Highly energetic light emitted by aged stars causes asteroids to keep spinning faster and faster to the point of disintegration, a new study finds. The findings help explain why white dwarf stars—remnants of Sunlike stars—often have metallic asteroid signatures in their light. As an aging star's powerful light pulverizes asteroids in the star's vicinity, a debris field is created, the study suggests. The aging star then transitions into a white dwarf, with the debris field coalescing into a disk around the new stellar remnant. Asteroidal material then rains down onto the white dwarf, polluting its surface and generating an observable metallic signature (Fig. 1). See also: Asteroid; Stellar evolution; White dwarf star
Asteroids represent leftover rocky bits and pieces from planetary formation and are a common cosmic occurrence, with around one million on record in our solar system. If including very small asteroids only a few meters or so in breadth, that total reaches into the hundreds of millions. The vast majority of asteroids in our solar system orbit in a belt between Mars and Jupiter. See also: Jupiter; Mars; Planet; Solar system
Similar to planets, asteroids have an inherent rotation rate as a result of their formation and history of interactions with other bodies. Gradually, the spin of these space rocks can change due to the YORP effect, named after four scientists (Yarkovsky, O'Keefe, Radzievskii, Paddack) who have contributed to its understanding since the late 19th century. Asteroids absorb electromagnetic radiation (in the form of sunlight) in one location and then emit that radiation back out to space from a different location. The resulting energy imbalance induces a torque upon the asteroid, increasing the asteroid's rotation rate over time. When its rotation rate reaches approximately a single revolution every two hours, the asteroid begins to break apart, possibly splitting into a double asteroid (Fig. 2). From there, the asteroid fragments continue to undergo the same process of starlight-triggered rotational increases, breaking into smaller and smaller chunks of rock. See also: Absorption of electromagnetic radiation; Binary and multiple asteroids; Electromagnetic radiation; Torque
In gauging the influence of the YORP effect on asteroid populations, the new study considered the dramatic changes that stars undergo across their billions of years of existence. As typical stars—including the Sun—which are known as main sequence stars near the end of their lives, they enter a giant branch phase. During this phase, which lasts only a few million years, stars greatly expand in size and increase in luminosity. The latter causes the YORP effect to rise dramatically, because the more intense the sunlight hitting the asteroid, the more significant the energy imbalance, and thus, torque, becomes. Asteroids that had been previously subjected to the YORP effect for billions of years without fragmenting now start to break down rapidly. In models run by astronomers, the YORP effect shattered all asteroids, excepting the littlest and most distant, in just one million years. That short span provides amplef time to demolish a solar system's asteroid population before the old star shrinks down into a white dwarf, a diminutive cosmic object the size of Earth. See also: Giant star; Planetary nebula
As for Earth, as well as other planets, it is too big and has too high an internal strength—owing to the chemical bonds of its constituents pressed tightly by gravity—to succumb to the YORP effect. Should Earth survive the Sun's distension into the giant branch phase, the planet would ultimately bear witness to destruction of the asteroid belt, along with the eventual formation of a rocky debris disk around a spent Sun. See also: Chemical bonding; Earth; Sun