Key to rapid planet formation

A team of LMU researchers has developed a new model to explain the formation of giant planets such as Jupiter, which furnishes deeper insights into the processes of planet formation and could expand our understanding of planetary systems.

Our solar system is our immediate cosmic neighborhood. We know it well: the Sun at the center; then the rocky planets Mercury, Venus, Earth, and Mars; and then the asteroid belt; followed by the gas giants Jupiter and Saturn; then the ice giants Uranus and Neptune; and finally the Kuiper belt with its comets. But how well do we really know our home? Previous theories have assumed that giant planets are formed by collisions and accumulations of asteroid-like celestial bodies, so-called planetesimals, and the subsequent accretion of gas over the course of millions of years. However, these models explain neither the existence of gas giants located far from their stars nor the formation of Uranus and Neptune.

From grain of dust to giant planet

Astrophysicists from LMU, the ORIGINS cluster, and MPS have developed the first ever model to incorporate all the necessary physical processes that play a role in planet formation. Using this model, they have shown that annular perturbations in protoplanetary disks, so-called substructures, can trigger the rapid formation of multiple gas giants. The results of the study match the latest observations and indicate that the formation of giant planets could happen more efficiently and quickly than previously thought.

With their model, the researchers demonstrate how millimeter-sized dust particles accumulate aerodynamically in the turbulent gas disk, and how this initial perturbation in the disk traps dust and prevents it from disappearing off in the direction of the star. This accumulation makes the growth of planets very efficient, as suddenly a lot of "building material" is available within a compact area and the right conditions for planet formation are present.

"When a planet gets large enough to influence the gas disk, this leads to renewed dust enrichment farther out in the disk," explains Til Birnstiel, Professor of Theoretical Astrophysics at LMU and member of the ORIGINS Cluster of Excellence. "In the process, the planet drives the dust -- like a sheepdog chasing its herd -- into the area outside its own orbit." The process begins anew, from inside to outside, and another giant planet can form. "This is the first time a simulation has traced the process whereby fine dust grows into giant planets," observes Tommy Chi Ho Lau, lead author of the study and doctoral candidate at LMU.

Variety of gas giants in our and other solar systems

In our solar system, the gas giants are situated at a distance of around 5 astronomical units (au) (Jupiter) to 30 au (Neptune) from the Sun. For comparison, the Earth is some 150 million kilometers from the Sun, which is equivalent to 1 au.

The study shows that in other planetary systems, a perturbation could set the process in motion at much larger distances and still happen very rapidly. Such systems have been observed frequently in recent years by the ALMA radio observatory, which has found gas giants in young disks at a distance beyond 200 au. However, the model also explains why our solar system apparently stopped forming additional planets after Neptune: the building material was simply used up.

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James Webb Space Telescope captures the end of planet formation

Scientists believe that planetary systems like our solar system contain more rocky objects than gas-rich ones. Around our sun, these include the inner planets -- Mercury, Venus, Earth and Mars -- the asteroid belt and the Kuiper belt objects such as Pluto.

Jupiter, Saturn, Uranus and Neptune, on the other hand, contain mostly gas. But scientists also have known for a long time that planet-forming disks start out with 100 times more mass in gas than solids, which leads to a pressing question: When and how does most of the gas leave a nascent planetary system?

A new study led by Naman Bajaj at the University of Arizona Lunar and Planetary Laboratory, published in the Astronomical Journal, provides answers. Using the James Webb Space Telescope, or JWST, the team obtained images from such a nascent planetary system -- also known as a circumstellar disk -- in the process of actively dispersing its gas into surrounding space.

"Knowing when the gas disperses is important as it gives us a better idea of how much time gaseous planets have to consume the gas from their surroundings," said Bajaj, a second-year doctoral student at UArizona's Lunar and Planetary Laboratory. "With unprecedented glimpses into these disks surrounding young stars, the birthplaces of planets, JWST helps us uncover how planets form."

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

"So, in short, the outcome of planet formation depends on the evolution and dispersal of the disk," Bajaj said.

At the heart of this discovery is the observation of T Cha, a young star -- relative to the sun, which is about 4.6 billion years old -- enveloped by an eroding circumstellar disk notable for a vast dust gap, spanning approximately 30 astronomical units, or au, with one au being the average distance between the Earth and the sun.

Bajaj and his team were able, for the first time, to image the disk wind, as the gas is referred to when it slowly leaves the planet-forming disk. The astronomers took advantage of the telescope's sensitivity to light emitted by an atom when high-energy radiation -- for example, in starlight -- strips one or more electrons from its nucleus. This is known as ionization, and the light emitted in the process can be used as a sort of chemical "fingerprint" -- in the case of the T Cha system, tracing two noble gases, neon and argon. The observations also mark the first time a double ionization of argon has been detected in a planet-forming disk, the team writes in the paper.

"The neon signature in our images tells us that the disk wind is coming from an extended region away from the disk," Bajaj said. "These winds could be driven either by high-energy photons -- essentially the light streaming from the star -- or by the magnetic field that weaves through the planet-forming disk."

In an effort to differentiate between the two, the same group, this time led by Andrew Sellek, a postdoctoral researcher at Leiden University in the Netherlands, performed simulations of the dispersal driven by stellar photons, the intense light streaming from the young star. They compared these simulations to the actual observations and found dispersal by high-energy stellar photons can explain the observations, and hence cannot be excluded as a possibility. That study concluded that the amount of gas dispersing from the T Cha disk every year is equivalent to that of Earth's moon. These results will be published in a companion paper, currently under review with the Astronomical Journal.

