Contact: Nikki Staab
nstaab@asu.edu
480-727-9329
Arizona State University
TEMPE, Ariz. – Quick: What’s the order of the planets in the solar system? Need a little help?
Maybe the following mnemonic rings a bell: “My Very Educated Mother Just Served Up Nine Pizzas.”
It’s useful for remembering the order of the planets today, but it wouldn’t have been as useful
in the past, and not just because the International Astronomical Union demoted Pluto to “dwarf
planet” last year.
The reason this mnemonic wouldn’t have worked is because the planets weren’t
always in the order they are today. Four billion years ago, early in the solar system’s evolution,
Uranus and Neptune switched places.
This is the result of recent work by Steve Desch, assistant professor in the School of Earth and Space Exploration at Arizona State University. The work appears in this week’s Astrophysical Journal. Desch based his conclusion on his calculations of the surface density of the solar nebula.
The solar nebula is the disk of gas and dust out of which all of the planets formed.
The surface density – or mass per area – of the solar nebula protoplanetary disk is a fundamental quantity needed to calculate everything from how fast planets grow to the types of chemicals they are likely to contain.
|
It’s very hard to observe the surface density in protoplanetary disks forming solar systems today,
both because they’re too far away and because most observations detect only dust and miss everything
larger than a baseball.
So for the last 30 years most researchers have relied on an estimate of the
surface density called the Minimum Mass Solar Nebula.
|
|
The idea is simple: take the rocky component
of each planet, add hydrogen and helium until it matches the Sun in composition, and spread the mass
over the area of each planet’s orbit.
The minimum mass solar nebula predicts disk masses not too different from what we can observe in
forming solar systems. But it also predicts low surface densities, with the mass too spread out to
form planets quickly.
“I was thinking about planet formation and noticing that all the current models failed to predict
how Jupiter could grow to its current size in the life time of the solar nebula,” Desch recounts.
“Given Jupiter’s composition and size, models predicted it would take many millions of years for it
to form, and billions of years for Uranus and Neptune – but our solar system isn’t that old,” says
Desch. “That’s when I ran across the Nice model.”
The Nice model (named for the city in France where it was developed) is based on sophisticated
numerical calculations of the planets’ orbits over millions of years. It explains several aspects
about the orbits of Jupiter, Saturn, Uranus and Neptune, as well as the Kuiper Belt of comets beyond,
by assuming the giant planets formed a lot closer together than they’re found today. Neptune, for
example, formed less than half the distance from the Sun that it orbits today. And in 50% of their
simulations, Uranus and Neptune switched places, although there was no way to determine whether they
did or not.
|
The fit varied by only a few percent from the planets’ masses, but only if Uranus and Neptune did
indeed switch places.
“Neptune had to form closer to the sun than Uranus or you don’t get the smooth
profile,” says Desch.
The new findings have other profound implications.
|
|
“The surface density of the solar nebula isn’t what
we originally thought – it is actually much higher – and this has implications for where we formed and
for how fast planets grow. A higher surface density of the solar nebula means that Uranus and Neptune
formed closer and faster, in only 10 million years instead of billions,” says Desch. That’s important
because Uranus and Neptune contain a few Earth masses of hydrogen and helium gas, and observations of
other protoplanetary disks show these gases don’t hang around for more than 10 million years.
In addition to demonstrating for the first time that all of the giant planets can grow within the
lifetime of the solar nebula, Desch also uncovered the reason behind the sharp variation in density
with distance from the Sun.
“The distribution of mass falls off very steeply because the outer edge is constantly being boiled away
through the process of photoevaporation, by the ultraviolet radiation of nearby massive stars.”
Prior to this, researchers had not considered the effects of photoevaporation on the mass distribution
of the solar system. Desch’s work shows that photoevaporation does move mass from the outer edge but
at a fixed rate so it keeps it from spreading out too much, thus aiding planet growth.
So it seems that 4 billion years ago, “My Very Educated Mother Just Served Nine Up Pizzas” would have
been the mnemonic to learn. Says Desch, “This reminds us that the solar system is a dynamic place.
For the first 650 million years of the solar system Neptune was closer to the sun than Uranus – that’s
15% of the history of the solar system. It looked completely different than we see it today.”
SOURCE:
Steven Desch, Steve.desch@asu.edu
Assistant Professor
School of Earth and Space Exploration
(480) 965-7742
MEDIA CONTACT:
Nikki Staab, nstaab@asu.edu
(480) 727-9329 direct line
(602) 710-7169 mobile
Arizona State University
School of Earth and Space Exploration
College of Liberal Arts and Sciences
Tempe, Arizona USA
www.sese.asu.edu
Support our site - learn how you can help!
| 
"NASA's New Horizons spacecraft will reach Pluto in 2015. In the meantime it has provided
new data on the planet Jupiter's atmosphere, rings, magnetosphere and its moons.
On its long way to Pluto and its moon Charon...
The spacecraft is working hard, testing its instruments before the primary goal - Pluto!"
It may appear as colored lights in the sky or a motionless green arc that stretches across
the sky.
This is a light phenomenon, a glowing, shimmering pattern, known as the “Aurora Borealis”,
occurring in northern latitudes. Its name originates from "Aurora", the Roman goddess of dawn and the
Greek name for north wind.
There is also the same phenomenon known as the "Aurora Australis", occurring in southern
latitudes and ... (with images and video)
Recent probes inside comets show it is
overwhelmingly likely that life began in space, according to a new paper by Cardiff University scientists.
A bold new mission to the Moon was launched by the Chinese National Space Administration (CNSA). Chang’e-1 blasted off
from the Xichang Satellite Launch Centre, Sichuan, atop a Long March 3A rocket on October 24, 2007.
Chang’e-1 represents the first step in the Chinese ambition to land robotic explorers on the Moon before 2020.
Chang’e-1 has four mission goals to accomplish. The first is to make three-dimensional images of many lunar
landforms and outline maps of major lunar geological structures ... (with video)
Astronomers have discovered the best “solar twin” to date, using the 2.7-meter Harlan J. Smith Telescope at
McDonald Observatory. 
Using the 3.6-m telescope an international team of astronomers from France, Switzerland, and Portugal
working in the European Southern Observatory (ESO) has discovered the most Earth-like planet outside our
Solar System. The planet's radius is estimated to be only 1.5 times
that of Earth. An unknown world seems to be covered with liquid oceans but may also be a rocky body
similar to Earth..."
"They are visible only at night, floating 50 miles above Earth around the polar regions. Their name is noctilucent ("night-shining" or "luminous at night") clouds. The clouds are wispy, thin and have grown brighter in recent years and they are definitely spreading. Their location is
an atmospheric layer known as "mesosphere" (middle sphere) extending between 50 and 85 km....
(wonderful images)..."
Scrutiny by NASA's newest Mars orbiter is helping scientists learn the stories of some of the
weirdest landscapes on Mars, as well as more familiar-looking parts of the Red Planet ... (with images)
"It resembles a shining UFO or a wide-brimmed hat but... this isn't what it looks like. Its marked
central bulge and its lane of dark interstellar dust give the galaxy its unusual resemblance.
This is a very beautiful compiled image of the Sombrero Galaxy hovering in space. Three separate
observatory images of the galaxy, photographed by NASA's... (wonderful images!)
"
"NASA scientists believe they have found a way to predict the color of plants on planets in other solar systems.
Green, yellow or even red-dominant plants may live on extra-solar planets..."
