The discovery of five archaic planets around one of the oldest stars in the galaxy has raised the possibility that the Milky Way might be home to extremely ancient forms of life.
Astronomers spotted the planets as they circled a star called Kepler 444, which lies 117 light years from Earth in the direction of Lyra, a constellation in the northern sky.
Researchers used variations in the brightness of the star to calculate its age and found that it was among the first generation of stars to illuminate the Milky Way 11.2bn years ago.
The sun, Earth and other planets of the solar system formed much more recently, about 4.5bn years ago, making the Kepler 444 planetary system more than twice as old as our own. By the time the Earth had formed, the planets around Kepler 444 were already older than the Earth is today.
The planets that orbit Kepler 444 are not hospitable to life as we know it. But the discovery of such profoundly old and almost certainly rocky planets suggests that other ancient worlds might lurk around other stars in more habitable reaches of the Milky Way.
“This tells us that these kinds of planets formed very early in the history of the galaxy. If some fraction formed in the habitable zones around their host stars, then you have environments where life may develop, and it could have been there for a very long time,” said Bill Chaplin , professor of astrophysics at the University of Birmingham.
Writing in The Astrophysical Journal , the scientists add: “Earth-size planets have formed throughout most of the Universe’s 13.bn year history, leaving open the possibility for the existence of ancient life in the galaxy.”
The worlds that orbit Kepler 444 are small, at least by planetary standards, and range in size from Mercury to Venus. They are extremely close to their parent star, with all five planets closer in than Mercury is to the sun. A year on each planet lasts fewer than 10 Earth days.
Kepler 444 is about three quarters the size and mass of the sun, and though 700C cooler, the planets circle so close to the star that their surfaces are permanently fried. The habitable zone around the star, where the temperature is just right for liquid water to flow, lies six times further out than the outermost of the five planets, said Chaplin.
Researchers measured the age of the star by using Nasa’s Kepler space telescope to observe minute changes in its brightness. The intensity of stars varies because sound waves trapped inside them make them contract and expand, as though they were breathing. When a star compresses it gets hotter and brighter. When it expands, it gets cooler and dimmer.
How a star’s brightness varies depends on the speed of sound waves within, and that is governed by the composition of its core. This material at the heart of the star changes as it grows old, for example, as the star converts its hydrogen fuel into helium.
“As you change the composition of the core, you change the speed at which sound waves moves through the star, and that affects the periods at which it resonates,” said Chaplin. “Because we can do this astroseismology, we can get very precise measurement of the age. It’s a very old star, over a 11bn years old.”
Astronomers are now keen to discover other ancient planetary systems, in the hope of pinpointing the beginning of the era of planetary formation in the universe.
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