̽��ֱ�� of Cambridge - comet

Scientists reveal structure of 74 exocomet belts orbiting nearby stars

Anonymous — Fri, 17 Jan 2025 08:00:00 +0000

̽��ֱ��crystal-clear images show light being emitted from these millimetre-sized pebbles within the belts that orbit 74 nearby stars of a wide variety of ages – from those that are just emerging to those in more mature systems like our own Solar System.

̽��ֱ��REASONS (REsolved ALMA and SMA Observations of Nearby Stars) study, led by Trinity College Dublin and involving researchers from the ̽��ֱ�� of Cambridge, is a milestone in the study of exocometary belts because its images and analyses reveal where the pebbles, and the exocomets, are located. They are typically tens to hundreds of astronomical units (the distance from Earth to the Sun) from their central star.

In these regions, it is so cold (-250 to -150 degrees Celsius) that most compounds are frozen as ice on the exocomets. What the researchers are therefore observing is where the ice reservoirs of planetary systems are located. REASONS is the first programme to unveil the structure of these belts for a large sample of 74 exoplanetary systems. ̽��ֱ��results are reported in the journal Astronomy & Astrophysics.

This study used both the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) in Hawai‘i to produce the images that have provided more information on populations of exocomets than ever before. Both telescope arrays observe electromagnetic radiation at millimetre and submillimetre wavelengths.

“Exocomets are boulders of rock and ice, at least one kilometre in size, which smash together within these belts to produce the pebbles that we observe here with the ALMA and SMA arrays of telescopes,” said lead author Luca Matrà from Trinity College Dublin. “Exocometary belts are found in at least 20% of planetary systems, including our own Solar System.”

“ ̽��ֱ��images reveal a remarkable diversity in the structure of belts,” said co-author Dr Sebastián Marino from the ̽��ֱ�� of Exeter. “Some are narrow rings, as in the canonical picture of a ‘belt’ like our Solar System’s Edgeworth-Kuiper belt. But a larger number of them are wide, and probably better described as ‘disks’ rather than rings.”

Some systems have multiple rings/disks, some of which are eccentric, providing evidence that yet undetectable planets are present and their gravity affects the distribution of pebbles in these systems.

“ ̽��ֱ��power of a large study like REASONS is in revealing population-wide properties and trends,” said Matrà.

For example, the study confirmed that the number of pebbles decreases for older planetary systems as belts run out of larger exocomets smashing together, but showed for the first time that this decrease in pebbles is faster if the belt is closer to the central star. It also indirectly showed – through the belts’ vertical thickness – that objects as large as 140 km across and even Moon-size objects are likely present in these belts.

“We have been studying exocometary belts for decades, but until now only a handful had been imaged,” said co-author Professor Mark Wyatt from Cambridge’s Institute of Astronomy. “This is the largest collection of such images and demonstrates that we already have the capabilities to probe the structures of the planetary systems orbiting a large fraction of the stars near to the Sun.”

“Arrays like the ALMA and SMA used in this work are extraordinary tools that are continuing to give us incredible new insights into the universe and its workings,” said co-author Dr David Wilner from the Center for Astrophysics | Harvard & Smithsonian “ ̽��ֱ��REASONS survey required a large community effort and has an incredible legacy value, with multiple potential pathways for future investigation.”

Reference:
L. Matrà et al. ‘REsolved ALMA and SMA Observations of Nearby Stars. REASONS: A population of 74 resolved planetesimal belts at millimetre wavelengths.’ Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202451397

Adapted from a Trinity College Dublin media release.

An international team of astrophysicists has imaged a large number of exocomet belts around nearby stars, and the tiny pebbles within them.

Luca Matra, Trinity College Dublin, and colleagues

Millimetre continuum images for the REASONS resolved sample of 74 exocomet belts

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

‘Bouncing’ comets could deliver building blocks for life to exoplanets

sc604 — Wed, 15 Nov 2023 00:10:19 +0000

In order to deliver organic material, comets need to be travelling relatively slowly – at speeds below 15 kilometres per second. At higher speeds, the essential molecules would not survive – the speed and temperature of impact would cause them to break apart.