While neon signatures had been detected in many other astronomical objects, they weren't known to originate in low-mass planet-forming disks until first discovered in 2007 with JWST's predecessor, NASA's Spitzer Space Telescope, by Ilaria Pascucci, a professor at LPL who soon identified them as a tracer of disk winds. Those early findings transformed research efforts focused on understanding gas dispersal from circumstellar disks. Pascucci is the principal investigator on the most recent observing project and a co-author on the publications reported here.

"Our discovery of spatially resolved neon emission -- and the first detection of double ionized argon -- using the James Webb Space Telescope could become the next step towards transforming our understanding of how gas clears out of a planet-forming disk," Pascucci said. "These insights will help us get a better idea of the history and impact on our own solar system."

In addition, the group has also discovered that the inner disk of T Cha is evolving on very short timescales of decades; they found that the spectrum observed by JWST differs from the earlier spectrum detected by Spitzer. According to Chengyan Xie, a second-year doctoral student at LPL who leads this in-progress work, this mismatch could be explained by a small, asymmetric disk inside of T Cha that has lost some of its mass in the short 17 years that have elapsed between the two observations.

"Along with the other studies, this also hints that the disk of T Cha is at the end of its evolution," Xie said. "We might be able to witness the dispersal of all the dust mass in T Cha's inner disk within our lifetime."

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Astronomers reveal a new link between water and planet formation

Researchers have found water vapour in the disc around a young star exactly where planets may be forming. Water is a key ingredient for life on Earth, and is also thought to play a significant role in planet formation. Yet, until now, we had never been able to map how water is distributed in a stable, cool disc -- the type of disc that offers the most favourable conditions for planets to form around stars. The new findings were made possible thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner.

"I had never imagined that we could capture an image of oceans of water vapour in the same region where a planet is likely forming," says Stefano Facchini, an astronomer at the University of Milan, Italy, who led the study published today in Nature Astronomy. The observations reveal at least three times as much water as in all of Earth's oceans in the inner disc of the young Sun-like star HL Tauri, located 450 light-years away from Earth in the constellation Taurus.

"It is truly remarkable that we can not only detect but also capture detailed images and spatially resolve water vapour at a distance of 450 light-years from us ," adds co-author Leonardo Testi, an astronomer at the University of Bologna, Italy. The 'spatially resolved' observations with ALMA allow astronomers to determine the distribution of water in different regions of the disc. "Taking part in such an important discovery in the iconic HL Tauri disc was beyond what I had ever expected for my first research experience in astronomy," adds Mathieu Vander Donckt from the University of Liege, Belgium, who was a master's student when he participated in the research.

A significant amount of water was found in the region where a known gap in the HL Tauri disc exists. Ring-shaped gaps are carved out in gas- and dust-rich discs by orbiting young planet-like bodies as they gather up material and grow. "Our recent images reveal a substantial quantity of water vapour at a range of distances from the star that include a gap where a planet could potentially be forming at the present time," says Facchini. This suggests that this water vapour could affect the chemical composition of planets forming in those regions.

Observing water with a ground-based telescope is no mean feat as the abundant water vapour in Earth's atmosphere degrades the astronomical signals. ALMA, operated by ESO together with its international partners, is an array of telescopes in the Chilean Atacama Desert at about 5000 metres elevation that was built in a high and dry environment specifically to minimise this degradation, providing exceptional observing conditions. "To date, ALMA is the only facility able to spatially resolve water in a cool planet-forming disc," says co-author Wouter Vlemmings, a professor at the Chalmers University of Technology in Sweden.*

"It is truly exciting to directly witness, in a picture, water molecules being released from icy dust particles," says Elizabeth Humphreys, an astronomer at ESO who also participated in the study. The dust grains that make up a disc are the seeds of planet formation, colliding and clumping into ever larger bodies orbiting the star. Astronomers believe that where it is cold enough for water to freeze onto dust particles, things stick together more efficiently -- an ideal spot for planet formation. "Our results show how the presence of water may influence the development of a planetary system, just like it did some 4.5 billion years ago in our own Solar System," Facchini adds.

With upgrades happening at ALMA and ESO's Extremely Large Telescope (ELT) coming online within the decade, planet formation and the role water plays in it will become clearer than ever. In particular METIS, the Mid-infrared ELT Imager and Spectrograph, will give astronomers unrivalled views of the inner regions of planet-forming discs, where planets like Earth form.

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Astronomers discover first step toward planet formation

Astronomers have gotten very good at spotting the signs of planet formation around stars. But for a complete understanding of planet formation, we also need to study examples where planet formation has not yet started. Looking for something and not finding it can be even more difficult than finding it sometimes, but new detailed observations of the young star DG Taurus show that it has a smooth protoplanetary disk without signs of planet formation. This successful non-detection of planet formation may indicate that DG Taurus is on the eve of planet formation.

Planets form in disks of gas and dust, known as protoplanetary disks, around protostars, young stars still in the process of forming. Planet growth is so slow that it's not possible to watch the evolution as it happens, so astronomers observe many protostars at slightly different stages of planet formation to build up a theoretical understanding.

This time an international research team led by Satoshi Ohashi at the National Astronomical Observatory of Japan (NAOJ) used the Atacama Large Millimeter/submillimeter Array (ALMA) to conduct high-resolution observations of a protoplanetary disk around a relatively young protostar, DG Taurus located 410 light-years away in the direction of the constellation Taurus. The team found that DG Taurus has a smooth protoplanetary disk, without any rings which would indicate that planets are forming. This led the team to believe that DG Taurus system will start forming planets in the future.

The team found that in this pre-planet-formation stage, the dust grains within 40 AU (about twice the size of the orbit of Uranus in the Solar System) of the central protostar are still small, while beyond this radius the dust grains have started to grow in size, the first step in planet formation. This is contrary to theoretical expectations that planet formation starts in the inner part of the disk.

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New exoplanet discovery builds better understanding of planet formation

An international team of scientists have discovered an unusual Jupiter-sized planet orbiting a low-mass star called TOI-4860, located in the Corvus constellation.

