Are we alone? Study hints at 300 mn habitable worlds in Milky Way galaxy

Before Nasa retired the Kepler space telescope in 2018, nine years of the telescope's observations revealed that there were billions of planets in our galaxy

At a time when we are looking for answers to the ultimate question – are we alone – and there is a widening space race amid the discovery of water on the Moon and parts of Mars, a new observation has piqued the interest of the astronomical community by suggesting that there could be nearly 300 million potentially habitable worlds within our galaxy.

A new research, published in The Astronomical Journal, observes that “half the stars similar in temperature to our Sun could have a rocky planet capable of supporting liquid water on its surface”.

The habitable zones are part of the exoplanet exploration done using the now-retired Kepler space telescope. According to scientists, some of these potentially habitable worlds could be our interstellar neighbours, barely 30 light years away, the closest one likely being about 20 light years from the Earth.

Exoplanets are planets that orbit a star other than the sun in our solar system. Astronomers have confirmed more than 4,000 exoplanets orbiting distant stars, with at least 1,000 more awaiting confirmation. The first such exoplanets were discovered in 1995. A recent statistical estimate places, on average, at least one planet around every star in the galaxy. That means there could be something on the order of a trillion planets in our galaxy alone, many of them of the Earth’s size.

"Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable," said Steve Bryson, lead author and a researcher at the National Aeronautics and Space Administration’s (Nasa's) Ames Research Center.

Using data from the Kepler, scientists looked at the exoplanets with 0.5 to 1.5 times the radius of the Earth, narrowing in on planets that are most likely rocky. They also focused on stars similar to our Sun in age and temperature, adjusting up to 1,500 degrees Fahrenheit. They combined the Kepler's final dataset of planetary signals with information about each star's energy output from an extensive data trove from the European Space Agency's Gaia mission.

The Gaia mission provided critical data around the amount of energy that falls on a planet from its host star. It should be noted that a planet's atmosphere plays a significant role as it provides information on how much light is needed to allow liquid water on a planet's surface. The researchers estimated an occurrence rate of about 50 per cent — that is, about half the Sun-like stars have rocky planets capable of hosting liquid water on their surfaces.

"We always knew defining habitability simply in terms of a planet's physical distance from a star, so that it's not too hot or cold, left us making a lot of assumptions," said Ravi Kopparapu, an author of the paper and a scientist at Nasa's Goddard Space Flight Center. "Gaia's data on stars allowed us to look at these planets and their stars in an entirely new way," he added.

According to Nasa’s Exoplanet Exploration, the finding is a significant step forward in Kepler's original mission to understand how many potentially habitable worlds exist in the Milky Way galaxy.

"Though this result is far from a final value, and water on a planet's surface is only one of the many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision," Bryson added.

The vastness of the Milky Way galaxy. (Nasa)

Even though more than 4,000 exoplanets have been confirmed in our galaxy, about a fifth of them in the Earth’s size range, the research is still in its nascent stage to find evidence of life on these surfaces.

With the Milky Way galaxy now pushing 14 billion years, the search continues.