The Berkeley Science Review: Articles

From Dust to Dawn
How solar systems arise (view PDF)
by Danae Schulz

A massively dense cloud of molecular hydrogen collapses under its own weight, and a star is born. From this violent beginning, many stars, including our own Sun, go on to exist calmly and happily as energy sources for planetary systems. How does a system transition from its tumultuous formation to a relatively peaceful state? When and how in that process do planets form? To put it simply, how did we get here? Geologists and paleontologists rely on fossils to learn about our planet's past, but the vacuum of space does not keep the same copious records as the stones of the earth. In the absence of such relics, astronomers are taking advantage of space's vast expanses to search for systems that may be analogous to our own but are earlier in their evolutionary process. Instead of looking at actual remnants of our solar system's past, they hope that putting a mirror up to younger systems will provide insight into our system's formation.

Dusty Disks
The journey from new star to mature solar system is long—it may take up to a billion years for a solar system to reach its final, relatively stable configuration—and as yet, not fully understood. However, scientists generally agree on a few basic steps along the way. Every new star is surrounded by a swirling cloud of dust and gas, a remnant of the violent collapse that led to its formation. Under the infl uence of gravity, this initially spherical cloud collapses to form a disk rotating around the star, rather the same way a ball of dough fl attens to form a pizza pie as the baker tosses it into the air. This disk of dust and gas is called a protoplanetary disk, and some of the material within it can eventually become a system of planets orbiting the newly formed star.

The physical process of building a fullgrown planet out of tiny dust particles requires intermediate stages, of which one is the formation of kilometer-sized bodies called planetesimals. It is believed that formation of rocky planets such as our own depends on productive collisions of planetesimals to form larger and larger bodies, but each of these collisions is just as likely to regenerate the dust of the disk. Researchers like Paul Kalas of the astronomy department therefore believe that studying planetesimals could provide insight into the mechanism of planet and solar system formation.

Detecting planetesimals themselves is quite difficult, but it is slightly easier to detect debris disks—rings of dust around the central star that result primarily from planetesimal collisions and are the remnants of planet formation. Our own solar system contains a debris disk in the form of the Kuiper Belt, a collection of relatively large objects traveling in an orbit just outside Neptune's. Compared to newer planetary systems, the Kuiper Belt is a relatively sparse debris disk—sufficient time has passed to allow much of the dust and debris of the protoplanetary disk to clear. Similarly sparse disks are difficult to detect; today's instruments typically are sensitive enough to detect debris disks only of stars younger than our Sun. In the search for the truth about our solar system's past, this condition is ideal.

Kalas and his group are currently searching the galaxy for as many debris disks as they can find. The goal of the project, as described by graduate student Holly Maness, is to "image enough debris disks so that we can begin to draw detailed conclusions about the dynamical structure and evolution of these systems as a whole." For example, once the disks have been imaged, researchers can identify whether it is likely that planets are present by analyzing the gravitational forces on the disk. The distribution of dust can also tell them where in the debris disk planets have formed and how big they are.

Can't See The Planets for the Dust
Recently, Kalas and his colleagues analyzed a debris disk orbiting the star Fomalhaut using data acquired by the Advanced Camera for Surveys (ACS), an instrument on board the Hubble Space Telescope. The study, focused on optical images, detected a striking ring of dust about 25 astronomical units (AUs, the distance between the Earth and the Sun) wide about 145 AU from the star. In contrast, the Kuiper Belt is about 20 AU wide but is only about 30 AU from the Sun. Interestingly, they found that the belt is not centered on Fomalhaut, but rather on a point about 15 AU away, indicating that some other body is exerting a gravitational infl uence on the ring. The inner boundary of the disk is also particularly well defined, and careful analysis indicates that these traits are likely caused by the presence of one or more planetary bodies exerting their influence on the debris disk.

Despite Kalas' success with Fomalhaut, observing distant debris disks is difficult at optical wavelengths because the stars they orbit are very bright, often obscuring the faint signal of the debris disk. However, small (sub-micron sized) particles within a disk refl ect some of the radiation emitted by the central star, primarily in the infrared region of the spectrum. A debris disk can therefore be found by looking for a star system with an excess of infrared radiation. Larger particles (approximately millimeter sized) can also absorb the star's energy and subsequently emit their own thermal radiation at longer wavelengths (in the radio region of the spectrum). Observing systems at different wavelengths, from radio to infrared, can therefore provide a relatively complete picture of all the particles in a debris disk. Now that they have identified one probable planetary system, Kalas and his group hope to find more debris disks to study in different systems.

Most of the work done so far on debris disks has focused on detecting smaller particles because the scattering signals at these shorter wavelengths are relatively strong. However, detecting larger particles is particularly important for understanding disk dynamics. Large grains are primarily infl uenced by gravity and therefore give the most direct information about the presence of massive bodies nearby. Small particles, on the other hand, are subject to additional forces besides simply gravity. Thus, clues about potential planets in their signal can be obscured by other fluctuations.

Unfortunately, the signals emitted at longer wavelengths are very faint and difficult to detect. One solution is to use many small, relatively inexpensive radio telescopes (large dish antenna used to detect these longer wavelength signals) set up in an array, instead of working with one gigantic, and very expensive, antenna. Observers use software to combine the data collected by the separate antennas and generate a map of the debris disk, providing information about both the size and the location of large dust grains and planetesimals within the disk. Two new arrays of antennas, CARMA and ALMA, have recently become available for use, providing exciting potential for new research (see sidebar).

