Stopping the Suicide Spiral

Ring Nebula
Planetary Protection: X-ray Super-Flares Aid Formation of "Solar Systems" Credit: NASA Chandra

Orion the Hunter is one of the most easily recognized constellations in the night sky, and lying just beneath his belt is the Orion Nebula, a nursery that cradles about 1400 newborn stars. Of these stars that gild Orion’s sword, about 30 of them will grow up to be similar to our own sun.

Half of the young suns in this cluster show evidence of being surrounded by planet-forming disks. In these gaseous envelopes, tiny grains grow into larger rocks, which eventually become the cores of both rocky and gaseous planets.

Astronomers using the Chandra X-Ray Observatory have discovered that the young stars in the Orion Nebula let loose an extraordinary amount of X-rays. They observed 41 powerful X-ray flares during 13 weeks of observation.

"These flares are incredibly strong," says Eric Feigelson of Penn State University, principal investigator for the international Chandra Orion Ultradeep Project. "Even the faintest of the X-ray events seen with Chandra is more powerful than the strongest events seen on the contemporary sun. And they also occur frequently: every few days there’s a big flare in a baby sun, while similar events occur on our sun once every few years."

Our sun is middle aged at about 4.5 billion years old, while the stars observed with Chandra are only between 1 and 10 million years old. Young stars tend to radiate more X-rays than older stars, because their magnetic fields are more unstable.

The X-ray flares, strobing like hot disco lights, may affect the dancing of prepubescent planets in the gas disks. "The disks should experience hundreds of millions of powerful flares while the planets are forming," says Feigelson. There is evidence that the disks absorb rather than reflect energy from the lower energy X-ray flares. This energy should ionize the gas, knocking electrons off the gas atoms and generating electric charges in the disk.

"It sort of resembles the painful shock you get if you short circuited wires on a plug in your house," says Feigelson.

When the ionized gas becomes coupled with this weak electric field, a smooth and calm disk can become turbulent. The scientists believe that this turbulence may somehow act as a protective device for newly forming planets. Without intervention, the gravitational interaction between planetary cores and the disk gas is expected to cause the core to quickly spiral into the star. Turbulence could explain why planetary cores don’t surrender to this gravity.

nebula
Orion as imaged by VLT.
Image Credit: VLT

Today, X-ray flares from our sun can disrupt communication and electricity on orbiting spacecraft and on Earth. But perhaps when the sun was much younger, such X-ray flares allowed the planets in our solar system to maintain their orbital positions.

"We used the Orion Nebula cluster as a virtual time machine in order to view the sun as it appeared four and a half billion years ago," says Scott Wolk of the Harvard-Smithsonian Center for Astrophysics. "We found that the average very young sun-like star has an X-ray flare about once a week. Such flares would have had a profound affect on the material in the solar system, and could even have helped protect Earth from rapidly spiraling in towards the sun and being destroyed."

At the moment this is only conjecture, since it is not known how turbulence in a gas disk affects planets or planetary formation. Feigelson says that studies on the effects of disk turbulence are ongoing.

An alternative theory of planetary formation says that disk turbulence could be responsible for forming gas giant planets. Alan Boss of the Carnegie Institution of Washington has suggested that, rather than slowly build up from dust grains, planets like Jupiter might develop due to instabilities in the proto-planetary disk. In his gravitational instability model, spiral arms form in a gas disk and then break up into clumps. These gaseous knots can turn into Jupiter-like planets in as little as a thousand years, rather than the millions of years it would take for planets to develop by dust grain accretion.

However, says Feigelson, "I’m personally worried about the earliest stages of making even pebbles in a turbulent region. It’s not clear how turbulence would collect things together or spread them apart."

Lower mass stars in the Orion Nebula – the ones that will become M or K-class stars someday – were found to have substantially weaker X-ray flares. A question the astronomers hope to answer is whether that means M or K stars will be less likely to hold onto planets. Without X-ray flares to create waves in the gas disk, will nascent planetary cores fall right into low-mass stars?

Most of the extrasolar planets discovered so far orbit stars that are the mass of our sun or greater. Astronomers have just begun the search for gas giant planets orbiting lower mass stars.


The results from the Chandra Orion Ultradeep Project will appear in an upcoming issue of The Astrophysical Journal Supplement.

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