other origins of life and civilizations in the Milky Way could have had a significant head start and should be significantly more advanced than we are.
Sasselov concludes that the statistical argument for Fermi's Paradox "holds true only if the timescale for the emergence of life is much shorter than the age of the universe, but not so if the two are comparable." The future for life in the Universe looks excellent, says Sasselov.
A preprint of the paper is available from arxiv.org/abs/1202.3436 Journal reference: Monthly Notices of the Royal Astronomical Society
The image at top of the page shows young star cluster RCW 38 aglow With a mysterious X-ray cloudcaptured by the Chandra X-ray Observatory on December 18, 2002. The star cluster RCW 38 is approximately 6,000 light years from Earth.
Image credit: David Sobral