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.
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!
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.
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.
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.
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.
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.
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.”
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.
[/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”).
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.
We’ve all wondered at some point or another what mysteries our Solar System holds. After all, the eight planets (plus Pluto and all those other dwarf planets) orbit within a very small volume of the heliosphere (the volume of space dominated by the influence of the Sun), what’s going on in the rest of the volume we call our home? As we push more robots into space, improve our observational capabilities and begin to experience space for ourselves, we learn more and more about the nature of where we come from and how the planets have evolved. But even with our advancing knowledge, we would be naive to think we have all the answers, so much still needs to be uncovered. So, from a personal point of view, what would I consider to be the greatest mysteries within our Solar System? Well, I’m going to tell you my top ten favourites of some more perplexing conundrums our Solar System has thrown at us. So, to get the ball rolling, I’ll start in the middle, with the Sun. (None of the following can be explained by dark matter, in case you were wondering… actually it might, but only a little…)
10. Solar Pole Temperature Mismatch
Why is the Sun’s South Pole cooler than the North Pole? For 17 years, the solar probe Ulysses has given us an unprecedented view of the Sun. After being launched on Space Shuttle Discovery way back in 1990, the intrepid explorer took an unorthodox trip through the Solar System. Using Jupiter for a gravitational slingshot, Ulysses was flung out of the ecliptic plane so it could pass over the Sun in a polar orbit (spacecraft and the planets normally orbit around the Sun’s equator). This is where the probe journeyed for nearly two decades, taking unprecedented in-situ observations of the solar wind and revealing the true nature of what happens at the poles of our star. Alas, Ulysses is dying of old age, and the mission effectively ended on July 1st (although some communication with the craft remains).
However, observing uncharted regions of the Sun can create baffling results. One such mystery result is that the South Pole of the Sun is cooler than the North Pole by 80,000 Kelvin. Scientists are confused by this discrepancy as the effect appears to be independent of the magnetic polarity of the Sun (which flips magnetic north to magnetic south every 11-years). Ulysses was able to gauge the solar temperature by sampling the ions in the solar wind at a distance of 300 million km above the North and South Poles. By measuring the ratio of oxygen ions (O6+/O7+), the plasma conditions at the base of the coronal hole could be measured.
Why are the Martian hemispheres so radically different? This is one mystery that had frustrated scientists for years. The northern hemisphere of Mars is predominantly featureless lowlands, whereas the southern hemisphere is stuffed with mountain ranges, creating vast highlands. Very early on in the study of Mars, the theory that the planet had been hit by something very large (thus creating the vast lowlands, or a huge impact basin) was thrown out. This was primarily because the lowlands didn’t feature the geography of an impact crater. For a start there is no crater “rim.” Plus the impact zone is not circular. All this pointed to some other explanation. But eagle-eyed researchers at Caltech have recently revisited the impactor theory and calculated that a huge rock between 1,600 to 2,700 km diameter can create the lowlands of the northern hemisphere (see Two Faces of Mars Explained).
Bonus mystery: Does the Mars Curse exist? According to many shows, websites and books there is something (almost paranormal) out in space eating (or tampering with) our robotic Mars explorers. If you look at the statistics, you would be forgiven for being a little shocked: Nearly two-thirds of all Mars missions have failed. Russian Mars-bound rockets have blown up, US satellites have died mid-flight, British landers have pock-marked the Red Planet’s landscape; no Mars mission is immune to the “Mars Triangle.” So is there a “Galactic Ghoul” out there messing with our ‘bots? Although this might be attractive to some of us superstitious folk, the vast majority of spacecraft lost due to The Mars Curse is mainly due to heavy losses during the pioneering missions to Mars. The recent loss rate is comparable to the losses sustained when exploring other planets in the Solar System. Although luck may have a small part to play, this mystery is more of a superstition than anything measurable (see The “Mars Curse”: Why Have So Many Missions Failed?).
8. The Tunguska Event
What caused the Tunguska impact? Forget Fox Mulder tripping through the Russian forests, this isn’t an X-Files episode. In 1908, the Solar System threw something at us… but we don’t know what. This has been an enduring mystery ever since eye witnesses described a bright flash (that could be seen hundreds of miles away) over the Podkamennaya Tunguska River in Russia. On investigation, a huge area had been decimated; some 80 million trees had been felled like match sticks and over 2,000 square kilometres had been flattened. But there was no crater. What had fallen from the sky?
