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Tucson, Ariz. - Since humans first developed critical consciousness, they have striven to understand the mysteries of our solar system. Prehistoric peoples built Stonehenge and other testaments to their understanding of the stars. Around 4000 B.C., Babylonian priests charted timetables of the constellations, while half way around the world early Mayan civilizations produced astounding astronomical feats. And from there, the historic record only avalanched.
Today, contemporary astronomers continue to pursue many of these age-old questions. And now, a wave of new findings may help elucidate how our solar system's Sun and planets formed about 4.5 billion years ago. Moreover, the findings may help expedite the search for extrasolar planetary systems similar to our own.
For several years now, modern astronomers have known about the existence of massive, unstable disks of gas and dust surrounding many stars. The disks revolve around the stars similar to the way the Earth revolves around the Sun. Early on, astronomers believed that the disks were found among stars of all ages. But a new study supports the notion that the disks tend to surround the younger stars in the galaxy. In fact, all stars that are less than 1 million years old seem to be encircled by these disks, according to Rice University astronomer Patrick Hartigan, who, along with Suzan Edwards of Smith College and Louma Ghandour of the University of Massachusetts, presented the new observations at the American Astronomical Society (AAS) meeting here.
The new observations may give a key to understanding the early stages of our solar system. "Now we have an evolutionary sequence among the youngest stars," Hartigan explains. On the basis of comparisons among the mass accretion rates of 42 young stars derived from ground-based observations, the team reports evidence supporting an emerging theory.
Here's how it works. When the disks are very young, some of the material of the disk spirals inward, crashing onto the stellar surface. From the energy released in these impacts, astronomers can determine how rapidly matter accretes onto the star along columns regulated by solar magnetic fields. Young stars accrete material the fastest, the team reports. "In the most extreme cases, stars swallow the equivalent of several times the mass of the Moon each week, and this feast continues for hundreds of thousands of years," Hartigan says.
Not all the material falls onto the central star, however. A fraction of the impact energy may drive collimated supersonic jets of material that shoot outward perpendicular to the plane of the disk into the interstellar medium. The astronomers found that greater mass losses occur most commonly from stars with large accretion rates; this observation suggests that energy of accretion powers stellar jets, though other scientists offer different explanations for the outflow. In addition, the team discovered a direct correlation between accretion and the collimated jets: When accretion stops, the outflow stops. How stars redirect accreting material into jets remains to be ascertained.
But these are not the only processes that may be at work. As the star and disk age, astronomers believe that some of the gas and dust congregates into planets. Initially, particles of the disk start clumping together, gradually growing in mass. During this process, gaps are created in the disk. Eventually, the gaps grow large enough to encompass the entire disk, and accretion onto the star stops. At this stage, which astronomers estimate to occur when the stars are about 1 million years old, the disks become transparent. If this explanation holds up, it would explain why the planets in our solar system all lie in roughly the same plane - the former plane of the disk.
To date, astronomers have not been able to locate other solar systems like ours. However, planetary-sized objects are believed to orbit around some pulsars.
Currently, there are a handful of models to explain exactly how and when planets might form in this evolutionary process, but astronomers have been unable to confirm or dismiss them with the current power of telescopes available - even from the Hubble Space Telescope. "The problem is we don't have the spatial resolution because the early congregations are just so small," Hartigan says.
Nonetheless, astronomers have some best guesses about what might regulate the gravitational and compressional processes that are entailed in one of the leading models. Various factors, such as the initial conditions of the star and the amount of rotation of the system, likely affect where the material accretes. For example, if there is a small amount of rotation in the young star, more material would likely be accreted on to the star and fewer planets would form in the system, Hartigan explains.
Meanwhile, other scientists have been hard at work on other pieces of this puzzle. For one, astrophysicist Alan P. Boss of Carnegie Institution's Department of Terrestrial Magnetism has developed a model that describes the dominant thermodynamic processes functioning in the disk.
On the basis of new calculations published last week in the journal Science, Boss says that in a given solar system, planets the size of Jupiter would likely form at radii similar to that of our Jupiter or at about 5 astronomical units (AU). And the distance depends only weakly on the mass of the central star.
"This is a somewhat surprising answer," Boss says, as the theoretical expectation would be that a Jupiter-like planet orbiting a low-mass star (of about a half of a solar mass) would be found at a distance closer to 1 AU.
Astronomers generally believe that Jupiter's core was formed by the accumulation of icy volatiles. When the early planet's mass was great enough, it captured gas from the disk of the central star to form the outer layers of the planet. In the past, scientists generally assumed that where a Jupiter-like planet could form would depend on the radius of ice condensation or distance where the temperature is about 160 degrees K.
But Boss' model has revealed that such a radius is not in free space about the star, because the formation of the planet occurs inside the dense disk. In other words, in a system with a central star that ranges down to one-half or even one-tenth the size of our Sun, a Jupiter-sized planet would be found at about 4-6 AU. Or, if there is another Jupiter-sized planet out there, it would have to be found at a distance of about 5 AU from its central star.
What's more, Boss' model complements the recent study by Hartigan and his colleagues. "If Jupiters are some day found at such radii from low-mass stars, our confidence in the currently understood theory of protoplanetary disks would be strengthened," Boss says. Much work remains to figure out how the disk evolves and to resolve the looming conundrums, such as how a Jupiter-sized planet could accrue enough mass fast enough to draw gas particles to its surface before the accretion process ceases. Generally, astronomers believe it would take 5-10 million years to accrue enough mass, but low-resolution observations indicate that accretion may stop after about 1 million years.
Another new finding may shed further light on the mechanism of how solar systems are formed. However, the observations would also limit the way in which such systems form. Andrea Ghez, an astronomer at the University of California at Los Angeles, reports that most stars likely form in pairs, or that there is a 90% probability that another star is found within 100 AU of a given newly formed star. In a survey of about 107 stars also released at the AAS meeting, Ghez and her colleagues found that 70% of stars were binary, with a margin of error of 20%. Previously, astronomers had thought the percentage of binaries was closer to 30-50%. Ghez's finding would mean that the probability is at least 50%.
Ghez says the finding "reduces the potential ways in which circumstellar disks could form." Many astronomers believe that in order for planets to form in binary systems, the tandem of stars must be very close together or at great distances from each other, or one must have a negligible mass in the system.
If most stars do indeed prove to be binary stars, "our Sun may be an oddball," Boss says.