Posted on by Ian O'Neill

The Sun as a White Dwarf Star

Dusty debris around an old white dwarf star (NASA)

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What will happen to all the inner planets, dwarf planets, gas giants and asteroids in the Solar System when the Sun turns into a white dwarf? This question is currently being pondered by a NASA researcher who is building a model of how our Solar System might evolve as our Sun loses mass, violently turning into an electron-degenerate star. It turns out that Dr. John Debes work has some very interesting implications. As we use more precise techniques to observe existing white dwarf stars with the dusty remains of the rocky bodies that used to orbit them, the results of Debes’ model could be used as a comparison to see if any existing white dwarf stars resemble how our Sun might look in 4-5 billion years time…

A comparison of the Sun in its yellow dwarf phase and red giant phase
A comparison of the Sun in its yellow dwarf phase and red giant phase
Today, our Sun is a healthy yellow dwarf star. If you want to be precise, it is a “G V star”. This yellow dwarf will happily burn 600 million tonnes of hydrogen per second in its core for 10 billion years, generating the light that is required to make our planet habitable. The Sun is approximately half-way through this hydrogen burning phase, so it’s OK, things aren’t going to change (for the Sun at least) for a long time yet.

But what happens then? What happens in 4-5 billion years when the supply of hydrogen runs out in the core? Although our Sun isn’t massive enough to entertain the thought of going out in a blaze of supernova glory, it will still go through an exciting, yet terrifying death. After evolving through the hydrogen-burning phase, the Sun will puff up into a huge red giant star as the hydrogen fuel becomes scarce, expanding 200 times the size it is now, probably swallowing the Earth. Helium, and then progressively heavier elements will be fused in and around the core. The Sun will never fuse carbon however, instead it will shed its outer layers forming a planetary nebula.

Once things calm down, a small sparkling jewel of a white dwarf star will remain. This tiny remnant will have a mass of around half that of our present Sun, but will be the size of the Earth. Needless to say, white dwarfs are very dense, intense gravitational pull countered not by fusion in the core (like all Main Sequence stars), but by electron degeneracy pressure.

Relative sizes of IK Pegasi A (left), IK Pegasi B (lower center; a white dwarf) and the Sun (NASA)
Relative sizes of IK Pegasi A (left), IK Pegasi B (lower center; a white dwarf) and the Sun (NASA)
When the Solar System reaches this phase in its evolution, what will it look like? What will become of the asteroids, gas giants, moons and rocky planets? I was very fortunate to chat with astrophysicist Dr John Debes, from NASA’s Goddard Space Flight Center, at January’s American Astronomical Society (AAS) conference in Long Beach (California) who is developing an n-body code simulating an evolving Solar System.

After the Sun has stopped hydrogen fusion in its core, it loses mass as it sheds its outer layers after the red giant phase and subsequent planetary nebula formation. It is estimated that the Sun will lose about 50% of its mass during this time, naturally affecting the Solar System as a whole. As the Sun loses mass, the outer planets (such as Jupiter) will drift outwards, increasing their orbital radii. In the simulation, Debes is very careful to ensure there is a gradual reduction in solar mass to ensure stability in the simulation.

What we are left with is an old Solar System, where little is left of the inner planets (it is likely that anything within the orbit of the Earth will have been swallowed by the Sun as it expanded through the red giant phase). Although the future white dwarf Solar System will seem very alien to present day, some things won’t change. Jupiter’s orbit might have receded with the drop in solar mass, it will remain a planetary heavyweight, causing disruption in asteroid orbits. Using known asteroid data, the motion of these chunks of rocks are allowed to evolve, and over millions of years, they may get thrown out of the Solar System, or more interestingly, pushed closer to the white dwarf. Once the whole system has settled down, resonances in the asteroid belt will become amplified; Kirkwood Gaps (caused by gravitational resonance with Jupiter) will widen, and according to Debes’ simulations, the edges of these gaps will become perturbed even more, making more asteroids available to be tidally disrupted and shredded to dust.

