̽��ֱ�� of Cambridge - planet formation

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.

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How did the building blocks of life arrive on Earth?

sc604 — Fri, 11 Oct 2024 18:00:00 +0000

Volatiles are elements or compounds that change into vapour at relatively low temperatures. They include the six most common elements found in living organisms, as well as water. ̽��ֱ��zinc found in meteorites has a unique composition, which can be used to identify the sources of Earth’s volatiles.

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge and Imperial College London, have previously found that Earth’s zinc came from different parts of our Solar System: about half came from beyond Jupiter and half originated closer to Earth.

“One of the most fundamental questions on the origin of life is where the materials we need for life to evolve came from,” said Dr Rayssa Martins from Cambridge’s Department of Earth Sciences. “If we can understand how these materials came to be on Earth, it might give us clues to how life originated here, and how it might emerge elsewhere.”

Planetesimals are the main building blocks of rocky planets, such as Earth. These small bodies are formed through a process called accretion, where particles around a young star start to stick together, and form progressively larger bodies.

But not all planetesimals are made equal. ̽��ֱ��earliest planetesimals that formed in the Solar System were exposed to high levels of radioactivity, which caused them to melt and lose their volatiles. But some planetesimals formed after these sources of radioactivity were mostly extinct, which helped them survive the melting process and preserved more of their volatiles.

In a study published in the journal Science Advances, Martins and her colleagues looked at the different forms of zinc that arrived on Earth from these planetesimals. ̽��ֱ��researchers measured the zinc from a large sample of meteorites originating from different planetesimals and used this data to model how Earth got its zinc, by tracing the entire period of the Earth’s accretion, which took tens of millions of years.

Their results show that while these ‘melted’ planetesimals contributed about 70% of Earth’s overall mass, they only provided around 10% of its zinc.

According to the model, the rest of Earth’s zinc came from materials that didn’t melt and lose their volatile elements. Their findings suggest that unmelted, or ‘primitive’ materials were an essential source of volatiles for Earth.

“We know that the distance between a planet and its star is a determining factor in establishing the necessary conditions for that planet to sustain liquid water on its surface,” said Martins, the study’s lead author. “But our results show there’s no guarantee that planets incorporate the right materials to have enough water and other volatiles in the first place – regardless of their physical state.”

̽��ֱ��ability to trace elements through millions or even billions of years of evolution could be a vital tool in the search for life elsewhere, such as on Mars, or on planets outside our Solar System.

“Similar conditions and processes are also likely in other young planetary systems,” said Martins. “ ̽��ֱ��roles these different materials play in supplying volatiles is something we should keep in mind when looking for habitable planets elsewhere.”

̽��ֱ��research was supported in part by Imperial College London, the European Research Council, and UK Research and Innovation (UKRI).

Reference:
Rayssa Martins et al. ‘Primitive asteroids as a major source of terrestrial volatiles.’ Science Advances (2024). DOI: 10.1126/sciadv.ado4121

Researchers have used the chemical fingerprints of zinc contained in meteorites to determine the origin of volatile elements on Earth. ̽��ֱ��results suggest that without ‘unmelted’ asteroids, there may not have been enough of these compounds on Earth for life to emerge.

Rayssa Martins/Ross Findlay

An iron meteorite from the core of a melted planetesimal (left) and a chondrite meteorite, derived from a ‘primitive’, unmelted planetesimal (right).

Yes

Neon sign identified by JWST gives clue to planet formation

Anonymous — Tue, 05 Mar 2024 15:06:11 +0000

Planetary systems like our Solar System seem to contain more rocky objects than gas-rich ones. Around our sun, these include the inner planets, the asteroid belt and the Kuiper belt. But scientists have known for a long time that planet-forming discs start with 100 times more mass in gas than in solids, which leads to a pressing question; when and how does most of the gas leave the disc/system?

JWST is helping scientists uncover how planets form, by advancing understanding of their birthplaces, the circumstellar discs surrounding young stars. In a new study published in the Astronomical Journal, a team of scientists including those from the ̽��ֱ�� of Leicester, the ̽��ֱ�� of Cambridge and led by the ̽��ֱ�� of Arizona, image for the first time an old planet-forming disc (still very young relative to the Sun) which is actively dispersing its gas content.

Knowing when the gas disperses is important as it constrains the time that is left for nascent planets to consume the gas from their surroundings.

