How Do Planets Form Around Stars?

We do not know how unique or unusual the Solar System is, but observations of other planetary systems are providing new ideas for how planets form and evolve. Astronomical observations of star-forming regions and young stars, together with hydrodynamic models of star formation, support the conclusion that stars—including the Sun—form by the gravitational collapse of a molecular cloud core composed of materials manufactured and reprocessed in many earlier generations of stars. Because the typical molecular cloud is rotating at the time of collapse, the developing star is surrounded by a rotating disk of gas and dust. Most disks around young stars, as viewed through telescopes, are approximately 99 percent gas and 1 percent dust, but even that

FIGURE 1.1 Hubble Space Telescope images of four protoplanetary disks around young stars in the Orion nebula, located 1,500 light-years from the Sun. The red glow in the center of each disk is a newly formed star approximately 1 million years old. The stars range in mass from 0.3 to 1.5 solar masses. Each image is of a region about 2.6 × 1011 km (400 AU) across and is a composite of three images taken in 1995 with Hubble’s Wide Field and Planetary Camera 2 (WFPC2), through narrow-band filters that admit the light of emission lines of ionized oxygen (represented by blue), hydrogen (green), and nitrogen (red). SOURCE: Mark McCaughrean, Max Planck Institute for Astronomy; C. Robert O’Dell, Rice University; and the National Aeronautics and Space Administration, <http://hubblesite.org/gallery/album/nebula_collection/pr1995045b/>.

small proportion of dust makes the disks opaque at visible wavelengths (Figures 1.1 and 1.2). Gas-giant planets, such as Jupiter and Saturn in our system, are believed to form in such circumstellar disks, but direct astronomical observations of planets forming have not yet been made.

Observations of planets around other nearby stars with masses similar to the Sun indicate that planet formation is a common outcome of star formation, but no star has yet been observed with a system of planets that looks anything like the Solar System. Over 200 extrasolar planets have been discovered by several indirect techniques (e.g., radial velocity of the host star, stellar transit, and microlensing) (Butler et al., 2006; <www.exoplanets.org>). Multiple planets are known to orbit some two dozen stars. The vast majority of these

FIGURE 1.2 Hubble Space Telescope WFPC2 image of Herbig-Haro 30, a prototype of a young (approximately 1-million-year-old) star surrounded by a thin, dark disk and emitting powerful bipolar jets of gas. The disk extends about 6 × 1010 km from left to right in the image, dividing the edge-on nebula in two. The central star is hidden from direct view, but its light reflects off the upper and lower surfaces of the flared disk to produce the pair of reddish nebulae. The gas jets, shown in green, are driven by accretion. SOURCE: Chris Burrows, Space Telescope Science Institute; John Krist, Space Telescope Science Institute; Kare Stapelfeldt, Jet Propulsion Laboratory; and colleagues; the WFPC2 Science Team; and the National Aeronautics and Space Administration, <http://hubblesite.org/gallery/album/entire_collection/pr1999005c/>.



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