Planetary Predictions Prove True
Australian scientist scores with forecast of Neptune moons - some say it's pluck, others luck. ASTRONOMY
SYDNEY — WEEKS before Voyager 2 reached Uranus in 1986, Andrew Prentice predicted two new moons would be found by the tiny spacecraft. He was right and pinpointed their orbits to within 3 percent. After nearly two decades of research on a controversial planetary theory, the plucky Australian scientist thought his work had been vindicated.
But the skeptics, mostly American scientists, weren't buying it.
This year, before the Voyager 2 swept past Neptune, Dr. Prentice again made some lunar predictions. Some were fairly close, others appear to be dead right. Prentice hopes that in the next few weeks and months, as the data on Neptune are analyzed, the doubters will be hushed.
The scientific heresy perpetrated by this mathematician at Monash University in Melbourne has been to rejuvenate a discarded theory about the origin of our solar system.
In 1796, Frenchman Pierre Simon de Laplace reckoned the solar system was once a giant spinning nebula or gaseous cloud. As it rotated faster and faster (``like an ice skater drawing in her arms during a spin,'' says Prentice), it collapsed under its own gravity.
The central nebula eventually became our sun. In its early life, it spun off concentric rings of gas. The hoops cooled and formed planets. But Laplace couldn't explain how the gas rings were shed and the theory fell from favor.
After earning a PhD at Oxford University in theoretical astrophysics, Prentice went to the University of Pittsburgh in 1971 and set about updating Laplace's theory. The collapsing solar cloud, he figures, produced powerful convection currents or ``supersonic turbulence.''
These forces periodically caused the sun to abruptly shed gas rings. The cloud contracted after these spasms, leaving the ``poor little gas rings out in space.'' They cooled rapidly and planets were formed as rocks and ice accreted.
But what's this theory got to do with moons around Neptune or Uranus?
``All the gaseous planets - Jupiter, Saturn, Uranus, and Neptune - have satellites [moons] going around them in the same direction. They look like miniature planetary systems and were formed in the same way,'' says Prentice. He notes that the distances of the planets from our sun roughly correlate to the moons orbiting the larger planets.
Using the revised Laplace theory and mathematical formulas, Prentice says he can predict where the gas rings are shed and the masses of the gas rings, and thereby determine the masses of the moons and their densities.
Prior to Voyager's encounter, the best telescopes on Earth had found only two of Neptune's moons. Using his model, Prentice predicted in the July 11 Newton Magazine (a Japanese publication, which printed a paper submitted in May) that the Voyager spacecraft would discover at least four new moons orbiting in the same direction Neptune turns and on the plane of its equator. The first would be found at a distance of five times Neptune's radius.
On July 7, a moon was discovered at 4.7 radii. Adjusting his calculations from that base, Prentice predicted in New Scientist (a British publication, Aug. 5 issue) that additional moons would be found at 3.2, 2.6, 2.35, and possibly as close as 2.2 radii.
Three more moons were found at 2.95, 2.5, and 2.1 radii. His calculations were off by 6 to 8 percent. Two moonlets were found at 2.0 and 1.93. He admits the pint-sized satellites were closer than expected.
On the basis of what's known about Pluto, Prentice predicted Neptune's largest known moon, Triton, would have a density 2.11 times that of water.
Scientists are recalculating Pluto's density and still running numbers on Triton, but Prentice's figure appears likely to be close to the mark.
Despite this remarkable record, Prentice's research papers have consistently been rejected by the respected US planetary science journal Icarus, and by the London-based Nature magazine.
The main obstacle to credibility is the supersonic turbulence theory.
``It's unphysical,'' says longtime critic Dr. Alastair G.W. Cameron of Harvard University's Center for Astrophysics. Supersonic convection currents would not be possible because the shock waves created at supersonic speeds dampen turbulence and it dissipates quickly, argues Professor Cameron.
``There's no basis for his predictions using the supersonic turbulence assumption. Prentice's predictions are based on observations or he's just lucky,'' says the Harvard scientist.
Cameron's comments about supersonic turbulence relate to the effects encountered when an aircraft reaches supersonic speeds and don't correlate with forming a solar system, replies Prentice. He compares the convection currents generated by his collapsing solar cloud to the bombing of Dresden during World War II.
``The city was an inferno with thermals shooting up at 150 meters [490 feet] per second. Twenty miles away you could feel the wind rushing in from behind you. Imagine what kind of unholy firestorm and tremendous convection forces are generated by a sun forming,'' says Prentice.
Next year, the US National Aeronautics and Space Administration will launch the Hubble Space Telescope. Observations of embryonic suns ought to support the supersonic turbulence theory, Prentice says.
And new data about the density and composition of Pluto and Triton may also reinforce the theory. ``Within six months to a year, existing planetary models which don't include the supersonic turbulence theory will be outdated,'' he asserts.
Prentice admits the theory may be flawed. ``I may not be on the center of the trail but I'm close to the ridge.'' And he's upset that because of a few key dissenting views, ``younger scientists aren't getting to hear about an alternative view in leading planetary journals.''
``If there's a better theory out there, why isn't someone using it to make accurate predictions?'' demands Prentice.
If correct, Prentice believes his model of planetary formation ought to apply elsewhere in the universe.
``It means all single stars, which make about 10 percent of the stars in our galaxy, should have planets. To find planets like Earth, with the right temperature, you need stars with a similar mass to our sun. I guess, then, at least 3 percent of all stars would have planets like the Earth.''