Dusty planetesimal ring orbiting star HR 4796 A

This image of a circumstellar dust ring was recently acquired by C. Thalmann and colleagues at the 8 meter Subaru telescope that the Japanese operate on the summit of mount Mauna Kea in Hawaii. Although the dust ring is nearly circular, the line-of-sight to the ring is oblique which makes the ring appear elongated. To reveal the faint ring, the astronomers must subtract a model image of the very bright central star, but imperfections in the star-subtraction results in the radial residuals seen in the image. Nonetheless the dust ring is still quite prominent despite those residuals, and its radius is about 80 AU. This dust is thought to be the result of collisions occurring among unseen planetesimals also orbiting within the ring. These planetesimals probably resemble the comets that inhabit our Kuiper Belt, which is the Sun's outermost debris belt that is of radius 45 AU. Another interesting feature of this image is that the dust ring's center is offset slightly from the star. Although the origin of this offset is uncertain, an unseen extra-solar planet is implicated, because its gravitational influence can displace the ring's center. But if there is an exoplanet here, it is too faint to be seen or is obscured by the residual starlight. For more information about this circumstellar dust ring, see this press release.

Planet formation and destruction via gravitational instability

This graphic shows a computer simulation of a circumstellar disk that orbits a young star, with each snapshot showing the state of the disk 1,500 years later. This disk is gravitationally unstable, which means that the disk's gravitational attraction for itself causes material to contract into spirals that can also clump up further to form Jupiter-mass protoplanets. The black circles in the above (click picture to zoom in) follow one such clump that first forms at a distance of 300 AU (ie 300 times the Sun-Earth distance) from the central star, which then spirals inwards due to its interactions with the disk. This simulation illustrates one of the difficulties in forming giant planets via gravitational instability, since the clumps that do form by this process also tend to get driven inwards by the disk, where they might accrete onto the central star. For additional details, see the preprint by Zhu and colleagues.

Spiral arms in disk suggests unseen planets

This image of the disk that is in orbit about the young star SAO 206462 suggests that this disk might be perturbed by one or more unseen planets. This image was acquired at the Japenese Subaru telescope in Hawaii by Carol Grady (Eureka Scientific). In this image, the central star is deliberately masked by the telescope's optics, which revealing a broad disk of gas and dust in orbit about the star. The size of this disk is at least twice the diameter of our Solar System. Planets are known to form in these circumstellar disks, and computer simulations of this process show that a young giant planet can also launch spiral density waves in such a disk. So this disk's spiral appearance does suggest that giant planets may have formed here. But keep in mind that this is not the only explanation. For instance, the gravity of passing star can also disturb a disk, and such a disturbance would wind-up over time and also resemble a spiral. But further study of this system may one day reveal whether the disk at SAO 206462 is indeed planet forming. See this press release for more details.

Mass transfer in circumstellar disks

The left graphic is a near-infrared image of the heirachical star system SR24. The lower object is the star SR24S, and the nebulosity surrounding it is a protoplanetary disk that is composed mostly of hydrogen gas plus some dust. The stellar contribution to this image has been subtracted, which is why the disk appear dark in its center. The upper object SR24N is actually an unresolved binary star; that binary also has its own circumbinary disk such that the SR24N stellar pair + disk orbits the SR24S star + disk. The right graphic is a computer simulation of this system, which reveals that these two disks can transfer mass between each other through a `bridge' that passes through this system's L1 Lagrange point, which is one of three sites in this system where gravity + Coriolis forces balance to zero. Observations such as this will hopefully reveal whether binary stars might one day form planets, or if their disks are too disturbed to produce planets. This image was acquire by Satoshi Mayami at the Subaru Telescope. A preprint on this work is also available.

Giant exoplanet confirmed orbiting in debris disk

Beta Pictoris b is a giant extrasolar planet, having a mass of about 10 Jupiter masses. It was first detected by direct imaging in 2003, but not seen again in followup images acquired in 2008, so the suspicion then was that this faint dot was just a background star, and not a planet that is actually bound to the star. However that expolanet was later recovered again in images acquired in late 2009 by Anne-Marie Lagrange; those images show that the dot seen above is indeed bound to Beta Pictoris, and orbits at a distance of about 10 AU from the star. So it seems that beta Pic b wasn't seen in 2008 because it was passing in front of or behind the very much brighter star.

Beta Pictoris is also known for its huge circumstellar debris disk; that disk is seen edge-on by astronomers at Earth, and the above graphic---which by the way is not a real telescopic image, but is probably the merger of two separate images---shows that the planet's orbit is coplanar with the debris disk. This in fact is to be expected, because such debris disks are composed of dust grains that are produced by collisions among unseen planetesimals, which are also the seeds from which planets form from. See this press release from the European Southern Observatory for more details.

Circumstellar debris disk orbiting HD 32297

This is the circumstellar debris disk that orbits the star HD 32997, imaged with the Palomar 5m telescope by Dimitri Mawet and colleagues. The star lies at the cross, but its light has been blocked by a phase mask coronograph, which is a device that shifts the phase some of that starlight so that the star's light waves interfere with itself destructively, effectively making the very bright star dissapear from this image. This is very useful, since it also reveals the light from the much fainter circumstellar material.

The colored blobs indicate that there is a ring or perhaps a disk of dust in orbit about this star, with that disk/ring seen nearly edge on. The dust grains are visible because they are reflecting starlight, and the colors indicate the intensity of that reflected light. Of particular interest to me is the asymmetry seen in this disk, with one side being brighter than the other by ~50%.

These dusty disks usually have rather short lifetimes, since dust grains destroy each other when the collide with each other. Consequently, other unseen `planetesimals' are implicated here, since collisions by these asteroidal or cometary bodies are needed to continually resupply the disk with the dust seen here. And since comets or asteroids are evidently forming in this system, it seems plausible that larger planets might have formed here, too. Additional details are also available in the paper by Mawet et al.

Shadows cast by disk-embedded planets

A recent theory paper by Hannah Jang-Condell (U. Maryland) examines the shadows that might be cast by recently-formed planets as they orbit within the circumstellar disk in which they formed. Her numerical models show that the planet's gravity will 'depress' the disk there. If that disk were then viewed by an astronomer at optical wavelengths, then that depressed spot would resemble a dark pothole, since that depression is not illuminated by the central star (see Figure). The exception is at the pothole's far side, which would instead appear as an illuminated bright spot. Note that extra-solar planets are difficult to see via direct imaging. However this work suggests a new technique that might be used to discover unseen planets indirectly---by searching for these planet's darkened potholes and dimples that they create in a planet-forming disk.