A new theory of how planets form finds havens of
stability amid violent turbulence in the swirling gas that surrounds a
young star. These protected areas are where planets can begin to form
without being destroyed. The theory will be published in the February
issue of the journal Icarus.
“This is another way to get a planet started. It marries the two main
theories of planet formation,” said Richard Durisen, professor of
astronomy and chair of that department at Indiana University
Bloomington. Durisen is a leader in the use of computers to model planet
formation.
Watching his simulations run on a computer monitor, it’s easy to imagine
looking down from a vantage point in interstellar space and watching the
process actually happen.
A green disk of gas swirls around a central star. Eventually, spiral
arms of yellow begin to appear within the disk, indicating regions where
the gas is becoming denser. Then a few blobs of red appear, at first
just hints but then gradually more stable. These red regions are even
denser, showing where masses of gas are accumulating that might later
become planets.
The turbulent gases and swirling disks are mathematical constructions
using hydrodynamics and computer graphics. The computer monitor displays
the results of the scientists’ calculations as colorful animations.
“These are the disks of gas and dust that astronomers see around most
young stars, from which planets form,” Durisen explained. “They’re like
a giant whirlpool swirling around the star in orbit. Our own solar
system formed out of such a disk.”
Scientists now know of more than 130 planets around other stars, and
almost all of them are at least as massive as Jupiter. “Gas giant
planets are more common than we could have guessed even 10 years ago,”
he said. “Nature is pretty good at making these planets.”
The key to understanding how planets are made is a phenomenon called
gravitational instabilities, according to Durisen. Scientists have long
thought that if gas disks around stars are massive enough and cold
enough, these instabilities happen, allowing the disk’s gravity to
overwhelm gas pressure and cause parts of the disk to pull together and
form dense clumps, which could become planets.
However, a gravitationally unstable disk is a violent environment.
Interactions with other disk material and other clumps can throw a
potential planet into the central star or tear it apart completely. If
planets are to form in an unstable disk, they need a more protected
environment, and Durisen thinks he has found one.
As his simulations run, rings of gas form in the disk at an edge of an
unstable region and grow more dense. If solid particles accumulating in
a ring quickly migrate to the middle of the ring, the core of a planet
could form much faster.
The time factor is important. A major challenge that Durisen and other
theorists face is a recent discovery by astronomers that giant gas
planets such as Jupiter form fairly quickly by astronomical standards.
They have to — otherwise the gas they need will be gone.
“Astronomers now know that massive disks of gas around young stars tend
to go away over a period of a few million years,” Durisen said. “So
that’s the chance to make gas-rich planets. Jupiter and Saturn and the
planets that are common around other stars are all gas giants, and those
planets have to be made during this few-million-year window when there
is still a substantial amount of gas disk around.”
This need for speed causes problems for any theory with a leisurely
approach to forming planets, such as the core accretion theory that was
the standard model until recently.
“In the core accretion theory, the formation of gas giant planets gets
started by a process similar to the way planets such as Earth
accumulate,” Durisen explained. “Solid objects hit each other and stick
together and grow in size. If a solid object grows to be about 10 times
the mass of Earth, and there’s also gas around, it becomes massive
enough to grab onto a lot of the gas by gravity. Once that happens, you
get rapid growth of a gas giant planet.”
The trouble is, it takes a long time to form a solid core that way —
anywhere from about 10 million to 100 million years. The theory may work
for Jupiter and Saturn, but not for dozens of planets around other
stars. Many of these other planets have several times the mass of
Jupiter, and it’s very hard to make such enormous planets by core
accretion.
The theory that gravitational instabilities by themselves can form gas
giant planets was first proposed more than 50 years ago. It’s recently
been revived because of problems with the core accretion theory. The
idea that vast masses of gas suddenly collapse by gravity to form a
dense object, perhaps in just a few orbits, certainly fits the available
time frame, but it has some problems of its own.
According to the gravitational instability theory, spiral arms form in a
gas disk and then break up into clumps that are in different orbits.
These clumps survive and grow larger until planets form around them.
Durisen sees these clumps in his simulations — but they don’t last
long.
“The clumps fly around and shear out and re-form and are destroyed over
and over again,” he said. “If the gravitational instabilities are strong
enough, a spiral arm will break into clumps. The question is, what
happens to them?”
Co-authors of the paper are IU doctoral student Kai Cai and two of
Durisen’s former students: Annie C. Mejia, postdoctoral fellow in the
Department of Astronomy, University of Washington; and Megan K. Pickett,
associate professor of physics and astronomy, Purdue University Calumet.
Durisen and his research group are supported by NASA’s Origins of Solar
Systems program. More information about the group is available at
http://westworld.astro.indiana.edu/