̽��ֱ��most likely place where comets can travel at the right speed are ‘peas in a pod’ systems, where a group of planets orbit closely together. In such a system, the comet could essentially be passed or ‘bounced’ from the orbit of one planet to another, slowing it down.

At slow enough speeds, the comet would crash on a planet’s surface, delivering the intact molecules that researchers believe are the precursors for life. ̽��ֱ��results, reported in the Proceedings of the Royal Society A, suggest that such systems would be promising places to search for life outside our Solar System if cometary delivery is important for the origins of life.

Comets are known to contain a range of the building blocks for life, known as prebiotic molecules. For example, samples from the Ryugu asteroid, analysed in 2022, showed that it carried intact amino acids and vitamin B3. Comets also contain large amounts of hydrogen cyanide (HCN), another important prebiotic molecule. ̽��ֱ��strong carbon-nitrogen bonds of HCN make it more durable to high temperatures, meaning it could potentially survive atmospheric entry and remain intact.

“We’re learning more about the atmospheres of exoplanets all the time, so we wanted to see if there are planets where complex molecules could also be delivered by comets,” said first author Richard Anslow from Cambridge’s Institute of Astronomy. “It’s possible that the molecules that led to life on Earth came from comets, so the same could be true for planets elsewhere in the galaxy.”

̽��ֱ��researchers do not claim that comets are necessary to the origin of life on Earth or any other planet, but instead they wanted to place some limits on the types of planets where complex molecules, such as HCN, could be successfully delivered by comets.

Most of the comets in our Solar System sit beyond the orbit of Neptune, in what is known as the Kuiper Belt. When comets or other Kuiper Belt objects (KBOs) collide, they can be pushed by Neptune’s gravity toward the Sun, eventually getting pulled in by Jupiter’s gravity. Some of these comets make their way past the Asteroid Belt and into the inner Solar System.

“We wanted to test our theories on planets that are similar to our own, as Earth is currently our only example of a planet that supports life,” said Anslow. “What kinds of comets, travelling at what kinds of speed, could deliver intact prebiotic molecules?”

Using a variety of mathematical modelling techniques, the researchers determined that it is possible for comets to deliver the precursor molecules for life, but only in certain scenarios. For planets orbiting a star similar to our own Sun, the planet needs to be low mass and it is helpful for the planet to be in close orbit to other planets in the system. ̽��ֱ��researchers found that nearby planets on close orbits are much more important for planets around lower-mass stars, where the typical speeds are much higher.

In such a system, a comet could be pulled in by the gravitational pull of one planet, then passed to another planet before impact. If this ‘comet-passing’ happened enough times, the comet would slow down enough so that some prebiotic molecules could survive atmospheric entry.

“In these tightly-packed systems, each planet has a chance to interact with and trap a comet,” said Anslow. “It’s possible that this mechanism could be how prebiotic molecules end up on planets.”

For planets in orbit around lower-mass stars, such as M-dwarfs, it would be more difficult for complex molecules to be delivered by comets, especially if the planets are loosely packed. Rocky planets in these systems also suffer significantly more high-velocity impacts, potentially posing unique challenges for life on these planets.

̽��ֱ��researchers say their results could be useful when determining where to look for life outside the Solar System.

“It’s exciting that we can start identifying the type of systems we can use to test different origin scenarios,” said Anslow. “It’s a different way to look at the great work that’s already been done on Earth. What molecular pathways led to the enormous variety of life we see around us? Are there other planets where the same pathways exist? It’s an exciting time, being able to combine advances in astronomy and chemistry to study some of the most fundamental questions of all.”

̽��ֱ��research was supported in part by the Royal Society and the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI). Richard Anslow is a Member of Wolfson College, Cambridge.

Reference:
R J Anslow, A Bonsor and P B Rimmer. ‘Can comets deliver prebiotic molecules to rocky exoplanets?’ Proceedings of the Royal Society A (2023). DOI: 10.1098/rspa.2023.0434

How did the molecular building blocks for life end up on Earth? One long-standing theory is that they could have been delivered by comets. Now, researchers from the ̽��ֱ�� of Cambridge have shown how comets could deposit similar building blocks to other planets in the galaxy.

