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.

Vesta's rugged surface

This image is a computerized rendering of what a hypothetical observer migth see when looking obliquely along the mountains and craters on the giant asteroid Vesta. Of course the Dawn spacecraft is in orbit about Vesta, and thus is always looking down upon the asteroid, so Dawn would never see anything like this. But Dawn has now collected enough measurements of Vesta surface topography to assemble a detailed computer model of the asteroid's surface. With that model, one can then calculate the view that any observer might see when looking in any direction about Vesta. Keep in mind that Vesta is only about 700 miles across, so the more distance mountains shown here would not be seen by that observer; they'd lie below the horizon and thus would be hidden. But that computerized view can be adjusted to remove the asteroid's curvature (ie, flatten the asteroid) while preserving its topography, as is seen here. See the Dawn website for more details about this computerized image of Vesta's rugged surface.

Four fascinating videos on science and math

Last week I was in a working meeting with a number of planetary scientists. When there was a lull in the meeting, I asked everybody to show me their favorite online science videos. Below are the best four, and all are quite remarkable.

The first is called Doodling in Math Class: Snakes and Graphs. This amazing video is by Vi Hart, and she shows how to sketch elaborate doodles that have a mathematical theme. The very clever (and funny) commentary provides tip on how to draw, plus some hints about the mathematics that are associated with each sketch.

The second is a 50 minute video by Astronaut Don Petit, who recorded several of the fluid-dynamics experiments that he performed while in the microgravity environment of the Space Station; see this youtube video. One interesting clip shows an air bubble that he creates inside a floating water bubble, which to me resembles a miniature planet that has a central core and an overlying mantle. He then injects little water bubbles into the air core, which careen around and around the core boundary until getting absorbed explosively by the mantle. Another video segment show how small bodies in the weightless environment of space can collide, merge, and form elaborate dendritic objects. This is in fact how planetesimals, which are the precursors to asteroids and planets, are thought to have formed. A fascinating video.

The next two videos are from the Slo Mo Guys. They use a high-speed camera to record ordinary events that are usually just a blur. Playing their movies very slowly then reveals lots of hidden events that are too fast to be seen by human eyes. In the first video, Giant 6ft Water Balloon, one of the Slo Mo guys leaps onto and breaks a large water-filled balloon. Slow motion video reveals how the entire balloon shivers as inertial waves propagate across the balloon and ultimately rip the balloon to shreds. In very cool slow mo.

But my favorite movie is Droplet Collisions at 5000fps, which provides an excellent analogy for how craters form and develop when an asteroid impacts a planet. Oh, and the Slo Mo guys are hilarious, too.

Earth's first Trojan asteroid

Earth's first trojan asteroid was recently detected by Martin Conners (Athabasca University) and colleagues using NASA's WISE satellite, which is surveying the sky at infrared wavelengths. Trojans are asteroids that reside in or near two sites that lead or trail a planet's orbit by 60 degrees, and these orbits are stablized by the combined gravities of the Sun, the planet, plus the centrifugal force that is due to the asteroid's orbital motion. This particular asteroid is known at 2010 TK7, and it is small, maybe a half mile across. This Trojan is indicated by the small white dot in the above graphic. The blue dots show the Earth's motion about the Sun, while the green dots show that 2010 TK7's motion can carry it quite far from the stable equilibrium site at 60 degrees, which is also known at the L4 Lagrange point. Indeed, orbit calculations by Paul Wiegert (University of Western Ontario) show that this is only a temporary Trojan, since Earth captured that object at its L4 point about 1500 years ago, and it will probably escape back into interplanetary space in a comparable amount of time. See Wiegert's very nice website on 2010 TK7 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.

Streaks at Martian crater suggest a watery flow

This is the upper edge of the Newton crater on Mars, imaged my the Mars Reconnaissance Orbiter (MRO). The dark streaks extending downslope are very interesting; they are only seen in spring and summer, and fade away during the colder seasons. These streaks might be due to a flow of subsurface water that is likely a salty brine due to long-term contact with the martian rocks. Salt lowers the freezing point, which might allow subsurface ice to melt and seep down the crater's slope. This might be the cause for the seasonal stains that are seen in the crater's soil. Additional details can be found at the MRO website, including this nice movie of streaks forming and then fading. If these streaks are in fact watery seeps, they will be of great biological interest because wet soil will be a natural place to look for microbial life on Mars.