The newly discovered gas giant, named TOI-4860 b, is an unusual planet for two reasons: stars of such low mass are not expected to host planets like Jupiter, and the planet appears to be particularly enriched by heavy elements.

The study, led by University of Birmingham astronomers, is published today (Friday 4th August) in a letter published within the Monthly Notices of the Royal Astronomical Society.

The planet was initially identified using NASA's Transiting Exoplanet Survey Satellite as a drop of brightness while transiting in front of its host star, but that data alone was insufficient to confirm that it was a planet.

The team used the SPECULOOS South Observatory, located in the Atacama Desert in Chile, to measure the planetary signal in several wavelengths and validated the planetary nature. The astronomers also observed the planet just before and after it disappeared behind its host star, noticing that there was no change in light, meaning the planet was not emitting any. Finally, the team collaborated with a Japanese group using the Subaru Telescope in Hawai'i. Together they measured the mass of the planet to fully confirm it.

Following this star and confirming its planet was the initiative of a group of PhD students within the SPECULOOS project.

George Dransfield, one of those PhD students, who recently submitted her thesis at the University of Birmingham, explains: "Under the canonical planet formation model, the less mass a star has, the less massive is the disc of material around that star.

"Since planets are created from that disc, high-mass planets like Jupiter, were widely expected not to form. However, we were curious about this and wanted to check planetary candidates to see if it was possible. TOI-4860 is our first confirmation and also the lowest mass star hosting such a high mass planet."

Amaury Triaud, Professor of Exoplanetology at the University of Birmingham, who led the study said: "I am ever thankful to the bright PhD students of our team for proposing to observe systems like TOI-4860. Their work has really paid off since planets like TOI-4860 are vital to deepening our understanding of planet formation.

"A hint of what might have happened is hidden in the planetary properties, which appear particularly enriched in heavy elements. We have detected something similar in the host star too, so it is likely that an abundance of heavy elements catalysed the planet formation process."

The new gas giant takes about 1.52 days to complete a full orbit around its host star, but because its host is a cold low mass star, the planet itself can be referred to as a 'Warm Jupiter'. This is a subclass of planet that holds particular interest for astronomers looking to build on their initial observations and learn more about how these kinds of planets are formed.

Mathilde Timmermans, another student of the SPECULOOS project, working at the University of Liege in Belgium concludes: "Thanks to its very short orbital period, and to the properties of its host star, the discovery of TOI-4860 b provides a brilliant opportunity to study the atmospheric properties of a warm Jupiter and learn more about how gas giants are formed."

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ALMA digs deeper into the mystery of planet formation

An international research team used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe disks around 19 protostars with a very high resolution to search for the earliest signs of planet formation. This survey was motivated by the recent findings that planet formation may be well-underway in the more-evolved proto-planetary disks, but until now there had been no systematic study to search for signs of planet formation in younger protostellar systems.

Planets form in a disk around a newborn star. These 'proto-planetary' disks only last a few million years, meaning that a forming planetary system only has this amount of time to finish its formation. However, it is still not clear just how rapidly planet formation begins within these disks. Recent ALMA observations have revealed that many proto-planetary disks have substructures such as gaps and rings, indicating that planets are already forming from the disk. "These previous results motivated us to examine even younger disks around protostars to answer the question, at what stage of star formation do planet forms," says Nagayoshi Ohashi at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), who led the team.

The team observed disks around 19 protostars located within about 650 light-years from the Earth. This is the first systematic study to investigate the detailed structure of disks around a large sample of protostars with high angular resolution. The observations clearly show that the disks around protostars are different from more-evolved proto-planetary disks. Among the 19 protostars, rings, and gaps, which are signs of planet formation, were observed only in a few disks. Moreover, the ring structures are less distinct than those seen in the proto-planetary disks.

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ALMA traces history of water in planet formation back to the interstellar medium

Scientists studying a nearby protostar have detected the presence of water in its circumstellar disk. The new observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) mark the first detection of water being inherited into a protoplanetary disk without significant changes to its composition. These results further suggest that the water in our Solar System formed billions of years before the Sun. The new observations are published today in Nature.

V883 Orionis is a protostar located roughly 1,305 light-years from Earth in the constellation Orion. The new observations of this protostar have helped scientists to find a probable link between the water in the interstellar medium and the water in our Solar System by confirming they have similar composition.

"We can think of the path of water through the Universe as a trail. We know what the endpoints look like, which are water on planets and in comets, but we wanted to trace that trail back to the origins of water," said John Tobin, an astronomer at the National Science Foundation's National Radio Astronomy Observatory (NRAO) and the lead author on the new paper. "Before now, we could link the Earth to comets, and protostars to the interstellar medium, but we couldn't link protostars to comets. V883 Ori has changed that, and proven the water molecules in that system and in our Solar System have a similar ratio of deuterium and hydrogen."

Observing water in the circumstellar disks around protostars is difficult because in most systems water is present in the form of ice. When scientists observe protostars they're looking for the water snow line or ice line, which is the place where water transitions from predominantly ice to gas, which radio astronomy can observe in detail. "If the snow line is located too close to the star, there isn't enough gaseous water to be easily detectable and the dusty disk may block out a lot of the water emission. But if the snow line is located further from the star, there is sufficient gaseous water to be detectable, and that's the case with V883 Ori," said Tobin, who added that the unique state of the protostar is what made this project possible.

V883 Ori's disk is quite massive and is just hot enough that the water in it has turned from ice to gas. That makes this protostar an ideal target for studying the growth and evolution of solar systems at radio wavelengths.