It’s possible that the molecules that led to life on Earth came from comets, so the same could be true for planets elsewhere in the galaxy

Richard Anslow

solarseven via Getty Images

Artist's impression of a meteor hitting Earth

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

Fast-moving gas flowing away from young star’s asteroid belt may be caused by icy comet vaporisation

sc604 — Mon, 30 Nov 2020 02:00:00 +0000

Astronomers have detected fast-moving carbon monoxide gas flowing away from a young, low-mass star: a unique stage of planetary system evolution which may provide insight into how our own solar system evolved and suggests that the way systems develop may be more complicated than previously thought.

Although it remains unclear how the gas is being ejected so fast, the team of researchers, led by the ̽��ֱ�� of Cambridge, believe it may be produced from icy comets being vaporised in the star’s asteroid belt. ̽��ֱ��results have been accepted for publication in the Monthly Notices of the Royal Astronomical Society and will be presented at the Five Years After HL Tau virtual conference.

̽��ֱ��detection was made with the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile, as part of a survey of young ‘class III’ stars, reported in an earlier paper. Some of these class III stars are surrounded by debris discs, which are believed to be formed by the ongoing collisions of comets, asteroids and other solid objects, known as planetesimals, in the outer reaches of recently formed planetary systems. ̽��ֱ��leftover dust and debris from these collisions absorbs light from their central stars and re-radiate that energy as a faint glow that can be studied with ALMA.

In the inner regions of planetary systems, the processes of planet formation are expected to result in the loss of all the hottest dust, and class IIII stars are those that are left with - at most - dim, cold dust. These faint belts of cold dust are similar to the known debris discs seen around other stars, similar to the Kuiper belt in our own solar system, which is known to host much larger asteroids and comets.

In the survey, the star in question, ‘NO Lup’, which is about 70% the mass of our sun, was found to have a faint, low-mass dusty disc, but it was the only class III star where carbon monoxide gas was detected, a first for this type of young star with ALMA. While it is known that many young stars still host the gas-rich planet-forming discs they are born with, NO Lup is more evolved, and might have been expected to have lost this primordial gas after its planets had formed.

While the detection of carbon monoxide gas is rare, what made the observation unique was the scale and speed of the gas, which prompted a follow-up study to explore its motion and origins.

“Just detecting carbon monoxide gas was exciting, since no other young stars of this type had been previously imaged by ALMA,” said first author Joshua Lovell, a PhD student from the Cambridge’s Institute of Astronomy. “But when we looked closer, we found something even more unusual: given how far away the gas was from the star, it was moving much faster than expected. This had us puzzled for quite some time.”

Grant Kennedy, Royal Society ̽��ֱ�� Research Fellow at the ̽��ֱ�� of Warwick, who led the modelling work on the study, came up with a solution to the puzzle. “We found a simple way to explain it: by modelling a gas ring, but giving the gas an extra kick outward,” he said. “Other models have been used to explain young discs with similar mechanisms, but this disc is more like a debris disc where we haven’t witnessed winds before. Our model showed the gas is entirely consistent with a scenario in which it’s being launched out of the system at around 22 kilometres per second, which is much higher than any stable orbital speed.”

Further analysis also showed that the gas may be produced during collisions between asteroids, or during periods of sublimation – the transition from a solid to a gaseous phase – on the surface of the star’s comets, expected to be rich in carbon monoxide ice.

There has been recent evidence of this same process in our own solar system from NASA’s New Horizons mission, when it observed the Kuiper Belt object Ultima Thule in 2019 and found sublimation evolution on the surface of the comet, which happened around 4.5 billion years ago. ̽��ֱ��same event that vaporised comets in our own solar system billions of years ago may have therefore been captured for the first time over 400 light years away, in a process that may be common around planet-forming stars, and have implications for how all comets, asteroids, and planets evolve.

