Monday, October 1, 2007
Puffy debris disks around three nearby stars could harbor Pluto-sized planets-to-be, a new computer model suggests.
The "planet embryos" are predicted to orbit three young, nearby stars, located within about 60 light years or less of our solar system. AU Microscopii (AU Mic) and Beta Pictoris (Beta Pic) are both estimated to be about 12 million years old, while a third star, Fomalhaut, is aged at 200 million years old.
If confirmed, the objects would represent the first evidence of a never-before-observed stage of early planet formation. Another team recently spotted "space lint" around a nearby star that pointed to an even earlier phase of planet building, when baseball-sized clumps of interstellar dust grains are colliding together.
The new finding will be detailed in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
Using NASA's Hubble Space Telescope, the researchers measured the vertical thickness of so-called circumstellar debris disks around the stars, and then used a computer model to calculate the size of planets growing within them.
The thickness of a debris disk depends on the size of objects orbiting inside it. The ring of dust thins as the star system ages, but if enough dust has clumped together to form an embryonic planet, it knocks the other dust grains into eccentric orbits. Over time, this can puff up what was a razor-thin disk.
The new model the researchers created predicts how large the bodies in a disk must be to puff it up to a certain thickness. The results suggest that each of the three stars studied is harboring a Pluto-sized embryonic planet.
Even though the disks are pretty thin, they turn out to be thick enough that there's something in there puffing them up.
At least one of the stars is thought to contain at least one other planet in addition to the circling Pluto-sized planet. The circumstellar disk of Fomalhaut contains a void that scientists think is being cleared out by a Neptune-sized world. The researchers think the embryonic planets predicted by their model are too small to clear gaps like this in the disk.
It is the large distances separating the planet embryos and their stars that have drawn the most criticism. Many find it hard to believe that any planet, even a diminutive Pluto-sized one, could form at such a far distance.
A NASA satellite has captured the first images of a collision between a comet and a solar hurricane. It is the first time scientists have witnessed such an event on another cosmic body.
The phenomenon was caused by a coronal mass ejection, a large cloud of magnetized gas cast into space by the sun. The collision resulted in the complete detachment of the plasma tail of Encke's comet. Observations of the comet reveal the brightening of its tail as the coronal mass ejection swept by and the tail's subsequent separation as it was carried away by the front of the ejection.
This is the first time we've witnessed a collision between a coronal mass ejection and a comet and the surprise of seeing the disconnection of the tail was the icing on the cake.
Encke's comet was traveling within the orbit of Mercury when a coronal mass ejection first crunched the tail then ripped it completely away. The comet is only the second repeating, or periodic, comet ever identified. Halley's comet was the first.
Scientists at the Naval Research Laboratory made the observations using the Heliospheric Imager in its Sun Earth Connection Coronal and Heliospheric Investigation telescope suite aboard the STEREO-A spacecraft. Coronal mass ejections are violent eruptions with masses greater than a few billion tons. They travel from 60 to more than 2,000 miles per second.
They have been compared to hurricanes because of the widespread disruption they can cause when directed at Earth. These solar hurricanes cause geomagnetic storms that can present hazards for satellites, radio communications and power systems. However, coronal mass ejections are spread over a large volume of space, mitigating their mass and power to create an impact softer than a baby's breath.
Scientists have been aware of the disconnection of the entire plasma tail of a comet for some time, but the conditions that lead to these events remained a mystery. It was suspected that coronal mass ejections could be responsible for some of the disconnected events, but the interaction between a coronal mass ejection and a comet never had been observed.
Preliminary analysis suggests the disconnection likely is triggered by what is known as magnetic reconnection, in which the oppositely directed magnetic fields around the comet are crunched together by the magnetic fields in the coronal mass ejection. The comet fields suddenly link together, reconnecting, to release a burst of energy that detaches the comet's tail. A similar process takes place in Earth's magnetosphere during geomagnetic storms, powering the aurora borealis and other phenomena.
Comets are icy leftovers from the solar system's formation billions of years ago. They usually reside in the cold, distant regions of the solar system. Occasionally, the gravitational tug from a planet, another comet or a nearby star sends a comet into the inner solar system, where the sun's heat and radiation vaporizes gas and dust from the comet to form its tail. Comets typically have two tails: one of dust and a fainter one of electrically conducting gas called plasma.