"This observation highlights the superb capabilities of the ALMA instrument in helping astronomers study something vitally important for life on Earth: water," said Joe Pesce, NSF Program Officer for ALMA. "An understanding of the underlying processes important for us on Earth, seen in more distant regions of the galaxy, also benefits our knowledge of how nature works in general, and the processes that had to occur for our Solar System to develop into what we know today."

To connect the water in V883 Ori's protoplanetary disk to that in our own Solar System, the team measured its composition using ALMA's highly sensitive Band 5 (1.6mm) and Band 6 (1.3mm) receivers and found that it remains relatively unchanged between each stage of solar system formation: protostar, protoplanetary disk, and comets. "This means that the water in our Solar System was formed long before the Sun, planets, and comets formed. We already knew that there is plenty of water ice in the interstellar medium. Our results show that this water got directly incorporated into the Solar System during its formation," said Merel van 't 'Hoff, an astronomer at the University of Michigan and a co-author of the paper. "This is exciting as it suggests that other planetary systems should have received large amounts of water too."

Clarifying the role of water in the development of comets and planetesimals is critical to building an understanding of how our own Solar System developed. Although the Sun is believed to have formed in a dense cluster of stars and V883 Ori is relatively isolated with no nearby stars, the two share one critical thing in common: they were both formed in giant molecular clouds.

"It is known that the bulk of the water in the interstellar medium forms as ice on the surfaces of tiny dust grains in the clouds. When these clouds collapse under their own gravity and form young stars, the water ends up in the disks around them. Eventually, the disks evolve and the icy dust grains coagulate to form a new solar system with planets and comets," said Margot Leemker, an astronomer at Leiden University and a co-author of the paper. "We have shown that water that is produced in the clouds follows this trail virtually unchanged. So, by looking at the water in the V883 Ori disk, we essentially look back in time and see how our own Solar System looked when it was much younger."

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'Forbidden' planet orbiting small star challenges gas giant formation theories

A team of astronomers led by Carnegie's Shubham Kanodia has discovered an unusual planetary system in which a large gas giant planet orbits a small red dwarf star called TOI-5205. Their findings which are published in The Astronomical Journal, challenge long-held ideas about planet formation.

Smaller and cooler than our Sun, M dwarfs are the most common stars in our Milky Way galaxy. Due to their small size, these stars tend to be about half as hot as the Sun and much redder. They have very low luminosities, but extremely long lifespans. Although red dwarfs host more planets, on average, than other, more massive types of stars, their formation histories make them unlikely candidates to host gas giants.

The newly discovered planet -- TOI 5205b -- was first identified as a potential candidate by NASA's Transiting Exoplanet Survey Satellite (TESS). Kanodia's team, which included Carnegie's Anjali Piette, Alan Boss, Johanna Teske, and John Chambers, then confirmed its planetary nature and characterized it using a variety of ground-based instruments and facilities.

"The host star, TOI-5205, is just about four times the size of Jupiter, yet it has somehow managed to form a Jupiter-sized planet, which is quite surprising!" exclaimed Kanodia, who specializes in studying these stars, which comprise nearly three-quarters of our galaxy yet can't be seen with the naked eye.

A small number of gas giants have been discovered orbiting older M dwarf stars. But until now no gas giant has been found in a planetary system around a low-mass M dwarf like TOI-5205. To grasp the size comparison here, a Jupiter-like planet orbiting a Sun-like star could be compared to a pea going around a grapefruit; for TOI-5205b, because the host star is so much smaller, it is more like a pea going around a lemon. In fact, when the Jupiter-mass TOI 5205b crosses in front of its host, it blocks about seven percent of its light -- one of the largest known exoplanet transits.

Planets are born in the rotating disk of gas and dust that surrounds young stars. The most commonly used theory of gas planet formation requires about 10 Earth masses of this rocky material to accumulate and form a massive rocky core, after which it rapidly sweeps up large amounts of gas from the neighboring regions of the disk to form the giant planet we see today.

The time frame in which this happens is crucial.

"TOI-5205b's existence stretches what we know about the disks in which these planets are born," Kanodia explained. "In the beginning, if there isn't enough rocky material in the disk to form the initial core, then one cannot form a gas giant planet. And at the end, if the disk evaporates away before the massive core is formed, then one cannot form a gas giant planet. And yet TOI-5205b formed despite these guardrails. Based on our nominal current understanding of planet formation, TOI-5205b should not exist; it is a "forbidden" planet."

The team demonstrated that the planet's very large transit depth makes it extremely conducive for future observations with the recently launched JWST, which could shed some light on its atmosphere and offer some additional clues about the mystery of its formation.

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Early planetary migration can explain missing planets

A new model that accounts for the interplay of forces acting on newborn planets can explain two puzzling observations that have cropped up repeatedly among the more than 3,800 planetary systems cataloged to date.

One puzzle known as the "radius valley" refers to the rarity of exoplanets with a radius about 1.8 times that of Earth. NASA's Kepler spacecraft observed planets of this size about 2-3 times less frequently than it observed super-Earths with radii about 1.4 times that of Earth and mini-Neptunes with radii about 2.5 times Earth's. The second mystery, known as "peas in a pod," refers to neighboring planets of similar size that have been found in hundreds of planetary systems. Those include TRAPPIST-1 and Kepler-223, which also feature planetary orbits of near-musical harmony.

"I believe we are the first to explain the radius valley using a model of planet formation and dynamical evolution that self-consistently accounts for multiple constraints of observations," said Rice University's André Izidoro, corresponding author of a study published this week in Astrophysical Journal Letters. "We're also able to show that a planet-formation model incorporating giant impacts is consistent with the peas-in-a-pod feature of exoplanets."

Izidoro, a Welch Postdoctoral Fellow at Rice's NASA-fundedCLEVER Planets project, and co-authors used a supercomputer to simulate the first 50 million years of the development of planetary systems using a planetary migration model. In the model, protoplanetary disks of gas and dust that give rise to young planets also interact with them, pulling them closer to their parent stars and locking them in resonant orbital chains. The chains are broken within a few million years, when the disappearance of the protoplanetary disk causes orbital instabilities that lead two or more planets to slam into one another.