“This fascinating star is shedding light on what kind of physical processes are shaping planetary systems shortly after they are born, just after they have emerged from being enshrouded by their protoplanetary disk,” said co-author Professor Mark Wyatt, also from the Institute of Astronomy. “While we have seen gas produced by planetesimals in older systems, the shear rate at which gas is being produced in this system and its outflowing nature are quite remarkable, and point to a phase of planetary system evolution that we are witnessing here for the first time.”

While the puzzle isn’t fully solved, and further detailed modelling will be required to understand how the gas is being ejected so quickly, what is sure is that this system is set to be the target of more intense follow-up measurements.

“We’re hoping that ALMA will be back online next year, and we’ll be making the case to observe this system again in greater detail,” said Lovell. “Given how much we have learned about this early stage of planetary system evolution with only a short 30-minute observation, there is still so much more that this system can tell us.”

References:
1: J.B. Lovell et al. ‘Rapid CO gas dispersal from NO Lup’s class III circumstellar disc.’ Paper presented at Five Years After HL Tau. 7-11 December 2020.

2: J.B. Lovell et al. ‘ALMA Survey of Class III stars: Early planetesimal formation and Rapid disc dispersal’, DOI: https://doi.org/10.1093/mnras/staa3335

A unique stage of planetary system evolution has been imaged by astronomers, showing fast-moving carbon monoxide gas flowing away from a star system over 400 light years away, a discovery that provides an opportunity to study how our own solar system developed.

Given how much we have learned about this early stage of planetary system evolution with only a short observation, there is still so much more that this system can tell us

Joshua Lovell

Institute of Astronomy

Artist's impression of No Lup system

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

Icy ring around young planetary system has similar chemical fingerprint to our solar system

sc604 — Thu, 18 May 2017 15:00:00 +0000

Earlier observations of the star, known as Fomalhaut and located 25 light years from Earth, were taken in 2012 by astronomers using the Atacama Large Millimetre/submillimetre Array (ALMA), located in Chile. ̽��ֱ��2012 results were gathered when the telescope was still under construction, and while they only revealed about half of the debris disc, the observations provided hints about the nature and possible origin of the disc.

̽��ֱ��new observations offer a far more complete view of this glowing band of debris, a band of rubble resulting from comets smashing together near the outer edges of the planetary system. ̽��ֱ��gases observed within the ring by the team suggest that there are chemical similarities between its icy contents and comets in our own solar system.

“We can finally see the well-defined shape of the disc, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance,” said Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics, and lead author on one of two papers accepted for publication in the Astrophysical Journal describing these observations.

Fomalhaut is a relatively nearby star system and one of only about 20 in which planets have been imaged directly. ̽��ֱ��entire system is approximately 440 million years old, or about one-tenth the age of our solar system. As revealed in the new ALMA image, a brilliant band of icy dust about two billion kilometres wide has formed approximately 20 billion kilometres from the star.

Debris discs are common features around young stars and represent a dynamic and chaotic period in the history of a solar system. Astronomers believe they are formed by the ongoing collisions of comets and other solid objects, known as planetesimals, in the outer reaches of a recently formed planetary system. ̽��ֱ��leftover debris from these collisions absorbs light from its central star and re-radiates that energy as a faint glow that can be studied with ALMA.

Using the new ALMA data and detailed computer modelling, the researchers could calculate the precise location, width, and geometry of the disc. These parameters confirm that such a narrow ring is likely produced through the gravitational influence of planets in the system.

̽��ֱ��new observations are also the first to definitively show “apocenter glow,” a phenomenon predicted in a 2016 paper by Margaret Pan, a scientist at the Massachusetts Institute of Technology and co-author on the new papers. Like all objects with elongated orbits, the dusty material in the Fomalhaut disc travels more slowly when it is farthest from the star. As the dust slows down, it piles up, forming denser concentrations in the more distant portions of the disc. These dense regions can be seen by ALMA as brighter millimetre-wavelength emission.

