Posted on by Jon Voisey

The Origin of Exoplanets

Artist's impression of the planet OGLE-TR-L9b. Credit: ESO/H. Zodet

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We truly live in an amazing time for exoplanet research. It was only 18 years ago the first planet outside our solar system was discovered. Fifteen since the first confirmation of one around a main sequence star. Even more recently, direct images have begun to sprout up, as well as the first spectra of the atmospheres of such planets. So much data is becoming available, astronomers have even begun to be able to make inferences as to how these extra solar planets could have formed.

In general, there are two methods by which planets can form. The first is via coaccretion in which the star and the planet would form from gravitational collapse independently of one another, but in close enough proximity that their mutual gravity binds them together in orbit. The second, the method through which our solar system formed, is the disk method. In this, material from a thin disk around a proto-star collapses to form a planet. Each of these processes has a different set of parameters that may leave traces which could allow astronomers to uncover which method is dominant. A new paper from Helmut Abt of Kitt Peak National Observatory, looks at these characteristics and determines that, from our current sampling of exoplanets, our solar system may be an oddity.

The first parameter that distinguishes the two formation methods is that of eccentricity. To establish a baseline for comparison, Abt first plotted the distribution of eccentricities for 188 main-sequence binary stars and compared that to the same type of plot for the only known system to have formed via the disk method (our Solar System). This revealed that, while the majority of stars have orbits with low eccentricity, this percentage falls off slowly as the eccentricity increases. In our solar system, in which only one planet (Mercury) has an eccentricity greater than 0.2, the distribution falls off much more steeply. When Abt constructed the distribution for the 379 planets with known eccentricity, it was nearly identical to that for binary stars.

A similar plot was created for the semi major axis of binary stars and our solar system. Again, when this was plotted for the known extra solar planets the distribution was similar to that of binary star systems.

Abt also inspected the configuration of the systems. Star systems containing three stars generally contained a pair of stars in a tight binary orbit with a third in a much larger orbit. By comparing the ratios of such orbits, Abt quantified the orbital spacing. However, instead of simply comparing to the solar system, he considered the analogous situation of formation of stars around the central mass of the galaxy and built a similar distribution in this manner. In this case, the results were ambiguous; Both modes of formation produced similar results.

Lastly, Abt considered the amount of heavy elements in the more massive body. It is widely known that most extra-solar planets are found around metal-rich stars. While there’s no reason planets forming in a disk couldn’t be formed around high mass stars, having a metal-rich cloud from which to form stars and planets is a requirement for the coaccretion model because it tends to accelerate the collapse process, allowing giant planets to fully form before the cloud was dissipated as the star became active. Thus, the fact that the vast majority of extra-solar planets exist around metal-rich stars favors the coaccretion hypothesis.

Taken together, this provides four tests for formation models. In every case, current observations suggest that the majority of planets discovered thus far formed from coaccretion and not in a disc. However, Abt notes that this is most likely due to statistical biases imposed by the sensitivity limits of current instruments. As he notes, astronomers “do not yet have the radial velocity sensitivity to detect disk systems like the solar system, except for single large planets, like Jupiter at 5 AU.” As such, this view will likely change as new generations of instruments become available. Indeed, as instruments improve to the point that three dimensional mapping becomes available, and orbital inclinations can be directly observed, astronomers will be able to add another test to determine the modes of formation.

EDIT: Following some confusion and discussion in the comments, I wanted to add one further note. Keep in mind this is only the average of all systems currently known that looks like coaccreted systems. While there are undoubtedly some in there that did form from disks, their rarity in the current data makes them not stand out. Certainly, we know of at least one system that fits a strong test for the disk method. This recent discovery by Kepler, in which three planets have been observed transiting their host star demonstrates that all of these planets must lie in a disk which does not conform to expectations of independent condensation. As more systems like this are discovered, we expect that the distributions of the tests described above will become bimodal, having components that match each formation hypothesis.

Posted on by Nancy Atkinson

Tight Binaries are ‘Death Stars’ for Planets

This plot of data from NASA's Spitzer Space Telescope tells astronomers that a dusty planetary smashup probably occurred around a pair of tight twin, or binary, stars. Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

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Astronomers studying double star systems where the two stars are extremely close have found a pattern of destruction. While there probably isn’t a Star Wars-like Death Star roaming the Universe, tight binary systems might provide the equivalent of Darth Vader’s favorite weapon. “This is real-life science fiction,” said Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics. “Our data tell us that planets in these systems might not be so lucky — collisions could be common. It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

Using the Spitzer Space Telescope, Drake and his team spotted a surprisingly large amount of dust around three mature, close-orbiting star pairs, that might be the aftermath of tremendous planetary collisions.