Artists concept of shredded asteroid around white dwarf (NASA/JPL-Caltech)
Artists concept of shredded asteroid around white dwarf (NASA/JPL-Caltech)
The AAS conference was full of amazing research into white dwarf observations. The reason for this is that there are many white dwarf candidates out there with dusty metallic absorption lines. This means that there used to be rocky bodies orbiting these stars, but became pulverised (by tidal shear) for astronomers to analyse. These white dwarf systems can give us a clue as to what mechanisms could be supplying the white dwarfs with dusty material, even giving us a glimpse into the future of our Solar System.

“We have a physical picture for the link between planetary systems and dusty white dwarfs,” Debes said when describing his model in relation to the mysterious dusty white dwarf observations. “Dusty white dwarfs are truly a mystery! We think we know what might be going on, but we don’t have a smoking gun yet.”

However, Debes is getting close to finding a possible smoking gun, he’s basing his model on some of the key characteristics of these ancient dusty remnants to see what the Solar System could look like in billions of years time.

So, where does this dust come from? As the asteroid orbits are perturbed by Jupiter, they may get close enough to be tidally disrupted. Get too close and they will get shredded by the gravitational shear created by the steep tidal radius of the compact white dwarf. The asteroid dust then settles into the white dwarf. The presence of this dust has a very obvious signature in the absorption lines of spectroscopic data, allowing researchers to infer an accretion rate for metal-rich white dwarfs. In Debes’ model, he has set the upper limit to 1016 g/year and a lower limit to 1013 g/year, consistent with observed estimates.

Spectra of G29-38. Could this resemble the spectra of the Sun after turning into a white dwarf? (NASA/Spitzer)
Spectra of G29-38. Could this resemble the spectra of the Sun after turning into a white dwarf? (NASA/Spitzer)
In his evolved Solar System model, Jupiter’s gravity controls this accretion rate, pushing asteroids toward the white dwarf and, by using a powerful supercomputer to track the perturbations and eventual shredding of known asteroids, there may be an opportunity to arrive at a profound conclusion. Debes is able to use his model to compare observations of known dusty white dwarfs with the simulated outcome of the Solar System. With reference to previous studies (in particularly Koester & Wilken, 2006 in the journal Astronomy & Astrophysics), Debes has found some similar white dwarf “Suns”.

“For G29-38, the canonical dusty white dwarf, they [Koester & Wilken] estimate a total mass of 0.55 solar masses–about what people believe the mass that our own sun will have remaining when it becomes a white dwarf,” Debes added. “But mass estimates are a bit uncertain–I’ve seen estimates ranging from 0.55-0.7 solar masses for this particular white dwarf.”

The Su<span>n's future? The whit</span>e dwarf G29-38 (NASA)
The Sun's future? The white dwarf G29-38 (NASA)
“Another good candidate is a DAZ [a metal-rich white dwarf] called WD 1257+278, which does not show dust but is spot on with the mass expected for the Sun–0.54 MSun,” said Debes. “Its accretion rate is also consistent with my model predictions so far assuming an asteroid belt mass and characteristic perturbation timescale that I found in my simulations.”

Debes is continuing to make his model more and more sophisticated, but already the results are promising. Most exciting is that we may already be observing white dwarfs, like G29-38 or WD 1257+278, giving us a tantalizing glimpse of what our Solar System will look like when the Sun becomes a white dwarf star, ripping apart any remaining asteroids and planets as they stray too close to the Sun’s tidal shear. However, it also raises the question: if white dwarfs like G29-38 are being fed by the remains of tidally-blended asteroids, are there massive planets shepherding asteroids in these white dwarf systems too?

Posted on by Anne Minard

Pluto’s Atmosphere Boasts Methane, Warmer Temps

Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

 

Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

 

Pluto is certainly frigid, but new research has revealed its atmosphere is a bit warmer.

