September 16, 2009 > TechKnow Talk: From Soup to Spheres: The Birth of Planets
TechKnow Talk: From Soup to Spheres: The Birth of Planets
One need only observe the crater-pocked surface of the moon to realize the solar system was once a very violent place. Out of this crowded, jostling maelstrom emerged our planets. Despite having these nearby examples to study, including one directly beneath our feet, astronomers continue to debate the details of how planets are formed.
Since the first planet beyond our solar system (exoplanet) was discovered in 1992, hundreds of others have been found. Studying these other planetary hatcheries has provided additional clues and supplemented scientific knowledge on the mechanics of planetary formation.
Shortly after the formation of our sun about 4.5 billion years ago, there remained a thin disk of gas and dust stretching out into space for a billion miles or more, remnants of the material from which the sun had coalesced (see the 9/17/08 TechKnow Talk for more).
While light elements, hydrogen and helium, comprised most of the disk, it also contained heavier elements such as carbon and silicon. In areas far from the heat of the newborn sun, gas and dust found in the inner regions was joined by abundant quantities of ice.
This demarcation, sometimes referred to as the "snow line" by astronomers, is important in influencing what type of planets formed. In the inner, warmer portion of the disk, rocky or "terrestrial" planets such as Earth evolved. Further out, where more solid, icy material is available, "gas giants" such as Jupiter were more likely to be created. This is thought to be true in all planetary systems, not just our own.
Let's turn first to the rocky planets. The traditional theory has been that these form when dust particles collide and agglomerate into rocks. This nucleus continues to grow as additional particles collide and stick. As the mass increases, the object sweeps more and more material out of the dust cloud, until its orbital path is relatively free of additional solid material.
However, recent research suggests a less sedate and orderly process. Depending on the size and density of the disk, several hundred objects may reach a mile or so in diameter via the process described above. At this size they have enough mass to accelerate their growth by gravitational attraction and by the larger area their increasing girths sweep clean with each orbit.
These begin to grow more quickly but also smash into each other frequently, breaking apart and re-forming, trading material back and forth until only a few large, rocky planets remain, settled into stable, non-intersecting orbits. The scarred surface of our moon may in part be testament to this chaotic period. In fact, most astronomers believe the moon was broken off the embryonic Earth during a collision with another planetary candidate.
There is less agreement on the processes leading to the formation of the much larger gas giant planets. Because water and some other materials that exist in liquid or gaseous states closer to the star are frozen beyond the snow line, more solid material is available in these outer orbits to support the creation of large objects composed of rock and ice.
The "core accretion theory" proposes the creation of a solid central core, as with the rocky planets. Fed by the abundant ice and rock available in these outer orbits, the core may quickly grow to five or ten times the mass of the Earth, developing sufficient gravitational pull to attract large amounts of gas. Like the rocky planets, these solid-cored gas balls grow quickly, but collide, split apart, and re-combine until only a few behemoths remain.
A second theory of gas giant formation, the "disk instability theory," suggests that a "whirlpool" or turbulence pattern in the disk may create a higher density region of gas, which exerts its gravitational influence to pull more and more gas into itself. As all the hydrogen and helium gas in the neighborhood swirls into this central body, it quickly grows to massive size. It then proceeds to clear its orbit of rocks and ice, building the central core.
To summarize, the core accretion theory begins with the rocky core and builds outward with gas, while the disk instability theory starts with the massive gas sphere, and collects the solid core as an afterthought. As exclusive as these theories seem, many hybrid models have been proposed, incorporating elements of each.
There is also some evidence that ice crystals forming under deep space conditions possess some unusual properties including electrical polarization, i.e., they may become tiny magnets. This would increase their attraction for each other, strengthen their bonds, and promote their agglomeration (accretion).
Astronomers do agree that planets, especially the gas giants, must form soon after the formation of the star, or lose their opportunity. The disk is only rich in planet-making materials for a few million years before they are consumed by the new sun or lost to space. This is the narrow window of time given a body to reach a viable size to contend for planetary status, though it may be hundreds of millions of years before its ultimate fate and orbital path are determined.
More planets outside our solar system are being discovered every week (373 planets around 315 stars so far at this writing). Until 2008, the presence and mass of these exoplanets was inferred from their gravitational effect on the parent stars. But last year the first direct photographic images of exoplanets were captured. All known exoplanets are gas giants; these are easier to detect, since they are so much more massive than rocky planets.
As the number of planetary systems available to study grows, astronomers must adjust theories to explain their observations. For example, there appear to be systems in which gas giants exist inside the snow line, close to their suns. Current theories do not adequately explain how these planets formed, though such observations lend credence to the disk instability model (as there may be insufficient solid material there to develop a giant-sized rocky core without the help of the massive gravity of a gas ball).
In March 2009, NASA launched the Kepler telescope into Earth orbit. This sensitive instrument is an exoplanet hunter, specifically seeking Earth-like, terrestrial planets. It measures tiny variations in the brightness of stars that may be accounted for by orbiting planets passing in front of them. Astronomers are impressed with the capability of Kepler so far and expect it to eventually find many exoplanets.
Though no Earth-like planets have yet been discovered, it is becoming clear that planetary systems are very common. Earth is probably not particularly unusual in its size and general composition, and it may be only a matter of time and better technology before we find a familiar-looking blue globe orbiting a distant star.