Drake is the principal investigator of the research, published in the Aug.19 issue of the Astrophysical Journal Letters.

The particular class of binary stars in the study are extremely close together. Named RS Canum Venaticorums, or RS CVns for short, they are separated by only about 3.2-million kilometers (two-million miles ), or two percent of the distance between Earth and our sun. The binaries orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other.

These stars are familiarly like our own Sun – about the same size and probably about a billion to a few billion years old — roughly the age of our sun when life first evolved on Earth. But these stars spin much faster, and, as a result, have powerful magnetic fields, and giant, dark spots. The magnetic activity drives strong stellar winds — gale-force versions of the solar wind — that slow the stars down, pulling the twirling duos closer over time.

This is not a good scenario for planetary survival.

As the stars cozy up to each other, their gravitational influences change, and this could cause disturbances to planetary bodies orbiting around both stars. Comets and any planets that may exist in the systems would start jostling about and banging into each other, sometimes in powerful collisions. This includes planets that could theoretically be circling in the double stars’ habitable zone, a region where temperatures would allow liquid water to exist. Though no habitable planets have been discovered around any stars beyond our sun at this point in time, tight double-star systems are known to host planets; for example, one system not in the study, called HW Vir, has two gas-giant planets.

“These kinds of systems paint a picture of the late stages in the lives of planetary systems,” said Marc Kuchner, a co-author from NASA Goddard Space Flight Center. “And it’s a future that’s messy and violent.”

The temperatures around these systems measured by Spitzer are about the same as molten lava. The astronomers says that dust normally would have dissipated and blown away from the stars by this mature stage in their lives. They conclude that something — most likely planetary collisions — must therefore be kicking up the fresh dust. In addition, because dusty disks have now been found around four, older binary systems, the scientists know that the observations are not a fluke. Something chaotic is very likely going on.

If any life forms did exist in these star systems, and they could look up at the sky, they would have quite a view. Marco Matranga, lead author of the paper, also from Harvard-Smithsonian said, “The skies there would have two huge suns, like the ones above the planet Tatooine in ‘Star Wars.'”

The research was published in the Aug.19 issue of the Astrophysical Journal Letters.

Source: JPL

Posted on by Nancy Atkinson

Astronomers Zoom in on Solar Systems in the Making

Young stars have a disk of gas and dust around them called a protoplanetary disk. Credit: NASA/JPL-Caltech

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For the first time, astronomers have observed in unprecedented detail the processes giving rise to stars and planets in nascent solar systems. Using both Keck telescopes on Mauna Kea in Hawaii outfitted with a specifically engineered instrument named ASTRA (ASTrometric and phase-Referenced Astronomy), Joshua Eisner from the University of Arizona and his colleagues were able to peer deeply into protoplanetary disks – swirling clouds of gas and dust that feed the growing star in its center and eventually coalesce into planets and asteroids to form a solar system. What they saw is providing insight into the way hydrogen gas from the protoplanetary disk is incorporated into the star.

In order to obtain the extremely fine resolution necessary to observe the processes that happen at the boundary between the star and its surrounding disk 500 light years from Earth, the team combined the light from the two Keck telescopes, which provides an angular resolution finer than Hubble’s. Eisner and his team also used a technique called spectro-astrometry to boost resolution even more. By measuring the light emanating from the protoplanetary disks at different wavelengths with both Keck telescope mirrors and manipulating it further with ASTRA, the researchers achieved the resolution needed to observe processes in the centers of the nascent solar systems.

“The angular resolution you can achieve with the Hubble Space Telescope is about 100 times too coarse to be able to see what is going on just outside of a nascent star not much bigger than our sun,” said Eisner. In other words, even a protoplanetary disk close enough to be considered in the neighborhood of our solar system would appear as a featureless blob.

With this new technique, the team was able to distinguish between the distributions of gas, mostly made up of hydrogen, and dust, thereby resolving the disk’s features.

“We were able to get really, really close to the star and look right at the interface between the gas-rich protoplanetary disk and the star,” said Eisner.

