Unusual Charon Closeup

by Paul Gilster on July 16, 2015

The latest view of Charon shows us a 390-kilometer strip of Pluto’s largest moon with a unique feature, clearly visible below. We are looking at what Jeff Moore (leader of the New Horizons Geology, Geophysics and Imaging team, calls “a large mountain sitting in a moat.” Moore is the first to admit that the scenario has geologists stumped.


Image: This new image of an area on Pluto’s largest moon Charon has a captivating feature — a depression with a peak in the middle, shown here in the upper left corner of the inset. The image shows an area approximately 390 kilometers from top to bottom, including few visible craters. Credit: NASA-JHUAPL-SwRI.

This view of Charon was taken at approximately 0630 EDT (1030 UTC) on July 14, 2015, about 1.5 hours before closest approach to Pluto, at a range of 79,000 kilometers. Again, notice the lack of craters here, reinforcing what we’re learning about Charon’s relatively young surface. I know we were all curious about Charon from the outset, but I don’t know anyone who thought we would be talking about geologically young features on either of these worlds. We have sharper versions coming — this image is heavily compressed, but the Long Range Reconnaissance Imager (LORRI) on New Horizons will be returning richer data.



First Post-Flyby Pluto Imagery

by Paul Gilster on July 15, 2015

I’m on the road and don’t have a lot of time for writing, but I want to go ahead and get these new Pluto images up. They’re now available on the NASA site, and were introduced at the news conference at JHU/APL that just concluded. I’ll also quote just a bit of the news release for each photo.


New close-up images of a region near Pluto’s equator reveal a giant surprise: a range of youthful mountains rising as high as 11,000 feet (3,500 meters) above the surface of the icy body.

The mountains likely formed no more than 100 million years ago — mere youngsters relative to the 4.56-billion-year age of the solar system — and may still be in the process of building, says Jeff Moore of New Horizons’ Geology, Geophysics and Imaging Team (GGI). That suggests the close-up region, which covers less than one percent of Pluto’s surface, may still be geologically active today.

This one I mis-typed in my Twitter coverage for those who were following it, but the correct number is 100 million years. Young mountains, and check that altitude! The lack of cratering implies a young surface, but we can rule out tidal effects as a driver for geology here. “This may cause us to rethink what powers geological activity on many other icy worlds,” says GGI deputy team leader John Spencer of the Southwest Research Institute in Boulder, Colo.


Remarkable new details of Pluto’s largest moon Charon are revealed in this image from New Horizons’ Long Range Reconnaissance Imager (LORRI), taken late on July 13, 2015 from a distance of 289,000 miles (466,000 kilometers).

A swath of cliffs and troughs stretches about 600 miles (1,000 kilometers) from left to right, suggesting widespread fracturing of Charon’s crust, likely a result of internal processes. At upper right, along the moon’s curving edge, is a canyon estimated to be 4 to 6 miles (7 to 9 kilometers) deep.

Again, a relatively young surface shaped by geological activity. And note the diffuse boundary at the dark region at the north pole, suggesting we’re looking at a thin film of material. “Underlying it is a distinct, sharply bounded, angular feature; higher resolution images still to come are expected to shed more light on this enigmatic region.”


The latest spectra from New Horizons Ralph instrument reveal an abundance of methane ice, but with striking differences from place to place across the frozen surface of Pluto.

“We just learned that in the north polar cap, methane ice is diluted in a thick, transparent slab of nitrogen ice resulting in strong absorption of infrared light,” said New Horizons co-investigator Will Grundy, Lowell Observatory, Flagstaff, Arizona. In one of the visually dark equatorial patches, the methane ice has shallower infrared absorptions indicative of a very different texture. “The spectrum appears as if the ice is less diluted in nitrogen,” Grundy speculated “or that it has a different texture in that area.”

We have so much data to come in the next sixteen months, and given the surprises we’ve already been dealt, it’s clear we’ll be talking about Pluto/Charon for some time. As one of the participants in the press briefing yesterday said, we’re not just re-writing the books now. We’re going to be writing entirely new books.