During the very early stages of planetary system formation, planets coalesce in a spinning disc of gas and tiny dust around the young star. These particles clump together, building up into bigger and bigger chunks called planetesimals. Over time, these planetesimals collide and stick together, eventually forming planets. ̽��ֱ��type, size, and location of planets that form depend on the amount of material available and how long it remains in the disc. So, the outcome of planet formation depends on the evolution and dispersal of the disc.

At the heart of this discovery is the observation of T Cha, a young star (relative to the Sun) enveloped by an eroding disc notable for its vast dust gap, approximately 30 astronomical units in radius. For the first time, astronomers have imaged the dispersing gas (aka winds) using the four lines of the noble gases neon (Ne) and argon (Ar), one of which is the first detection in a planet-forming disc. ̽��ֱ��images of [Ne II] show that the wind is coming from an extended region of the disc. ̽��ֱ��team is also interested in knowing how this process takes place, so they can better understand the history and impact on our solar system.

Scientists have been trying to understand the mechanisms behind the winds in protoplanetary discs for over a decade. ̽��ֱ��observations by JWST represent a huge step-change in the data they have to work with, compared to previous data from ground-based telescopes.

“We first used neon to study planet-forming discs more than a decade ago, testing our computational simulations against data from Spitzer, and new observations we obtained with the ESO VLT,” said co-author Professor Richard Alexander from the ̽��ֱ�� of Leicester. “We learned a lot, but those observations didn’t allow us to measure how much mass the discs were losing.

“ ̽��ֱ��new JWST data are spectacular, and being able to resolve disc winds in images is something I never thought would be possible. With more observations like this still to come, JWST will enable us to understand young planetary systems as never before.”

“These winds could be driven either by high-energy stellar photons (the star's light) or by the magnetic field that weaves the planet-forming disc,” said Naman Bajaj from the ̽��ֱ�� of Arizona, the study’s lead author.

To differentiate between the two, the same group, this time led by Dr Andrew Sellek of Leiden Observatory and previously of the Institute of Astronomy at the ̽��ֱ�� of Cambridge, performed simulations of the dispersal driven by stellar photons. They compare these simulations to the actual observations and find dispersal by high-energy stellar photons can explain the observations, and hence cannot be excluded as a possibility.

“ ̽��ֱ��simultaneous measurement of all four lines by JWST proved crucial to pinning down the properties of the wind and helped us to demonstrate that significant amounts of gas are being dispersed,” said Sellek.

To put it into context, the researchers calculate that the mass dispersing every year is equivalent to that of the moon! These results will be published in a companion paper, currently under review at the Astronomical Journal.

̽��ֱ��[Ne II] line was discovered towards several planet-forming discs in 2007 with the Spitzer Space Telescope and soon identified as a tracer of winds by team member Professor Ilaria Pascucci at the ̽��ֱ�� of Arizona; this transformed research efforts focused on understanding disc gas dispersal. Now the discovery of spatially resolved [Ne II] - as well as the first detection of [Ar III] - using the James Webb Space Telescope, could become the next step towards transforming our understanding of this process.

̽��ֱ��implications of these findings offer new insights into the complex interactions that lead to the dispersal of the gas and dust critical for planet formation. By understanding the mechanisms behind disc dispersal, scientists can better predict the timelines and environments conducive to the birth of planets. ̽��ֱ��team's work demonstrates the power of JWST and sets a new path for exploring planet formation dynamics and the evolution of circumstellar discs.

Reference:
Naman S Bajaj et al. ‘JWST MIRI MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind.’ ̽��ֱ��Astronomical Journal (2024). DOI: 10.3849/1538-3881/ad22e1

Adapted from a ̽��ֱ�� of Leicester press release.

̽��ֱ��winds that help to form planets in the gaseous discs of early solar systems have been imaged for the first time by the James Webb Space Telescope (JWST) using the noble gases neon and argon.

ESO/M. Kornmesser

Artist's impression of the surroundings of the supermassive black hole in NGC 3783

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Scientists have new tool to estimate how much water might be hidden beneath a planet’s surface

cmm201 — Wed, 15 Mar 2023 12:25:33 +0000

Scientists from the ̽��ֱ�� of Cambridge now have a way to estimate how much water a rocky planet can store in its subterranean reservoirs. It is thought that this water, which is locked into the structure of minerals deep down, might help a planet recover from its initial fiery birth.

