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Planet Formation

Phil Armitage has been studying the formation and migration of planets around stars outside our solar system for more than a decade. He and his fellow planetary scientists know that during star formation, about 10% of the available mass doesn't end up in the star; this material becomes the building blocks of planets. In searches for planets circling more than a thousand nearby stars, astronomers had discovered more than 1730 planets by June of 2014. Many of these planets are like Jupiter and Saturn, but some are much smaller, down to masses just a few times that of the Earth. The smaller planets appear to have a rocky composition like Earth’s.

Few extrasolar planets travel in the nearly circular orbits characteristic of Earth and its neighbors. Armitage and his group have investigated mechanisms that produce extra solar planetary systems like ours and different from the Solar System. For instance, they have considered the possibility that large planets form at a similar distance from their star as Jupiter, and then migrate either toward or away from the star. They have studied two mechanisms that could cause such a migration: (1) the interaction of a large planet with the protoplanetary disk in which it was formed and (2) the interaction between two or more massive planets. Both processes appear to be at work.

Giant gas and ice planets form quickly and are often closely packed. This close packing can make them violently unstable. Giant planet collisions can send planets rocketing out of a star system or cause them to have wildly eccentric orbits, setting the state for additional crashes. 

Some gas planets can end up much closer to their star than similar planets in our solar system. They wind up in blistering proximity to their Sun-like parents, orbiting them in 1.2 to 8 days. There is no way these planets could have formed so close to their stars, where they are literally “roasted” by the heat from the star. Some giant gas planets, especially the largest ones, fare even worse than the roasters. They fly all the way into the stars where they are incinerated.

Interactions and collisions of giant gas planets also influence the evolution of inner rocky planets like Earth and Mars as well as the planetoids and debris found beyond the zone where the giant planets form. If the outer planets are relatively well behaved (as was likely in the early Solar System), then the inner planets are free to evolve slowly via collisions with other rocky bodies, possibly into habitable planets. However, violent collisions of giant planets can not only destroy nascent rocky planets, but also eject all the rocky material needed for their formation and growth.

Luckily for us, the Solar System’s four terrestrial planets, including Earth, survived because of the relatively stable orbits of Jupiter, Saturn, Uranus, and Neptune during the 100 million years it took to form the inner planets. Only 15–25% of planetary systems around Sun-like stars are as fortunate, according to a recent study by Armitage and his colleagues. Most such systems not only formed in calm environments, but also still have large disks of gas and other debris in orbits beyond their giant planets. These debris disks mirror the favorable conditions for the formation of rocky planets so well that bright cold dust emission around Sun-like stars could well serve as signposts of terrestrial planet formation.

The Solar System is an exception, however. The Kuiper belt has a relatively meager collection of dust, rocks, and planetesimals (rocky bodies a few kilometers or more in diameter). Its small size is evidence of a somewhat unique history. About 700 million years after the birth of the Solar System, Uranus and Neptune moved far enough away from Jupiter and Saturn to interact with the then much-larger Kuiper belt. This interaction brought Uranus and Neptune into their present orbits. It also destabilized countless planetesimals, hurtling many into outer space and others straight into the heart of the Solar System. This bombardment cratered the Moon and must have inflicted even more damage on the Earth and Mars as well as on the moons of the giant planets. Fortunately for us, the inner planets remained stable. Lulls in the rain of comets and asteroids may even have allowed primitive life forms to survive on Earth.

Having garnered this basic picture of planetary formation around the Sun, Armitage now wants to understand how planetary systems form around stars. He’d  like to figure out the relationship between the origins of hundreds of “nearby” planetary systems in our Galaxy and the history of the Solar System. The solution to these challenges lies in figuring out how to correctly calculate the origin and evolution of a protoplanetary disk starting from the fundamental laws of nature, or “first principles” that govern our Universe. 

Armitage will need to use the information that he and his colleagues have been gathering for years. This information will include an estimate of the variation of the density (with distance from the star) of the disk of gas and dust; an estimate of dust concentration relative to the hydrogen gas that makes up most of the disk; an approximation of the variations in temperature with distance from the central star; and information about how temperature and density gradients affect the interaction of the disk with magnetic fields. He also has to account for the formation of planets inside the disk of gas and dust, their evolution inside a transition disk where gas and dust are being blown away and gravity begins to affect the behavior of new planets, and what happens to the planets when they are solely under the influence of gravity after the disk has disappeared.

Because he wants to answer the question of how giant planets grow up in a real environment, he hopes his new model will reveal why giant gas and ice planets do not typically form far away from the central star—even though protoplanetary disks can extend out to hundreds of AU (astronomical units) from their star. One AU is the distance from the Sun to the Earth, and the planet Neptune at 30 AU is the most distant large body in the Solar System. Around other stars, Jupiter-sized planets are uncommon at distances of 100 AU or more.

Armitage’s experience modeling the Solar System and other star systems has prepared him for the task ahead of modeling the creation of a star system from scratch. As part of this research, he and his colleagues are already making predictions for how Chile’s new radio telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), can identify calm and turbulent parts of a disk, information that will help them refine their model to more closely mirror what actually happens during the formation of a new planetary system. The researchers expect to be working on this project for years to come. 

The group is currently conducting a series of computer simulations of the outer disks around young stars and using new information coming in from ALMA to adjust and enhance its simulations to better reflect actual conditions that lead to the formation of planets around young stars. The researchers were recently able to simulate turbulence strong enough to cause gas in the disk to fall into the star. They found that a relatively weak magnetic field (tens to hundreds of micro-Gauss) threaded through the disk was necessary to simulate ALMA's observations. And, they figured out that the magnetic fields in the disk were likely left over from the process of star formation. 

Jeffrey Linsky investigates the atmospheres of exoplanets. He studies ultraviolet (UV) spectra obtained by the Hubble Space Telescope of M dwarf stars (cool stars), which almost always have exoplanets. He and his colleagues hope to better understand how the interaction of UV radiation from stars with planetary atmospheres drives photochemistry. Linsky is also looking at ways to determinine whether oxygen in an exoplanet's atmosphere is due to photochemistry or the existence of alien life forms. 

Recently, Linsky and his colleagues from CU's Center for Astrophysics and Space Astronomy (CASA) have come up with a strategy for determining whether an exoplanet's atmosphere contains oxygen, ozone, or other molecules that could have been produced by Earth-like organisms such as plants. They propose analyzing spectral lines from the host star's light to determine if the same molecules could exist in the atmosphere without life on the planet. For instance, Lyman-alpha radiation could produce oxygen molecules in an exoplanet's atmosphere via the photodissociation of water and carbon dioxide. 

Linsky and his fellow researchers have developed a new technique to estimate the amount of Lyman-alpha radiation coming from cool stars like the Sun. In the future, this new method will allow astrophysicists to determine whether photodissociation is responsible for part, or all, of the oxygen in an exoplanet atmosphere.