Astronomers using the European Southern Observatory’s Very Large Telescope have found unexpectedly large amounts of methane in Pluto’s atmosphere, which evidently helps it stay about 40 degrees warmer than the dwarf planet’s surface. The atmosphere warms to -180 degrees Celsius (-356 degrees Fahrenheit), compared to a surface that’s usually -220 degrees Celsius (-428 degrees Fahrenheit).

“With lots of methane in the atmosphere, it becomes clear why Pluto’s atmosphere is so warm,” said Emmanuel Lellouch of the Observatoire de Paris in France. Lellouch is lead author of the paper reporting the results, which is in press at the journal Astronomy and Astrophysics.

Pluto, which is about a fifth the size of Earth, is composed primarily of rock and ice and orbits about 40 times further from the Sun than the Earth.

It has been known since the 1980s that Pluto also has a thin, tenuous atmosphere. Abundant nitrogen, along with traces of methane and probably carbon monoxide, are held to the surface by an atmospheric pressure only about one hundred thousandth of that on Earth, or about 0.015 millibars. As Pluto moves away from the Sun, during its 248 year-long orbit, its atmosphere gradually freezes and falls to the ground. In periods when it is closer to the Sun — as it is now — the temperature of Pluto’s solid surface increases, causing the ice to sublimate into gas.

Until recently, only the upper parts of the atmosphere of Pluto could be studied. By observing stellar occultations, a phenomenon that occurs when a Solar System body blocks the light from a background star, astronomers were able to demonstrate that Pluto’s upper atmosphere was some 50 degrees warmer than the surface. Those observations couldn’t shed any light on the atmospheric temperature and pressure near Pluto’s surface. But unique, new observations made with the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO’s Very Large Telescope, have now revealed that the atmosphere as a whole, not just the upper atmosphere, has a mean temperature much less frigid than the surface.

Usually, air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth’s surface, which, in turn, warms the layer of the atmosphere directly above it. Under certain conditions, this situation is inverted so that the air is colder near the surface of the Earth. Meteorologists call this an inversion layer, and it can cause smog build-up.

Most, if not all, of Pluto’s atmosphere is thus undergoing a temperature inversion: the temperature is higher, the higher in the atmosphere you look. The change is about 3 to 15 degrees per kilometer (.62 miles). On Earth, under normal circumstances, the temperature decreases through the atmosphere by about 6 degrees per kilometer.

The reason why Pluto’s surface is so cold is linked to the existence of Pluto’s atmosphere, and is due to the sublimation of the surface ice; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto.

The CRIRES observations also indicate that methane is the second most common gas in Pluto’s atmosphere, representing half a percent of the molecules. “We were able to show that these quantities of methane play a crucial role in the heating processes in the atmosphere and can explain the elevated atmospheric temperature,” said Lellouch.

Two different models can explain the properties of Pluto’s atmosphere. In the first, the astronomers assume that Pluto’s surface is covered with a thin layer of methane, which will inhibit the sublimation of the nitrogen frost. The second scenario invokes the existence of pure methane patches on the surface.

“Discriminating between the two will require further study of Pluto as it moves away from the Sun,” says Lellouch. “And of course, NASA’s New Horizons space probe will also provide us with more clues when it reaches the dwarf planet in 2015.”

LEAD IMAGE CAPTION: Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

Source: ESO

Posted on by Anne Minard

Jupiter, Saturn Plowed Through Asteroids, Study Says

Asteroids
Artist's depiction of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra

 

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When Mars and Jupiter migrated to their present orbits around 4 billion years ago, they left scars in the asteroids belt that are still visible today.

The evidence is unveiled in a new paper in this week’s issue of the journal Nature, by planetary scientists David Minton and Renu Malhotra from the University of Arizona in Tucson.  

The asteroid belt has long been known to harbor gaps, called Kirkwood gaps, in distinct locations. Some of these gaps correspond to unstable zones, where the modern-day gravitational influence of Jupiter and Saturn eject asteroids. But for the first time, Minton and Malhotra have noticed that some clearings don’t fit the bill.