Vesta is groovy

The giant asteroid Vesta, as seen by the Dawn spacecraft, which has been orbiting Vesta since July 15. See the Dawn website for more information. This movie of Vesta's rotation is also interesting...evidently, Vesta is quite `groovy'.

Last shuttle re-entry, as seen from Space Station

Shuttle Atlantis' final return to Earth, as seen from the International Space Station. This picture comes from the NASA Image of the Day.

Hubble finds new moon orbiting Pluto

Mark Showalter (SETI Institute) just discovered a fourth moon orbiting Pluto, designated P4 for now. It is quite small, about 15 miles across, and its orbit resides between satellites Nix and Hydra. This discovery was quite accidental, because Showalter was observing the Pluto system to look for signs of any planetary rings there; see the Hubble website for more details. Since Pluto seems ripe with satellites, we might expect the New Horizons spacecraft, which flies by Pluto in 2015, to find even more tiny satellites.

Dawn spacecraft to orbit Vesta tonight!

The Dawn spacecraft goes into orbit about the giant asteroid Vesta tonight, July 15, about 10pm Pacific time or 1am Eastern time July 16. See the Dawn website for more.

Cassini monitors monstor storm on Saturn

Cassini snapped this picture of a monster storm on Saturn on Feb. 25, 2011. This storm started in December 2010, and has since wrapped itself all the way around the planet, spanning an area about 8 times that of the Earth's total surface. The thin dark band at the equator is Saturn's rings seen edge on, and the broader dark bands below are the shadows cast by the rings.

The image below is color coded to indicate the altitude of the various cloud layers in the storm. Blue, white, and yellow indicate clouds at higher altitudes, with green, red, and brown colors showing clouds that are deeper in this gas giant planet's atmosphere. Check the CICLOPS website for more information and Cassini images of Saturn.

Dawn gets closer Vesta

The Dawn spacecraft's view of Vesta's rugged surface gets a little sharper as the spacecraft approaches this very large asteroid. Dawn is still 100,000 miles away from Vesta, but it arrives and goes into orbit about that giant asteroid soon on July 16! Meanwhile, keep an eye on the Dawn website for the latest images that will get more dramatic over time as the spacecraft nears.

Helene at Saturn

The Cassini spacecraft captured this closeup image of Helene on June 18, 2011. Helene is a small satellite of Saturn about 20 miles across. Helene is a coorbital satellite, which means that it shares an orbit with the much larger satellite Dione that is ~30 times larger and ~30,000 times more massive. Helene resides at Dione's L4 Lagrange point, which is a stable niche in Dione's orbit that leads that satellite by 60 degrees. The other stable niche is of course the L5 Lagrange point that trails Dione by 60 degrees. The orbit of a coorbital satellite is analogous to the Trojan asteroids that lead or trail Jupiter by 60 degrees in its orbit about the Sun.

No one knows how a coorbital satellite like Helene came to reside in such a special orbit. But it is conceivable that a coorbital satellite is debris that was excavated when the larger satellite Dione was stuck by a comet long ago. If this scenario is correct, then a lucky fraction of that debris managed to find and settle into one or both Lagrange points where it could have reassembled into a small coorbital satellite like Helene.

To see more images of Helene, as well as the rest of the Saturnian system, visit Cassini's CICLOPS website.

The Dawn spacecraft approaches asteroid Vesta

The is the Dawn spacecraft's view of the asteroid Vesta. Dawn is the NASA mission that will visit two asteroids, beginning with Vesta in a few weeks, and then Ceres in 2015. Dawn will go into orbit about Vesta on July 16 to study that asteroid's surface for about a year. Vesta is about 300 miles across and is the fourth largest asteroid. Vesta's surface is composed of basaltic rock, or lava, which makes this asteroid quite unique and very interesting. Evidently Vesta was volcanically active in the past, likely when it first formed 4.5 billion years ago.

In July 2012, Dawn will then fire up its ion engine and depart Vesta for its 3 year trip to asteroid Ceres, which is the largest asteroid, one that appears to be quite rich in water. Meanwhile, keep an eye on the Dawn mission website for better pictures of Vesta that will soon get much more interesting when the spacecraft goes into orbit.