Using the same dataset, but focusing on distinct millimetre-wavelength signals naturally emitted by molecules in space, the researchers also detected vast stores of carbon monoxide gas in precisely the same location as the debris disc.

“These data allowed us to determine that the abundance of carbon monoxide plus carbon dioxide around Fomalhaut is about the same as found in comets in our own solar system,” said Dr Luca Matrà of Cambridge’s Institute of Astronomy, and lead author of the team’s second paper. “This chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this planetary system and our own.” Matrà and his colleagues believe this gas is either released from continuous comet collisions or the result of a single, large impact between ‘supercomets’ hundreds of times more massive than Hale-Bopp.

̽��ֱ��presence of this well-defined debris disc around Fomalhaut, along with its curiously familiar chemistry, may indicate that this system is undergoing its own version of the Late Heavy Bombardment, a period approximately four billion years ago when the Earth and other planets were routinely struck by swarms of asteroids and comets left over from the formation of our solar system.

“Twenty years ago, the best millimetre-wavelength telescopes gave the first fuzzy maps of sand grains orbiting Fomalhaut. Now with ALMA’s full capabilities the entire ring of material has been imaged,” said Paul Kalas, an astronomer at the ̽��ֱ�� of California at Berkeley and principal investigator on these observations. “One day we hope to detect the planets that influence the orbits of these grains.”

Adapted from a National Radio Astronomy Observatory press release.

References:
Meredith A. MacGregor et al. 'A Complete ALMA Map of the Fomalhaut Debris Disk.' arXiv:1705.05867
L. Matrà et al. 'Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO2 ice abundance in exocomets and Solar System comets.' arXiv:1705.05868

An international team of astronomers, including researchers from the ̽��ֱ�� of Cambridge, has made the most detailed image of the ring of dusty debris surrounding a young star and found that the ice content of colliding comets within it is similar to comets in our own solar system.

̽��ֱ��chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this planetary system and our own.

Luca Matra

ALMA (ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton (NRAO/AUI/NSF)

Composite image of the Fomalhaut star system. ̽��ֱ��ALMA data, shown in orange, reveal the distant and eccentric debris disk in never-before-seen detail. ̽��ֱ��central dot is the unresolved emission from the star, which is about twice the mass of our sun.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

First evidence of icy comets orbiting a sun-like star

sc604 — Thu, 19 May 2016 09:20:14 +0000

An international team of astronomers have found evidence of ice and comets orbiting a nearby sun-like star, which could give a glimpse into how our own solar system developed.

Using data from the Atacama Large Millimeter Array (ALMA), the researchers, led by the ̽��ֱ�� of Cambridge, detected very low levels of carbon monoxide gas around the star, in amounts that are consistent with the comets in our own solar system.

̽��ֱ��results, which will be presented today at the ‘Resolving Planet Formation in the era of ALMA and extreme AO’ conference in Santiago, Chile, are a first step in establishing the properties of comet clouds around sun-like stars just after the time of their birth.

Comets are essentially ‘dirty snowballs’ of ice and rock, sometimes with a tail of dust and evaporating ice trailing behind them, and are formed early in the development of stellar systems. They are typically found in the outer reaches of our solar system, but become most clearly visible when they visit the inner regions. For example, Halley’s Comet visits the inner solar system every 75 years, some take as long as 100,000 years between visits, and others only visit once before being thrown out into interstellar space.

It’s believed that when our solar system was first formed, the Earth was a rocky wasteland, similar to how Mars is today, and that as comets collided with the young planet, they brought many elements and compounds, including water, along with them.

̽��ֱ��star in this study, HD 181327, has a mass about 30% greater than the sun and is located 160 light years away in the Painter constellation. ̽��ֱ��system is about 23 million years old, whereas our solar system is 4.6 billion years old.

“Young systems such as this one are very active, with comets and asteroids slamming into each other and into planets,” said Sebastián Marino, a PhD student from Cambridge’s Institute of Astronomy and the paper’s lead author. “ ̽��ֱ��system has a similar ice composition to our own, so it’s a good one to study in order to learn what our solar system looked like early in its existence.”