“What we found was that many regions are depleted in asteroids relative to other regions, not just in the previously known Kirkwood gaps that are explained by the current planetary orbits,” Minton wrote in an email. In an editorial accompanying the paper, author Kevin Walsh added, “Qualitatively, it looks as if a snow plough were driven through the main asteroid belt, kicking out asteroids along the way and slowing to a stop at the inner edge of the belt.” 

Walsh hails from the Observatoire de la Côte d’Azur in France. In his News and Views piece, he explains that the known Kirkwood gaps, discovered by Daniel Kirkwood in 1867, “correspond to the location of orbital resonances with Jupiter — that is, of orbits whose periods are integer ratios of Jupiter’s orbital period.” For example, if an asteroid orbited the Sun three times for every time Jupiter did, it would be in a 3:1 orbital resonance with the planet, he wrote. Objects in resonance with a giant planet have inherently unstable orbits, and are likely to be ejected from the solar system. When planets migrated, astronomers believe objects in resonance with them also shifted, affecting different parts of the asteroid belt at different times. 

“Thus, if nothing has completely reshaped the asteroid belt since the planets settled into their current orbits, signatures of past planetary orbital migration may still remain,” Walsh wrote. And that’s exactly what Minton and Malhotra sought.

The asteroid belt easily gave up its secrets, showing the lingering evidence of planetary billiards on the inner edge of the asteroid belt and at the outer edge of each Kirkwood gap. The new finding, based on computer models, lends additional support to the theory that the giant planets — Jupiter, Saturn, Uranus and Neptune — formed twice as close to the sun as they are now and in a tighter configuration, and moved slowly outward. 

“The orbit of Pluto and other Kuiper belt objects that are trapped in [orbits that resonate] with Neptune can be explained by the outward migration of Neptune,” Minton and Malhotra write in the new study. “The exchange of angular momentum between planetesimals and the four giant planets caused the orbital migration of the giant planets until the outer planetesimal disk was depleted.”  Planetesimals are rocky and icy objects left over from planet formation.

“As Jupiter and Saturn migrated,” the authors continue, they wreaked havoc on the young asteroid belt, “exciting asteroids into terrestrial planet-crossing orbits, thereby greatly depleting the asteroid belt population and perhaps also causing a late heavy bombardment in the inner Solar System.”

The late heavy bombardment is proposed to have occurred about 3.9 billion years ago, or 600 million years after the birth of the Solar System, and it’s believed to account for many of the Moon’s oldest craters. Walsh said a reasonable next step, to corroborate the theory about the newly described clearings in the asteroid belt, is to link them chronologically with the bombardment.

LEAD PHOTO CAPTION: Artist’s depiction of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra

Source: Nature

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

Posted on by Ian O'Neill

Google and NASA are Working on an Interplanetary Internet

The Interplanetary Internet concept

[/caption]In an initiative energized by Google Vice-President and Chief Internet Evangelist Vint Cerf, the International Space Station could be testing a brand new way of communicating with Earth. In 2009, it is hoped that the ISS will play host to an Interplanetary Internet prototype that could standardize communications between Earth and space, possibly replacing point-to-point single use radio systems customized for each individual space mission since the beginning of the Space Age.

This partnership opens up some exciting new possibilities for the future of communicating across vast distances of the Solar System. Manned and robotic space craft will be interconnected via a robust interplanetary network without the problems associated with incompatible communication systems…

“The project started 10 years ago as an attempt to figure out what kind of technical networking standards would be useful to support interplanetary communication,” Cerf said in a recent interview. “Bear in mind, we have been flying robotic equipment to the inner and outer planets, asteroids, comets, and such since the 1960’s. We have been able to communicate with those robotic devices and with manned missions using point-to-point radio communications. In fact, for many of these missions, we used a dedicated communications system called the Deep Space Network (DSN), built by JPL in 1964.”

Indeed, the DSN has been the backbone of interplanetary communications for decades, but an upgrade is now required as we have a growing armada of robotic missions exploring everything from the surface of Mars to the outermost regions of the Solar System. Wouldn’t it be nice if a communication network could be standardized before manned missions begin moving beyond terrestrial orbit?

