The search for the youngest planet ever discovered leads astronomers far beyond our own solar system, which has been stable for approximately 4.6 billion years. The planets orbiting our sun are considered middle-aged, making the study of planetary infancy possible only through the detection of exoplanets. Observing these newborn celestial bodies is a technical challenge, as they are often obscured by the dense clouds of gas and dust from which they are still emerging. These rare detections offer a direct window into the earliest moments of planet formation, providing a glimpse into the conditions that shape planetary systems across the galaxy.
Setting the Clock: Understanding Planetary Formation Timelines
In astronomical terms, a young planet is measured in millions of years, not billions, and its formation is governed by two main theoretical pathways. The most widely accepted model, core accretion, begins with solid materials in the disk surrounding a young star colliding to form a rocky core. This process is slow, requiring millions of years for the core to become massive enough to rapidly pull in a large gaseous atmosphere. A competing theory, gravitational instability, proposes that planets can form much faster, with dense clumps of gas and dust in the outer disk collapsing directly under their own gravity. This rapid process can form massive gas giants in only a few thousand years.
These formation events take place within a protoplanetary disk, a structure of gas and dust that orbits a newly formed star. The host star is typically in the T-Tauri phase, a pre-main-sequence stage lasting less than ten million years, characterized by extreme magnetic activity and irregular brightness variations. The survival of the disk is the limiting factor for gas giant formation, as the gas component begins to dissipate quickly, often clearing out within five to ten million years.
Identifying the Youngest Known Planet System
The current record holder for the youngest fully-formed transiting planet is TIDYE-1b, with an estimated age of only three million years. This world, orbiting an orange dwarf star approximately 520 light-years from Earth, is categorized as either a super-Earth or a sub-Neptune, with a diameter about eleven times greater than Earth’s. Its detection was important because young planets are typically hidden by the remnants of their natal disks. The planet was found using the transit method, where it passes in front of its star, causing a slight dimming of the starlight.
TIDYE-1b is considered a rarity because its relatively short 8.8-day orbit and young age challenge models suggesting planet formation takes much longer. While TIDYE-1b is a confirmed, fully-formed transiting planet, other worlds are observed while still forming. For instance, the PDS 70 system hosts two directly imaged protoplanets, PDS 70b and PDS 70c, orbiting a star that is about 5.4 million years old. These worlds are still actively accreting material from their surrounding disk.
How Astronomers Date Distant Worlds
Astronomers determine a planet’s age by dating its host star, since the planet must be younger than or the same age as the star it orbits. For young stellar associations, the Lithium Depletion Boundary (LDB) technique is used. This method relies on the fact that lithium is easily destroyed by nuclear fusion in a star’s interior at a predictable point in its evolution. Astronomers observe a stellar cluster and identify the lowest-mass stars that have just begun to burn their lithium, establishing the boundary between lithium-rich and lithium-poor stars.
The luminosity of this boundary correlates directly with the cluster’s age, making the LDB method accurate for stars younger than about 500 million years. For older, main-sequence stars, a different technique called gyrochronology is used. This method measures the star’s rotation rate, relying on the principle that stars slow their spin over time due to magnetic braking. By measuring the rotation period, scientists can estimate the star’s age based on established rotation-age relationships.
Why Finding Newborn Planets Matters
The study of newborn planets provides data to test and refine planet formation theories. Observing planets like TIDYE-1b and the PDS 70 worlds allows scientists to investigate planetary migration, where a planet’s orbit shrinks, moving it closer to its star. Models suggest that much of this migration occurs early, during the first few million years while the protoplanetary disk is still present.
Young planets offer an opportunity to study the primordial state of planetary atmospheres before significant evolution has occurred. Comparative studies of young and old exoplanet populations reveal that gas-rich planets start “puffy” and shrink over millions of years as they cool and lose atmospheric mass. Observing a planet’s atmosphere while it is still forming can reveal its chemical composition and provide clues about whether it formed close to its star or migrated inward.