The Old Ones were already ancient when the Earth was born. Five small planets orbit an 11.2 billion-year-old star, making them about 80 per cent as old as the universe itself. That means our galaxy started building rocky planets earlier than we thought.
"Now that we know that these planets can be twice as old as Earth, this opens the possibility for the existence of ancient life in the galaxy," says Tiago Campante at the University of Birmingham in the UK.
NASA's Kepler space telescope spotted the planets around an orange dwarf star called Kepler 444, which is 117 light years away and about 25 per cent smaller than the sun.
Orange dwarfs are considered good candidates for hosting alien life because they can stay stable for up to 30 billion years, compared to the sun's 10 billion years, the time it takes these stars to consume all their hydrogen. For context, the universe is currently 13.8 billion years old.
Metal light
Since, as far as we know, life begins by chance, older planets would have had more time to allow life to get going and evolve. But it was unclear whether planets around such an old star could be rocky – life would have a harder time on gassy planets without a solid surface.
The first stars to form in the universe were made of just hydrogen and helium, and forged heavier elements in their interior before exploding. The next generation of stars emerged from their debris, and incorporated those heavier elements into their cores and whatever planets they formed. This means thatin general, older stars have fewer metals.
Until recently, planet-hunters assumed that stars needed metals to form planets, partly because the first planets they discovered all orbited metal-rich stars, and partly because planets themselves are made of heavier stuff than hydrogen and helium. But a 2012 survey of Kepler planets showed that low-metal stars could host relatively small planets.
"We knew beforehand that small planets could exist around stars of any metallicity, but it was not really well known if we could go down to Earth-sized planets," Campante says.
Kepler 444's planets are all smaller than Earth, ranging from 0.4 to 0.74 times Earth's radius. Kepler data suggests that planets tend to be rocky when they're smaller than 1.7 Earth radii, and gaseous when they're bigger, making the Kepler 444 worlds almost certainly rocky. But they orbit scorchingly close to the star: the furthest, Kepler-444f, orbits once every 9.7 days, and the closest, Kepler-444b, every 3.6 days. The length of their orbits are all multiples of each other, meaning they eclipse each other regularly and every so often line up all in a row.
Planets align
"You can imagine if you are standing on the surface of the outermost planet, at some points during the orbit you could look in the direction of the star and see all the other four planets aligned," Campante says. "It must be amazing."
To find out how old the star is, Campante and his colleagues used a technique called astroseismology to measure the age of the star very precisely. With the help of the Kepler telescope's entire four-year data set, the team watched Kepler 444's brightness change over time. These fluctuations reflect vibrations within the star, which tell you its mean density. Because a star converts hydrogen to helium in its core as it ages, changing its density, knowing a star's density tells you how old it is.
This technique gave Kepler 444 an age of 11.2 billion years, plus or minus 1 billion years. That makes it the oldest known system of terrestrial planets in the galaxy – when Earth formed, these planets were already older than our planet is today. (The previous record-holder, a red dwarf known as Kapteyn's star, hosts larger planets that are probably mini-Neptunes.)
"These planets mean it only took the universe a couple billion years to figure out how to build rocky planets, and they've been around for a really long time," says Travis Metcalfe at the Space Science Institute in Boulder, Colorado. While Kepler 444's planets are too hot for life, its age suggests there might be cooler, older worlds elsewhere. "If life needs a long time to develop or lots of places to try to develop, having rocky planets this early in the history of the galaxy means planets with advanced civilisations should be everywhere."