Using ALMA, the astronomers observed the star, which is surrounded by a ring of dust caused by the collisions of comets, asteroids and other bodies. It’s likely that this star has planets in orbit around it, but they are impossible to detect using current telescopes.

“Assuming there are planets orbiting this star, they would likely have already formed, but the only way to see them would be through direct imaging, which at the moment can only be used for very large planets like Jupiter,” said co-author Luca Matrà, also a PhD student at Cambridge’s Institute of Astronomy.

In order to detect the possible presence of comets, the researchers used ALMA to search for signatures of gas, since the same collisions which caused the dust ring to form should also cause the release of gas. Until now, such gas has only been detected around a few stars, all substantially more massive than the sun. Using simulations to model the composition of the system, they were able to increase the signal to noise ratio in the ALMA data, and detect very low levels of carbon monoxide gas.

“This is the lowest gas concentration ever detected in a belt of asteroids and comets – we’re really pushing ALMA to its limits,” said Marino.

“ ̽��ֱ��amount of gas we detected is analogous to a 200 kilometre diameter ice ball, which is impressive considering how far away the star is,” said Matrà. “It’s amazing that we can do this with exoplanetary systems now.”

̽��ֱ��results have been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

Reference:
S. Marino et al. ‘Exocometary gas in the HD 181327 debris ring.’ Paper presented to the Resolving Planet Formation in the era of ALMA and extreme AO conference, Santiago, May 16-20, 2016. http://www.eso.org/sci/meetings/2016/Planet-Formation2016/program.html

Inset image: ALMA image of the ring of comets around HD 181327 (colours have been changed). ̽��ֱ��white contours represent the size of the Kuiper Belt in the Solar System. Credit: Amanda Smith, ̽��ֱ�� of Cambridge.

Astronomers have found the first evidence of comets around a star similar to the sun, providing an opportunity to study what our solar system was like as a ‘baby’.

̽��ֱ��system has a similar ice composition to our own, so it’s a good one to study in order to learn what our solar system looked like early in its existence.

Sebastián Marino

Amanda Smith, ̽��ֱ�� of Cambridge

Illustration of the dust ring surrounding HD 181327

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Galactic gas caused by colliding comets suggests mystery ‘shepherd’ exoplanet

fpjl2 — Thu, 06 Mar 2014 19:05:00 +0000

Astronomers exploring the disc of debris around the young star Beta Pictoris have discovered a compact cloud of carbon monoxide located about 8 billion miles (13 billion kilometers) from the star. This concentration of poisonous gas – usually destroyed by starlight – is being constantly replenished by ongoing rapid-fire collisions among a swarm of icy, comet-like bodies.

In fact, to offset the destruction of carbon monoxide (CO) molecules around the star, a large comet must be getting completely destroyed every five minutes, say researchers.

They suggest the comet swarm is most likely frozen debris trapped and concentrated by the gravity of an as-yet-unseen exoplanet.

This mystery ‘shepherd’ exoplanet – so-called for its capacity to corral the swarms of comets through its gravitational pull, like Jupiter in our own solar system – is likely to be about the size of Saturn.

"Detailed dynamical studies are now under way, but at the moment we think this shepherding planet would be around Saturn's mass and positioned near the inner edge of the CO belt," said Mark Wyatt, from Cambridge’s Institute of Astronomy, who proposed the shepherd model – currently the favoured hypothesis because it explains so many puzzling features of the Beta Pictoris disc.

"We think the Beta Pictoris comet swarms formed when the hypothetical planet migrated outward, sweeping icy bodies into resonant orbits."

Paradoxically, the presence of carbon monoxide – so harmful to humans on Earth – could indicate that the Beta Pictoris planetary system may eventually be a good habitat for life. If there is CO in the comets, then there is likely also water ice – meaning that the cometary bombardment this system’s planets are probably undergoing could also be providing them with life-giving water.

̽��ֱ��findings are published today in the journal Science Express.