“When we launch a spacecraft with a unique set of sensors onboard, we often end up writing special communication and application software that is adapted to that spacecraft’s sensor systems and manipulators,” Cerf said in response to the challenges space missions face each time they are designed.

The Internet uses standard TCP/IP protocols so billions of online entities are always compatible. Although there are limitations to the Internet, it has proven to be a highly flexible and scalable system, so with the help of Google, NASA hopes to push the Internet beyond Earth. “The Interplanetary Internet project is primarily about developing a set of communication standards and technical specifications to support rich networking in space environments,” Cerf added.

This all sounds very interesting, but the challenges with building such a system require some novel techniques. How do you deal with the limitation of the speed of light? After all, it can take light 40 minutes to travel to-and-from Mars, and up to 12 hours to Pluto and back. How do you cater for planetary rotation? The transmitters/receivers won’t always be on the correct side of the planet. What happens if a satellite signal is blocked by a planet, the Sun or a moon?

Vint Cerf says the disruption of data transmission has to be confronted with a delay- and disruption-tolerant networking system, otherwise known as DTN. “It will allow us to maintain communications more effectively, getting much more data because we don’t have to be in direct line of sight with the ultimate recipient in order to transfer data,” he said.

DTN will be based on store-and-forward methods used by TCP/IP systems; if there is a disruption in signal, the transmitting station will hold data packets until the signal is re-established. However, DTN will be more robust, catering for long transmission lag-times (such as the many-hour light transmission times between Earth and the outer Solar System). “We have to cope with the fact that there is a really high potential for delay and disruption in the system,” he added.

Standard TCP/IP protocol should also work seamlessly with the DTN, allowing planetary missions to have their own distributed Internet whilst using DTN as a link through interplanetary space.

This has obvious applications for future manned missions to Mars, after all, can you imagine the first colonists without their own blog?

Source: Technology Review

Posted on by Nancy Atkinson

New Eye on the Outer Solar System Launches Successfully

The Interstellar Boundary Explorer. Credit: NASA

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There’s a new spacecraft in Earth orbit, with a really “far out” mission: to map the outer solar system. NASA’s Interstellar Boundary Explorer mission, or IBEX launched successfully from the Kwajalein Atoll in the Pacific Ocean at 1:47 p.m. EDT, Sunday, from an Orbital Sciences Pegasus XL launch vehicle. IBEX will be the first spacecraft to image and map dynamic interactions taking place in the outer solar system. The two Voyager probes sent back a limited amount of information about the region of space where our solar system ends and interstellar space begins. But beyond that, not much is known about this area. The region is about three times further from the sun than the orbit of planet Pluto. “No one has seen an image of the interaction at the edge of our solar system where the solar wind collides with interstellar space,” said IBEX Principal Investigator David McComas of the Southwest Research Institute in San Antonio. “We know we’re going to be surprised.”

The spacecraft separated from the third stage of its Pegasus launch vehicle at 1:53 p.m. and immediately began powering up components necessary to control onboard systems. The operations team is continuing to check out spacecraft subsystems.

“After a 45-day orbit raising and spacecraft checkout period, the spacecraft will start its exciting science mission,” said IBEX mission manager Greg Frazier of NASA’s Goddard Space Flight Center in Greenbelt, Md.

“The heliosphere’s boundary region is enormous, and the Voyager crossings of the termination shock, while historic, only sampled two tiny areas 10 billion miles (16 billion km) apart,” NASA scientist Eric Christian said.

Voyager 1 passed the inner boundary in 2004 and Voyager 2 crossed over last year.

The solar wind, a stream of electrically conducting gas continuously moving outward from the sun at 1 million mph (1.6 million kph), blows against this interstellar material and forms a huge protective bubble around the solar system. This bubble is called the heliosphere.

As the solar wind reaches far beyond the planets to the solar system’s outer limits, it encounters the edge of the heliosphere and collides with interstellar space. A shock wave is present at this boundary.