̽��ֱ��researchers developed a model that can predict the proportion of water-rich minerals inside a planet. These minerals act like a sponge, soaking up water which can later return to the surface and replenish oceans. Their results could help us understand how planets can become habitable following intense heat and radiation during their early years.

Planets orbiting M-type red dwarf stars — the most common star in the galaxy — are thought to be one of the best places to look for alien life. But these stars have particularly tempestuous adolescent years — releasing intense bursts of radiation that blast nearby planets and bake off their surface water.

Our Sun’s adolescent phase was relatively short, but red dwarf stars spend much longer in this angsty transitional period. As a result, the planets under their wing suffer a runaway greenhouse effect where their climate is thrown into chaos.

“We wanted to investigate whether these planets, after such a tumultuous upbringing, could rehabilitate themselves and go on to host surface water,” said lead author of the study, Claire Guimond, a PhD student in Cambridge’s Department of Earth Sciences.

̽��ֱ��new research, published in the Monthly Notices of the Royal Astronomical Society, shows that interior water could be a viable way to replenish liquid surface water once a planet’s host star has matured and dimmed. This water would likely have been brought up by volcanoes and gradually released as steam into the atmosphere, together with other life-giving elements.

Their new model allows them to calculate a planet’s interior water capacity based on its size and the chemistry of its host star. “ ̽��ֱ��model gives us an upper limit on how much water a planet could carry at depth, based on these minerals and their ability to take water into their structure,” said Guimond.

̽��ֱ��researchers found that the size of a planet plays a key role in deciding how much water it can hold. That’s because a planet’s size determines the proportion of water-carrying minerals it is made of.

Most of a planet’s interior water is contained within a rocky layer known as the upper mantle — which lies directly below the crust. Here, pressure and temperature conditions are just right for the formation of green-blue minerals called wadsleyite and ringwoodite that can soak up water. This rocky layer is also within reach of volcanoes, which could bring water back to the surface through eruptions.

̽��ֱ��new research showed that larger planets — around two to three times bigger than Earth — typically have drier rocky mantles because the water-rich upper mantle makes up a smaller proportion of their total mass.

̽��ֱ��results could provide scientists with guidelines to aid their search for exoplanets that might host life, “This could help refine our triaging of which planets to study first,” said Oliver Shorttle, who is jointly affiliated with Cambridge’s Department of Earth Sciences and Institute of Astronomy. “When we’re looking for the planets that can best hold water you probably do not want one significantly more massive or wildly smaller than Earth.”

̽��ֱ��findings could also add to our understanding of how planets, including those closer to home like Venus, can transition from barren hellscapes to a blue marble. Temperatures on the surface of Venus, which is of a similar size and bulk composition to Earth, hover around 450oC and its atmosphere is heavy with carbon dioxide and nitrogen. It remains an open question whether Venus hosted liquid water at its surface 4 billion years ago. “If that’s the case, then Venus must have found a way to cool itself and regain surface water after being born around a fiery sun,” said Shorttle, “It’s possible that it tapped into its interior water in order to do this.”

Reference:
Guimond, C. M., Shorttle, O., & Rudge, J. F. 'Mantle mineralogy limits to rocky planet water inventories'. Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad148

In the search for life elsewhere in the Universe, scientists have traditionally looked for planets with liquid water at their surface. But, rather than flowing as oceans and rivers, much of a planet’s water can be locked in rocks deep within its interior.

We wanted to investigate whether these planets, after such a tumultuous upbringing, could rehabilitate themselves and go on to host surface water

Claire Guimond

NASA

Water worlds

̽��ֱ��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.

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Astronomers use ‘little hurricanes’ to weigh and date planets around young stars

sc604 — Fri, 06 Jan 2023 08:25:43 +0000

Researchers from the ̽��ֱ�� of Cambridge and the Institute for Advanced Study have developed a technique, which uses observations of these ‘hurricanes’ by the Atacama Large Millimeter/submillimetre Array (ALMA) to place some limits on the mass and age of planets in a young star system.

Pancake-like clouds of gases, dust and ice surrounding young stars – known as protoplanetary discs - are where the process of planet formation begins. Through a process known as core accretion, gravity causes particles in the disc to stick to each other, eventually forming larger solid bodies such as asteroids or planets. As young planets form, they start to carve gaps in the protoplanetary disc, like grooves on a vinyl record.

