Astronomers have spotted the oldest known set of planets in the Milky Way, a quintet of hot and presumably rocky worlds that is more than twice as old as our solar system. Further study of the ancient system may shed light on the early days of planetary formation in the galaxy.
The parent star of the planetary system is Kepler-444, a sunlike star about 117 light-years from Earth, says Bill Chaplin, an astrophysicist at the University of Birmingham in the United Kingdom.
Chaplin and his colleagues analyzed several years’ worth of data gathered by NASA’s Kepler mission, a space telescope that surveys a region of the galaxy for signs of Earth-size and smaller worlds. In Kepler-444, they seem to have struck a jackpot. In all, five planets pass in front of the star, creating minieclipses that betray the presence and size of the planets as well as how quickly they orbit their parent star. All the planets lie within 12 million kilometers of Kepler-444 and circle it in 10 days or less. (Mercury, at its closest, swings about 46 million km from the sun.)
That’s too close to the star to be in the “Goldilocks zone” of habitability, Chaplin says, so the planets likely don’t host life. Indeed, the orbs have surface temperatures much hotter than Mercury, so any atmospheres or oceans probably have long since been boiled away, leaving nothing but a scorched, rocky surface. The closest planet of the five to Kepler-444 is about the size of Mercury, and the farthest is slightly smaller than Venus or Earth. The other three are about the size of Mars, Chaplin says. “Finding five planets all smaller than Earth in the same distant system, that’s pretty incredible,” says Andrew Vanderburg, an astrophysicist at Harvard University who was not involved with the work.
Kepler has previously found multiplanet systems around other stars, Chaplin says. What’s unusual about this new find is Kepler-444’s great age: The star is about 11.2 billion years old, the team reports online today in The Astrophysical Journal. (By comparison, the universe is about 13.8 billion years old, and our own solar system clocks in at about 4.6 billion years of age.)
To assess Kepler-444’s age, the team analyzed subtle variations in the star’s brightness, as revealed in data samples taken as often as once every minute for a year. Those variations allow astrophysicists to calculate the speed of sound inside the star, which in turn enables the researchers to infer the ratio of hydrogen and helium inside the star—the key to determining how far along the star is in its evolution. The astronomers assume that, as in our solar system, the planets formed quickly after the star did. “By the time the Earth formed, the planets in this system were already older than our planet is today,” Chaplin says.
The immense age of the Kepler-444 system suggests that Earth-class planets could have formed very early in the life of the universe. Spectroscopy of Kepler-444 reveals that the star is iron-poor, so presumably the planets are as well. The planets, Chaplin says, are likely predominantly made of lighter than iron elements such as carbon, nitrogen, silicon, and sulfur.
Further analyses of this ancient system, and others like it, will help scientists better model how and when planets formed in our galaxy and throughout the universe, says Natalie Batalha, an astrobiologist at NASA’s Ames Research Center in Mountain View, California.