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.

A pulsar planet with a diamond core?

PSR J1719−1438 is a pulsar, which is a rapidly spinning neutron star. Such an object is called a pulsar because its powerful magnetic field shoot jets of energetic particles out along its magnetic poles. This also makes the spinning neutron star appear to pulse as its jet sweeps past an astronomer on Earth.

Slight variations in the timing of the neutron star's pulses can indicate the presence of an unseen planet that is also orbiting the pulsar. Indeed, the first known extra-solar planet was discovered via pulsar timing variations. Matthew Bailes (Swinbourne University in Australia) discovered the timing variations in pulsar PSR J1719−1438, and they are due to a Jupiter-mass planet in a very close two-hour orbit about the neutron star. He and his colleagues also show that this planet must be very small and dense to avoid having been ripped apart by the neutron star's gravitational tide. This planet's minimum density is about 20 times that of Jupiter's, and the planet's core is likely made of carbon. If so, then carbon at the center of this very dense planet will have crystallized, possibly into one giant planetary core-sized diamond. See this press release for more details.

Free floating planets might outnumber the stars in the Galaxy

A recent gravitational microlensing survey indicates that there may be twice as many free floating planets in our Galaxy than stars. Gravitational microlensing is the brightening that occurs when a dim but massive object passes along the line of sight to a more distant brighter object. According to Einstein's theory of relativity, mass bends spacetime, so the path followed by a light ray is deflected (ie lensed) if passing near enough to a star or a planet. So an astronomer observing a lensed star will see it brighten for a month or two if a very dim star (such as a white dwarf or neutron star) passes near the line of sight (LOS). This also occurs if a planet passes near the LOS, but the lower mass planet has a smaller gravitational influence, so the lensing event is briefer, only a few days.

This is illustrated in the above figure, which shows an otherwise steady star brightening by 40% during three days. These planetary microlensing events are quite rare, so astronomers must continuously monitor millions of stars just to detect 10 such microlensing events in one year. From the observed frequency of these microlensing events, it can be shown that most of the lensing objects are free-floating Jupiter-mass planets that are not bound to any star. But this unusual finding is consistent with some models of planet formation, which predict that when multiple planets form around a star, the planets' gravitational interactions can eject one or more planets from the system. Those ejected bodies are free-floating planets, and their fate is to roam the Galaxy unseen, except in these microlensing surveys. These results were obtained by astrophysicists T. Sumi and K. Kamiya (Osaka Japan) and others, with further details reported in their preprint.

HR 8799 hosts jumbo planetary system

Christian Marois (Herzberg Institute of Astrophysics) and colleagues discovered a fourth giant planet orbiting HR 8799. The new planet, 'e' in the above image, lies inwards of planets b,c,d. This is typical when using imaging to hunt for extra-solar planets, since it is the closer-in planets that are harder to find, due to obscuration by the bright central star. A simulated star was actually removed from this image; the fuzzy blob seen in the center is due to imperfections in this star-subtraction process. These planets lie about 15 to 70 AU away from the central star, and have masses of ~5 to 10 Jupiter masses. So this is a jumbo planetary system, since the orbits and masses are several times larger than that the Solar System's. See the abstract of their paper in Nature for more details.

A densely packed planetary system at HD 10180

C. Lovis and colleagues at the European Southern Observatory in Chile used the 3.6m telescope there to discover 5 or more planets orbiting the star HD 10180. The five certain planets are all Neptune-class, having masses of 12-25 times that of Earth. Also, all orbit rather close to the star, at distances of 0.06-1.4 AU. There is also tentative evidence for an Earth-mass planet orbiting at r=0.02 AU, and a possible 3-Neptune-mass planet orbiting at r=3.4 AU. Ellipses in the above figure show the planets current orbits, while the colored regions indicate their range of motions over time that are due to their mutual gravities. As the figure shows, this system is quite dense with planets, which is actually rather typical for multi-planets extra-solar planetary systems. See the preprint for more details.

HD 10180, a five-exoplanet system

Astronomers used the HARPS radial-velocity instrument (at the European Southern Observatory) to detect five exoplanets orbiting the star HD 10180. Two additional planets are also suspected, but their existence is uncertain due to their weak signal. These five make this exo-planetary system one of the most abundant discovered to date. The five planets are Neptune-class, and they all reside at distances interior to Mar's orbit about the Sun. The above graphic is an artist's rendition of this system. A press release is also available.

A big expoplanet in a wide orbit

The circled dots indicate the 8 Jupiter mass planet that was discovered by David Lafrenière and colleagues as it orbits the star 1RXS J160929.1-210524. A preprint is also available. These infrared images of the system are acquired at wavelengths of 3 (left) and 4 (right) microns. This giant exoplanet orbits about 300 AU away from the primary star (cross), which is quite remarkable, because current models of planet formation generally do not produce such large planets orbiting at such great distances from the star. However, binary stars do exist at these separations, so one might wonder whether these objects really are large exoplanets, or are perhaps rather small stars.

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.

Image of planet orbiting Fomalhaut

This remarkable image shows the motion of the Jupiter-mass planet as it orbits the star Fomalhaut (see inset). This optical image was acquired by Paul Kalas (UC Berkeley) and colleagues using the Hubble Space Telescope. Interestingly, this star also harbors a dusty circumstellar debris disk. The planet orbits just inside a gap within this disk, and its gravity is responsible for keeping that gap clear of dust. Note also that the debris disk appears to resemble a ring, due to the disk's inner edge being illuminated by the central star. The dust in these debris disks is thought to be generated by collisions among unseen planetesimals (eg, asteroids or comets) that also probably orbit within this disk. The ellipse in this graphic has a radius of 30 AU (1 AU = Sun-Earth distance), which is the radius of Neptune's orbit. The radius of the disk's inner edge is about 140 AU, which is about 3 times larger than the size of our Solar System's Kuiper Belt, which is also a belt of comets orbiting just beyond Neptune.