Even a relatively small planet – as small as one-tenth the mass of Jupiter according to some recent calculations – may be capable of creating such gaps. As these ‘super-Neptune’ planets can orbit their star at a distance greater than Pluto orbits the Sun, traditional methods of exoplanet detection cannot be used.

In addition to the grooves, observations from ALMA have shown other distinct structures in protoplanetary discs, such as banana- or peanut-shaped arcs and clumps. It had been thought that at least some of these structures were also driven by planets.

“Something must be causing these structures to form,” said lead author Professor Roman Rafikov from Cambridge’s Department of Applied Mathematics and Theoretical Physics, and the Institute for Advanced Study in Princeton, New Jersey. “One of the possible mechanisms for producing these structures – and certainly the most intriguing one – is that dust particles that we see as arcs and clumps are concentrated in the centres of fluid vortices: essentially little hurricanes that can be triggered by a particular instability at the edges of the gaps carved in protoplanetary discs by planets.”

Working with his PhD student Nicolas Cimerman, Rafikov used this interpretation to develop a method to constrain a planet’s mass or age if a vortex is observed in a protoplanetary disc. Their results have been accepted for publication in two separate papers in the Monthly Notices of the Royal Astronomical Society.

“It’s extremely difficult to study smaller planets that are far away from their star by directly imaging them: it would be like trying to spot a firefly in front of a lighthouse,” said Rafikov. “We need other, different methods to learn about these planets.”

To develop their method, the two researchers first theoretically calculated the length of time it would take for a vortex to be produced in the disc by a planet. They then used these calculations to constrain the properties of planets in discs with vortices, basically setting lower limits on the planet’s mass or age. They call these techniques ‘vortex weighing’ and ‘vortex dating’ of planets.

When a growing planet becomes massive enough, it starts pushing material from the disc away, creating the tell-tale gap in the disc. When this happens, material on the outside of the gap becomes denser than material on the inside of the gap. As the gap gets deeper and the differences in density become large, an instability can be triggered. This instability perturbs the disc and can eventually produce a vortex.

“Over time, multiple vortices can merge together, evolving into one big structure that looks like the arcs we’ve observed with ALMA,” said Cimerman. Since the vortices need time to form, the researchers say their method is like a clock that can help determine the mass and age of the planet.

“More massive planets produce vortices earlier in their development due to their stronger gravity, so we can use the vortices to place some constraints on the mass of the planet, even if we can’t see the planet directly,” said Rafikov.

Using various data points such as spectra, luminosity and motion, astronomers can determine the approximate age of a star. With this information, the Cambridge researchers calculated the lowest possible mass of a planet that could have been in orbit around the star since the protoplanetary disc formed and was able to produce a vortex that could be seen by ALMA. This helped them put a lower limit on the mass of the planet without observing it directly.

By applying this technique to several known protoplanetary discs with prominent arcs, suggestive of vortices, the researchers found that the putative planets creating these vortices must have masses of at least several tens of Earth masses, in the super-Neptune range.

“In my daily work, I often focus on the technical aspects of performing the simulations,” said Cimerman. “It’s exciting when things come together and we can use our theoretical findings to learn something about real systems.”

“Our constraints can be combined with the limits provided by other methods to improve our understanding of planetary characteristics and planet formation pathways in these systems,” said Rafikov. “By studying planet formation in other star systems, we may learn more about how our own Solar System evolved.”

̽��ֱ��research was supported in part by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).

References:
Roman R Rafikov and Nicolas P Cimerman. ‘Vortex weighing and dating of planets in protoplanetary discs.’ Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3692 or DOI: 10.48550/arXiv.2301.01789

Nicolas P Cimerman and Roman R Rafikov. ‘Emergence of vortices at the edges of planet-driven gaps in protoplanetary discs.’ Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3507

Little ‘hurricanes’ that form in the discs of gas and dust around young stars can be used to study certain aspects of planet formation, even for smaller planets which orbit their star at large distances and are out of reach for most telescopes.

