Dirty stars make good solar system hosts (w/ Video)

Oct 06, 2009

Some stars are lonely behemoths, with no surrounding planets or asteroids, while others sport a skirt of attendant planetary bodies. New research published this week in The Astrophysical Journal Letters explains why the composition of the stars often indicates whether their light shines into deep space, or whether a small fraction shines onto orbiting planets. When a star forms, collapsing from a dense cloud into a luminous ball, it and the disk of dust and gas orbiting it reflect the composition of that original cloud and the elements within it. While some clouds are poor in heavier elements, many have a wealth of these elements. These are the dirty stars that are good solar system hosts.

"When you observe stars, the ones with more heavy elements have more planets," says co-author Mordecai-Mark Mac Low, Curator of Astrophysics at the American Museum of Natural History. "In other words, what's in the disk reflects what's in the star. This is a common sense result." Observation of distant solar systems shows that exoplanets, or planets that orbit stars other than the Sun, are much more abundant around stars that have a greater abundance of elements heavier than helium, like iron and oxygen. These elements are the ones that can turn into rocks or ice.

This video is not supported by your browser at this time.
This clip shows the pebble density as it evolves in the computer simulation. The black regions have no pebbles, blue regions have a moderate density, and bright regions have high density of pebbles. The square represents a small part of the disk of gas and dust that surrounds the star before the planets form, referred to as the protoplanetary disk, seen from above. The drifting pebbles first concentrate in an elongated filament. The filament then breaks into seven gravitationally bound mini-planets. This is only possible because the abundance of heavy elements in the simulated disk is slightly higher than the solar value. Disks that are less dirty, referred to as lower metallicity or fewer heavy elements, would not be able to form planets in these simulations. Credit: Anders Johansen/Sterrewacht Leiden or Leiden Observatory

The new simulations by Mac Low and his colleagues Anders Johansen (Leiden Observatory in the Netherlands) and Andrew Youdin (Canadian Institute of Theoretical Astrophysics at the University of Toronto) compute just how planets and other bodies form as pebbles clump into mini-planets referred to as planetesimals. Their current work hinges on their previously published research (in Nature in 2007) that explains why rocks orbiting a star within the more slowly-revolving gas disk are not quickly dragged into the star itself because of the headwinds they feel.

Like bicyclists drafting behind the leader in the Tour de France, the rocks draft behind each other, so that in orbits with more rocks, they feel less drag and drift towards the star more slowly. Rocks orbiting further out drift into those orbits, until there are so many that gravity can form them into mini-planets. This concentration of orbiting rocks in a gas disk is called a "streaming instability" and is the theoretical work of co-author Youdin. "It's a run-away process. When a small group of rocks distorts the flow of gas, many others rush to line up like lazy cyclists and matter accumulates very quickly," he says.

The team was able to build this mechanism—drag leading to clumping—into a three-dimensional simulation of gas and solid rocks orbiting a star. Their results show that when pebbles, made of heavy elements, constitute less than one percent of the gas mass, clumping is weak. But if the fraction of pebbles is increased slightly, the clumping increases dramatically and quickly results in the accretion of sufficient material to make larger-scale planetesimals. These mini-planets work as planetary building blocks, merging over millions of years to form planets. In short, clumping of pebbles, when the fraction of solids in the gas is high enough, is the recipe for mini-planet formation, a crucial intermediate step in forming planets.

"There is an extremely steep transition from not being able to make planets at all to easily making planets, by increasing the abundance of heavy elements just a little," says lead author Johansen. "The probability of having planets almost explodes."

Youdin adds that "There's an inherent advantage in being born rich, in terms of solid rocks. But less advantaged systems, like our own Solar System, can still make planets if they work to marshal their resources and hang onto their solids as the gas evaporates away. So the Sun is middle-class, rather than rich." The Sun's abundance of heavy elements suggests its protoplanetary disk (the disk from which the Solar System formed) had close to the critical ratio of pebbles to gas; if the abundance of heavy elements had been slightly less, planetesimals and would have been far less likely to form, and we would not be here to study the question.

The results of this paper will be presented on October 8, 2009 at a meeting of the Division of Planetary Sciences of the American Astronomical Society in Puerto Rico.

Source: American Museum of Natural History

Explore further: Image: NGC 6872 in the constellation of Pavo

add to favorites email to friend print save as pdf

Related Stories

Planetary systems can form around binary stars

Jan 10, 2006

New theoretical work shows that gas-giant planet formation can occur around binary stars in much the same way that it occurs around single stars like the Sun. The work is presented today by Dr. Alan Boss of ...

Planets by the Dozen

May 09, 2008

You know the planets of our solar system, each a unique world with its own distinctive appearance, size, and chemistry. Mars, with its bitter-cold, rusty red sands; Venus, a fiery world shrouded in thick clouds ...

Interiors of extrasolar planets: A first step

May 30, 2006

A team of European astronomers, led by T. Guillot (CNRS, Observatoire de la Côte d’Azur, France), will publish a new study of the physics of Pegasids (also known as hot Jupiters) in Astronomy & Astrophysics. They f ...

Recommended for you

Image: NGC 6872 in the constellation of Pavo

14 hours ago

This picture, taken by the NASA/ESA Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2), shows a galaxy known as NGC 6872 in the constellation of Pavo (The Peacock). Its unusual shape is caused ...

Measuring the proper motion of a galaxy

14 hours ago

The motion of a star relative to us can be determined by measuring two quantities, radial motion and proper motion. Radial motion is the motion of a star along our line of sight. That is, motion directly ...

Gravitational waves according to Planck

Sep 22, 2014

Scientists of the Planck collaboration, and in particular the Trieste team, have conducted a series of in-depth checks on the discovery recently publicized by the Antarctic Observatory, which announced last ...

Infant solar system shows signs of windy weather

Sep 22, 2014

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have observed what may be the first-ever signs of windy weather around a T Tauri star, an infant analog of our own Sun. This may help ...

Finding hints of gravitational waves in the stars

Sep 22, 2014

Scientists have shown how gravitational waves—invisible ripples in the fabric of space and time that propagate through the universe—might be "seen" by looking at the stars. The new model proposes that ...

How gamma ray telescopes work

Sep 22, 2014

Yesterday I talked about the detection of gamma ray bursts, intense blasts of gamma rays that occasionally appear in distant galaxies. Gamma ray bursts were only detected when gamma ray satellites were put ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

omatumr
1 / 5 (1) Oct 07, 2009
"When a star forms, . . .

collapsing from a dense cloud into a luminous ball," it will be an event unlike that which produced the Sun.

With kind regards,
Oliver K. Manuel