Some planets were already billions of years old when Earth was a mere twinkle in our sun’s eye.
The Kepler space telescope has unearthed the oldest known solar system. Five tiny rocky worlds snuggle up to the 11.2-billion-year-old Kepler 444, a cool red star more than twice as old as our sun. Because planets form at the same time as the stars they orbit, the discovery implies that the universe has been churning out rocky planets throughout its entire history, providing ample time for alien life to develop and perhaps flourish (SN: 2/7/15, p. 7).
Kepler 444 itself, however, is not the best place for life to get going. The star’s planets, all between the sizes of Mercury and Earth, are too close to it for liquid water to endure on their surfaces. The longest year for any of the planets is less than 10 days long, astronomers report online January 27 in the Astrophysical Journal.
“Planets this small have never been found around a star this old,” says study leader Tiago Campante, an astronomer at the University of Birmingham in England. Old stars don’t have as many of the elements essential to planet formation — such as carbon, silicon and iron — as stars that formed later. Astronomers once thought that planets could form only around stars rich in these ingredients. But researchers last year found a mega-Earth — a rocky planet roughly as massive as Neptune — around the 10.4-billion-year-old star Kepler 10 (SN: 7/12/14, p. 10). And Kepler 444 now confirms suspicions that Earth-sized planets can form around a wide variety of stars.
Campante and colleagues discovered the planets, which sit about 116 light-years away in the constellation Lyra, by sifting through data from the Kepler space telescope (SN: 2/7/15, p. 9). Kepler spent four years staring at roughly 150,000 stars, looking for the silhouettes of planets as they pass in front of their suns.
To nail down the age of Kepler 444, the researchers measured the frequency of waves rippling across the surface of the star, which show up as tiny fluctuations in starlight. As stars age, the frequency of these waves drops. By comparing the frequency of the flickering light with calculations that describe how stars evolve, Campante’s team deduced that Kepler 444 is 11.2 billion years old.
Finding planets around a star that old isn’t terribly surprising, says Joshua Winn, an astrophysicist at MIT. “It’s not like there’s any kind of theoretical objection to having planets form so quickly.” What’s groundbreaking, he says, is the precise measurement of the star’s pulsation. Before Kepler, astronomers could make these measurements only for giant stars. “It’s probably the case that many known planets are this old,” he says. But until now, researchers had no way of knowing that.
The discovery shows that a key necessity for life was present relatively early in the universe: a solid surface to call home. “Once you have the rocky planet and the ingredients for life,” says Lisa Kaltenegger, an astronomer at Cornell University, “then you could have life.” While the known planets around Kepler 444 are inhospitable, planets with more temperate climates may have been forming around other stars at the same time. “We just need to find them now.”
Kepler 444 was already 6.6 billion years old when Earth was forming. That’s 2 billion years older than Earth is now. “By the time Earth formed,” Campante says, “these planets had a huge head start.” Modern humans arose in just the last 50,000 years or so, he says, “Imagine having a head start of a few billion years.”