Pluto: Encounter and Aftermath

by Paul Gilster on July 15, 2015

Exoplanet hunter Greg Laughlin (UC-Santa Cruz), who could make a living as a poet (if it were possible to make a living as a poet) wrote recently of his hope for a Pluto image “ that will become a touchstone, a visual shorthand for distance, isolation, frigidity and exile.” We haven’t seen that one yet, but I suspect we will with one of the images we’re still to receive showing New Horizons’ view of a receding crescent Pluto again being folded into the deep.

Last night’s reacquisition of the New Horizons’ signal sets us up for many weeks of data return, and provides a triumphant exclamation point on the flyby. Our spacecraft punched right through the orbital plane of Pluto’s system and emerged unscathed. The joy and festivity apparent on those actually at JHU/APL and the wild and celebratory conversations on social media bring home how popular this diminutive spacecraft has become. What an accomplishment, and even now I’m wondering what advances in technology could do in an outer system follow-up.

Clyde Tombaugh’s Apparatus

But I also found myself thinking of that portion of Clyde Tombaugh’s ashes that are now further from us than Pluto. Recently I mentioned Michael Byers’ fine novel Percival’s Planet (Henry Holt, 2010), which contains a fictional account of the discovery of Pluto, seen through the eyes of Byers’ protagonist, an astronomer named Alan Barber, who works with the same equipment Tombaugh uses. Weaving fictional characters in with historical personages like Tombaugh, Vesto Slipher and Percival Lowell, Byers re-creates the era and the task. Here he’s talking about the ‘blink comparator’ methods employed in the hunt for ‘Planet X’:


It is dreadful work, the blinking. You have two exposures of the same area of sky — long exposures a week apart or so; then the two exposed plates, ten inches square, are placed side by side in the big new Bosch, all brass fittings and an urgent smell of electricity and heated gas. Looking through an eyepiece you can see a very small portion of one of these plates — an area roughly the size of a nickel, showing about two hundred stars. Then you hit a switch and the comparator will show you the identical area of the other plate. And if you have managed against all odds to expose your two plates identically — if you’ve got the differential right, and the timing, and moreover if the weather hasn’t been hazy one night and clear the next, and if the telescope hasn’t slipped or jarred or just been slightly misaimed for some reason, and then if you’ve managed in the basement darkroom to develop both plates the same way — well, then you will see the same two hundred stars, looking the same way, appear again in the eyepiece as the blinker shows you the second plate.

The trick is to find a ‘star’ that vanishes or brightens or does something odd between one plate and another. You might be looking at a Cepheid variable, or you might find the track of an asteroid, or if you really get lucky, maybe you’ll find Planet X. Credit Clyde Tombaugh with a magnificent persistence. You can see from the discovery plates just how tricky this work was.


Next Steps in the Outer System

It was fifty years ago yesterday that Mariner 4 reached Mars, a fitting time to think about how far we have come and where we might go next. In What About the Next Pluto Mission?, Centauri Dreams regular Andrew LePage tackles the question of Pluto follow-ups, noting that given how infrequently we have low-energy launch opportunities, and considering the flight times necessary to reach Pluto, we should start thinking about an encore right now. We have a launch opportunity at the end of 2028 and the beginning of 2029 that LePage finds attractive. But there are other options depending on trajectory that he’s careful to analyze.

A follow-on mission, even a flyby, would benefit from advances in technology, remote sensing and miniaturization, allowing for more data to be collected, but LePage also speculates on the possibility of a design that would allow us to multiply our observing chances:

Depending on the available payload margin and, just as importantly, the new mission’s budget, it may prove possible to carry several lightweight but very capable sub-probes, with masses perhaps on the order of tens of kilograms, that the main spacecraft could deploy weeks or months before its 2039 Pluto encounter. These lightweight sub-probes could be directed to make observations of Pluto, Charon, or its other moons at closer range or under different viewing conditions than might be possible with the main spacecraft. This tactic would add flexibility to mission planning as well as the quantity and quality of the scientific data returned. While a traditional entry probe would be of little use in the thin atmosphere of Pluto (which has an estimated surface pressure on the order of a few microbars), a properly equipped sub-probe could be aimed to fly hundreds or maybe even tens of kilometers above Pluto’s surface to directly sample its atmosphere and any aerosol or cloud layers that may exist.