̽��ֱ��clump was discovered when an international team of astronomers, led by ALMA-based ESO astronomer Bill Dent, along with Wyatt, used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to map the millimeter-wavelength light from dust and carbon monoxide molecules in the disc surrounding Beta Pictoris, a star located about 63 light-years away and only 20 million years old.

Beta Pictoris is considered one of the best examples of a typical young solar system, and hosts one of the closest and brightest debris discs known – making it an ideal laboratory for studying the early development of planetary systems. ̽��ֱ��latest findings could help us understand what conditions were like during the formation of our own solar system.

Much of the carbon monoxide is concentrated in a single clump located about 8 billion miles (13 billion kilometers) from the star, or nearly three times the distance between the planet Neptune and the sun. ̽��ֱ��total amount of the gas observed exceeds 200 million billion tons – equivalent to about one-sixth the mass of Earth’s oceans, say researchers.

̽��ֱ��presence of all this gas is a clue that something interesting is going on because ultraviolet starlight breaks up CO molecules in about 100 years, much faster than the main cloud can complete a single orbit around the star. “So unless we are observing Beta Pictoris at a very unusual time, then the carbon monoxide we observed must be continuously replenished,” said Bill Dent, ESO astronomer based at ALMA and lead author on the paper.

̽��ֱ��researchers calculate that a large comet must be completely destroyed every five minutes, and only an unusually massive and compact swarm of comets could support such an astonishingly high collision rate.

"Although toxic to us, carbon monoxide is one of many gases found in comets and other icy bodies," said team member Aki Roberge, an astrophysicist at NASA’s Goddard Space Flight Center. "In the rough-and-tumble environment around a young star, these objects frequently collide and generate fragments that release dust, icy grains and stored gases."

Because we view the disc nearly edge-on, the ALMA data cannot determine whether the carbon monoxide belt has a single concentration of gas or two on opposite sides of the star. Further studies of the gas cloud's orbital motion will clarify the situation, but current evidence favors a two-clump scenario, which in turn points to a shepherding planet.

In our own solar system, Jupiter's gravity has trapped thousands of asteroids in two groups, one leading and one following it as it travels around the sun. A giant planet located in the outer reaches of the Beta Pictoris system likewise could corral comets into a pair of tight, massive swarms.

Astronomers have already directly imaged one giant exoplanet, Beta Pictoris b, with a mass several times greater than Jupiter, orbiting much closer to the star. While it would be unusual for a giant planet to form up to 10 times farther away, as required to shepherd the massive comet clouds, the hypothetical planet could have formed near the star and migrated outward as the young disc underwent changes. Indeed, this outward motion is needed to corral the comets.

A brief animation expanding on this can be viewed here.

If, however, the gas actually turns out to form a single clump, Wyatt’s recently graduated Cambridge PhD student Alan Jackson, also a co-author on the paper, suggested an even more violent alternative scenario. A crash between two Mars-sized icy planets about half a million years ago would account for the comet swarm, with frequent ongoing collisions among the fragments gradually releasing carbon monoxide gas.

Either way, Beta Pictoris clearly has a fascinating story to tell, say the scientists, one that could provide insight into the early development of our own solar system.

Latest research has uncovered a massive clump of carbon monoxide in a young solar system. ̽��ֱ��gas is the result of near constant collisions of icy comets – suggesting vast swarms of tightly packed comets in thrall to the gravitational pull of an as-yet-unseen exoplanet.

We think the Beta Pictoris comet swarms formed when the hypothetical planet migrated outward

Mark Wyatt

NASA's Goddard Space Flight Center/F.Reddy

At the outer fringes of the system, the gravitational influence of a hypothetical giant planet (bottom left) captures comets into a dense, massive swarm (right) where frequent collisions occur.

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Yes

Low-mass planets make good neighbours for debris discs

tdk25 — Tue, 27 Nov 2012 14:00:24 +0000

Astronomers have detected massive debris discs around 61 Virginis and Gilese 581, two nearby stars that are known to host “super-Earth” planets - so-called because their mass is between that of Earth and Neptune. Debris discs are belts of comets and asteroids orbiting the star.

