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Three Theories of Planet Formation Busted, Expert Says

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January 23, 2013 in Offbeat


Explosion of strange new worlds shows “theory has struck out.”

An illustration of a planet with a highly inclined orbit found around the star HAT-P-11. Image courtesy NAOJ

Richard A. Lovett
for National Geographic News

Published February 22, 2011

The more new planets we find, the less we seem to know about how planetary systems are born, according to a leading planet hunter.

With the tally of confirmed planets orbiting other stars now more than 500, planet hunters are heading for a golden age of discovery, said Geoffrey Marcy of the University of California, Berkeley. (Explore an interactive of the known planets.)

But that bonanza has been a headache for theoreticians, he said, because many of the newly discovered star systems defy existing models of how planets form. (Related: “New Planet Found; Star’s Fourth World Stumps Astronomers.”)

Current theory holds that planets are made from disks of gas and dust left over after star birth.

In our solar system, it’s long been thought that the large, gassy planets such as Jupiter and Saturn initially took shape in the far reaches and then migrated inward, as gravitational drag from leftover gas and dust eroded their orbits.

The migration process halted when most of the gas and dust had been swept up to make various objects, leaving the planets more or less where we find them today.

In theory, other stars with planets should have gotten similar starts. But according to Marcy, theory has implications not born out in reality.

Implication #1: All planetary orbits should be roughly circular.

It’s possible some planets are born with eccentric orbits, moving around their stars in elongated ovals. But as a migrating planet spirals closer toward its star, gravitational drag should smooth out its orbit, like an object circling a drain, Marcy said.

The eight planets of our solar system all have roughly circular orbits, and models of planet-forming disks suggest most other star systems should be the same.

In reality, though, only about one in three of the known exoplanets has a circular or near-circular orbit.

Implication #2: With minor exceptions, everything in a star system should orbit in the same plane and in the same direction.

The eight planets of our solar system orbit in the same direction along what’s called the ecliptic, a flat plane that’s nearly aligned with the sun’s equator. This makes sense if planets take shape inside the flat disks of material that rotate around newborn stars.

Models are based on the notion that gravitational drag in these disks is the main influence on planets as they migrate. Based on this theory, planets should stay in the ecliptic and continue to follow their stars’ rotations.

However, about one in three exoplanets’ orbits are “misaligned.” Some orbit in the opposite directions as their stars’ rotations, and others are tilted out of the ecliptic, like weather satellites crossing over Earth’s Poles rather than the Equator.

“Orbital inclinations are all over the map,” Marcy said.

Implication #3: Neptune-size planets should be rare across the universe.

Theories of gas drag also say that planets between three times Earth’s mass and Jupiter’s mass should be relatively rare. That’s because models suggest that migration speed is proportional to the mass of a planet, said astronomer Alessandro Morbidelli of the Laboratoire Cassiopee in Nice, France.

Planets smaller than Earth can easily survive in the disk because they migrate slowly. Planets between an Earth mass to Uranus mass migrate so fast that they should be engulfed by the central star. Planets that grow fast enough to become gas giants eat up all the gas around them, slowing their migration speeds and giving them a chance to survive.

Based on what planet-hunters are finding, though, UC Berkeley’s Marcy argues that there are too many Neptune-size worlds for theory to be right. (See “Six New Planets: Mini-Neptunes Found Around Sunlike Star.”)

The size range where there should be the fewest planets—3 to 15 times the size of Earth—are actually the most common. Planets substantially smaller than this are still too hard to detect for accurate statistics.

“Theory has struck out,” Marcy said last month at a meeting of the American Astronomical Society in Seattle, Washington.

New Finds Will Continue to Boggle Theory

Marcy thinks part of the problem is that theoreticians have paid too much attention to interactions with gas and dust and not enough attention to interactions between planets.

“This might be the dominant source of migrations, slingshot[ing] them into eccentric orbits and high inclinations,” he said.

Meanwhile, he said, the next generation of planet-hunting instruments will probably add a plethora of weird new exoplanets “that will give the theoreticians yet more reasons to tear out their hair.”

Read more @news.nationalgeographic.com

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2 responses to Three Theories of Planet Formation Busted, Expert Says

  1. with the amount of objects in the universe is it any surprise that we are finding strange things that dont conform to our limited understanding of how planets and stars are accreted, we are like novices trying to figure out a chess game and only seeing 5% of the moves in the game, we may never know how the universe creates all its wonders but (unless we destory our civilisation) we will be continuously learning all we can about astrophysics and the creation of the cosmos

    • Exactly. It appears most modern natural science is convincing the public that they know more than they actually do. Still these scientists are so naive to believe man can figure out the formula for the Universe even though we don’t know what everything is. This dark matter crap is ridiculous, in my opinion it’s nothing more than unaccounted for variables. I am honestly not sure now if the “mainstream scientific community” is simply arrogant or they do it as a publicity stunt to gain confidence/funding. I’d much rather take a humble approach and accept that mankind in all reality understands very little. It is awesome being able to achieve a greater understanding of the cosmos but no matter how high mankind’s knowledge reaches there will always be more to far more to grasp.

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