This mystery is still an open case, although researchers are pinning their bets of some form of “airburst” when a comet or meteorite entered the atmosphere, exploding above the ground. A recent cosmic forensic study retraced the steps of a possible asteroid fragment in the hope of finding its origin and perhaps even finding the parent asteroid. They have their suspects, but the intriguing thing is, there is next-to-no meteorite evidence around the impact site. So far, there doesn’t appear to be much explanation for that, but I don’t think Mulder and Scully need be involved (see Tunguska Meteoroid’s Cousins Found?).
7. Uranus’ Tilt
Why does Uranus rotate on its side? Strange planet is Uranus. Whilst all the other planets in the Solar System more-or-less have their axis of rotation pointing “up” from the ecliptic plane, Uranus is lying on its side, with an axial tilt of 98 degrees. This means that for very long periods (42 years at a time) either its North or South Pole points directly at the Sun. The majority of the planets have a “prograde” rotation; all the planets rotate counter-clockwise when viewed from above the Solar System (i.e. above the North Pole of the Earth). However, Venus does the exact opposite, it has a retrograde rotation, leading to the theory that it was kicked off-axis early in its evolution due to a large impact. So did this happen to Uranus too? Was it hit by a massive body?
Some scientists believe that Uranus was the victim of a cosmic hit-and-run, but others believe there may be a more elegant way of describing the gas giant’s strange configuration. Early in the evolution of the Solar System, astrophysicists have run simulations that show the orbital configuration of Jupiter and Saturn may have crossed a 1:2 orbital resonance. During this period of planetary upset, the combined gravitational influence of Jupiter and Saturn transferred orbital momentum to the smaller gas giant Uranus, knocking it off-axis. More research needs to be carried out to see if it was more likely that an Earth-sized rock impacted Uranus or whether Jupiter and Saturn are to blame.
6. Titan’s Atmosphere
Why does Titan have an atmosphere? Titan, one of Saturn’s moons, is the only moon in the Solar System with a significant atmosphere. It is the second biggest moon in the Solar System (second only to Jupiter’s moon Ganymede) and about 80% more massive than Earth’s Moon. Although small when compared with terrestrial standards, it is more Earth-like than we give it credit for. Mars and Venus are often cited as Earth’s siblings, but their atmospheres are 100 times thinner and 100 times thicker, respectively. Titan’s atmosphere on the other hand is only one and a half times thicker than Earth’s, plus it is mainly composed of nitrogen. Nitrogen dominates Earth’s atmosphere (at 80% composition) and it dominates Titans atmosphere (at 95% composition). But where did all this nitrogen come from? Like on Earth, it’s a mystery.
Titan is such an interesting moon and is fast becoming the prime target to search for life. Not only does it have a thick atmosphere, its surface is crammed full with hydrocarbons thought to be teeming with “tholins,” or prebiotic chemicals. Add to this the electrical activity in the Titan atmosphere and we have an incredible moon with a massive potential for life to evolve. But as to where its atmosphere came from… we just do not know.
5. Solar Coronal Heating
Why is the solar atmosphere hotter than the solar surface? Now this is a question that has foxed solar physicists for over half a century. Early spectroscopic observations of the solar corona revealed something perplexing: The Sun’s atmosphere is hotter than the photosphere. In fact, it is so hot that it is comparable to the temperatures found in the core of the Sun. But how can this happen? If you switch on a light bulb, the air surrounding the glass bulb wont be hotter than the glass itself; as you get closer to a heat source, it gets warmer, not cooler. But this is exactly what the Sun is doing, the solar photosphere has a temperature of around 6000 Kelvin whereas the plasma only a few thousand kilometres above the photosphere is over 1 million Kelvin. As you can tell, all kinds of physics laws appear to be violated.
However, solar physicists are gradually closing in on what may be causing this mysterious coronal heating. As observational techniques improve and theoretical models become more sophisticated, the solar atmosphere can be studied more in-depth than ever before. It is now believed that the coronal heating mechanism may be a combination of magnetic effects in the solar atmosphere. There are two prime candidates for corona heating: nanoflares and wave heating. I for one have always been a huge advocate of wave heating theories (a large part of my research was devoted to simulating magnetohydrodynamic wave interactions along coronal loops), but there is strong evidence that nanoflares influence coronal heating too, possibly working in tandem with wave heating.
