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A Primer on Planet-forming Disks and Flares in Orion

May 10, 2005 ::

Chandra X-ray Image of Orion Nebula

Scientists have recently used Chandra to observe powerful flares on young Sun-like stars in the Orion Nebula. These results show that solar flares may have played an important role in shaping our solar system. Here, we explain more of the science behind these conclusions and give some other details about these deep Chandra observations.

How do we know there are disks around the young suns in Orion?

Hubble Optical Image of Orion

The strongest evidence for planet-forming disks in Orion comes from the images of "proplyds" (short for "protoplanetary disks"). Some of the proplyds in Orion are visible in optical light as disks in silhouette against the bright nebula. In some cases the proplyds are being bombarded with energetic radiation from the brightest stars in Orion, causing them to lose material in a bright, comet-like tail. The Hubble Space Telescope (HST) has obtained dramatic high-resolution images of both types of proplyd.

VLT Infrared Image of Orion Nebula

Infrared radiation can give strong but indirect evidence for the presence of disks. Since disks are composed of gas and dust, and dust glows at infrared wavelengths, young stars with unusually high infrared radiation are often thought to have disks.

Among the 30 young Sun-like stars in Orion observed with Chandra, four are associated with proplyds and thirteen show evidence for disks because of excess infrared radiation.

What is the evidence that planets might form in these disks?

Illustration of our Solar System

Although there is no direct evidence that planets are forming in the Orion Nebula disks, there are compelling hints right here in our solar system that disks are crucial for planet formation. All planets orbiting the Sun, with the exception of Pluto, lie in an almost exactly flat plane. The belt of comets outside of Neptune's orbit, known as the Kuiper Belt, also lies in this plane. These are strong clues that the planets were created from a disk, with the comets as disk debris.

There are other important links as well. For example, the Sun, like most stars, is expected to have formed in a cluster like the Orion Nebula. This shows that if planets survived the Sun's development in a cluster, they can do so in Orion.

Also, the age of the stars in Orion is another piece of evidence. The stars in Orion are young -- between about one and 10 million years. Astronomers think planet-forming disks do not survive much longer than 10 million years. Therefore, stars and disks in Orion are the right age for planets to be forming in Orion.

Indeed, Chandra observations of Orion may show indirect evidence for an early stage of planet formation. The proplyds in the cluster are tilted by different amounts with respect to Chandra's line of sight, with some viewed nearly edge-on. In edge-on proplyds, the X-rays must pass through the material in the disk to reach Chandra's detectors, and thus are a probe of the gas in the disk. In at least one such case, the gas in the disk appears to show a lack of metallic elements. These metals may have been locked up into large dust grains, which is a necessary step in planet formation.

How large are the flares on the young suns in Orion?

Illustration of Large Flares

X-ray flares on the young suns in Orion are much more powerful than the strongest flares seen from the Sun today. One of the flares observed by Chandra is thought to have extended out to almost 20 times the radius of the star. With this super-flare, there may have been direct contact between the inner part of the disk and the loop of the flare, a phenomenon that may channel material from the disk onto the young star. By comparison, the largest flares on mature stars like the Sun usually extend out to about one solar radius, or about 20 times smaller than the giant flare seen in Orion.

What are some other possible effects of powerful X-ray flares from young stars?

Besides affecting planet formation, X-ray flares in the young Orion stars may be able to solve long-standing mysteries about the composition of ancient meteorites here in our own solar system. A mystery has surrounded certain meteorites that contain an enormous number of flash-melted chondrules (small, glassy spheres). Scientists have had a difficult time explaining how these chondrules would have developed in a calm stellar environment. Nor can they easily determine the origin of so-called calcium and aluminum-rich inclusions (CAIs) also found in some meteorites.

Animation of X-ray Flares from a "Young Sun"

The enormously powerful X-ray flares seen in Orion with Chandra might provide the solution. Flares of this magnitude can create thermal flashes or shock waves in the protoplanetary disk. This, in turn, could have melted dustballs into chondrules or CAIs, which were eventually swept up into meteorites. Thus, the Sun in its highly-energetic youth may be responsible for some of the meteorite material we find today.

What are the other 1370 X-ray sources in Orion?

Chandra X-ray Image of Orion Nebula, Full-Field

A few of the X-ray sources are massive stars, including Theta Ori C, a star about 40 times more massive than the Sun and easily the brightest X-ray source in Orion. However, most of the sources are stars that are less massive than the Sun. The lightest of these are nine brown dwarfs, objects with less than a tenth of the mass of the Sun which will never become massive enough to begin hydrogen burning like the Sun.

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