Tuesday, March 20, 2012

The planetary pileups of popular solar system orbits


Some orbits are apparently more popular than others in young solar systems emerging around baby stars - which often results in "planet pileups" and "planet deserts."


Sophisticated computer simulations have revealed a rather plausible explanation for a phenomenon that has long puzzled astronomers. 

Essentially, rather than occupying orbits at regular distances from a star, giant gas planets similar to Jupiter and Saturn appear to prefer residing within certain regions of mature solar systems while staying clear of others.

The planet pileups of popular solar system orbits"Our results show that the final distribution of planets does not vary smoothly with distance from the star," explained Ilaria Pascucci, an assistant professor at the University of Arizona's Lunar and Planetary Laboratory. "Instead, it has clear 'deserts' - deficits of planets - and 'pileups' of planets at particular locations."


Pascucci and Richard Alexander of the University of Leicester identified high-energy radiation from baby sun-like stars as the likely force that carves gaps in protoplanetary disks, the clouds of gas and dust that swirl around young stars and provide the raw materials for planets. The gaps then act as barricades, corralling planets into certain orbits.

Of course, the exact locations of those gaps depend on the planets' mass, but they generally occur in an area between 1 and 2 astronomical units from the star. One astronomical unit, or AU, marks the average distance from the Earth to the sun. 

According to conventional wisdom, a solar system starts out from a cloud of gas and dust. At the center of the prospective solar system, material clumps together, forming a young star. As the baby star grows, its gravitational force increases as well, and it attracts dust and gas from the surrounding cloud.

Accelerated by the growing gravitation of its star, the cloud spins faster and faster, and eventually flattens into what is called a protoplanetary disk. Once the bulk of the star's mass has formed, it is still fed material by its protoplanetary disk, but at a significantly lower rate.

"For a long time, it was assumed that the process of accreting material from the disk onto the star was enough to explain the thinning of the protoplanetary disk over time," said Pascucci. "Our new results suggest that there is another process at work that takes material out of the disk."

That process, called photo-evaporation, works by high-energy photons streaming out of the star and heating the dust and gas on the surface of the protoplanetary disk.

"The disk material that is very close to the star is very hot, but it is held in place by the star's strong gravity," Alexander noted. "Further out in the disk where gravity is much weaker, the heated gas evaporates into space."

However, even further out in the disk, the radiation emanating from the star is not intense enough to heat the gas sufficiently to cause much evaporation. Yet at a distance between 1 and 2 AU, the balancing effects of gravitation and heat clear a gap.

While studying protoplanetary disks, Pascucci also discovered that gas on the surface of the disk was gravitationally unbound and leaving the disk system via photoevaporation. These were the first observations proving that photoevaporation does occur in real systems.

Encouraged by those findings, Alexander and Pascucci subsequently used the ALICE High Performance Computing Facility at the University of Leicester to simulate protoplanetary discs undergoing accretion of material to the central star that took the effects of photo-evaporation into account.

"We don't yet know exactly where and when planets form around young stars, so our models considered developing solar systems with various combinations of giant planets at different locations and different stages in time," Alexander said.

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Monday, March 12, 2012

A Strong Backhand Slap from End of Solar Storm


The solar storm that seemed to be more fizzle than fury got much stronger early Friday before fading again.

At its peak, it was the most potent solar storm since 2004, space weather forecasters said.

No power Relevant Products/Services outages or other technological disturbances were reported from the solar storm that started to peter out late Friday morning.

Solar storms, which can't hurt people, can disturb electric Relevant Products/Services grids, GPS systems, and satellites. They can also spread colorful Northern Lights further south than usual, as the latest storm did early Friday.

And more storms are coming. The federal government's Space Weather Prediction Center says the same area of the sun erupted again Thursday night, with a milder storm expected to reach Earth early Sunday.

The latest storm started with a flare on Tuesday, and had been forecast to be strong and direct, with one scientist predicting it would blast Earth directly like a punch in the nose. But it arrived Thursday morning at mild levels -- at the bottom of the government's 1-5 scale of severity. It strengthened to a level 3 for several hours early Friday as the storm neared its end. Scientists say that's because the magnetic part of the storm flipped direction.

"We were watching the boxer, expecting the punch. It didn't come," said physicist Terry Onsager at the National Oceanic and Atmospheric Administration's space weather center Relevant Products/Services in Boulder, Colo. "It hit us with the back of the hand as it was retreating."

