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BOX 2-2 The Origin of Planets
After literally centuries of speculation as to how our own planetary system formed, the past two
decades of ground- and space-based astronomy have resolved the general question of planetary origin:
planets form in the disks of gas, dust, and ice that commonly surround newly born stars.
That such disks are seen around more than 80% of the youngest stars in nearby stellar nurseries
strongly implies that planets are a frequent outcome of star formation. But the details of how planets
form within disks are still being revealed by current astronomical techniques including imaging from
Hubble, Spitzer, and the largest ground-based telescopes, plus theoretical studies including computer
modeling. Disks start out being dominated by gas—the hydrogen and helium of the primordial cosmos
salted with the heavy elements out of which planets and life are composed – and evolve with time into
thinner dust-only structures. While most if not all stars like our Sun may possess disks early in their
histories, how many of these turn into planetary systems is not known.
Over the past decade facilities such as NASA’s Spitzer Space Telescope and the federally
supported CARMA, SMA, and VLA telescopes, and various space- and ground-based coronographic
instruments have advanced our understanding of disk properties and evolution considerably. The next
decade of astronomical facilities should have the capability to see the effects of young planets
embedded within the disks from whence they arose.
Is the typical outcome of planet formation gas giant worlds with panoplies of satellites, like
Jupiter and Saturn, or rocky worlds like the Earth with atmospheres and surface liquids stabilized by
being suitably near to stable parent stars like the Sun, or some completely different kind of object that
is not represented in our Solar System The answer to this question will require a complete census of
planetary systems in the nearby portion of our galaxy. By compiling the statistics of planetary sizes,
masses, and orbits for a range of planetary systems around stars of different masses, compositions, and
ages, it will be possible to gain deep insight into the processes by which worlds such as our own come
into being.
FIGURE 2‐2‐1 Images of dust disks around young stars. Left: image taken with the Hubble Space Telescope of
disk around the young, 5‐million‐year‐old star HD141569. Credit: NASA, M. Clampin (STScI), H. Ford (JHU), G.
Illingworth (UCO/Lick), J. Krist (STScI), D. Ardila (JHU), D. Golimowski (JHU), the ACS Science Team and ESA.
Right: edge‐on view of disk around AU Mic, a nearby 10‐20 million year old star. Reproduced by permission of
AAS. Michael P. Fitzgerald et al., 2007, ApJ, 670, 536.
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