Planetary migration models have been used to study planetary systems that have retained their resonant orbital chains. For example, Izidoro and CLEVER Planets colleagues used a migration model in 2021 to calculate the maximum amount of disruption TRAPPIST-1's seven-planet system could have withstood during bombardment and still retained its harmonious orbital structure.

In the new study, Izidoro partnered with CLEVER Planets' investigators Rajdeep Dasgupta and Andrea Isella, both of Rice, Hilke Schlichting of the University of California, Los Angeles, and Christian Zimmermann and Bertram Bitsch of the Max Planck Institute for Astronomy in Heidelberg, Germany.

"The migration of young planets towards their host stars creates overcrowding and frequently results in cataclysmic collisions that strip planets of their hydrogen-rich atmospheres," Izidoro said. "That means giant impacts, like the one that formed our moon, are probably a generic outcome of planet formation."

The research suggests planets come in two "flavors," super-Earths that are dry, rocky and 50% larger than Earth, and mini-Neptunes that are rich in water ice and about 2.5 times larger than Earth. Izidoro said new observations seem to support the results, which conflict with the traditional view that both super-Earths and mini-Neptunes are exclusively dry and rocky worlds.

Based on their findings, the researchers made predictions that can be tested by NASA's James Webb Space Telescope. They suggest, for instance, that a fraction of planets about twice Earth's size will both retain their primordial hydrogen-rich atmosphere and be rich in water.

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Martian meteorite upsets planet formation theory

A new study of an old meteorite contradicts current thinking about how rocky planets like the Earth and Mars acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases as they form. The work is published June 16 in Science.

A basic assumption about planet formation is that planets first collect these volatiles from the nebula around a young star, said Sandrine Péron, a postdoctoral scholar working with Professor Sujoy Mukhopadhyay in the Department of Earth and Planetary Sciences, University of California, Davis.

Because the planet is a ball of molten rock at this point, these elements initially dissolve into the magma ocean and then degass back into the atmosphere. Later on, chondritic meteorites crashing into the young planet deliver more volatile materials.

So scientists expect that the volatile elements in the interior of the planet should reflect the composition of the solar nebula, or a mixture of solar and meteoritic volatiles, while the volatiles in the atmosphere would come mostly from meteorites. These two sources -- solar vs. chondritic -- can be distinguished by the ratios of isotopes of noble gases, in particular krypton.

Mars is of special interest because it formed relatively quickly -- solidifying in about 4 million years after the birth of the Solar System, while the Earth took 50 to 100 million years to form.

"We can reconstruct the history of volatile delivery in the first few million years of the Solar System," Péron said.

Meteorite from Mars' interior

Some meteorites that fall to Earth come from Mars. Most come from surface rocks that have been exposed to Mars' atmosphere. The Chassigny meteorite, which fell to Earth in north-eastern France in 1815, is rare and unusual because it is thought to represent the interior of the planet.

By making extremely careful measurements of minute quantities of krypton isotopes in samples of the meteorite using a new method set up at the UC Davis Noble Gas Laboratory, the researchers could deduce the origin of elements in the rock.

"Because of their low abundance, krypton isotopes are challenging to measure," Péron said.

Surprisingly, the krypton isotopes in the meteorite correspond to those from chondritic meteorites, not the solar nebula. That means that meteorites were delivering volatile elements to the forming planet much earlier than previously thought, and in the presence of the nebula, reversing conventional thinking.

"The Martian interior composition for krypton is nearly purely chondritic, but the atmosphere is solar," Péron said. "It's very distinct."

The results show that Mars' atmosphere cannot have formed purely by outgassing from the mantle, as that would have given it a chondritic composition. The planet must have acquired atmosphere from the solar nebula, after the magma ocean cooled, to prevent substantial mixing between interior chondritic gases and atmospheric solar gases.

The new results suggest that Mars' growth was completed before the solar nebula was dissipated by radiation from the Sun. But the irradiation should also have blown off the nebular atmosphere on Mars, suggesting that atmospheric krypton must have somehow been preserved, possibly trapped underground or in polar ice caps.

"However, that would require Mars to have been cold in the immediate aftermath of its accretion," Mukhopadhyay said. "While our study clearly points to the chondritic gases in the Martian interior, it also raises some interesting questions about the origin and composition of Mars' early atmosphere."

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Discovery of 30 exocomets in a young planetary system

For the past thirty years, the star β Pictoris has fascinated astronomers because it enables them to observe a planetary system in the process of formation. It is made up of at least two young planets, and also contains comets, which were detected as early as 1987. These were the first comets ever observed around a star other than the Sun.

Now, an international research team headed by Alain Lecavelier des Etangs, CNRS researcher at the Institut d'Astrophysique de Paris (CNRS/Sorbonne Université)1, has discovered 30 such exocomets and determined the size of their nuclei1, which vary between 3 and 14 kilometres in diameter. The scientists were also able to estimate the size distribution of the objects, i.e. the proportion of small comets to large ones. This is the first time this distribution has been measured outside our Solar System, and it is strikingly similar to that of comets orbiting the Sun. It shows that, just like the comets of the Solar System, the exocomets of β Pictoris were shaped by a series of collisions and breakups.

This work sheds new light on the origin and evolution of comets in planetary systems. Since a part of Earth's water probably originated in comets, scientists are seeking to understand their impact on the characteristics of planets. Their findings, published in Scientific Reports on April 28, 2022, are the outcome of 156 days of observation of the β Pictoris system using NASA's Transiting Exoplanet Survey Satellite (TESS).