Moreover, there are various ‘energetically favorable’ launch windows in the early 2030s that could get flyby spacecraft to Uranus or Neptune, a follow-up to the grand work of the Voyagers. LePage suggests three separate missions to be launched toward Pluto, Uranus and Neptune in the 2028-2034 timeframe. Dedicated orbiters are, of course, the best choice for maximizing data return, but fast flybys give us the chance to get to the outer system again before any orbiters we choose to send arrive, which presumably wouldn’t be any earlier than mid-century.

The Allure of Sedna

Meanwhile, in At Pluto, the End of a Beginning, Lee Billings makes the case for renewed study of the outer system with his usual elegance. Even before we see the images and data we will be receiving over the next 16 months, we can take heart from the remarkable number of interesting features we’ve found:

Pluto bears a bright polar cap of methane and nitrogen ice, and mottled regions at its equator that signal strange and complex geology. Charon, by contrast, harbors a mysteriously dark polar region apparently bereft of bright ice, and an impact-generated chasm deeper and longer than Earth’s own Grand Canyon. More and better images will soon stream down from New Horizons’ far-distant memory banks, no doubt filled with even greater wonders – perhaps signs of ice volcanoes, or of ancient frozen seas, or of things so strange and unexpected they cannot yet be imagined. The only thing unimaginable is that they will contain nothing tantalizing enough to someday call us back.

All the questions seem to be multiplying. Why is Eris more massive than Pluto, a question that resonates even as we debate whether or not Pluto may not in fact be a bit larger? Surely there are major differences in composition, but why did these occur? And on beyond Eris there remains the king of outer system puzzles. Sedna’s orbit, which goes out thirty times as far as Neptune’s at aphelion, seems to imply gravitational nudges from something else. Is there, Billings asks, a planet as large as several Earth masses waiting to be discovered? And we can add, is Sedna itself a capture from a primordial stellar flyby, an object from another star?


Image: An artist’s impression of the view from Sedna, with points of interest labeled. Credit: Adolf Schaller/NASA.

The questions abound, and we’ll start to tackle at least of few of them when New Horizons goes on to visit a Kuiper Belt Object, assuming an extended mission is approved (it’s hard to see it being rejected at this point). And closer in, we have only fragmentary looks at major satellite systems like those of Uranus and Neptune. Billings and LePage are pointing to what we need to be thinking about as we plan the next steps beyond New Horizons, a process that, given the length of time it takes to develop a mission and a spacecraft, we should have already begun.



Closest Approach!

by Paul Gilster on July 14, 2015


Closest approach for New Horizons was at 0749:57 EDT (1149:57 UTC), with closest approach to Charon at about 0806 EDT. Mission operations manager Alice Bowman told the media briefing that we arrived at Pluto 72 seconds early and 70 kilometers closer than the aiming point, all of which was well within mission specs. Nice work.

I’ve found Twitter the best place to keep up, along with NASA TV for the media briefings. The #PlutoFlyby hashtag has been so active that it’s sometimes hard to read the messages, a heartening demonstration of the powerful sentiment this mission invokes. I also track @New Horizons2015, @NASANewHorizons, @AlanStern and, of course, @elakdawalla — Emily Lakdawalla’s work has been definitive. The Twitterverse has been exploding.

And here is the latest image, showing 4 kilometers per pixel, about 1000 times higher than Hubble can provide. Much better still to come. Here we’re sixteen hours from closest approach, at a distance of 766,000 kilometers. Note the varying areas of brightness, with very bright terrain just north of the equator. It’s a surface, says Alan Stern, that shows a history of impact and surface activity, but we have so much yet to learn as more data arrive.