It’s extremely difficult to study smaller planets that are far away from their star by directly imaging them: it would be like trying to spot a firefly in front of a lighthouse. We need other, different methods to learn about these planets

Roman Rafikov

ALMA (ESO/NAOJ/NRAO)

ALMA image of the protoplanetary disc around HL Tauri

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Study of ‘polluted’ white dwarfs finds that stars and planets grow together

sc604 — Mon, 14 Nov 2022 15:52:52 +0000

A study of some of the oldest stars in the Universe suggests that the building blocks of planets like Jupiter and Saturn begin to form while a young star is growing. It had been thought that planets only form once a star has reached its final size, but new results, published in the journal Nature Astronomy, suggest that stars and planets ‘grow up’ together.

̽��ֱ��research, led by the ̽��ֱ�� of Cambridge, changes our understanding of how planetary systems, including our own Solar System, formed, potentially solving a major puzzle in astronomy.

“We have a pretty good idea of how planets form, but one outstanding question we’ve had is when they form: does planet formation start early, when the parent star is still growing, or millions of years later?” said Dr Amy Bonsor from Cambridge’s Institute of Astronomy, the study’s first author.

To attempt to answer this question, Bonsor and her colleagues studied the atmospheres of white dwarf stars – the ancient, faint remnants of stars like our Sun – to investigate the building blocks of planet formation. ̽��ֱ��study also involved researchers from the ̽��ֱ�� of Oxford, the Ludwig-Maximilians-Universität in Munich, the ̽��ֱ�� of Groningen and the Max Planck Institute for Solar System Research, Gottingen.

“Some white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards,” said Bonsor.

Normally, the interiors of planets are out of reach of telescopes. But a special class of white dwarfs – known as ‘polluted’ systems – have heavy elements such as magnesium, iron, and calcium in their normally clean atmospheres.

These elements must have come from small bodies like asteroids left over from planet formation, which crashed into the white dwarfs and burned up in their atmospheres. As a result, spectroscopic observations of polluted white dwarfs can probe the interiors of those torn-apart asteroids, giving astronomers direct insight into the conditions in which they formed.

Planet formation is believed to begin in a protoplanetary disc – made primarily of hydrogen, helium, and tiny particles of ices and dust – orbiting a young star. According to the current leading theory on how planets form, the dust particles stick to each other, eventually forming larger and larger solid bodies. Some of these larger bodies will continue to accrete, becoming planets, and some remain as asteroids, like those that crashed into the white dwarfs in the current study.

̽��ֱ��researchers analysed spectroscopic observations from the atmospheres of 200 polluted white dwarfs from nearby galaxies. According to their analysis, the mixture of elements seen in the atmospheres of these white dwarfs can only be explained if many of the original asteroids had once melted, which caused heavy iron to sink to the core while the lighter elements floated on the surface. This process, known as differentiation, is what caused the Earth to have an iron-rich core.

“ ̽��ֱ��cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the earliest stages of the planetary system but decay away in just a million years,” said Bonsor. “In other words, if these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly.”

̽��ֱ��study suggests that the early-formation picture is likely to be correct, meaning that Jupiter and Saturn had plenty of time to grow to their current sizes.

“Our study complements a growing consensus in the field that planet formation got going early, with the first bodies forming concurrently with the star,” said Bonsor. “Analyses of polluted white dwarfs tell us that this radioactive melting process is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets.

“This is just the beginning – every time we find a new white dwarf, we can gather more evidence and learn more about how planets form. We can trace elements like nickel and chromium and say how big an asteroid must have been when it formed its iron core. It’s amazing that we’re able to probe processes like this in exoplanetary systems.”

Amy Bonsor is a Royal Society ̽��ֱ�� Research Fellow at the ̽��ֱ�� of Cambridge. ̽��ֱ��research was supported in part by the Royal Society, the Simons Foundation, and the European Research Council.

Reference:
Amy Bonsor et al. ‘Rapid formation of exoplanetesimals revealed by white dwarfs.’ Nature Astronomy (2022). DOI: 10.1038/s41550-022-01815-8

A team of astronomers have found that planet formation in our young Solar System started much earlier than previously thought, with the building blocks of planets growing at the same time as their parent star.

Some white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards

Amy Bonsor

Amanda Smith

Study of ‘polluted’ white dwarfs finds that stars and planets grow together

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Microscopic view on asteroid collisions could help us understand planet formation

cmm201 — Thu, 24 Feb 2022 10:00:43 +0000

A team of researchers, led by the ̽��ֱ�� of Cambridge, combined dating and microscopic analysis of the Chelyabinsk meteorite — which fell to Earth and hit the headlines in 2013 — to get more accurate constraints on the timing of ancient impact events.