Protoplanetary disks form in stellar nurseries when clouds of gas molecules and dust particles begin to collapse under the influence of gravity.

Initially rotating slowly, the cloud’s growing mass and gravity cause it to become more dense and more compact. The preservation of rotational momentum speeds up the cloud as it shrinks, much like a figure skater spins faster as she tugs in her arms. The centrifugal force flattens the cloud into a spinning disk of swirling gas and dust, eventually giving rise to planets orbiting their star in roughly the same plane.

Astronomers know that stars acquire mass by incorporating some of the hydrogen gas in the disk that surrounds them, in a process called accretion, which can happen in one of two ways.

In one scenario, gas is swallowed as it washes up right to the fiery surface of the star.

In the second, much more violent scenario, the magnetic fields sweeping from the star push back the approaching gas and cause it to bunch up, creating a gap between the star and its surrounding disk. Rather than lapping at the star’s surface, the hydrogen atoms travel along the magnetic field lines as if on a highway, becoming super-heated and ionized in this process.

“Once trapped in the star’s magnetic field, the gas is being funneled along the field lines arching out high above and below the disk’s plane,” Eisner explained. “The material then crashes into the star’s polar regions at high velocities.”

In this inferno, which releases the energy of millions of Hiroshima-sized atomic bombs every second, some of the arching gas flow is ejected from the disk and spews out far into space as interstellar wind.

“We want to understand how material accretes onto the star,” Eisner said. “This process has never been measured directly.”

Eisner’s team pointed the telescopes at 15 protoplanetary disks with young stars varying in mass between one half and 10 times that of our sun.

“We could successfully discern that in most cases, the gas converts some of its kinetic energy into light very close to the stars” he said, a tell-tale sign of the more violent accretion scenario.

“In other cases, we saw evidence of winds launched into space together with material accreting on the star,” Eisner added. “We even found an example – around a very high-mass star – in which the disk may reach all the way to the stellar surface.”

The solar systems the astronomers chose for this study are still young, probably a few million years old.

“These disks will be around for a few million years more,” Eisner said. “By that time, the first planets, gas giants similar to Jupiter and Saturn, may form, using up a lot of the disk material.”

More solid, rocky planets like the Earth, Venus or Mars, won’t be around until much later.

“But the building blocks for those could be forming now,” he said, which is why this research is important for our understanding of how solar systems form, including those with potentially habitable planets like Earth.

“We are going to see if we can make similar measurements of organic molecules and water in protoplanetary disks,” he said. “Those would be the ones potentially giving rise to planets with the conditions to harbor life.”

The team’s paper was published in the Astrophysical Journal

Paper: Eisner et al. Spatially and Spectrally Resolved Hydrogen Gas within 0.1 AU of T Tauri and Herbig Ae/Be Stars.

Source: University of Arizona

Posted on by Nancy Atkinson

Wild and Crazy Multi-Planetary System Surprises Astronomers

Epsilon Andromedae. Illustration Credit: NASA, ESA, and A. Feild (STScI) Science Credit: NASA, ESA, and B. McArthur, University of Texas at Austin, McDonald Observatory.

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Astronomers are finding that not only are there a wide range of different extrasolar planets, but there are different types of planetary systems, as well. “We’re not in Kansas anymore as far as solar systems go,” said Barbara McDonald from the University of Texas’ McDonald Observatory, at the American Astronomical Society meeting in Miami, Florida today. “The exciting thing is, we found another multi-planet system that is not at all like our own.”

A close look at the Upsilon Andromedae system with the Hubble Space Telescope, the Hobby-Eberly Telescope and other ground-based telescopes shows a whacky system where planets are out of tilt and have highly inclined orbits. The astronomers also found another planet, and also another star – this is likely a binary star system.

Even with Pluto’s inclined orbit, our solar system looks like an ocean of calm compared to Upsilon Andromedae.

Comparison of solar systems. Credit: HubbleSite

McDonald said these surprising findings will impact theories of how multi-planet systems evolve, and it shows that some violent events can happen to disrupt planets’ orbits after a planetary system forms.

“The findings mean that future studies of exoplanetary systems will be more complicated,” she said. “Astronomers can no longer assume all planets orbit their parent star in a single plane.” says Barbara McArthur of The University of Texas at Austin’s McDonald Observatory.