̽��ֱ��study, which was carried out using the European Space Agency’s Herschel Space Observatory, also reveals that debris discs are preferentially found in planetary systems with low-mass planets than in those hosting high-mass planets. This suggests that debris discs may survive more easily in the absence of planets with a very high mass, and highlights the importance of debris discs in the study of planet formation.

̽��ֱ��formation of planets, around a newly-born star, is a dynamic process than can last hundreds of millions of years. Debris discs are a by-product of the process. They consist of everything orbiting a star that is not a planet: asteroids, comets, planetesimals and the dust that derives from them. In our own Solar System, the debris disc is mainly concentrated in two belts - the asteroid belt (between the orbits of Mars and Jupiter) and the Kuiper Belt beyond the orbit of Neptune.

Debris discs were first detected in other systems during the 1980s. Several hundred are now known. Astronomers are currently using the Herschel Observatory to search for discs around a variety of stars in our Galaxy - the Milky Way - deeper and more thoroughly than was previously possible. By exploiting the telescope’s unprecedented sensitivity and resolution, it is possible to detect very faint discs and image them in great detail.

One survey using Herschel, known as DEBRIS (an abbreviation of Disc Emission via a Bias-free Reconaissance in the Infrared/Submillimetre) has now produced two studies which detected discs around a handful of nearby stars, known to host planets. These all appear to be systems with super-Earths - planets with a mass between that of our own and Neptune, which means that their mass is relatively low.

̽��ֱ��results hint that the presence of debris discs which are bright enough to be detected with current observatories could be related to whether their parent star has low-mass planets in orbit around it.

“One of the debris discs surrounds the star 61 Virginis, which is very similar to our Sun in terms of its mass, temperature and age,” Mark Wyatt, from the ̽��ֱ�� of Cambridge’s Institute of Astronomy and leader of the analysis of G-type stars in the DEBRIS survey, said. G-type stars are of the same spectral type as the Sun.

61 Virginis is also known to host at least two planets. These have masses equivalent to about five and 18 times the mass of Earth and orbit their parent star in positions much closer than Mercury is to the Sun.

“ ̽��ֱ��debris disc extends well beyond the orbits of the system’s known planets,” Wyatt said. “Since planets and debris discs occupy such different scales, one would not necessarily expect a correlation between their properties. However, material in the debris disc is also a fossil from the epoch of planet formation so it may carry information about the processes that contributed to build up the planetary system.”

Wyatt and his collaborators took a closer look at the 60 G-type stars that are nearest to the Sun. From this sample, they found 11 with planets. Five host high-mass planets, the remaining six host low-mass planets. Of the latter group, four showed debris discs, whereas this was true of none of the high-mass planet systems. This suggests that the presence of high-mass planets may hinder the survival of debris discs.

A similar result has also emerged from a second study based on M-type stars in the DEBRIS survey. These are stars with very low masses and temperatures and are the most abundant kind in the Milky Way. Until now, only one M-type star was known to possess a debris disc - the very young star AU Mic, which is about 12 million years old.

Given the lower surface temperature of these stars, astronomers expect them to retain debris discs more easily than hotter stars, where the radiation pressure may drive the debris away. However, M-type stars have a different internal structure from their higher-mass counterparts, which creates very intense magnetic fields and leads them to radiate plenty of X-rays. It is possible that both effects may disperse a debris disc.

̽��ֱ��study found a new debris disc around an M-type star, known as Gilese 581. ̽��ֱ��star is more than two billion years old, suggesting that debris discs can actually survive for a long time around M-type stars. Gilese 581 also hosts at least four planets - all with low masses at a “super-Earth” level.

Astronomers using the Herschel Space Observatory have detected massive debris discs around two nearby stars hosting low-mass planets. ̽��ֱ��discovery suggests that debris discs may survive more easily in planetary systems without high-mass planets.

Material in the debris disc is a fossil from the epoch of planet formation so it may carry information about the processes that contributed to build up the planetary system.

Mark Wyatt

ESA.

An image of the star Gilese 581 (bottom of image), with an illustration of the debris disc superimposed to show its position.

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