Evolution of Planetary Systems





A star-forming region in the nearby Orion Nebula, located just beyond the boundaries of the Local Bubble, approximately 450 parsecs (1,465 light years) from the Solar System. Courtesy NASA/JPL-Caltech/Hubble Space Telescope.

Most stars form out of vast clouds of molecular hydrogen in clusters of 100 or more (Lada & Lada 2003). These hydrogen clouds disperse quickly, often within a few million years. The newly formed stars then pursue their own orbits around the Galactic Core in the main sequence phase of their evolution. Some remain in so-called open clusters along with their stellar siblings. Others travel in loose groups comprising former cluster members, all sharing a common motion through space. These stellar congregations are variously known as moving groups, associations, or dynamical streams. The Local Bubble contains several, including the Beta Pictoris Moving Group, the AB Doradus Moving Group, the Ursa Major Moving Group, and the Tucana-Horologium Association (Lopez-Santiago et al. 2006, Zuckerman & Song 2004, King et al. 2003). These groups tend to break up within one billion years.






Perhaps 80% of young stars are surrounded by dusty clouds of gas, variously known as nebulae, protoplanetary disks, or proplyds (Haisch et al. 2001). These clouds rapidly flatten into extended rotating disks and disperse on time scales of 1 to 10 million years. In most disks, solids condense out of the gases and begin to assemble rocky or icy bodies.

The viscous gobs in this photograph are proplyds surrounding newborn stars in the Orion Nebula. Courtesy NASA/C.R. O'Dell/Rice University.






The proplyd flattens as it spins, and the central star begins to shine through the dust. The outer regions of the nebula cool first, with volatiles freezing out of the gases as the ice line moves inward. Courtesy NASA/JPL-Caltech.






As the primordial nebula flattens into a disk, solids begin clumping together, first as pebbles or snowballs and eventually as planetesimals, which are rocky or icy bodies a kilometer or more in diameter. This artist's impression depicts a protoplanetary disk at the epoch when planetesimals begin to form. Courtesy NASA/JPL-Caltech.






Planetesimals collide as their orbits cross, forming larger and larger objects. When the resulting planetary embryos reach a minimum mass several times that of Earth, they rapidly accrete gas from the protoplanetary disk. This phase of "runaway growth" opens a gap in nebula and breaks it into rings. Planetary cores that manage to accrete large quantities of gas and achieve stable orbits evolve into gas giant planets. Courtesy NASA/JPL-Caltech.






Eventually most of the original disk is dispersed, with fields of dust and debris remaining in belts beyond the influence of planetary perturbations. Debris rings analogous to the Asteroid and Kuiper Belts have been detected in many estrasolar systems. Courtesy T. Pyle/SSC/ NASA/JPL-Caltech.






Under favorable conditions the evolutionary process results in a system of stable planets. Depending on the mass and spectral type of the host star, a planetary system can endure for several hundred million years (for hot, massive A-type stars) or tens of billions of years (for cool, low mass M-type stars). Courtesy NASA/JPL-Caltech.






When a Sun-like star burns through all the hydrogen in its core, it begins a new evolutionary phase. Nuclear fusion consumes first the hydrogen in its outer layers and then the helium in its core. Its diameter expands by a factor of 10 or more as it radiates far more heat and light. The star is now said to have left the main sequence and entered the giant or red giant phase. Among the brightest red giant stars in the Solar neighborhood are Arcturus and Aldebaran. The image above depicts such a star as seen from a desolate planetary companion. Courtesy Jeff Bryant.






Eventually, when the star has exhausted all its nuclear fuel, it throws off mass in a series of explosive contractions. In some cases the result is a roughly spherical gas cloud known as a planetary nebula surrounding the stellar core. One of the best-known examples of such an object is the Cat's Eye Nebula, shown above in a photograph by the Hubble Space Telescope (courtesy NASA/STScI). The distance to the Cat's Eye Nebula has recently been calculated as approximately 1000 parsecs (3,260 light years). Planetary nebulae are extremely short-lived phenomena, at least in astronomical terms, since they dissipate in a matter of millennia to leave behind a white dwarf.








After an aging star has thrown off a large proportion of its mass, the remaining core is exposed as a hot, extremely dense object known as a white dwarf star. Both red giants and white dwarfs are common in the local Solar neighborhood. Planetary systems can evidently survive the cataclysmic events that lead to the formation of a white dwarf, since the nearby star GJ 86 is accompanied by a close-in gas giant planet as well as a white dwarf star orbiting at about 20 AU (i.e., the distance of the planet Uranus from Sol).

The image above shows the white dwarf G29-38, which is surrounded by a debris ring created by the disintegration of one of its surviving comets or asteroids. G29-38 is located at distance of 13.6 parsecs (44 light years) in the direction of Pisces (Debes et al. 2005). Courtesy NASA/JPL-Caltech.



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