With all these maps, Kalas hopes to further dissect their physical characteristics and identify more potential planetary systems, further developing our picture of solar system formation. But how did these debris disks come to be in the first place?

An Aging Star
On a solar system's journey from an infant protoplanetary disk to a more mature system with a debris disk, it is believed to pass through an intermediate phase called a transition disk. Most stars that have been observed appear to have no disk at all (our own sun would fall into this category). Of the few stars with identified disks of any sort (protoplanetary, debris, or transition), only about 10 percent of them appear to have transition disks. The rarity of observed transition disks is further evidence that they are only a brief stopover on the way to a fully developed star system where all the material from the disk has cleared. The transitional period is also thought to coincide with the time that planets are beginning to form, so understanding them is essential for understanding planet formation. UC Berkeley astronomer Eugene Chiang hopes that studying disks at this point in their evolution can lead to a better understanding of the physical conditions that exist at the time of planet formation and of the development of solar systems in general.

Transition disks arise as the star's gravity pulls in, or accretes, some of the material from the swirling disk of dust and gas that surrounded it at birth. As this process proceeds, it leaves behind a ring of material—the transition disk. Such disks can be identified by a deficit of emission originating from near the star, refl ecting the fact that the area around the star is relatively devoid of dust—a hole has formed. These holes can be very large, reaching 10 AU in size (roughly the distance between Saturn and the Sun).

So, how do the holes get there? Chiang has developed a theory that explains why the inner portion of a protoplanetary disk is continuously eaten away, leaving behind a transition disk that eventually develops into a planetary system and remnant debris disk. Elementary physics dictates that material in orbit around a star should stay in orbit unless some external force causes it to change that orbit. That is, each particle of gas or dust within the protoplanetary disk should just continue to orbit around the star without changing its trajectory. However, temperatures close to the star are so high that the gas surrounding the star can be photoionized: the heat from the star kicks electrons off of gas molecules, giving them a net positive charge. This charge makes the dust particles susceptible to the force of the star's strong magnetic field, which pulls material toward the star. A challege to understanding how this process creates transition disks comes when the majority of the dust and gas is far from the central star. In this scenario, temperatures are much too low to allow direct photoionization to occur; thus, it appears that the star should no longer be able to accrete any material, and the protoplanetary disk should never transition fully into a debris disk. Since evolved debris disks are observed, more distant material must be accreted somehow.

Only a small amount of the gas in the disk needs to become ionized to start the process, because charged gas particles can collide with neutral particles and ionize them. So the problem becomes how to ionize enough gas to initiate this domino effect. The solution? The gas is ionized not by the star's heat, but by X-rays emitted from the star. These X-rays contain enough energy to ionize the atoms they collide with, thereby setting off a domino effect of ionization. Chiang and his group are not the first to propose that X-rays, rather than thermal energy, could ionize a disk's gas, but their study was the first to apply this idea to transition disks and show that it creates a viable model of the accretion process.

The X-rays are absorbed by the outermost layer of dust and therefore cannot penetrate very far into the disk. Just as the rain that falls gets absorbed by the Earth's crust rather than traveling to the planet's core, the radiation from the star that hits the rim of the disk never makes it past the first layer of dust. Therefore, only the material on the rim is subject to ionization and subsequent interaction with the star's magnetic field. Once the material at the rim has been pulled to the star, the X-rays can work on a newly exposed layer of material. Thus, the star slowly eats away at the inner rim of the disk, evacuating the material one layer at a time, moving from the inner edge of the donut to the outer edge. This explains how the disk dissipates over time.

Aside from this qualitative understanding of how the disk transition process can occur, Chiang's theory also provides a quantitative relationship between the size of a given hole and the rate of the inward gas diffusion. It turns out that as the size of the hole increases, more and more dust rains down on the star. As the hole grows, the X-rays must travel further to ionize the gas, which could be expected to slow the diffusion rate. However, a larger hole also presents more surface area for the X-rays to interact with, and Chiang hypothesizes that this is the cause of the increased diffusion rate.

A competing theory argues that formation of the holes within transitional disks is the result of a planet evacuating material from the disk as it progresses through its orbit. This theory depends on the presence of a planet to create the hole in the first place. Chiang's theory, however, does not require the presence of a planet to hold true, but at the same time, the presence of a planet would not alter the explanation for how the protoplanetary disk gets eaten away. Because it is unclear whether fully formed planets exist within transitional disk systems, the theorists might have to wait for Kalas and other observational teams to gather enough data to support one theory over the other.

A Life Through the Ages
From protoplanetary disk to emptying transitional disks to dusty debris disks and onwards, forming planetary systems is a long road, and it is just the beginning of a star's, and planet's, known life. Just as physiologists have long since realized that in order to understand the body and its development, one must study it in all its stages, from infancy to old age; astronomers have now come to the same conclusion. Sophisticated technology and complex mathematical and physical studies are beginning to help us understand how our planet and solar system may have come into existence, providing small parts of the answer to the question of how we got here. Still, astronomers are continuing their quest for new phases in a star's development to better understand the life history of our own solar system and all the myriad solar systems that exist within our galaxy, or even the universe.

Danae Schulz is a graduate student in molecular and cell biology.

Want to know more? Check out:
Paul Kalas's circumstellar disk learning site: astro.berkeley.edu/~kalas/disksite

CARMA telescope: mmarray.org

ALMA telescope: nrao.edu/index.php/about/facilities/alma

Geoff Marcy's exoplanets page: exoplanets.org

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