The New Horizons team saw the image above for the first time this morning around 0545 EDT.


Image: Members of the New Horizons science team react to seeing the spacecraft’s last and sharpest image of Pluto. Credit: NASA / JHU/APL.

And now we wait. Stern estimates no more than two chances in 10,000 that we’ll lose New Horizons due to impact with debris, but until we get tonight’s signal, this writer at least is going to be on edge. After all, we’re crossing the orbital plane.



New Horizons Countdown

by Paul Gilster on July 13, 2015

We’re under the 24 hour mark for the Pluto flyby. NASA will offer a news briefing for New Horizons (check NASA TV), covering mission status and what to expect during flyby, at 1030 EDT (1430 UTC) today, a schedule change that moves the time up by half an hour. On Tuesday morning, the agency will present a live program called Arrival at Pluto Countdown starting at 0730 (1130 UTC). Remember that closest approach to Pluto is scheduled to occur at approximately 0749 (1149 UTC) on Tuesday, when the spacecraft comes within 12,500 kilometers of the surface. Gathering data, the spacecraft will be out of communication for much of that day.


Image: Pluto as seen from New Horizons on July 11, 2015. Credit: NASA/JHUAPL/SWRI

You can check NASA’s television coverage and media activities here, but I’ll also send you to Emily Lakdawalla’s page at The Planetary Society, where the indefatigable reporter has gathered in one place everything known about the schedule and other sources of information. Lakdawalla also offers a list of all the planned downlinks of image data during close approach. It’s interesting to see that Pluto will appear larger than the LORRI (Long-Range Reconnaissance Imager) field of view for less than 24 hours during the close approach.

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Image: On July 11, 2015, New Horizons captured a world that is growing more fascinating by the day. For the first time on Pluto, this view reveals linear features that may be cliffs, as well as a circular feature that could be an impact crater. Rotating into view is the bright heart-shaped feature that will be seen in more detail during New Horizons’ closest approach on July 14. The annotated version includes a diagram indicating Pluto’s north pole, equator, and central meridian. Credit: NASA/JHUAPL/SWRI

What we’ll all be waiting for with no little degree of tension is the downlink scheduled for the night of July 14, which will demonstrate that after its period of intense data-taking, New Horizons has survived the encounter and will be able to transmit stored data back to Earth. Assuming that all goes well, NASA has scheduled release of close-up images for the media briefing at 1500 (1900 UTC) on July 15.

Here is what happens next, as described by Emily Lakdawalla:

Following closest approach, on Wednesday and Thursday, July 15 and 16, there will be a series of “First Look” downlinks containing a sampling of key science data. Another batch of data will arrive in the “Early High Priority” downlinks over the subsequent weekend, July 17-20. Then there will be a hiatus of 8 weeks before New Horizons turns to systematically downlinking all its data. Almost all image data returned during the week around closest approach will be lossily compressed — they will show JPEG compression artifacts. Only the optical navigation images are losslessly compressed.

Data transmission takes quite a while, with the entire dataset (lossily compressed) being downlinked starting in mid-September and continuing over a ten week period. In November, the spacecraft will downlink the entire science data set compressed without loss, a process that will take a year to complete. We’ll be keeping up with New Horizons for a long time, and here’s hoping that will include a KBO encounter and an upload of the One Earth Message.

Off on a Comet

And let’s not forget Comet 67P/Churyumov–Gerasimenko, which for over a year now has been under study by the European Space Agency’s Rosetta spacecraft. We’re now one month away from perihelion for the comet, the point at which the comet is closest to the Sun in its orbit, and therefore likely to show the greatest amount of surface activity. When Rosetta reached the comet, both were 540 million kilometers from the Sun. That distance has now closed to 195 million kilometers, and will reach 186 million km by the 13th of August.


Image: The orbit of Comet 67P/Churyumov–Gerasimenko and its approximate location around perihelion, the closest the comet gets to the Sun. The positions of the planets are correct for 13 August 2015. Credit: ESA.