Their study, published in Communications Earth & Environment, looked at how minerals within the meteorite were damaged by different impacts over time, meaning they could identify the biggest and oldest events that may have been involved in planetary formation.

“Meteorite impact ages are often controversial: our work shows that we need to draw on multiple lines of evidence to be more certain about impact histories – almost like investigating an ancient crime scene,” said Craig Walton, who led the research and is based at Cambridge’s Department of Earth Sciences.

Early in our Solar System’s history, planets including the Earth formed from massive collisions between asteroids and even bigger bodies, called proto-planets.

“Evidence of these impacts is so old that it has been lost on the planets — Earth, in particular, has a short memory because surface rocks are continually recycled by plate tectonics,” said co-author Dr Oli Shorttle, who is based jointly at Cambridge’s Department of Earth Sciences and Institute of Astronomy.

Asteroids, and their fragments that fall to Earth as meteorites, are in contrast inert, cold and much older— making them faithful timekeepers of collisions.

̽��ֱ��new research, which was a collaboration with researchers from the Chinese Academy of Sciences and the Open ̽��ֱ��, recorded how phosphate minerals inside the Chelyabinsk meteorite were shattered to varying degrees in order to piece together a collision history.

Their aim was to corroborate uranium-lead dating of the meteorite, which looks at the time elapsed for one isotope to decay to another.

“ ̽��ֱ��phosphates in most primitive meteorites are fantastic targets for dating the shock events experienced by the meteorites on their parent bodies,” said Dr Sen Hu, who carried out the uranium-lead dating at Beijing’s Institute of Geology and Geophysics, Chinese Academy of Sciences.

Previous dating of this meteorite has revealed two impact ages, one older, roughly 4.5-billion-year-old collision and another which occurred within the last 50 million years.

But these ages aren’t so clear-cut. Much like a painting fading over time, successive collisions can obscure a once clear picture, leading to uncertainty among the scientific community over the age and even the number of impacts recorded.

̽��ֱ��new study put the collisions recorded by the Chelyabinsk meteorite in time order by linking new uranium-lead ages on the meteorite to microscopic evidence for collision-induced heating seen inside their crystal structures. These microscopic clues build up in the minerals with each successive impact, meaning the collisions can be distinguished, put in time order and dated.

Their findings show that minerals containing the imprint of the oldest collision were either shattered into many smaller crystals at high temperatures or strongly deformed at high pressures.

̽��ֱ��team also described some mineral grains in the meteorite that were fractured by a lesser impact, at lower pressures and temperatures, and which record a much more recent age of less than 50 million years. They suggest this impact probably chipped the Chelyabinsk meteorite off its host asteroid and sent it hurtling to Earth.

Taken together, this supports a two-stage collision history. “ ̽��ֱ��question for us was whether these dates could be trusted, could we tie these impacts to evidence of superheating from an impact?” said Walton. “What we’ve shown is that the mineralogical context for dating is really important.”

Scientists are particularly interested in the date of the 4.5-billion-year-old impact because this is about the time we think the Earth-Moon system came to being, probably as a result of two planetary bodies colliding.

̽��ֱ��Chelyabinsk meteorite belongs to a group of so-called stony meteorites, all of which contain highly shattered and remelted material roughly coincident with this colossal impact.

̽��ֱ��newly acquired dates support previous suggestions that many asteroids experienced high energy collisions between 4.48 – 4.44 billion years ago. “ ̽��ֱ��fact that all of these asteroids record intense melting at this time might indicate Solar System re-organisation, either resulting from the Earth-Moon formation or perhaps the orbital movements of giant planets.”

Walton now plans to refine dating over the window of the Moon-forming impact, which could tell us how our own planet came to being.

Reference:
Walton, C.R. et al. ‘Ancient and recent collisions revealed by phosphate minerals in the Chelyabinsk meteorite.’ Communications Earth & Environment (2022). DOI: 10.1038/s43247-022-00373-1

A new way of dating collisions between asteroids and planetary bodies throughout our Solar System’s history could help scientists reconstruct how and when planets were born.