Similar to our Sun in its properties, Upsilon Andromedae lies about 44 light-years away. It’s a little younger, more massive, and brighter than the Sun. For just over a decade, astronomers have known that three Jupiter-type planets orbit the yellow-white star Upsilon Andromedae.

But after over a thousand combined observations, McDonald and her team uncovered hints that a fourth planet, e, orbits the star much farther out. They were also able to determine the exact masses of two of the three previously known planets, Upsilon Andromedae c and d. Much more startling, though, is that not all planets orbit this star in the same plane. The orbits of planets c and d are inclined by 30 degrees with respect to each other. This research marks the first time that the “mutual inclination” of two planets orbiting another star has been measured.

“Most probably Upsilon Andromedae had the same formation process as our own solar system, although there could have been differences in the late formation that seeded this divergent evolution,” McArthur said. “The premise of planetary evolution so far has been that planetary systems form in the disk and remain relatively co-planar, like our own system, but now we have measured a significant angle between these planets that indicates this isn’t always the case.”

Until now the conventional wisdom has been that a big cloud of gas collapses down to form a star, and planets are a natural byproduct of leftover material that forms a disk. In our solar system, there’s a fossil of that creation event because all of the eight major planets orbit in nearly the same plane. The outermost dwarf planets like Pluto are in inclined orbits, but these have been modified by Neptune’s gravity and are not embedded deep inside the Sun’s gravitational field.

So what knocked the Upsilon Andromedae system around?

“Possibilities include interactions occurring from the inward migration of planets, the ejection of other planets from the system through planet-planet scattering, or disruption from the parent star’s binary companion star, Upsilon Andromedae B,” McArthur said.

Or, the companion star – a red dwarf less massive and much dimmer than the Sun — could be the culprit. is.

“We don’t have any idea what its orbit is,” said team member Fritz Benedict. “It could be very eccentric. Maybe it comes in very close every once in a while. It may take 10,000 years.” Such a close pass by the secondary star could gravitationally perturb the orbits of the planets.”

The two different types of data combined in this research were astrometry from the Hubble Space Telescope and radial velocity from ground-based telescopes.

Astrometry is the measurement of the positions and motions of celestial bodies. McArthur’s group used one of the Fine Guidance Sensors (FGSs) on the Hubble telescope for the task. The FGSs are so precise that they can measure the width of a quarter in Denver from the vantage point of Miami. It was this precision that was used to trace the star’s motion on the sky caused by its surrounding — and unseen — planets.

Radial velocity makes measurements of the star’s motion on the sky toward and away from Earth. These measurements were made over a period of 14 years using ground-based telescopes, including two at McDonald Observatory and others at Lick, Haute-Provence, and Whipple Observatories. The radial velocity provides a long baseline of foundation observations, which enabled the shorter duration, but more precise and complete, Hubble observations to better define the orbital motions.

The fact that the team determined the orbital inclinations of planets c and d allowed them to calculate the exact masses of the two planets. The new information told us that our view as to which planet is heavier has to be changed. Previous minimum masses for the planets given by radial velocity studies put the minimum mass for planet c at 2 Jupiters and for planet d at 4 Jupiters. The new, exact masses, found by astrometry are 14 Jupiters for planet c and 10 Jupiters for planet d.

“The Hubble data show that radial velocity isn’t the whole story,” Benedict said. “The fact that the planets actually flipped in mass was really cute.”

The fourth planet is so far out, that its signal does not reveal the curvature of its orbit.

The 14 years of radial velocity information compiled by the team uncovered hints that a fourth, long-period planet may orbit beyond the three now known. There are only hints about that planet because it’s so far out that the signal it creates does not yet reveal the curvature of an orbit. Another missing piece of the puzzle is the inclination of the innermost planet, b, which would require precision astrometry 1,000 times greater than Hubble’s, a goal attainable by a future space mission optimized for interferometry.

Sources: HubbleSite, AAS Press conference

Posted on by Steve Nerlich

Astronomy Without A Telescope – The Nice Way To Build A Solar System

When considering how the solar system formed, there are a number of problems with the idea of planets just blobbing together out of a rotating accretion disk. The Nice model (and OK, it’s pronounced ‘niece’ – as in the French city) offers a better solution.

In the traditional Kant/Laplace solar nebula model you have a rotating protoplanetary disk within which loosely associated objects build up into planetesimals, which then become gravitationally powerful centres of mass capable of clearing their orbit and voila planet!