“Perihelion is an important milestone in any comet’s calendar, and even more so for the Rosetta mission because this will be the first time a spacecraft has been following a comet from close quarters as it moves through this phase of its journey around the Solar System,” notes Matt Taylor, ESA’s Rosetta project scientist.

“We’re looking forward to reaching perihelion, after which we’ll be continuing to monitor how the comet’s nucleus, activity and plasma environment changes in the year after, as part of our long-term studies.”

This ESA news release has more about Rosetta’s progress. Post-Pluto/Charon, we have a summer full of fascinating science ahead with Rosetta and, of course, Dawn’s continuing work at Ceres. But first let’s get New Horizons through that flyby.



Charon: A Rugged, Cratered Surface

by Paul Gilster on July 12, 2015

A chasm in Charon’s southern hemisphere turns out to be longer and deeper than Earth’s Grand Canyon, says William McKinnon (Washington University, St. Louis), deputy lead scientist with New Horizon’s Geology and Geophysics investigation team.

“This is the first clear evidence of faulting and surface disruption on Charon. New Horizons has transformed our view of this distant moon from a nearly featureless ball of ice to a world displaying all kinds of geologic activity.”


Image: Chasms, craters, and a dark north polar region are revealed in this image of Pluto’s largest moon Charon taken by New Horizons on July 11, 2015. Credit: NASA/JHUAPL/SWRI.

The most prominent crater, near Charon’s south pole, is almost 100 kilometers across, and evidently the result of a geologically recent impact. This NASA news release adds that the darkness of the crater floor may be the result of a different kind of icy material being exposed, less reflective than the ices on the surface. Another possibility: The ice of the crater floor has a larger grain size, reflecting less sunlight. This would be the result of ice melting during the impact event and re-freezing into larger grains.


Image: This annotated version of the Charon imagery includes a diagram showing Charon’s north pole, equator, and central meridian, with the features highlighted. Credit: NASA/JHUAPL/SWRI.

Meanwhile, the dark region near Charon’s north pole bears watching, with more detailed images coming up on the 14th.



Last Look at Pluto’s ‘Far Side’

by Paul Gilster on July 11, 2015

The side of Pluto that always faces its large moon Charon is the side that New Horizons won’t see when it makes its close flyby on July 14. That makes the image below what principal investigator Alan Stern is calling “the last, best look that anyone will have of Pluto’s far side for decades to come.”


Image: New Horizons’ last look at Pluto’s Charon-facing hemisphere reveals intriguing geologic details that are of keen interest to mission scientists. This image, taken early the morning of July 11, 2015, shows newly-resolved linear features above the equatorial region that intersect, suggestive of polygonal shapes. This image was captured when the spacecraft was 2.5 million miles (4 million kilometers) from Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Four dark spots seem to be connected to the dark belt in Pluto’s equatorial region, their fairly regular spacing a source of considerable curiosity. The large areas are estimated to be roughly 480 kilometers across, with irregular boundaries between light and dark terrain. Jeff Moore (NASA Ames) gives a glimpse of what’s ahead:

“When we combine images like this of the far side with composition and color data the spacecraft has already acquired but not yet sent to Earth, we expect to be able to read the history of this face of Pluto.”

And of course, as this JHU/APL news release reminds us, we’ll soon be seeing the encounter hemisphere from as close as 12,500 kilometers.



New Horizons: Detecting Geology

by Paul Gilster on July 11, 2015

Pluto’s surface is beginning to be revealed, with the first signs of geological features, as principal investigator Alan Stern explains:

“Among the structures tentatively identified in this new image are what appear to be polygonal features; a complex band of terrain stretching east-northeast across the planet, approximately 1,000 miles long; and a complex region where bright terrains meet the dark terrains of the whale. After nine and a half years in flight, Pluto is well worth the wait.”