How did dust formed at intense temperatures appear in frozen comets? Comets are the icy, dusty nomads of the Solar System. Thought to have evolved in the outermost reaches of space, in the Kuiper Belt (around the orbit of Pluto) or in a mysterious region called the Oort Cloud, these bodies occasionally get knocked and fall under the weak gravitational pull of the Sun. As they fall toward the inner Solar System, the Sun’s heat will cause the ice to vaporize, creating a cometary tail known as the coma. Many comets fall straight into the Sun, but others are more lucky, completing a short-period (if they originated in the Kuiper Belt) or long-period (if they originated in the Oort Cloud) orbit of the Sun.
But something odd has been found in the dust collected by NASA’s 2004 Stardust mission to Comet Wild-2. Dust grains from this frozen body appeared to have been formed a high temperatures. Comet Wild-2 is believed to have originated from and evolved in the Kuiper Belt, so how could these tiny samples be formed in an environment with a temperature of over 1000 Kelvin?
The Solar System evolved from a nebula some 4.6 billion years ago and formed a large accretion disk as it cooled. The samples collected from Wild-2 could only have been formed in the central region of the accretion disk, near the young Sun, and something transported them into the far reaches of the Solar System, eventually ending up in the Kuiper Belt. But what mechanism could do this? We are not too sure (see Comet Dust is Very Similar to Asteroids).
3. The Kuiper Cliff
Why does the Kuiper Belt suddenly end? The Kuiper Belt is a huge region of the Solar System forming a ring around the Sun just beyond the orbit of Neptune. It is much like the asteroid belt between Mars and Jupiter, the Kuiper Belt contains millions of small rocky and metallic bodies, but it’s 200-times more massive. It also contains a large quantity of water, methane and ammonia ices, the constituents of cometary nuclei originating from there (see #4 above). The Kuiper Belt is also known for its dwarf planet occupant, Pluto and (more recently) fellow Plutoid “Makemake”.
The Kuiper Belt is already a pretty unexplored region of the Solar System as it is (we wait impatiently for NASA’s New Horizons Pluto mission to arrive there in 2015), but it has already thrown up something of a puzzle. The population of Kuiper Belt Objects (KBOs) suddenly drops off at a distance of 50 AU from the Sun. This is rather odd as theoretical models predict an increase in number of KBOs beyond this point. The drop-off is so dramatic that this feature has been dubbed the “Kuiper Cliff.”
We currently have no explanation for the Kuiper Cliff, but there are some theories. One idea is that there are indeed a lot of KBOs beyond 50 AU, it’s just that they haven’t accreted to form larger objects for some reason (and therefore cannot be observed). Another more controversial idea is that KBOs beyond the Kuiper Cliff have been swept away by a planetary body, possibly the size of Earth or Mars. Many astronomers argue against this citing a lack of observational evidence of something that big orbiting outside the Kuiper Belt. This planetary theory however has been very useful for the doomsayers out there, providing flimsy “evidence” for the existence of Nibiru, or “Planet X.” If there is a planet out there, it certainly is not “incoming mail” and it certainly is notarriving on our doorstep in 2012.
So, in short, we have no clue why the Kuiper Cliff exists…
2. The Pioneer Anomaly
Why are the Pioneer probes drifting off-course? Now this is a perplexing issue for astrophysicists, and probably the most difficult question to answer in Solar System observations. Pioneer 10 and 11 were launched back in 1972 and 1973 to explore the outer reaches of the Solar System. Along their way, NASA scientists noticed that both probes were experiencing something rather strange; they were experiencing an unexpected Sun-ward acceleration, pushing them off-course. Although this deviation wasn’t huge by astronomical standards (386,000 km off course after 10 billion km of travel), it was a deviation all the same and astrophysicists are at a loss to explain what is going on.
One main theory suspects that non-uniform infrared radiation around the probes’ bodywork (from the radioactive isotope of plutonium in its Radioisotope Thermoelectric Generators) may be emitting photons preferentially on one side, giving a small push toward the Sun. Other theories are a little more exotic. Perhaps Einstein’s general relativity needs to be modified for long treks into deep space? Or perhaps dark matter has a part to play, having a slowing effect on the Pioneer spacecraft?
How do we know the Oort Cloud even exists? As far as Solar System mysteries go, the Pioneer anomaly is a tough act to follow, but the Oort cloud (in my view) is the biggest mystery of all. Why? We have never seen it, it is a hypothetical region of space.