It’s generally agreed now that this just can’t work since a growing planetesimal, in the process of constantly interacting with protoplanetary disk material, will have its orbit progressively decayed so that it will spiral inwards, potentially crashing into the Sun unless it can clear an orbit before it has lost too much angular momentum.

The Nice solution is to accept that most planets probably did form in different regions to where they orbit now. It’s likely that the current rocky planets of our solar system formed somewhat further out and have moved inwards due to interactions with protoplanetary disk material in the very early stages of the solar system’s formation.

It is likely that within 100 million years of the Sun’s ignition, a large number of rocky protoplanets, in eccentric and chaotic orbits, engaged in collisions – followed by the inward migration of the last four planets left standing as they lost angular momentum to the persisting gas and dust of the inner disk. This last phase may have stabilised them into the almost circular, and only marginally eccentric, orbits we see today.

The hypothesized collision between 'Earth Mk 1' and Theia may have occurred late in rocky planet formation creating the Earth as we know it with its huge Moon of accreted impact debris

Meanwhile, the gas giants were forming out beyond the ‘frost line’ where it was cool enough for ices to form. Since water, methane and CO2 were a lot more abundant than iron, nickel or silicon – icy planetary cores grew fast and grew big, reaching a scale where their gravity was powerful enough to hold onto the hydrogen and helium that was also present in abundance in the protoplanetary disk. This allowed these planets to grow to an enormous size.

Jupiter probably began forming within only 3 million years of solar ignition, rapidly clearing its orbit, which stopped it from migrating further inward. Saturn’s ice core grabbed whatever gases Jupiter didn’t – and Uranus and Neptune soaked up the dregs. Uranus and Neptune are thought to have formed much closer to the Sun than they are now – and in reverse order, with Neptune closer in than Uranus.

And then, around 500 million years after solar ignition, something remarkable happened. Jupiter and Saturn settled into a 2:1 orbital resonance – meaning that they lined up at the same points twice for every orbit of Saturn. This created a gravitational pulse that kicked Neptune out past Uranus, so that it ploughed in to what was then a closer and denser Kuiper Belt.

The result was a chaotic flurry of Kuiper Belt Objects, many being either flung outwards towards the Oort cloud or flung inwards towards the inner solar system. These, along with a rain of asteroids from a gravitationally disrupted asteroid belt, delivered the Late Heavy Bombardment which pummelled the inner solar system for several hundred million years – the devastation of which is still apparent on the surfaces of the Moon and Mercury today.

Then, as the dust finally settled around 3.8 billion years ago and as a new day dawned on the third rock from the Sun – voila life!

Posted on by Ryan Anderson

Mysterious Alien Dust Hints at Violent Planet Formation

Image credit: Lynette Cook for Gemini Observatory/AURA
An artist's depiction of two colliding rocky bodies. Such a collision is the most likely source for the warm dust in the HD 131488 system. Image credit: Lynette Cook for Gemini Observatory/AURA

An artist’s rendition of colliding planets, the most likely explanation for the warm dust observed around HD 131488. Image credit: Lynette Cook for Gemini Observatory/AURA

Five-hundred light years away, worlds are colliding, and they’re made of nothing we’ve ever seen.

Last week at the 215th American Astronomical Society meeting, UCLA astronomers announced that they had found warm dust – evidence for the violent collision of rocky planets – around a star called HD 131488. The strange thing is, the composition of the dust has little in common with the composition of rocky bodies in any other known system.

“Typically, dust debris around other stars, or our own Sun, is of the olivine, pyroxene, or silica variety, minerals commonly found on Earth,” said Dr. Carl Melis, who led the research as a graduate student at UCLA. “The material orbiting HD 131488 is not one of these dust types. We have yet to identify what species it is – it really appears to be a completely alien type of dust.”

The warm dust in the HD 131488 system is concentrated in an area close to the star, where temperatures are similar to those on Earth. The researchers concluded that the most likely source for dust in that part of the system would be the collision of two rocky planetary bodies. Only five other stars like HD 131488 with dust in their terrestrial planet zone are known. “Interestingly, all five of these stars have ages in the range of 10-30 million years,” Melis said. “This finding indicates that the epoch of final catastrophic mass accretion for terrestrial planets, the likes of which could have resulted in the formation of the Earth-Moon system in our own Solar System, occurs in this narrow age range for stars somewhat more massive than the Sun.”