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Image: Tantalizing signs of geology on Pluto are revealed in this image from New Horizons taken on July 9, 2015 from 3.3 million miles (5.4 million kilometers) away. At this range, Pluto is beginning to reveal the first signs of discrete geologic features. This image views the side of Pluto that always faces its largest moon, Charon, and includes the so-called “tail” of the dark whale-shaped feature along its equator. (The immense, bright feature shaped like a heart had rotated from view when this image was captured. Among the structures tentatively identified in this new image are what appear to be polygonal features; a complex band of terrain stretching east-northeast across the planet, approximately 1,000 miles long; and a complex region where bright terrains meet the dark terrains of the whale. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

The annotated image below includes a reference globe showing Pluto’s orientation, with equator and central meridian in bold.

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And the photo below speaks for itself. It’s been nine years. Pluto at last.


Image: Science team members react to the latest New Horizons data from Pluto at the Johns Hopkins University Applied Physics Lab on July 10, 2015. Left to right: Cathy Olkin, Jason Cook, Alan Stern, Will Grundy, Casey Lisse, and Carly Howett. Photo by Michael Soluri.



New Horizons: Flyby Schedule, Images

by Paul Gilster on July 10, 2015

New Horizons makes its closest approach to Pluto, at approximately 12,500 kilometers above the surface, at 0749 EDT (1149 UTC) on Tuesday July 14. Be aware that for much of that day, we’ll be out of communication with the spacecraft while it’s busy gathering data. About 2102 EDT (0102 UTC on the 15th), we should receive a confirmation of a successful flyby — the spacecraft is scheduled to send a preprogrammed signal that it has survived the close approach. Then the data flow begins and will continue for months.

NASA offers the schedule for the flyby here, with information on NASA TV coverage. We should be looking at close-up images of Pluto and hearing early reactions from the science team by mid-afternoon of Wednesday the 15th. And of course it will be possible to follow the mission on Facebook or on Twitter (also #PlutoFlyby). The nail-biting time will be the wait on the 14th for the signal announcing a successful transit of the system. It doesn’t take a large object to silence a spacecraft moving at 14 kilometers per second and we can only hope for the best.

Screenshot from 2015-07-10 08:44:39


Meanwhile, the imagery keeps improving, with both Pluto and Charon beginning to swim out of the image-processing mist. We’re seeing bright and dark features on Pluto’s surface, while Charon presents a more uniform light gray terrain with a large dark polar region. This NASA news release speculates on the possibility of impact craters on Charon, quoting Jeff Moore (NASA Ames): “If we see impact craters on Charon, it will help us see what’s hidden beneath the surface. Large craters can excavate material from several miles down and reveal the composition of the interior.”

Image: Pluto from the New Horizons’ Long Range Reconnaissance Imager (LORRI), July 8, 2015. Most of the bright features around Pluto’s edge are a result of image processing, but the bright sliver below the dark “whale,” which is also visible in unprocessed images, is real.


The differences between Pluto and Charon are increasingly apparent, with Charon lacking the reddish color of the former, a hue that has reminded some of us of early, blurry photos of Mars. While frozen ices like nitrogen, carbon dioxide and methane have been detected on Pluto, Charon’s surface seems to be mostly frozen water and ammonia compounds. Unlike Pluto, Charon has no atmosphere, and its interior is composed of rock and ice in equal measures, whereas Pluto’s interior is predominantly rock. The lower surface contrast of Charon has been abundantly clear as New Horizons returns images of the two dissimilar worlds.

Image: Image of Charon from the New Horizons’ Long Range Reconnaissance Imager (LORRI), July 8, 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

As New Horizons nears its closest approach, the Cassini orbiter will take images of Pluto/Charon from its distant station around Saturn, and beginning July 23, the Spitzer Space Telescope will take seven days of observations at infrared wavelengths. The plan is to study ices on Pluto’s surface. New Horizons will also be backed up by Kepler observations, with the K2 mission focusing on Pluto for a three month period beginning in October.

