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These Earth-Sized Planets Are Over Twice as Old as Ours

And the fact they exist around such an old star suggests there could be ancient life out there with a few billion years’ headstart on us.
​Illustration of Kepler-444 orbited by five planets. Image: Tiago Campante/Peter Devine

​There's an extrasolar system in our galaxy that's home to five Earth-sized planets, and it's over 10 billion years old. That makes it the oldest-known system of planets of this size, and also suggests the possibility that life a lot older than us could exist out there somewhere.

A new paper published in the Astrop​hysical Journal details a star named Kepler-444, which acts as the Sun for our galactic pals. The researchers, led by Tiago Campante at the University of Birmingham in the UK, put the star's age at 11.2 billion years old, which means it was formed in the relatively early years of the universe (which is around 13.8 billion years old). Our Earth is a comparative baby at only 4.5 billion years old.

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"We thus show that Earth-size planets have formed throughout most of the Universe's 13.8-billion-year history, leaving open the possibility for the existence of ancient life in the Galaxy," the researchers write of their findings.

These planets are special for several reasons; to Campante, the most important is their age. "When Earth formed, these five planets were already older than the Earth is today," he told me in a phone call. The researchers used NASA's Kep​ler mission and dated the star using a technique called asteros​eismology, which can determine precise details of a star's size, mass, density, and age by studying their oscillations.

It's unexpected to find planets of this kind in such an old system. Because of its age, Kepler-444 doesn't have as many metals as newer stars—older stars tend to stick to the basic elements like hydrogen and helium.

"You need metals in order to have form planets so if you lack them, partially at least, you would expect planetary systems to be rare," explained Campante. Gas giants, for instance, are known to form around metal-rich stars.

But their finding suggests the same isn't true for smaller planets: Around metal-poor Kepler-444 they found not just one planet, but five. "This is the observational confirmation that planets the size of Earth—and even smaller, because all of them are smaller—can in fact form around these old metal-poor stars," he said. "Earth-sized" here is a rough guide; the five planets are between the sizes of Mercury and Venus.

So could they harbour technically advanced life that's had over twice as long to develop than our own planet's offerings?

In this case, probably not. Campante explained that the planets lie too close to their parent star for water to exist in liquid form, which is generally considered essential for life. But, the discovery nevertheless suggests that there could be other ancient planetary systems out there that might harbour more attractive conditions.

"Think about the possibility now—we're just hypothesising here—of the existence of ancient civilisations in the galaxy, that have a few billion years of head start relevant to us," said Campante. "It opens questions obviously that technically developed civilisations could be there with a few billion years head start."

He added that stars this old aren't as abundant as younger stars. "But I think now astronomers will start turning their telescopes towards these ancient corners of the galaxy."