“Every six months, we will make global sky maps of where these atoms come from and how fast they are traveling. From this information, we will be able to discover what the edge of our bubble looks like and learn about the properties of the interstellar cloud that lies beyond the bubble,” physicist Herb Funsten of the U.S. Department of Energy’s Los Alamos National Laboratory.

Sources: NASA, Reuters

Posted on by Nancy Atkinson

Where Are All the Kuiper Belt Objects?

Occultations of KBOs. Credit: TAOS

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A group of astronomers spent two years photographing portions of the sky to look for small chunks of rock and ice orbiting beyond Neptune, in the Kuiper Belt region of our Solar System. The survey targeted Kuiper Belt objects (KBOs) with sizes between 2 miles (3 km) and 17 miles (28 km). The researchers are surprised, as well as a little disappointed with the results. They came up empty. Nada. Not a single KBO within those parameters was spotted. But these researchers from the Taiwanese-American Occultation Survey (TAOS) are ‘glass-half-full’ types, and say that defeat can provide as much information as a successful search, so they are making the most of their data. What this means is that there are less KBOs out there than previously thought.

Since the KBO’s this group searched for are too small to see directly, the survey watched for stars to dim as KBOs passed in front of and occulted them. After accumulating more than 200 hours of data watching for stellar flickers lasting a second or less, TAOS did not spot any occultations.

Here’s a movie that illustrates their search.

The Kuiper Belt contains objects in a range of sizes: a few very large ones (like the dwarf planets Pluto, Eris, Makemake and Haumea) and many more smaller ones. The commonness of a given size tells us information about the history of planet formation and dynamics. In particular, the size distribution of KBOs reflects a history of agglomeration, in which colliding objects tended to stick together, followed by destructive collisions, where collisional velocities were high enough to shatter the rocks involved.

Astronomers questioned whether they would find more and more objects as sizes decreased further, or whether the distribution leveled out. The fact that no occultations were seen sets a stringent upper limit on the number density of KBOs between 2 and 17 miles in diameter. The outer solar system, therefore, appears not as crowded as some theories suggest, perhaps because small KBOs have already stuck together to form larger bodies or frequent collisions have ground down small KBOs into even smaller bits below the threshold of the survey.

The paper announcing this result, co-authored by CfA director Charles Alcock, was published in the October 1 issue of the Astrophysical Journal Letters.

Sources: TAOS, CfA

Posted on by Nancy Atkinson

Did Our Solar System Start With a “Little Bang?”

Artist illustration of supernova. Credit: NASA

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What prompted the formation of our little corner of the universe – our sun and planetary system? For several decades, scientists have thought that the Solar System formed as a result of a shock wave from an exploding star—a supernova—that triggered the collapse of a dense, dusty gas cloud, which then contracted to form the Sun and the planets. But detailed models of this formation process have only worked under the simplifying assumption that the temperatures during the violent events remained constant. That, of course, is very unlikely. But now, astrophysicists at the Carnegie Institution’s Department of Terrestrial Magnetism (DTM) have shown for the first time that a supernova could indeed have triggered the Solar System’s formation under the more likely conditions of rapid heating and cooling. So have these new findings resolved this long-standing debate?

“We’ve had chemical evidence from meteorites that points to a supernova triggering our Solar System’s formation since the 1970s,” remarked lead author, Carnegie’s Alan Boss. “But the devil has been in the details. Until this study, scientists have not been able to work out a self-consistent scenario, where collapse is triggered at the same time that newly created isotopes from the supernova are injected into the collapsing cloud.”

Short-lived radioactive isotopes—versions of elements with the same number of protons, but a different number of neutrons—found in very old meteorites decay on time scales of millions of years and turn into different (so-called daughter) elements. Finding the daughter elements in primitive meteorites implies that the parent short-lived radioisotopes must have been created only a million or so years before the meteorites themselves were formed. “One of these parent isotopes, iron-60, can be made in significant amounts only in the potent nuclear furnaces of massive or evolved stars,” explained Boss. “Iron-60 decays into nickel-60, and nickel-60 has been found in primitive meteorites. So we’ve known where and when the parent isotope was made, but not how it got here.”

