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Milky Way may harbour over six billion Earth-like planets

Image credit: NASA AMES/ W STENZEL.

As many as six billion Earth-like planets are estimated to reside in our galaxy, researchers have said, although only a tiny fraction of these may be home to alien life.

To be considered Earth-like, a planet must be rocky, roughly Earth-sized and orbiting Sun-like (G-type) stars.

It also has to orbit in the habitable zones of its star - the range of distances from a star in which a rocky planet could host liquid water and potentially life on its surface.

“My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” said researcher Michelle Kunimoto, co-author of the new study from University of British Columbia.

“Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimise future missions dedicated to finding exoplanets.”

According to UBC astronomer Jaymie Matthews: “Our Milky Way has as many as 400 billion stars, with seven per cent of them being G-type. That means less than six billion stars may have Earth-like planets in our Galaxy.”

Previous estimates of the frequency of Earth-like planets range from roughly 0.02 potentially habitable planets per Sun-like star, to more than one per Sun-like star.

Typically, planets like Earth are more likely to be missed by a planet search than other types, as they are so small and orbit so far from their stars.

That means that a planet catalogue represents only a small subset of the planets that are actually in orbit around the stars searched. Kunimoto used a technique known as ‘forward modelling’ to overcome these challenges.

Despite the relatively large number of possible habitable planets, another team of researchers from the University of Nottingham has calculated that there may only be around 36 advanced alien races in our galaxy.

They calculated the number of species with the capacity to send radio signals into space by assuming that Earth-like planets would support life and it would take around five billion years for them to evolve.

The research team said they decided to take a new approach to the issue, as calculating possible alien races in the past had been difficult.

“The classic method for estimating the number of intelligent civilisations relies on making guesses of values relating to life, whereby opinions vary quite substantially,” the study’s lead author Dr Tom Westby said.

“Our new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilisations in our galaxy.”

Christopher Conselice, professor of astrophysics, who led the research, said: “There should be at least a few dozen active civilisations in our galaxy under the assumption that it takes five billion years for intelligent life to form on other planets, as on Earth. The idea is looking at evolution, but on a cosmic scale.”

The researchers say the average distance to such civilisations is around 17,000 light years.

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