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One hundred billion planets across the Milky Way

Paul Sutherland, Feature writer
Jan 5, 2013, 0:00 UTC

Sen—Our Milky Way galaxy contains at least one hundred billion planets, a new scientific study reveals. The conclusion, following a cosmic census by US astronomers, boosts the chances of alien life elsewhere in the Universe.

More than 850 alien worlds, or exoplanets, have been detected orbiting other stars in the past 20 years. That's an impressive number, though well short of the one hundred billion assessment. So how did the team at the California Institute of Technology (Caltech) reach their astonishing figure?

The answer is they looked at a solar system of five planets that have been found orbiting a star labelled Kepler-32. Two of the planets had been discovered earlier by other astronomers and the remaining three by the Caltech researchers.

They analysed the make-up of the Kepler-32 planetary system, which has a type of star called an M dwarf as its central sun, and then compared it to other stars with planets detected by the Kepler space telescope. Because the planets pass in transit across the face of the star, the fluctuations in its starlight gave away detailed information about the planets' nature and composition.

Around three-quarters of all stars in the Milky Way are thought to be M dwarfs and planets that have been discovered orbiting others of this type appear to be similar to those belonging to Kepler-32.

This allowed the Caltech team to reach their estimate for the size of our galaxy's planetary population - and they say that if anything, it is likely to be an underestimate because they only calculated for planets orbiting close in to their host stars and not the outer worlds.

Caltech assistant professor of planetary astronomy John Johnson, who co-authored the study, said: "There's at least 100 billion planets in the galaxy—just our galaxy. That's mind-boggling."

He added: "I usually try not to call things 'Rosetta stones,' but this (Kepler-32) is as close to a Rosetta stone as anything I've seen. It's like unlocking a language that we're trying to understand—the language of planet formation."

Kepler space telescopeArtwork showing how the Kepler observatory is working in deep space. Credit: NASA

Colleague Dr Jonathan Swift, lead author of the paper which will appear in the Astrophysical Journal, said: "It's a staggering number, if you think about it. Basically there's one of these planets per star."

Planetary systems like Kepler-32's differ from our own Solar System. The star is smaller and cooler than the Sun and its five planets lie so close to it that they would all orbit well within the orbit of Mercury. It is thought they formed further out but migrated inwards.

However, the fact the host stars are cooler means that some of their planets might lie within their so-called habitable zones, where water can exist as a liquid, despite being so close in.

The new stellar census supports earlier findings, including a NASA study that suggested in 2010 that nearly one in every four stars like the Sun could host planets the size of our own. That study was made over five years by astronomers from the University of California, using the W M Keck Observatory on Hawaii.