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Classifying exoplanet atmospheres opens new field of study

An international team of researchers examined data for 25 exoplanets and found some links among the properties of the atmospheres, including the thermal profiles and chemical abundances in them. This marks the first time exoplanet atmospheres have been studied as populations, rather than individually. These findings will help establish a generalized theory of planet formation which will improve our understanding of all planets, including the Earth.

Today there are more than 3000 confirmed exoplanets, planets orbiting stars other than the Sun. Because they are far away from Earth, it is difficult to study them in detail. Determining the characteristics of even one exoplanet has been a noteworthy accomplishment.

In this research, astronomers used archival data for 25 hot Jupiters, gas giant planets that orbit close to their host stars. The data included 600 hours of observations from the Hubble Space Telescope and more than 400 hours of observations from the Spitzer Space Telescope.

One of the characteristics investigated by the team was the presence or absence of a "thermal inversion." Planetary atmospheres trap heat, so in general the temperature increases as you probe deeper into the atmosphere. But some planets show a thermal inversion where an upper layer of the atmosphere is warmer than the layer beneath it. On Earth, the presence of ozone causes a thermal inversion. The team found that almost all of the hot Jupiters with a thermal inversion also showed evidence for hydrogen anion (H-) and metallic species such as titanium oxide (TiO), vanadium oxide (VO), or iron hydride (FeH). Conversely, exoplanets without these chemicals almost never had thermal inversions. It is difficult to draw conclusions based on correlation alone, but since these metallic species are efficient absorbers of stellar light, one theory holds that when these chemicals are present in the upper atmosphere, they absorb light from the host star and cause the temperature to increase.

Masahiro Ikoma at the National Astronomical Observatory of Japan, a co-investigator in this study, explains, "The theory of gas giant formation proposed by my students and I predicted diversity in the composition of hot Jupiter atmospheres, and helped to motivate this systematic survey of atmospheric characteristics."

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Even dying stars can still give birth to planets

Planets are usually not much older than the stars around which they revolve. Take the Sun: it was born 4.6 billion years ago, and not long after that, Earth came into the world. But KU Leuven astronomers have discovered that a completely different scenario is also possible. Even if they are near death, some types of stars can possibly still form planets. If this is confirmed, theories on planet formation will need to be adjusted.

Planets such as Earth, and all other planets in our solar system, were formed not long after the Sun. Our Sun started to burn 4.6 billion years ago, and in the next million years, the matter around it clumped into protoplanets. The birth of the planets in that protoplanetary disc, a gigantic pancake made of dust and gas, so to speak, with the Sun in the middle, explains why they all orbit in the same plane.

But such discs of dust and gas needn't necessarily only surround newborn stars. They can also develop independently from star formation, for example around binary stars of which one is dying (binary stars are two stars that orbit each other, also called a binary system). When the end approaches for a medium-sized star (like the Sun), it catapults the outer part of its atmosphere into space, after which it slowly dies out as a so-called white dwarf. However, in the case of binary stars, the gravitational pull of the second star causes the matter ejected by the dying star to form a flat, rotating disc. Moreover, this disc strongly resembles the protoplanetary discs that astronomers observe around young stars elsewhere in the Milky Way.

This we already knew. However, what is new is that the discs surrounding so-called evolved binary stars not uncommonly show signs that could point to planet formation, as discovered by an international team of astronomers led by KU Leuven researchers. What's more, their observations show that this is the case for one in ten of these binary stars. "In ten per cent of the evolved binary stars with discs we studied, we see a large cavity (a void/opening, ed.) in the disc," says KU Leuven astronomer Jacques Kluska, first author of the article in the journal Astronomy & Astrophysics in which the discovery is described. "This is an indication that something is floating around there that has collected all matter in the area of the cavity."

Second-generation planets

The clean-up of the matter could be the work of a planet. That planet might not have formed at the very beginning of one of the binary stars' life, but at the very end. The astronomers moreover found further strong indications for the presence of such planets. "In the evolved binary stars with a large cavity in the disc, we saw that heavy elements such as iron were very scarce on the surface of the dying star," says Kluska. "This observation leads one to suspect that dust particles rich in these elements were trapped by a planet." By the way, the Leuven astronomer doesn't rule out the possibility that in this way, several planets can be formed around these binary stars.

The discovery was made when the astronomers were drawing up an inventory of evolved binary stars in our Milky Way. They did that based on existing, publicly available observations. Kluska and his colleagues counted 85 of such binary star pairs. In ten pairs, the researchers came across a disc with a large cavity on the infrared images.

Current theories put to the test

If new observations confirm the existence of planets around evolved binary stars, and if it turns out the planets were only formed after one of the stars had reached the end of its life, the theories on planet formation will need to be adjusted. "The confirmation or refutation of this extraordinary way of planet formation will be an unprecedented test for the current theories," according to Professor Hans Van Winckel, head of the KU Leuven Institute of Astronomy.

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ALMA animation of circling twin young stars

Researchers analyzed the accumulated data from the Atacama Large Millimeter/submillimeter Array (ALMA) and depicted the motion of a young twin star system XZ Tauri over three years. This first-ever "ALMA Animation" of twin stars sheds new light on the origins of the binary stars and the planets to be formed around them.

"This achievement was made possible by the high resolution and rich archival data of ALMA," says Takanori Ichikawa, the first author of the research paper and a former graduate student at Kagoshima University, Japan. "This research utilized three years of observation data. The results show the feasibility of a new research method using radio astronomical animations instead of conventional images. I hope that this method will help to clarify various astronomical phenomena in the future."

The Sun is a single star, but the Universe is full of binary stars, which are two stars orbiting each other. During its youth, each young star in a binary system is surrounded by a protoplanetary disk composed of molecular gas and dust. This disk is known to be the site of planet formation. Many planets associated with binary stars have actually been detected, but how the disks are formed in binary star systems and how planets are formed in those systems is still a mystery.