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.

Propellers and wakes in Saturn's rings

This figure shows results from an Nbody simulation of a small patch in Saturn's rings; click figure to zoom in. Small dots represent meter sized ring particles, while the circle at the center is a 150m moonlet that is embedded in the ring. All bodies are travelling to the right as they orbit Saturn, but keep in mind that those nearer Saturn (which is far downwards in this figure) orbit faster, so ring particles in the lower x<0 half of this figure are drifting towards the right side of the moonlight, while those in the upper x>0 half are drifting left of the moonlet. This Nbody simulation was performed by Shugo Michikoshi and Eiichiro Kokubo, and their results are detailed in this preprint.

The upper figure shows what happens in a low mass ring having a surface density of 60 grams/cm^2. As particles drift past the moonlet they receive a kick due to the moonlet's gravity, which in turn opens a propeller-shaped gap in the ring. The Cassini spacecraft has in fact observed many such propellers orbiting in Saturn's A ring, like the one seen below; see the CICLOPS website for more details about this image. Curiously, the model predicts that the propeller should appear as a dark gap in the ring, while the Cassini image below shows that a propeller is bright. The meaning of this is unclear, but it may indicate that the propeller gap is also filled with sunlight-reflecting dust grains that are produced as ring particles collide near the moonlet.

The lower Nbody simulation (in the lower half of the top graphic) shows results for a high mass ring of surface density 400 grams/cm^2. In this case, the higher ring gravity cause the ring particles to condense into ropy or taffy-like structures that are known as wakes. These wakes dominate the ring's appearance and completely wash-out the propeller that the moonlet is trying to form. The fact that propellers are seen in Saturn's A ring, while none have been observed in Saturn's B ring, suggest that the A ring is a relatively low mass ring that allows moonlets to form propellers, while the B ring is massive and full of gravitating wakes that inhibit any such propellers.

Messenger's first image from Mercury orbit

The planet Mercury acquired its first artificial satellite on March 17, when the Messenger spacecraft went into orbit about the planet. Two weeks later, the spacecraft acquired its first image from Mercury orbit, with 75,000 more images to be acquired during the next year. See this press release for more details and a much larger image, or visit the Messenger website.

Rare elongated crater on Mars

ESA's Mars Express imaged this elongated crater on Mars. Note that most craters are circular, even when the impactor strikes the planet at a shallow angle. However a train of interplanetary debris can leave an elongated scar, which might account for the crater seen here. But accounting for the origin of that hypothetical debris train can be problematic---perhaps this is debris from a comet or asteroid that was tidally disrupted by Mars? Or perhaps this debris is from a tidally disrupted satellite that had spiraled inwards and onto the planet due to the martian tide. Although this might seem farfetched, this in fact will be the ultimate fact of the Martian satellite Phobos, which will eventually impact Mars in tens of million years, due to its slow orbital decay that is driven by the martian tidal forces. See the Mars Express website for more details.

Monster solar flare imaged by Solar Dynamics Observatory

This massive solar flare erupted from the Sun's surface on February 24, and was imaged by NASA's Solar Dynamics Observatory. Check the SDO website for more info, plus a very dramatic movie of this explosion in space.

Stardust sees impact site on comet Tempel 1

The stardust spacecraft flew by comet Tempel 1 last night, and acquired the close-up image of the surface that is seen on the left. Recall that this comet was first visited by the Deep Impact spacecraft in 2005, which shot that comet with a one-ton projectile; the high-res image on the left shows the impact site prior to the collision. One of Deep Impact's science goals was to observe the resulting crater, but the impact generated so much dust, the crater was never seen by Deep Impact. Fortunately, the Stardust spacecraft could maneuvered so that it would encounter comet Tempel 1 six years later. Stardust acquired the fuzzy low-res image on the right, which does indeed show the impact site, with a shallow 100m crater just inside the smaller circle. Note also that the impact obliterated the dark region that is seen at 10 o'clock along the yellow circle (left), in the pre-impact image. The Deep Impact crater has been observed at last!

ISS transits the Sun during solar eclipse

Thierry Legault captured this amazing picture of the International Space Station as it transits the Sun during a partial solar eclipse by the Moon on January 4.