Cross-sectional view of one-half of a solar-mass target cloud being struck by a supernova shock front that is traveling downward. Credit: Carnigie Institution for Science
Cross-sectional view of one-half of a solar-mass target cloud being struck by a supernova shock front that is traveling downward. Credit: Carnigie Institution for Science

Previous models by Boss and former DTM Fellow Prudence Foster showed that the isotopes could be deposited into a pre-solar cloud if a shock wave from a supernova explosion slowed to 6 to 25 miles per second and the wave and cloud had a constant temperature of -440 °F (10 K). “Those models didn’t work if the material was heated by compression and cooled by radiation, and this conundrum has left serious doubts in the community about whether a supernova shock started these events over four billion years ago or not,” remarked Harri Vanhala, who found the negative result in his Ph.D. thesis work at the Harvard-Smithsonian Center for Astrophysics in 1997.

Using an adaptive mesh refinement hydrodynamics code, FLASH2.5, designed to handle shock fronts, as well as an improved cooling law, the Carnegie researchers considered several different situations. In all of the models, the shock front struck a pre-solar cloud with the mass of our Sun, consisting of dust, water, carbon monoxide, and molecular hydrogen, reaching temperatures as high as 1,340°F (1000 K). In the absence of cooling, the cloud could not collapse. However, with the new cooling law, they found that after 100,000 years the pre-solar cloud was 1,000 times denser than before, and that heat from the shock front was rapidly lost, resulting in only a thin layer with temperatures close to 1,340°F (1000 K). After 160,000 years, the cloud center had collapsed to become a million times denser, forming the protosun. The researchers found that isotopes from the shock front were mixed into the protosun in a manner consistent with their origin in a supernova.

“This is the first time a detailed model for a supernova triggering the formation of our solar system has been shown to work,” said Boss. “We started with a Little Bang 9 billion years after the Big Bang.”

Source: Carnegie Institution for Science

Posted on by Fraser Cain

Evidence of Our Violent Early Solar System

Meteorite. Image credit: NASA/JPL/Cornell Click to enlarge
A U of T scientist has found unexpectedly ?young? material in meteorites ? a discovery that breaks open current theory on the earliest events of the solar system.

A paper published today in the August issue of Nature reports that the youngest known chondrules ? the small grains of mineral that make up certain meteorites ? have been identified in the meteorites known as Gujba and Hammadah al Hamra.

Researchers who have studied chondrules generally agree that most were formed as a sudden, repetitive heat, likely from a shock wave, condensed the nebula of dust floating around the early Sun. Thinking that an analysis of the chondrules in Gujba and Hammadah al Hamra would be appropriate for accurately dating this process, U of T geologist Yuri Amelin, together with lead author Alexander Krot of the University of Hawaii, studied the chondrules? mineralogical structure and determined their isotopic age. ?It soon became clear that these particular chondrules were not of a nebular origin,? says Amelin. ?And the ages were quite different from what was expected. It was exciting.?

Amelin explains that not only were these chondrules not formed by a shock wave, but rather emerged much later than other chondrules. ?They actually post-date the oldest asteroids,? he says. ?We think these chondrules were formed by a giant plume of vapour produced when two planetary embryos, somewhere between moon-size and Mars-size, collided.?

What does this mean in the grand scheme of things? The evolution of the solar system has traditionally been seen as a linear process, through which gases around the early sun gradually cooled to form small particles that eventually clumped into asteroids and planets. Now there is evidence of chondrules forming at two very distinct times, and evidence that embryo planets already existed when chondrules were still forming. ?It moves our understanding from order to disorder,? Amelin admits. ?But I?m sure that as new data is collected, a new order will emerge.?

Financial support for this project was provided by NASA and the Canadian Space Agency.

Original Source: University of Toronto

Posted on by Fraser Cain

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