At least with the Kuiper Belt, we can observe the large KBOs and we know where it is, but the Oort Cloud is too far away (if it really is out there). Firstly, the Oort Cloud is predicted to be over 50,000 AU from the Sun (that’s nearly a light year away), making it about 25% of the way toward our nearest stellar neighbour, Proxima Centauri. The Oort Cloud is therefore a very long way away. The outer reaches of the Oort Cloud is pretty much the edge of the Solar System, and at this distance, the billions of Oort Cloud objects are very loosely gravitationally bound to the Sun. They can therefore be dramatically influenced by the passage of other nearby stars. It is thought that Oort Cloud disruption can lead to icy bodies falling inward periodically, creating long-period comets (such as Halley’s comet).
In fact, this is the only reason why astronomers believe the Oort Cloud exists, it is the source of long-period icy comets which have highly eccentric orbits emanating regions out of the ecliptic plane. This also suggests that the cloud surrounds the Solar System and is not confined to a belt around the ecliptic.
So, the Oort Cloud appears to be out there, but we cannot directly observe it. In my books, that is the biggest mystery in the outermost region of our Solar System…
This protoplanetary disk in the Orion Nebula has a mass more than one hundredth that of the sun, the minimum needed to form a Jupiter-sized planet. Image credit: Bally et al 2000/Hubble Space Telescope & Eisner et al 2008/CARMA, SMA)
The Orion Nebula shines brilliantly, as it is packed with over 1,000 young stars in a region just a few light-years wide. With all those stars, there’s probably the potential for thousands of planets to one day form from the dust and gas surrounding these stars, right? Actually, according to a new study, fewer than 10 percent of stars in the Orion Nebula have enough surrounding dust to make a planet the size of Jupiter. And that doesn’t bode well for the planet-forming abilities of most stars, at least in forming planets the size of Jupiter or larger. “We think that most stars in the galaxy are formed in dense, Orion-like regions, so this implies that systems like ours may be the exception rather than the rule,” said Joshua Eisner lead author of the study from the University of California Berkeley. This finding is also consistent with the results of current planet searches, which are finding that only about 6 percent of stars surveyed have planets the size of Jupiter or larger.
In the observations of Orion’s central region of more than 250 known stars, the findings showed that only about 10 percent emit the wavelength radiation typically emitted by a warm disk of dust, (1.3-millimeter). Even fewer – less than 8 percent of stars surveyed – were found to have dust disks with masses greater than one-hundredth the mass of the sun, which is thought to be the lower mass limit for the formation of Jupiter-sized planets. The average mass of a protoplanetary disk in the region was only one-thousandth of a solar mass, the researchers calculated.
The study was done using the Combined Array for Research in Millimeter Astronomy (CARMA) in California, and the Submillimeter Array (SMA) atop Mauna Kea in Hawaii. Both facilities observe at millimeter wavelengths, which is ideal for piercing the clouds of dust and gas surrounding young stars to see their dense, dusty disks.
Four billion years ago our own sun may have been in a dense, open cluster like Orion. Because open clusters like Orion eventually become gravitationally unbound, they disperse over the course of billions of years, and as a result, the sun’s birth neighbors are long gone.
Eisner said studying star clusters like the Orion Nebula Cluster “helps our understanding of the typical mode of star and planet formation.”
However, another survey of the Taurus cluster, which is a lower-density star-forming region showed that more than 20 percent of its stars have enough mass to form planets. The difference is probably related to the tightly packed, hot stars of the Orion cluster, said John Carpenter, colleague of Eisner’s in the study.
“Somehow, the Orion cluster environment is not conducive to forming high mass disks or having them survive long, presumably due to the ionization field from the hot, massive OB stars , which you might expect would photoevaporate dust and lead to small disk masses,” he said.
How planets form is one of the major questions in astronomy. Only recently have we been able to study the disks of dust and gas surrounding other stars in an effort to understand the process of how planets coalesce and form from these “protoplanetary” materials. But this is a difficult task at best, given the observational distances. “This is a vast topic with many challenges,” said David Wilner from the Harvard-Smithsonian Center for Astronomy at his talk at the American Astronomical Society meeting this week. “But over the course of the past few decades with observations of nearby star systems, we’ve come to a basic outline of the process of solar system formation.”
