Kepler-444: Ancient Star System Challenges Planet Formation Theories
For decades, scientists assumed rocky planets like Earth were latecomers in the cosmos. They believed such worlds could arise only after generations of stars seeded the galaxy with heavy elements. However, an ancient star system named Kepler-444 is upending that assumption. It contains a compact assembly of small, rocky planets and is roughly 11 billion years old. This extreme age is forcing astronomers to rewrite the timeline of planet formation.
Astronomers discovered Kepler-444 in 2015 using data from NASA’s Kepler space telescope. The system lies about 119 light-years away in the constellation Lyra. Its primary star—a slightly smaller, cooler sun than our own—hosts five known planets. All five race around in under ten days. The star also has two faint companion stars bound to it, making for a complex triple-star arrangement. Such an architecture is unusual. Yet even more extraordinary is the system’s extreme age. Kepler-444’s planets formed when the Milky Way was in its infancy, making this the oldest known collection of Earth-sized worlds.
By the time Earth formed, Kepler-444’s planets were already ancient.
Discovery of an Unlikely System
In 2015, a team at the University of Birmingham announced the discovery of Kepler-444’s planets after spotting telltale patterns in the Kepler telescope data. The spacecraft had detected subtle dips in the star’s light each time a planet passed in front of it. This technique, known as the transit method, revealed five planets tucked extremely close to their star. The discovery immediately stood out. Here was a star more than twice the Sun’s age harboring multiple Earth-sized planets — something unprecedented at the time. It broke records for planetary antiquity. More importantly, it opened a new window into how planetary systems might survive over billions of years.
Scientists quickly recognized that Kepler-444 was more than an outlier; it was an opportunity. By examining such an ancient system, researchers could study a living fossil from the early era of planet formation. As one astronomer put it, this find wasn’t just about adding another entry to the exoplanet catalog. It was about probing how planets form and endure across cosmic time.
Ancient Worlds, New Timeline
Conventional wisdom long held that the universe needed to churn through several generations of stars before it could produce enough “metals” (elements heavier than helium). Only then, the thinking went, would there be sufficient material to build Earth-like planets. Kepler-444 upends that idea. Its planets took shape when the universe was barely one-fifth of its current age. This means planet building was well underway while the cosmos was still very young. That was billions of years earlier than scientists once expected.
“We now know that Earth-sized planets have formed throughout most of the universe’s history,” says Tiago Campante, lead author of the study that unveiled Kepler-444. His team’s finding showed that planet formation was possible nearly as soon as it could be. For context, Kepler-444’s planets began circling their sun roughly seven billion years before our solar system even existed. Such timing vastly extends the timeline for when and where planets can arise.
Notably, Kepler-444’s parent star is relatively metal-poor—a natural consequence of having formed so early in the galaxy’s life. Yet this star still managed to spawn and retain a family of five rocky worlds. That reality challenges the old assumption that a high abundance of heavy elements is a prerequisite for making planets. It turns out small terrestrial planets can emerge even around stars that lack a rich stockpile of metals. In fact, subsequent exoplanet surveys have confirmed that while giant planets prefer metal-rich stars, smaller planets are less picky.
A Compact Cosmic Laboratory
Beyond its ancient birthdate, the configuration of Kepler-444 is remarkable. All five known planets crowd incredibly close to the primary star, each completing an orbit in fewer than ten days. For comparison, Mercury—the innermost planet of our solar system—takes 88 days to loop the Sun. In fact, the entire Kepler-444 planetary family would fit comfortably within Mercury’s orbit. Moreover, these planets are tiny, ranging from roughly the size of Mercury up to slightly smaller than Venus. This tightly packed setup is rare, making Kepler-444 a fascinating outlier in exoplanet surveys.
In addition to the planets, the Kepler-444 system includes two companion stars circling the primary. This triple-star arrangement adds another layer of complexity to an already exotic system. One of the companion stars travels on a highly elongated orbit. It swings far out and then back toward the primary star. Yet despite this gravitational dance, the inner five planets have remained in stable orbits for billions of years. That stability in such a tumultuous environment is surprising. It offers astronomers a natural laboratory. Here, they can test how planets form and persist under the disruptive influence of multiple stars.
Interestingly, the five tiny planets do not line up in neat orbital resonances as seen in some compact systems. Instead, their spacings are irregular, hinting that past gravitational interactions were chaotic. Scientists suspect that gravitational tugs—perhaps from the companion stars or from the planets’ own early movements—shaped this unusual planetary dance. Every aspect of this system’s architecture is teaching researchers something new about the range of possible planetary systems.
Challenging How Planets Form
Kepler-444’s existence is spurring fresh debate about the processes that build planets. One big question is how the planet-forming disk around the primary star produced five worlds in the first place. The two companion stars would have posed significant gravitational challenges during the planets’ birth. The companions’ gravity may have disrupted or even truncated the disk. This interference could have prevented the formation of Jupiter-like giants, leaving only material close to the star. Some astronomers propose that the planets formed very quickly in the inner disk. This would have happened before the companion stars could interfere. Others suggest these planets formed farther out and later migrated inward to their current orbits. Each scenario tests the limits of current planet-formation models.
Meanwhile, astronomers continue to probe Kepler-444 for new clues. Recent observations with advanced instruments have refined our picture of Kepler-444. Data from the Keck Observatory’s spectrograph and the Gaia space telescope helped pin down the orbits of the companion stars. Those precise measurements reveal how the triple-star system has remained dynamically stable. Intriguingly, some analyses hint at a possible sixth planet lurking in the data. This unseen world, if confirmed, would add another twist to the ancient puzzle. Each new discovery forces theorists to refine their models. They must ensure our ideas of planet formation can accommodate the reality of systems like Kepler-444.
Shifting Paradigms in Planetary Science
In the broader context of astronomy, Kepler-444 serves as a striking reminder that planetary systems can defy expectations. It demonstrates that planets can form and survive in conditions once thought too extreme. Such worlds can arise in the universe’s youth and even amid the gravitational chaos of multi-star environments. As a result, scientists are revising theories to account for worlds born under such circumstances. Survey teams are also eyeing the skies for other ancient planetary survivors. The debate over how and when planets form has gained new momentum. But one thing is certain: Kepler-444 has expanded the boundaries of what we thought possible.
