Only 8% of the universe’s habitable worlds have formed so far (AAAS)

There are likely hundreds of millions of Earth-like planets in the Milky Way today, but that’s a small fraction of the number that may form throughout the universe in the future, a new study suggests. Using data from the Hubble Space Telescope, researchers estimated the rates of past star and planet formation in the universe, which is now about 13.8 billion years old. They then combined that information with data from previous surveys that estimated the amounts of hydrogen and helium left over from the big bang that still haven’t collapsed to form stars. At the time our solar system formed about 4.6 billion years ago, only about 39% of the hydrogen and helium in our galaxy had collapsed into clouds that then evolved into stars, they say. That means that the remaining 61% is available to form future solar systems that may include Earth-like planets in their habitable zones, the researchers report online today in Monthly Notices of the Royal Astronomical Society. In the universe as a whole, the researchers suggest, only 8% of its original starmaking gases was locked up in stars by Earth’s first birthday. The rest will, over the remaining trillions of years of the universe’s lifetime, coalesce into stars whose solar systems will contain a myriad of Earth-like planets

A galactic vampire - The Milky Way is not as young as it looks (The Economist, 21st Feb)

AS EVERY horror fan knows, the secret of eternal youth is to suck the lifeblood of others. If you are a galaxy, that lifeblood is hydrogen gas, from which stars form. And it seems that some galaxies are indeed able to maintain a youthful appearance by sucking great clouds of the stuff in from intergalactic space.
According to Felix Lockman of America’s National Radio Astronomy Observatory, the Milky Way, humanity’s home galaxy, is one such vampire. Dr Lockman is part of a team that has been using the Green Bank radio telescope in West Virginia, which boasts the world’s largest steerable dish, to study the process.

The mystery to be solved is why, of the visible matter in the Milky Way, around 15% is still in the form of gas and dust. The galaxy is over 13 billion years of age, almost as old as the universe itself. In theory, that is plenty of time for all of its gas to have condensed into stars, as has happened in a class of galaxies known as ellipticals. But the Milky Way is a spiral galaxy, in which active star-formation continues at the rate of about one newborn a year. This must mean that the galactic supply of hydrogen is somehow being replenished. And Dr Lockman and his colleagues think they know how.

In the 1960s a strange intergalactic gas cloud was discovered near the Milky Way by an astronomer called Gail Smith. Smith’s cloud, as it is known, is an elongated structure almost 10,000 light-years from end to end. At roughly a tenth of the diameter of the Milky Way’s disc, that is big even by galactic standards. Smith’s cloud was a puzzle. No other such object had ever been seen. It was therefore put aside by astronomers, for it is hard to do useful science when you have only one example of something. Dr Lockman and his colleagues have now rectified that neglect. Their measurements show the cloud is on a collision course with the Milky Way. It will hit one of the arms of the spiral in about 30m years, and then be absorbed into the galaxy, probably triggering a burst of star formation in the process.
This suggested to the team that they had found the mechanism of eternal galactic youth: galaxies eat gas clouds. If that were true it would imply that such clouds must be quite common.
And that is what theory predicts. Recent calculations about how matter is distributed in the universe suggest that a good number of such clouds should, indeed, be out there. These calculations concern not the hydrogen itself, but the real fabric of the universe: a still-mysterious substance known as dark matter that interacts with the familiar, atomic, sort only through the force of gravity. The calculations suggest that there should be dark-matter globs of the right mass to attract interstellar gas clouds as big as Smith’s. And when Dr Lockman used Green Bank to search for such clouds, he found ten in just one small volume of nearby space.
The mystery of the ever-youthful Milky Way thus seems to be solved. Spiral galaxies are constantly rejuvenated by collisions with things like Smith’s cloud. Eventually, the supply of these clouds will run out, and with it the elixir of galactic life. But that will not be for many billions of years. Meanwhile, the universe’s spiral galaxies will keep on keeping young.

Secrets of Molecule that Helped Build the Universe Exposed

A new study of one of the most fundamental molecules in the universe has given scientists clues into how the very first stars were formed.

For the first time, researchers have calculated the vibration patterns of a compound called H3+ (also known as a triatomic hydrogen ion), which consists of three hydrogen atoms sharing two electrons. Knowing how the molecule can vibrate allows scientists to predict which wavelengths of lightit will emit, giving them a way to identify its signature in astronomical observations.

H3+ is important because it is thought to have been prevalent in the universe just after the Big Bang that started things off around 13.7 billion years ago.

"Most of the universe consists of hydrogen in various forms," University of Arizona chemist Ludwik Adamowicz said in a statement, "but the H3+ ion is the most prevalent molecular ion in interstellar space. It's also one of the most important Molecules in existence."[Wacky Physics: The Coolest Little Particles in Nature]

H3+'s vibration and light-emitting qualities may have enabled it to transfer heat away from the first stars as they were in the process of forming, allowing them to coalesce without overheating and bursting apart.

"There wouldn't be any star formation if there weren't molecules that slowly cool down the forming star by emitting light," said Michele Pavanello, who was a University of Arizona graduate student when he worked on the project. "Astronomers think that the only molecule that could cool down a forming star in that particular time is H3+."


Adamowicz and Pavanello used a computer simulation to model the behavior of H3+, based on quantum mechanics.

"One has to involve a large amount of computations at the quantum mechanical level to predict those vibrations," Adamowicz said. "The role of theory is essentially to simulate those vibrations in the computer and then describe how the molecule is swinging or dancing."

Their simulations predicted numerous potential vibrations that would cause H3+ to emit photons of specific wavelengths, or energies. If telescope observations of a particular cloud in space reveal light of these wavelengths, then astronomers will know the cloud contains H3+.

The calculations should also help scientists understand the complicated physics of how stars form, especially the earliest stars in the universe.

"The only way we can predict how the stars form is if we know very well what the cooling abilities of H3+ are, and we cannot know its cooling ability until we know its vibrational spectrum," Pavanello said. "We need to know what these energy levels are. With this paper, we have pinpointed the energy levels up to a certain energy threshold that is already good enough to generate accurate predictions of the cooling ability of H3+."