“K2 observations will expand the time coverage of the speedy New Horizons flyby of Pluto, making observations of the dwarf planet-moon system every 30 minutes,” said Steve Howell, project scientist for Kepler/K2 at NASA’s Ames Research Center in Moffett Field, California. “We are excited to turn the planet-hunting Kepler spacecraft’s attention to this distant solar system object to provide additional scientific insight into this far-off, mysterious world, itself a miniature solar system of five moons in orbit about Pluto.”


Image: New Horizons was about 3.7 million miles (6 million kilometers) from Pluto and Charon when it snapped this portrait late on July 8, 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

And we’ve already had one interesting study performed by the SOFIA airborne observatory, an infrared telescope mounted on a 747 aircraft. On June 28, Pluto occulted a distant star, creating a backlighting that could be studied to provide a baseline measurement of Pluto’s atmosphere. All of this, plus the data that the Hubble Space Telescope has gathered about Pluto’s smaller moons, will provide datasets that complement the abundant science return of New Horizons.


Image: This is the same image of Pluto and Charon from July 8, 2015; color information obtained earlier in the mission from the Ralph instrument has been added. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.



Detection of Pebbles in a Circumstellar Disk

by Paul Gilster on July 9, 2015

Not long ago we looked at a new paper from Alan Boss that modeled interactions in young protoplanetary disks (A Disruptive Pathway for Planet Formation). The idea here is that as dust grains and larger objects bump into each other on the way to forming planetesimals, a mechanism must exist to keep them from spiraling into their star. Boss’ models show explosive phases in young stars that lead to gravitational instabilities of the sort needed to scatter these small objects outward and preserve their prospects for forming into planetesimals, and perhaps one day, planets.

Watching infant solar systems form is akin to studying embryology in animal species, a chance to understand the myriad interactions that affect growth and set it in particular directions. Now we have work out of the University of St. Andrews, recently presented at the National Astronomy Meeting in Llandudno, Wales, that announces the discovery of a ring of small rocks circling the star DG Tauri, a 2.5 million year old object some 450 light years from Earth.


Image: An artist’s impression of the belt of ‘pebbles’ in orbit around the star DG Tauri. The inset is a close up view of a section of the belt. Credit: J. Ilee. Adapted from original work by ESO/L. Calçada/M. Kornmesser, ALMA (ESO/NAOJ/NRAO)/L. Calçada (ESO).

The work of Jane Greaves and Anita Richards (University of Manchester) is based on data from the e-MERLIN array of radio telescopes centered in Jodrell Bank (Cheshire) and extending over southern England to form an interferometer, giving it the resolution of a single large telescope. The instrumentation proved up to the considerable challenge, as Greaves relates:

“The extraordinarily fine detail we can see with the e-MERLIN telescopes was the key to this discovery. We could zoom into a region as small as the orbit of Jupiter would be in the Solar System. We found a belt of pebbles strung along a very similar orbit – just where they are needed if a planet is to grow in the next few million years. Although we thought this was how planets must get started, it’s very exciting to actually see the process in action!”

The observations, as this Royal Astronomical Society news release explains, were made at a wavelength of about 4.6 cm. They revealed a signature that requires chunks of rock at least a centimeter in size. This is a useful finding, for as we’ve seen in the work of Alan Boss, we’re in the process of tuning up our computer models of protoplanetary disks and their interactions. Now we can identify, at least in some systems, the location of pebble-like material that will one day accrete into larger objects. A deeper analysis of young disks should emerge from all this.


Image: An e-MERLIN map of the star DG Tauri. The yellow and red areas show what is thought to be a ring of pebble-sized clumps in orbit around the star. Credit: J. Greaves / A. Richards / JCBA.

Studying the results will be, among others, a group called the Planet Earth Building Blocks Legacy e-MERLIN Survey (known by its fitting acronym — PEBBLeS). The team plans to extend studies like this to a number of stars that are in the process of forming their own solar systems. Right now we’re using equipment sensitive to regions as small as Jupiter’s orbit, but the logical goal is to move in five times closer to witness the formation of planets like our own. The researchers believe that upgrades to e-MERLIN and the coming capabilities of the Square Kilometer Array will make such observations possible.