Our work shows that we need to draw on multiple lines of evidence to be more certain about impact histories – almost like investigating an ancient crime scene

Craig Walton

False-colour image of impact recrystallised phosphate mineral in Chelyabinsk meteorite

Yes

‘Slushy’ magma ocean led to formation of the Moon’s crust

sc604 — Thu, 13 Jan 2022 14:00:00 +0000

̽��ֱ��scientists, from the ̽��ֱ�� of Cambridge and the Ecole normale supérieure de Lyon, have proposed a new model of crystallisation, where crystals remained suspended in liquid magma over hundreds of millions of years as the lunar ‘slush’ froze and solidified. ̽��ֱ��results are reported in the journal Geophysical Research Letters.

Over fifty years ago, Apollo 11 astronauts collected samples from the lunar Highlands. These large, pale regions of the Moon – visible to the naked eye – are made up of relatively light rocks called anorthosites. Anorthosites formed early in the history of the Moon, between 4.3 and 4.5 billion years ago.

Similar anorthosites, formed through the crystallisation of magma, can be found in fossilised magma chambers on Earth. Producing the large volumes of anorthosite found on the Moon, however, would have required a huge global magma ocean.

Scientists believe that the Moon formed when two protoplanets, or embryonic worlds, collided. ̽��ֱ��larger of these two protoplanets became the Earth, and the smaller became the Moon. One of the outcomes of this collision was that the Moon was very hot – so hot that its entire mantle was molten magma, or a magma ocean.

“Since the Apollo era, it has been thought that the lunar crust was formed by light anorthite crystals floating at the surface of the liquid magma ocean, with heavier crystals solidifying at the ocean floor,” said co-author Chloé Michaut from Ecole normale supérieure de Lyon. “This ‘flotation’ model explains how the lunar Highlands may have formed.”

However, since the Apollo missions, many lunar meteorites have been analysed and the surface of the Moon has been extensively studied. Lunar anorthosites appear more heterogeneous in their composition than the original Apollo samples, which contradicts a flotation scenario where the liquid ocean is the common source of all anorthosites.

̽��ֱ��range of anorthosite ages – over 200 million years – is difficult to reconcile with an ocean of essentially liquid magma whose characteristic solidification time is close to 100 million years.

“Given the range of ages and compositions of the anorthosites on the Moon, and what we know about how crystals settle in solidifying magma, the lunar crust must have formed through some other mechanism,” said co-author Professor Jerome Neufeld from Cambridge’s Department of Applied Mathematics and Theoretical Physics.

Michaut and Neufeld developed a mathematical model to identify this mechanism.

In the low lunar gravity, the settling of crystal is difficult, particularly when strongly stirred by the convecting magma ocean. If the crystals remain suspended as a crystal slurry, then when the crystal content of the slurry exceeds a critical threshold, the slurry becomes thick and sticky, and the deformation slow.

This increase of crystal content occurs most dramatically near the surface, where the slushy magma ocean is cooled, resulting in a hot, well-mixed slushy interior and a slow-moving, crystal-rich lunar ‘lid’.

“We believe it’s in this stagnant ‘lid’ that the lunar crust formed, as lightweight, anorthite-enriched melt percolated up from the convecting crystalline slurry below,” said Neufeld. “We suggest that cooling of the early magma ocean drove such vigorous convection that crystals remained suspended as a slurry, much like the crystals in a slushy machine.”

Enriched lunar surface rocks likely formed in magma chambers within the lid, which explains their diversity. ̽��ֱ��results suggest that the timescale of lunar crust formation is several hundreds of million years, which corresponds to the observed ages of the lunar anorthosites.

Serial magmatism was initially proposed as a possible mechanism for the formation of lunar anorthosites, but the slushy model ultimately reconciles this idea with that of a global lunar magma ocean.

̽��ֱ��research was supported by the European Research Council.

Jerome Neufeld is also affiliated with the Department of Earth Sciences. He is a Fellow of Trinity College.

Reference:
Chloé Michaut and Jerome A Neufeld. ‘Formation of the lunar primary crust from a long-lived slushy magma ocean.’ Geophysical Research Letters (2022). DOI: 10.1029/2021GL095408

Adapted from an ENS-Lyon press release.

Scientists have shown how the freezing of a ‘slushy’ ocean of magma may be responsible for the composition of the Moon’s crust.

Cooling of the early magma ocean drove such vigorous convection that crystals remained suspended as a slurry, like the crystals in a slushy machine.

Jerome Neufeld

NASA/Goddard Space Flight Center

Magma ocean and first rocky crust on the Moon

Yes

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Public Domain