The Origin of Stars and Planets
Looking up on a clear night from a dark location, we see that the sky is full of stars. Telescopic observations by Galileo revealed that the Milky Way’s white band traversing high across the summer and fall sky can be resolved into countless stars. Gazing upon the winter constellation of Orion, the sharp eye will note the fuzzy Orion Nebula (see in Box 2.4 Figure 2.4.3) with its nursery of stars born “yesterday” in cosmic time—not long after the first humans walked. Nearby is the famed Pleiades star cluster—formed when dinosaurs still roamed Earth. In contrast, some stars of our galaxy are nearly as old as the universe itself. The story of how successive generations of stars form out of the gas and dust in the interstellar medium in both benign and exotic environments is fundamental to our understanding of, on the larger scale, the galaxies in which stars reside and, on the smaller scale, the planetary systems they might host.
This looping animation shows an artist’s impression of how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disc of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 to 2016. Image credit: NASA/JPL-Caltech. AN animation by Ade Ashford.
What was it about the Sun’s birth environment or its star formation process that determined the final properties of our solar system versus that of other planetary systems? (See Box 2.2.) How and on what timescale did the solar mass build up, and how much gas and dust were left over for planet formation? How rapidly did the high-energy radiation of young stars disperse their gas disks, ending the phase of major planet formation? Do all environments yield the same mass distribution of stars, and what determines the lower and upper mass limits in the distribution (Figure 2.8)? What is the star formation history of our galaxy in particular, and of galaxies in general? Does star formation regulate itself, or are there external factors at work?
A key aim of studies in the next decade is to understand, through both observations and theory, the process of star formation over cosmic time. Beginning near
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 (Figure 2.2.1).
That such disks are seen around more than 80 percent 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. Although 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 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.
home, detailed spectroscopic measurements at short radio wavelengths will track the internal dynamics of the dust-enshrouded molecular clouds that fragment and seed the star-forming cores within a few hundred light-years of our Sun (see Figure 2.2.1 in Box 2.2). Given the importance of high-mass stars to the production and dispersal of heavy elements, understanding their proportion in both the benign and the more extreme star-forming environments is critical to tracking the heavy-element history of the universe.