Growing exoplanets

Astronomers weren’t around when the solar system formed, but ever-more-sophisticated computer simulations give them an opportunity to explore young planetary environments. In “How do you make a giant exoplanet?” author Alan P. Boss suggests that the most widely accepted model of planet formation, core accretion, seems to have some shortcomings. Notably, upper limits to Jupiter’s core mass, gleaned from Galileo spacecraft data, seem smaller than theory suggests. Moreover, models have difficulty forming planets like Uranus and Neptune in the few million years protoplanetary disks are thought to endure.

Boss thinks something else is at work. His simulations show that dense, self-gravitating clumps easily form in the gaseous disks around young stars, and they do so rapidly enough to form objects like Uranus and Neptune. Density variations come and go in the spinning disk of gas. Some clumps grow large enough that their own gravity attracts more gas to them, and they continue to grow. A few survive as potential gas-giant cores.

It’s possible that modifications to core-accretion theory, or a hybrid model in which both gravitational instability and core accretion play roles, will prove better able to build planetary systems. Perhaps different astrophysical environments favor different mechanisms. Computer models help scientists sort this out.

Planetary migration, which leads to the shrinking or expansion of orbits, is an important process in a young solar system. Planetary cores excite spiral waves in the star’s accretion disk, but this change in the disk’s gas density gives the cores a gravitational twist. The cores lose orbital energy and spiral in toward the star. This phenomenon, called Type I planetary migration, was first recognized in the late 1970s, but it rose to prominence in the mid-1990s when astronomers began discovering “hot Jupiters” — giant planets orbiting their stars much closer than Mercury orbits the Sun. Planetary orbits can shrink or expand due to gravitational interactions with a disk, other planets, or even a population of small bodies, such as asteroids or Kuiper Belt objects.

As a planet grows beyond about 10 percent of Jupiter’s mass, its interaction with the protoplanetary disk strengthens. The planet steals orbital energy from the part of the disk inside its orbit and deposits orbital energy in the disk outside its orbit. The effect is to repel gas from a zone centered on the planet’s orbit, creating a gap in the disk. The planet may still accrete material by herding gas flowing in from the gap’s edges, but its growth rate diminishes once the gap forms. By the time the planet reaches Jupiter’s mass, the gap is almost entirely evacuated.

Once a gap forms, the planet’s orbital destiny is locked with the disk’s. If internal stresses in the disk cause it to expand away from or toward the star, the planet moves with it, all the while remaining within its gap. This Type II migration can move a planet in either direction. Scattering lots of small bodies, such as asteroids or Kuiper Belt objects, also can produce this type of orbital change. In fact, astronomers think Uranus and Neptune may have moved outward, too. Once they grew massive enough, the two planets began scattering objects in the Kuiper Belt, which expanded their orbits.