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November 4, 2013 8:00 pm
Our Milky Way galaxy contains more than 20bn Earth-like planets with temperatures that could sustain life, according to the most comprehensive survey to date of habitable planets beyond the solar system.
Statistical analysis of observations by Kepler, Nasa’s planet-hunting satellite, over three years suggests that 22 per cent of stars have Earth-sized planets in the “Goldilocks zone” that is neither too hot nor too cold for biological processes.
“What this means is, when you look up at the thousands of stars in the night sky, the nearest Sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing,” said Erik Petigura of the University of California, Berkeley.
“I feel tingly inside – this is such a special moment,” added Geoffrey Marcy, a UC Berkeley astronomy professor who has been involved in the search for “extrasolar planets” or exoplanets, since the first one was detected in 1995. “The discovery is a great leap towards the possibility of life, perhaps intelligent life, existing elsewhere.”
Telescopes are not yet sensitive enough to image Earth-like planets elsewhere in the galaxy. Instead, the Kepler space observatory detects them from a minute diminution in the brightness of their parent star – typically by one part in 10,000 – as they transit in front of it.
The smaller the planet and the further out from the star, the harder it is to spot, so the vast majority of the 1,000 or so exoplanets detected so far have been much heavier and much hotter than Earth. Only 10 are a terrestrial size and the right temperature for liquid water to exist on the surface.
The discovery is a great leap towards the possibility of life, perhaps intelligent life, existing elsewhere
- Geoffrey Marcy, astronomy professor, University of California, Berkeley
The UC Berkeley team, working with Nasa scientists, used a statistical technique to extrapolate from these 10 planets – based on the number of stars sampled by Kepler and the small chance that a planet is geometrically aligned to transit as seen from Earth – to reach their remarkable conclusion. Their paper appears in Proceedings of the National Academy of Sciences.
However, the researchers warned that not all 20bn Earth-like planets would be suitable homes for life. “Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbour liquid water suitable for living organisms,” Prof Marcy said. “We don’t know what range of planet types and their environments are suitable for life.”
There may be something exceptional about the geometry of the solar system, for instance the almost circular orbits of the planets, the position of Jupiter and the existence of the moon, that has allowed life to evolve on Earth for almost 4bn years.
Kepler, launched in 2009, stopped working in May but there is a huge trove of data still to be analysed over the next two or three years, said Natalie Batalha, mission scientist of Nasa Ames Research Centre. Meanwhile, Nasa and the European Space Agency are planning future planet-hunting observatories sensitive enough to make direct observations of Earth-like bodies around distant stars.
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