"In order to study the formation of planets in binary star systems, it is important to accurately determine the orbital motion of the two stars and the tilt of the individual protoplanetary disks," explains Shigehisa Takakuwa, a professor at Kagoshima University.

Researchers have suggested two formation mechanisms for binary systems; one is a breakup of a single large gaseous disk, and the other is fragmentation of the larger molecular cloud due to violent turbulence. In the former case, astronomers suppose that the orbit of the binary stars and the individual disks should be on the same plane. On the other hand, in the latter case, the orbital plane of the binary stars and the plane of the disks are expected to be different. This is a major issue that will affect the final orbits of the planets in binary systems.

The research team dug into the ALMA data archive and obtained the data for the young XZ Tau system taken in 2015, 2016, and 2017. They carefully analyzed the data and for the first time made an animation of the orbital motion of the binary stars, which shows that XZ Tau B moved 3.4 astronomical units (3.4 times the radius of Earth's orbit) around XZ Tau A during these three years.

The team figured out the three-dimensional structure of the orbit. In addition, analyzing the doppler effect and the distribution of the radio waves from the disk around each star in the XZ Tau system, they found that those disks are significantly misaligned with respect to each other, and also not in the same plane as the binary orbit.

Previous observations with ALMA had found examples of young binary stars with protoplanetary disks tilted with respect to each other (Note). However, this is the first time that the orbital motion of a binary system has been clarified, showing that the inclination is different from those of the circumstellar disks. These results support the idea that the XZ Tau system was formed via molecular cloud fragmentation.

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HR 8799 super-Jupiters’ days measured for the first time, gives a new spin on unraveling planet formation mystery

Astronomers have captured the first-ever spin measurements of HR 8799, the famed system that made history as the very first exoplanetary system to have its image taken.

Discovered in 2008 by two Maunakea Observatories in Hawaii -- W. M. Keck Observatory and the international Gemini Observatory, a Program of NSF's NOIRLab -- the HR 8799 star system is located 129 light-years away and has four planets more massive than Jupiter, or super-Jupiters: HR 8799 planets b, c, d, and e. None of their rotation periods had ever been measured, until now.

The breakthrough was made possible by a Caltech and Keck Observatory-led science and engineering team that has developed an instrument capable of observing known imaged exoplanets at spectral resolutions that are detailed enough to allow astronomers to decipher how fast the planets are spinning.

Using the state-of-the-art Keck Planet Imager and Characterizer (KPIC) on the Keck II telescope atop Hawaii Island's Maunakea, astronomers found that the minimum rotation speeds of HR 8799 planets d and e clocked in at 10.1 km/s and 15 km/s, respectively. This translates to a length of day that could be as short as three hours or could be up to 24 hours such as on Earth depending on the axial tilts of the HR 8799 planets, which are currently undetermined. For context, one day on Jupiter lasts nearly 10 hours; its rotation speed is about 12.7 km/s.

As for the other two planets, the team was able to constrain the spin of HR 8799 c to an upper limit of less than 14 km/s; planet b's rotation measurement was inconclusive.

The findings are KPIC's first science results, which have been accepted for publication in The Astronomical Journal.

"With KPIC, we were able to obtain the highest spectral resolution observations ever conducted of the HR 8799 exoplanets," says Jason Wang, an astronomer at Caltech and lead author of the study. "This allows us to study them with finer granularity than ever before and unlocks the key to gaining a deeper understanding of not just how these four planets formed, but how gas giants in general develop throughout the universe."

DIZZY DATA UNVEILS PLANETS' PAST

How fast a planet spins gives insight into its formation history. Created out of gas and dust kicked up by a newborn star, baby planets start spinning faster as they accumulate more material and grow -- a process called core accretion. It is believed that planetary magnetic fields then slow and cap their rotation speed. After the fully-formed planet is finished accreting and cools off, it spins back up.

"The spins of HR 8799 planets d and e are consistent with the theory that the planets' magnetic fields put a brake on their spins in their natal years," says Wang. "The spin measurements also hint at the notion that lower mass planets spin faster because they are less affected by magnetic braking, which might tell us something important about how they form. I find this tantalizing."

Wang stresses this possible trend is unconfirmed; to validate it requires more KPIC spin measurements of lower mass companions. The team's goal is to find a common link between the rotation periods of the HR 8799 planets, the giant planets in our own solar system, Jupiter and Saturn, and other known super-Jupiters and brown dwarfs.

"With enough spin measurements, we'll be able to identify trends that would reveal how the physical processes driving planet formation work," says co-author Jean-Baptiste Ruffio, a David and Ellen Lee Postdoctoral Scholar Research Associate in Astronomy at Caltech. "This is something that people have already started doing, but KPIC is allowing us to do this for the smallest, faintest, and closest imaged alien worlds."

KPIC'S FIRST LIGHT SUCCESS

Commissioned between 2018 to 2020, KPIC's specialty is detecting exoplanets and brown dwarfs that orbit so close to their host stars that the glare from the starlight makes it difficult to 'see' these celestial bodies from Earth. The instrument filters unwanted starlight by way of an innovative fiber injection unit that routes light from the Keck II telescope adaptive optics (AO) system into the Observatory's Near-Infrared Spectrograph (NIRSPEC).

KPIC's first light results are outlined in a technical paper that has been accepted in the Journal of Astronomical Telescopes, Instruments, and Systems (JATIS).

"KPIC is a game-changer in the field of exoplanet characterization," says KPIC Principal Investigator Dimitri Mawet, Professor of Astronomy at Caltech. "It allows us to measure a planet's length of day, orbit, and molecular makeup of its atmosphere."