There are a couple of hurdles to overcome in studying protoplanetary disks. First, the bulk of the disk mass is cold and dark, as the molecular hydrogen doesnâ€™t radiate. These areas are probed only through a couple of minor constituents: thermal emission from dust and scattered light from the star.
Second, the amount of “stuff” astronomers are looking at is actually fairly small. Usually, the amount of protoplanetary material is about 1/100th the mass of the star, and about 1/4000th of a degree in the sky.
Through observations of many systems with several telescopes, we can see these disk systems in a variety of wavelengths in an effort to see both the star and the disk components. Wilner said there are two properties that are particularly important to know: Disk masses in general, as the luminosity is directly proportional to the mass, and second is the disk lifetime. From current knowledge, the dust disk disperses by 50% in 3 million years, and 90% by 5 million years.
As an example, Milner discussed the Rho Ophiuchi nebula, (image above), located near the constellations Scorpius and Ophiuchus, about 407 light years away from Earth.
“The Rho Oph cloud is spectacular, with beautiful dark regions that are columns of gas and dust extinguishing the background star field. This is the material that is forming stars and planets.”
Wilner said the steps in solar system formation are as follows: first the formation of a primordial proto-star disk, then the protoplanetary disk, and then debris disk within a planetary system.
But the main problems in our understanding lies in that astronomers haven’t yet actually seen all the steps in this process, and can’t prove directly that these early disks go on to form the planets. There are several clues, such as that gaps form in the dust around clumps of materials, similar to the gaps in the rings of Saturn around moons.
For the past 15 years protoplanetary disks have been studied with various interferometers at the Keck Observatory on Mauna Kea at various wavelengths from .87 microns to 7 mm. And the past five years the Spitzer Space Telescope has lent its infrared capabilities to further our knowledge to our current understanding. But soon, a new telescope in the high Chilean desert might provide the resolution needed to offer a glimpse at not only the gaps in the disks, but a new window on how materials around emerging planets may form moons. The Atacama Large Millimeter/submillimeter Array (ALMA), will operate at wavelengths of 0.3 to 9.6 millimeters.
Wilner obviously looks forward to putting observational capabilities of this array to work. Scheduled to be completed in 2012, ALMA will help fill in the “gaps” of our knowledge about planetary formation.
Source: AAS Meeting presentation, with clarification from Chris Lintott
For every backyard astronomer, we know 4.5 billion years ago, both Venus and Earth were formed with nearly the same radius, mass, density and chemical composition. Venus is like Earth’s evil twin, but why is the climate on both worlds so widely varied? Scientists analysing the data from the orbiting European Venus Express spacecraft are finally putting the pieces of the geological and climatological puzzle together as they take a closer look at Venusian evolution.
Today, Professor Fred Taylor of Oxford University presented the scenario in a talk at the Royal Astronomical Society National Astronomy Meeting in Belfast. According to the studies, Venus appeared to have evolved very rapidly compared to the Earth during the early formation of the solar system. Thanks to data obtained from the Venus Express, it would appear our wicked sister planet once had significant volume of water covering the surface… Oceans which were lost in a very short geological timescale. As the water disappeared, the geological evolution of the surface of Venus slowed quickly – unable to develop plate tectonics like the Earth. Biological evolution could never happen. If, at one time, Venus mirrored Earth in climate and habitability terms, then it evolved too quickly at first, then too slowly.
‘They may have started out looking very much the same,’ said Professor Taylor, ‘but increasingly we have evidence that Venus lost most of its water and Earth lost most of its atmospheric carbon dioxide.’
Here on Earth, carbon dioxide is captive plant life, minerals and the crust itself. Not to harp on global warming, but the release CO2 back into the atmosphere is a source of climatic change. On Venus, the majority of the carbon dioxide resides it its atmosphere, leaving the surface temperature at a searing 450 degrees Celsius. This slows or stops geological as well as biological evolution.
‘The interesting thing is that the physics is the same in both cases’ said Prof Taylor. ‘The great achievement of Venus Express is that it is putting the climatic behaviour of both planets into a common framework of understanding.’
But, we haven’t heard the last from Venus Express just yet. Due to operate until May 2009, scientists involved in the project are already busy applying for an extension until 2011.
‘We have plans for joint operations with the Japanese spacecraft called Venus Climate Orbiter that will arrive in December 2010’, said Taylor. ‘Together, we can do things neither could do alone to crack some of the remaining puzzles about Venus.’