The team also discovered a unique second dusty region in the outer reaches of the HD 131488 system, comparable to the location of Pluto and other Kuiper Belt objects in our own solar system.
Image Credit: Lynette Cook for Gemini Observatory/AURA

Top: Illustration depicting the location of the warm and cold dust rings in the HD131488 system. Bottom: Comparable regions in our own solar system, with the orbits of the outer planets for scale. Image Credit: Lynette Cook for Gemini Observatory/AURA

“The hot dust almost certainly came from a recent catastrophic collision between two large rocky bodies in HD 131488’s inner planetary system,” Melis said. “The cooler dust, however, is unlikely to have been produced in a catastrophic collision and is probably left over from planet formation that took place farther away from HD 131488.”

“…for some reason stars that have large amounts of orbiting warm dust do not also show evidence for the presence of cold dust. HD 131488 dramatically breaks this pattern,” said Dr. Benjamin Zuckerman, a co-author on the paper and a professor of physics and astronomy at UCLA.

With its unusual dust composition and unique combination of warm and cold dust regions, the HD 131488 system is now under intense scrutiny. Melis and colleagues plan to continue trying to determine the composition of the dust, and will search for other stars with the dusty evidence for planet formation.

Source: Gemini Observatory

Posted on by Anne Minard

Planet Precursors May be Sized Like Trucks, Not Towns

Credit: NASA/JPL-Caltech/T. Pyle (SSC-Caltech)

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A typical model has planets forming from collisions of material swirling around stars. But new laboratory experiments indicate the colliding bodies may be much smaller than most people have thought.

Lead author Oliver Tschauner, of the University of Nevada in Las Vegas, and his colleagues have synthesized a mineral called wadsleyite that naturally exists only in meteorites and deep below the Earth’s crust. It’s believed to be the most abundant mineral in the Earth between the depths of 410 and 520 km (254 to 323 miles).

The conditions where wadsleyite forms are known from long-duration, high-pressure experiments, but the only confirmed natural occurrence is in shocked meteorites, which are remnants of the early solar system. The researchers found small quantities of wadsleyite after a high-pressure laboratory collision between thin layers of magnesium oxide and fused quartz. They suggest the mineral formed in approximately one one-millionth of a second.

On the basis of their experiments, the group inferred that the wadsleyite in ancient meteorites could be generated by collisions between bodies one to five meters (three to 16 feet) in diameter, rather than one to five kilometers (.6 to three miles).

“Based on the present results we suggest that the interpretation of the high-grade shock-metamorphic record in meteorites needs a re-evaluation,” the authors write.

Source: PNAS

Posted on by Anne Minard

History of Iron Yields New Insight Into Earth’s Deepest Reaches

Credit: Louise Kellogg, modified by James Rustad & Qing-zhu Yin/UC Davis

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Earth may have given up its innermost secrets to a pair of California geochemists, who have used extensive computer simulations to piece together the earliest history of our planet’s core.

This schematic of Earth’s crust and mantle shows the results of their study, which found extreme pressures would have concentrated iron’s heavier isotopes near the bottom of the mantle as it crystallized from an ocean of magma.

World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago
World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago

By using a super-computer to virtually squeeze and heat iron-bearing minerals under conditions that would have existed when the Earth crystallized from an ocean of magma to its solid form 4.5 billion years ago, the two scientists — from the University of California at Davis — have produced the first picture of how different isotopes of iron were initially distributed in the solid Earth.

The discovery could usher in a wave of investigations into the evolution of Earth’s mantle, a layer of material about 1,800 miles deep that extends from just beneath the planet’s thin crust to its metallic core.

“Now that we have some idea of how these isotopes of iron were originally distributed on Earth,” said lead study author James Rustad, “we should be able to use the isotopes to trace the inner workings of Earth’s engine.”

A paper describing the study by Rustad and co-author Qing-zhu Yin was posted online by the journal Nature Geoscience on Sunday, June 14, in advance of print publication in July.

Sandwiched between Earth’s crust and core, the vast mantle accounts for about 85 percent of the planet’s volume. On a human time scale, this immense portion of our orb appears to be solid. But over millions of years, heat from the molten core and the mantle’s own radioactive decay cause it to slowly churn, like thick soup over a low flame. This circulation is the driving force behind the surface motion of tectonic plates, which builds mountains and causes earthquakes.

