Some orbits more popular than others in solar systems

March 19, 2012
Computer simulations suggest high-energy radiation from baby sun-like stars are likely to create gaps in young solar systems, leading to pile-ups of planets in certain orbits. Credit: NASA/JPL-Caltech

( -- In young solar systems emerging around baby stars, some orbits are more popular than others, resulting in “planet pile-ups” and “planet deserts."

Computer simulations have revealed a plausible explanation for a phenomenon that has puzzled astronomers: Rather than occupying orbits at regular distances from a star, giant gas planets similar to Jupiter and Saturn appear to prefer to occupy certain regions in mature solar systems while staying clear of others.

"Our results show that the final distribution of planets does not vary smoothly with distance from the star, but instead has clear ‘deserts' – deficits of planets – and ‘pile-ups' of planets at particular locations," said Ilaria Pascucci, an assistant professor at the University of Arizona's Lunar and Planetary Laboratory.

"Our models offer a plausible explanation for the pile-ups of observed recently detected in exoplanet surveys," said Richard Alexander of the University of Leicester in the United Kingdom.

Alexander and Pascucci 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.

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. The findings are to be published in the journal Monthly Notices of the Royal Astronomical Society.

According to conventional wisdom, a 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 grows 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 much 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," Pascucci explained. "Our new results suggest that there is another process at work that takes material out of the disk."

Pascucci presented the findings at the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas on March 19. 

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 said. "Further out in the disk where gravity is much weaker, the heated gas evaporates into space."

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. But at a distance between 1 and 2 AU, the balancing effects of gravitation and heat clear a gap, the researchers found.

While studying protoplanetary disks, Pascucci found that gas on the surface of the disk was gravitationally unbound and leaving the disk system via photoevaporation, as Alexander had previously predicted. "These were the first observations proving that photoevaporation does occur in real systems," she said.

Encouraged by those findings, Alexander and Pascucci then 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 , so our models considered developing solar systems with various combinations of giant planets at different locations and different stages in time," Alexander said.

The experiments revealed that just as observations of real solar systems have shown, giant planets migrate inward before they finally settle on a stable around their star. This happens because as the star draws in material from the protoplanetary disk, the planets are dragged along, like a celebrity caught in a crowd of fans.

However, the researchers discovered that once a giant planet encounters a gap cleared by photo-evaporation, it stays put.

"The planets either stop right before or behind the gap, creating a pile-up," Pascucci said. "The local concentration of planets leaves behind regions elsewhere in the disk that are devoid of any planets. This uneven distribution is exactly what we see in many newly discovered solar systems."

Once surveys for discovering extrasolar planet systems such as the Kepler Space Telescope project become more sensitive to outer giant planets, Alexander and Pascucci expect to find more and more evidence for the pileup of giant planets around 1 AU.

Pascucci said. "As we discover more exoplanets, we will be able to test these predictions in detail and learn more about the conditions under which form."

Explore further: Gas giants jump into planet formation early

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not rated yet Mar 19, 2012
Does the balance point between photoenergetic effects (which fall off with the square of distance from the surface of the star) and the gravitational effects (which ALSO fall off with the square of distance from the surface of the star) always occur at 1-2 AU for all stars (including red giants), or just for sun-like G-2 stars?
Simple celestial mechanics would suggest that the most massive planet would form near the mean angular moment arm, and that other planets would find their stablest orbits near harmonic resonances with the more massive stars, such as the 5:2 resonance of Jupiter and Saturn.
It's not as if gravity can tell what the composition of a planet is - just its mass.
5 / 5 (1) Mar 19, 2012
"Does the balance point..."? Beats me, but I would think it would vary with the star's mass. While both gravity and radiation are inverse-square effects, gravity varies directly with mass, while stellar luminosity increases much faster than mass. A star with half Sol's mass, and the same age and composition, would have half the gravity at a given distance, but only about 1/28 the luminosity. So the balance point would almost have to change.
not rated yet Mar 19, 2012
It could explain the ancient model of J.Kepler, based on geometry of particle packing.
not rated yet Mar 19, 2012
While both gravity and radiation are inverse-square effects, gravity varies directly with mass, while stellar luminosity increases much faster than mass.

A countereffect would be that with dense protoplanetary disks the radiation intensity drops faster than with the square of the distance (since the disk absorbs some of the photons en route)

Plenty of variables here. Should be interesting to see whether simulations with various starting conditions (amount of gas, angular momentum, element distribution in the disk) result in similar or disimilar orbits.
not rated yet Mar 19, 2012
That is also possible, and would have to be taken into consideration. And, since disks vary in density, even with the same stellar mass, there's more uncertainty. In any case it's unlikely that all stellar masses would produce gaps at the same distance.
1.7 / 5 (6) Mar 20, 2012
It's all very well to find ways to describe/explain why the currently observed exoplanets seem to congregate in specific regions.
However, one is left wondering just how true it can be since the whole simulation theory is based on the nebular model.
This is the same model that fails dismally in describing our own solar system which is right here with us, so why should we trust it to be accurate with things we have zero contact with(except via light).
To name a few challenges:
The sun is tilted 7 degrees to the ecliptic. Major conundrum for a spinning disc scenario.
The sun should have the majority of the angular momentum - but that resides in the planets, specifically the gas giants. Another major fail for spinning disc theory.
Venus spins the wrong way. Major fail for the model.
Uranus is rolling along on it's side. How does this happen in a nebular model?
The gas giants would take longer than the life of the solar system to form, so they shouldn't exist. Major fail.
Too many fails.
5 / 5 (5) Mar 20, 2012
"The sun is tilted 7 degrees to the ecliptic. Major conundrum for a spinning disc scenario."

Not really. The primary plane of the system is determined by Jupiter's gravity, not Earth, and Jupiter's final orbit was determined by perturbations while it was forming. Seven degrees isn't much.

Angular momentum? Most of the momentum in the disk stayed in the disk, not in the Sun, so should be in the planets' orbits, right where it is.

Uranus? Very simple. All it takes is one collision to change the axis of rotation.

Age? Numerous simulations show that a gas giant can form in less than 10 million years.
not rated yet Mar 20, 2012
Kinedryl, That link makes for some interesting reading. Certainly more so than fixing SQL servers :)

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