Forecasters can predict a solar storm's speed and strength, but not the direction of its magnetic field. If it is northward, like Earth's, the jolt of energy flows harmlessly around the planet, Onsager said. A southerly direction can cause power outages and other problems.
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Thursday's storm came in northerly, but early Friday switched to the fierce southerly direction. The magnetic part of the storm spent several hours at that strong level, so combined with strong radiation and radio levels, it turned out to be the strongest solar storm since November 2004, said NOAA lead forecaster Bob Rutledge.

Skywatchers reported to NOAA shimmering colorful auroras in Michigan, Wisconsin and Seattle -- areas that don't normally see the Northern Lights -- Rutledge said. Other space weather enthusiasts reported auroras in Alaska, Minnesota, and North Dakota and in the southern hemisphere in Australia and New Zealand.

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Wednesday, February 01, 2012

Solar Storm Blasts Electrons from Earth's Van Allen Belts

Scientists say they have solved the mystery of why electrically-charged particles intent in radiation belts thousands of kilometers above the Earth suddenly vanish and then reappear during periods of heightened solar activity.

NASA-funded researchers at the University of California, Los Angeles (UCLA) tracked the electrons using data collected at once with 11 different spacecraft.

Their findings show that when bursts of solar energy released by storms on the sun strike Earth’s magnetic field, they send electrons in the so-called Van Allen radiation belts hurtling into external space. Within a few days, the depleted radiation rings once again swell with a whole new crop of the sun’s highly-charged electrons, which are so active that they move at almost the speed of light.

The UCLA researchers note that the highly charged particles that escape the Van Allen belts forever stream outward, rather than raining down into Earth’s atmosphere as some theories suggest.
The Van Allen belts are a method of bubble-shaped rings of radiation that encircle the planet. Earth's protective magnetic field holds the Van Allen belts in their spot several tens of thousands of kilometers above its surface, and protects the planet from deadly solar, cosmic and other types of space radiation.

The Van Allen belts are named after late NASA astrophysicist James Van Allen, who confirmed occurrence of the radiation rings in 1958. The revolutionary scientist died in 2006 at the age of 91.

Tuesday, January 17, 2012

"Earth's Twin" May Be in Our Solar System: Saturn's Moon, Titan --Hosts a Layered Atmosphere


Saturn's moon Titan may be more similar to an Earth-like world than previously thought, possessing a layered atmosphere just like our planet. Titan has long held interest for scientists because of its promise, as the only known moon in the solar system that has a dense atmosphere, there has been hope that it might host some form of life. Information provided by three separate spacecraft missions sent to the area has created more speculation about the moon, which is roughly twice the size of our own (which, quite inexplicably, still has no name) but is nine times farther away from the sun and a freezzing -180°C.

The first mission was Voyager 1, which flew by in 1981, followed by Cassini in 2004, and the next year by the Huygens probe, which actually landed on its surface. Despite the massive amounts of data collected by all three vehicles and the dense athmosphere, scientists have still not been able to get conclusive evidence on what is going on with Titan’s atmosphere.

To clear up some of the mystery, the two researchers --Benjamin Charnay, a planetary scientist at France's National Center of Scientific Research and colleague, S├ębastien Lebonnois-- put together a three dimensional computer model that incorporates information collected from all three space vehicles that includes among other things, chemical compositions, dune movement and measurements of wind and cloud formations and were able to conclude that Titan’s atmosphere very clearly has at least one boundary, which is the part of an atmosphere that is impacted by the surface (friction, heat, etc.) and vice-versa.

But they also found evidence that there appears to be a second boundary as well that is likely caused by changes in seasonal air circulation. The lowest layer is most influenced by a planet or moon's surface, and has greatest influence on the surface with clouds and winds, as well as by sculpting dunes found on Titan.

Earth's boundary layer, which is between 1,650 feet and 1.8 miles (500 meters and 3 kilometers) thick, is controlled mostly by solar heat warming the planet's surface. Since Titan is more distnat from the sun, its boundary layer might behave quite differently. Titan's atmosphere is thick and opaque, obscuring our knowledge about its lower layers. "This layer is very important for the climate and weather — we live in the terrestrial boundary layer," said Charnay.

Their simulations revealed the lower atmosphere of Titan appears separated into two layers that are both distinct from the upper atmosphere in terms of temperature. The lowermost boundary layer is shallow, only about 2,600 feet (800 meters) deep and, like Earth's, changes on a daily basis. The layer above, which is 1.2 miles (2 kilometers) deep, changes seasonally.

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