One source of information providing insight into the physics of this viscous mass are the four stable forms, or isotopes, of iron that can be found in rocks that have risen to Earth’s surface at mid-ocean ridges where seafloor spreading is occurring, and at hotspots like Hawaii’s volcanoes that poke up through the Earth’s crust. Geologists suspect that some of this material originates at the boundary between the mantle and the core some 1,800 miles beneath the surface.

“Geologists use isotopes to track physico-chemical processes in nature the way biologists use DNA to track the evolution of life,” Yin said.

Because the composition of iron isotopes in rocks will vary depending on the pressure and temperature conditions under which a rock was created, Yin said, in principle, geologists could use iron isotopes in rocks collected at hot spots around the world to track the mantle’s geologic history. But in order to do so, they would first need to know how the isotopes were originally distributed in Earth’s primordial magma ocean when it cooled down and hardened.

Yin and Rustad investigated how the competing effects of extreme pressure and temperature deep in Earth’s interior would have affected the minerals in the lower mantle, the zone that stretches from about 400 miles beneath the planet’s crust to the core-mantle boundary. Temperatures up to 4,500 degrees Kelvin in the region reduce the isotopic differences between minerals to a miniscule level, while crushing pressures tend to alter the basic form of the iron atom itself, a phenomenon known as electronic spin transition.

The pair calculated the iron isotope composition of two minerals under a range of temperatures, pressures and different electronic spin states that are now known to occur in the lower mantle. The two minerals, ferroperovskite and ferropericlase, contain virtually all of the iron that occurs in this deep portion of the Earth.

The calculations were so complex that each series Rustad and Yin ran through the computer required a month to complete.

Yin and Rustad determined that extreme pressures would have concentrated iron’s heavier isotopes near the bottom of the crystallizing mantle.

The researchers plan to document the variation of iron isotopes in pure chemicals subjected to temperatures and pressures in the laboratory that are equivalent to those found at the core-mantle boundary. Eventually, Yin said, they hope to see their theoretical predictions verified in geological samples generated from the lower mantle.

Source: Eurekalert

Posted on by Nicholos Wethington

Planet-Forming Disk Discovered Orbiting Binary System

An artist's conception of the V4046 Sagittarii binary star system, which is home to a molecular gas cloud that may contain planets. Credit: David A. Aguilar (CfA)

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Science fiction is lousy with examples of planets that orbit a system of two suns. Tatooine, in the Star Wars saga, is endowed with a pair of suns to light up the sky, as is the planet Magrathea in The Hitchhiker’s Guide to the Galaxy. It would indeed be quite a spectacle to wake up to more than one Sun every day for us who have only one. This sight may entirely be possible to view around the young binary star system V4046 Sagittarii, as new images from the Smithsonian’s Submillimeter Array (SMA) have confirmed the existence of a molecular cloud – which could harbor, or later produce planets –  orbiting the twin stars. This is the first time that evidence of planetary formation around a binary system of stars has been uncovered.

“We believe that V4046 Sagittarii provides one of the clearest examples yet discovered of a Keplerian, planet-forming disk orbiting a young star system,” said David Wilner of the Harvard-Smithsonian Center for Astrophysics in a press release issued today at the American Astronomical Society (AAS) meeting in Pasadena, Calif.

The disk has traces of carbon monoxide and hydrogen cyanide, gases that are telltale signs of planetary formation. It also lies between 30 -300 Astronomical Units from the central binary star system, a distance at which it is likely that our own giant planets Jupiter and Saturn formed, as well as the Kuiper belt objects. The two stars that make up the V4046 Sagittarii binary system are both approximately the mass of the Sun, and separated by a distance of 5 solar diameters.

Joel Kastner of the Rochester (NY) Institute of Technology, the lead scientist on the study, said in the press release“It’s a case of seeing is believing….We had the first evidence for this rotating disk in radio telescope observations of V4046 Sagittarii that we made last summer. But at that point, all we had were molecular spectra, and there are different ways to interpret the spectra. Once we saw the image data from the SMA, there was no doubt that we have a rotating disk here.”

Could this be the view of a sunset from a planet orbiting V4046 Sagittarii?
Could this be the view of a sunset from a planet orbiting V4046 Sagittarii?

