The first stars, which formed 400 million years after the Big Bang, were massive, short-lived beasts. But their deaths scattered the heavier elements into the Universe, which eased the formation of smaller stars like our Sun. The heavier elements were also essential for the formation of another feature of our Solar System: the planets.

But just because the raw materials for planets were in place, it doesn't mean that the conditions were appropriate for their formation. But a new discovery, made using data from the Kepler mission, suggests what the authors call the "era of planet formation" didn't take too long. The discovery is the Kepler-444 system, which contains five small rocky planets that formed over 11 billion years ago.

As far as the Kepler data was concerned, Kepler-444 was simply a single star with evidence of transiting planets. But an international team of researchers took a closer look with Hawaii's Keck I telescope, which has a significantly higher resolving power than Kepler. And there they spotted a separate object, orbiting with a period of 430 years. An examination of the light from this revealed that it was a binary containing two red dwarfs. That makes Kepler-444 a three-star system.

The third star, the largest of the set, is still somewhat smaller than the Sun (about three-quarters of its mass), putting it in the category called "K dwarfs."

Kepler had seen evidence of five separate transiting planets, but hadn't been able to confirm their existence. By examining the system more closely, the authors are able to conclude that these are very likely to be real planets. To begin with, the host star is unusually fast moving; if there were a background object creating effects that look like transits, its position relative to Kepler-444 would have shifted over time. An examination of archival footage shows nothing of the sort.

The planets themselves are also at what are called resonance points, in that their gravitational interactions should stabilize their orbits. And, if placed around one of the neighboring red dwarfs, the orbits would become unstable. Based on all of these features, the authors conclude that all five signals from Kepler-444 represent planets.

It's a tightly packed system, with the most distant planet completing an orbit in less than ten days and the closest needing only 3.6 days. All of the planets are also small, with radii ranging from 0.4 times that of Earth up to 0.7 times; they increase in size as you get farther from their host star. Because of their small size, the authors conclude that the bodies are almost certainly rocky, and likely Earth-like in composition.

That's also what you'd predict based on the composition of the host star, which has very low levels of heavier elements (technically termed "metal poor"). You might think that heavier elements would be needed to form rocky planets, but it has turned out that you need lots of them to form gas giants. That's because gas giants form by the quick growth of a large, rocky body that's then able to pull in lots of gas via gravity.

Metal poor stars are also typically old, as they've formed before multiple rounds of star birth and death have produced lots of heavier elements. To find out how old, the authors turned to asteroseismology, the study of the equivalent of seismic waves humming through the star, causing it to pulse. Certain types of pulses occur with a regularity that changes with the age of the star.

Thanks to a detailed analysis (which involved, in part, a software package called "Asteroseismology Made Easy"), the authors came up with an estimate of Kepler-444's age: 11.2 billion years. That's only about two billion years after the deaths of the first stars and means these planets were seven billion years old by the time the Earth formed.

That suggests that the era of planet formation probably began shortly after the elements were created that could support it. Which has implications for the presence of life elsewhere in the galaxy; even if you assume life is very improbable, that many billions of years can skew the odds pretty dramatically.

Astrophysical Journal, 2015. DOI: 10.1088/0004-637X/799/2/170. (About DOIs).