The team of astronomers from the Harvard Smithsonian Center for Astrophysics and the Rochester Institute of Technology used the 30-meter radio telescope operated by the Institut de Radio Astronomie Millimetrique (IRAM) to pin down the the composition of the cloud, then used images from the Submillimeter Array to further confirm the finding. Both telescopes are sensitive to light in the submillimeter spectrum, which emanates from cold interstellar material such as gas and dust.

This new finding bodes well for the possibility that many other binary star systems harbor planets, and gives astronomers a new place to search for planets outside of our own solar system. Even better, V4046 Sagittarii is only 240 light-years away from our solar system, meaning that there’s a good chance that astronomers can image any planets that have already formed in the disk.

Source: AAS, Harvard-Smithsonian Center for Astrophysics

Posted on by Ian O'Neill

Plutoid Eris is Changing… But We Don’t Know Why

The mysterious Eris and moons. Credit: NASA

[/caption]Eris, the largest dwarf planet beyond Neptune, is currently at its furthest point in its orbit from the Sun (an aphelion of nearly 100 AU). At this distance Eris doesn’t receive very much sunlight and any heating of the Plutoid will be at a minimum. However, two recent observations of Eris have shown a rapid change in the surface composition of the body. Spectroscopic analysis suggests the concentration of frozen nitrogen has dramatically altered during the two years Eris had been at this furthest point from the Sun. This is very unexpected, there should be very little change in nitrogen concentration at this point in its 557 year orbit.

So what is going on with this strange Plutoid? Is there a mystery mechanism affecting the surface conditions of this frozen moon? Could there be some cryovolcanic process erupting? Or is the explanation a little more mundane?

“We’re really scratching our heads,” says Stephen Tegler of Northern Arizona University in Flagstaff, author of the new Eris research (to be published in the journal Icarus). Tegler and his team analysed spectroscopic data from the 6.5 metre MMT observatory in Arizona and compared their 2007 results with a similar observation campaign by the 4.2 metre William Herschel Telescope in Spain two years earlier in 2005.

During that two year period, the scientists wouldn’t have thought there would be much difference in the two datasets. After all, the reflected sunlight off the surface of Eris should reveal a similar surface composition, right? Actually, the results couldn’t be more surprising. It would appear that within two years, having not changed its distance from the Sun significantly, the surface composition has changed a lot. Normally, this would be expected if a planetary body approaches or travels away from the Sun; the increase or decrease in solar energy would change the weather conditions on the surface. But this situation does not apply to Eris, there is little chance that the Sun could influence the weather on the surface of Eris to any degree (or, indeed, if Eris even has “weather”).

//neo.jpl.nasa.gov/orbits/2003ub313.html'>NASA's Near Earth Program</a>

So what have the researchers deduced from the comparison of the 2005/2007 data? It would appear the spectroscopic methane lines have become diluted by an increased quantity of nitrogen. This means that the 2005 results showed a higher concentration of nitrogen near the surface, whereas the 2007 results show a higher concentration below the surface. For a dwarf planet to demonstrate a very fast change in surface composition appears to show some very dynamic process is at work.

So what could have caused this change? In the case of a dynamic weather process, “it’s very hard to imagine that something that dramatic would be happening on a relatively short time scale,” says Mike Brown of Caltech, a scientist not involved with the research. Another possibility is that 2003ub313 is a cryovolcanic body. Cryovolcanoes can erupt on icy moons or bodies in the Kuiper belt, but rather than spewing molten rock (magma), they erupt volatiles like ammonia, water or (in this case) nitrogen and methane. The ejected cryomagma then condenses into a solid, thus changing the surface composition of the icy body.

But it is not known whether Eris is warm enough for such a process to work. More information on trans-Neptunian object (TNO) cryovolcanism will be examined when NASA’s New Horizons mission reaches Eris’ smaller cousin Pluto in 2015. “If a shrimpy little body like Pluto can do it, Eris can too,” said co-author William Grundy of Lowell Observatory in Flagstaff, Arizona.

However, there is a possibility that the surface composition of Eris hasn’t changed at all. The 2005 and 2007 observations may have been analysing two different regions on the dwarf planet, thus the difference in surface composition (after all, the Plutoid has a rotation period of 26 hours, they would have almost definitely have seen different parts of Eris). So the next step for the researchers is to carry out an extended campaign throughout an “Eris day” to see if the surface composition is in fact patchy, which would be an interesting discovery in itself.

Publication: arXiv:0811.0825v1 [astro-ph]
Original source: New Scientist