KPIC made strong detections of water and carbon monoxide, but no methane, in three of the four HR 8799 planets -- c, d, and e -- which is consistent with what is known of the planets' atmospheres.

"It's exciting to see KPIC's superpower manifest," says Keck Observatory AO Scientist/Engineer Jacques Delorme, lead author of the JATIS paper. "Because this is the first technology of its kind, we didn't know if KPIC was going to work as well as it did. Now that we have successfully demonstrated its capabilities, we can move on to Phase 2 of the project to further improve the instrument's overall performance."

"We have yet to unlock KPIC's full science potential," says Caltech Lead Instrument Scientist Nemanja Jovanovic, co-author of the technical paper. "Through more instrument upgrades, we hope to observe exoplanets in the near future with such a high degree of detail, that we'll be able to study weather phenomena and map clouds of gas giant planets."

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Study of young chaotic star system reveals planet formation secrets

A team of scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) to study the young star Elias 2-27 have confirmed that gravitational instabilities play a key role in planet formation, and have for the first time directly measured the mass of protoplanetary disks using gas velocity data, potentially unlocking one of the mysteries of planet formation. The results of the research are published today in two papers in The Astrophysical Journal.

Protoplanetary disks -- planet-forming disks made of gas and dust that surround newly formed young stars -- are known to scientists as the birthplace of planets. The exact process of planet formation, however, has remained a mystery. The new research, led by Teresa Paneque-Carreño -- a recent graduate of the Universidad de Chile and PhD student at the University of Leiden and the European Southern Observatory, and the primary author on the first of the two papers -- focuses on unlocking the mystery of planet formation.

During observations, scientists confirmed that the Elias 2-27 star system -- a young star located less than 400 light-years away from Earth in the constellation Ophiuchus -- was exhibiting evidence of gravitational instabilities which occur when planet-forming disks carry a large fraction of the system's stellar mass. "How exactly planets form is one of the main questions in our field. However, there are some key mechanisms that we believe can accelerate the process of planet formation," said Paneque-Carreño. "We found direct evidence for gravitational instabilities in Elias 2-27, which is very exciting because this is the first time that we can show kinematic and multi-wavelength proof of a system being gravitationally unstable. Elias 2-27 is the first system that checks all of the boxes."

Elias 2-27's unique characteristics have made it popular with ALMA scientists for more than half a decade. In 2016, a team of scientists using ALMA discovered a pinwheel of dust swirling around the young star. The spirals were believed to be the result of density waves, commonly known to produce the recognizable arms of spiral galaxies -- like the Milky Way Galaxy -- but at the time, had never before been seen around individual stars.

"We discovered in 2016 that the Elias 2-27 disk had a different structure from other already studied systems, something not observed in a protoplanetary disk before: two large-scale spiral arms. Gravitational instabilities were a strong possibility, but the origin of these structures remained a mystery and we needed further observations," said Laura Pérez, Assistant Professor at the Universidad de Chile and the principal investigator on the 2016 study. Together with collaborators, she proposed further observations in multiple ALMA bands that were analyzed with Paneque-Carreño as a part of her M.Sc. thesis at Universidad de Chile.

In addition to confirming gravitational instabilities, scientists found perturbations -- or disturbances -- in the star system above and beyond theoretical expectations. "There may still be new material from the surrounding molecular cloud falling onto the disk, which makes everything more chaotic," said Paneque-Carreño, adding that this chaos has contributed to interesting phenomena that have never been observed before, and for which scientists have no clear explanation. "The Elias 2-27 star system is highly asymmetric in the gas structure. This was completely unexpected, and it is the first time we've observed such vertical asymmetry in a protoplanetary disk."

Cassandra Hall, Assistant Professor of Computational Astrophysics at the University of Georgia, and a co-author on the research, added that the confirmation of both vertical asymmetry and velocity perturbations -- the first large-scale perturbations linked to spiral structure in a protoplanetary disk -- could have significant implications for planet formation theory. "This could be a 'smoking gun' of gravitational instability, which may accelerate some of the earliest stages of planet formation. We first predicted this signature in 2020, and from a computational astrophysics point of view, it's exciting to be right."

Paneque-Carreño added that while the new research has confirmed some theories, it has also raised new questions. "While gravitational instabilities can now be confirmed to explain the spiral structures in the dust continuum surrounding the star, there is also an inner gap, or missing material in the disk, for which we do not have a clear explanation."

One of the barriers to understanding planet formation was the lack of direct measurement of the mass of planet-forming disks, a problem addressed in the new research. The high sensitivity of ALMA Band 6, paired with Bands 3 and 7, allowed the team to more closely study the dynamical processes, density, and even the mass of the disk. "Previous measurements of protoplanetary disk mass were indirect and based only on dust or rare isotopologues. With this new study, we are now sensitive to the entire mass of the disk," said Benedetta Veronesi -- a graduate student at the University of Milan and postdoctoral researcher at École normale supérieure de Lyon, and the lead author on the second paper. "This finding lays the foundation for the development of a method to measure disk mass that will allow us to break down one of the biggest and most pressing barriers in the field of planet formation. Knowing the amount of mass present in planet-forming disks allows us to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form."

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How planets form controls elements essential for life

The prospects for life on a given planet depend not only on where it forms but also how, according to Rice University scientists.

Planets like Earth that orbit within a solar system's Goldilocks zone, with conditions supporting liquid water and a rich atmosphere, are more likely to harbor life. As it turns out, how that planet came together also determines whether it captured and retained certain volatile elements and compounds, including nitrogen, carbon and water, that give rise to life.

In a study published in Nature Geoscience, Rice graduate student and lead author Damanveer Grewal and Professor Rajdeep Dasgupta show the competition between the time it takes for material to accrete into a protoplanet and the time the protoplanet takes to separate into its distinct layers -- a metallic core, a shell of silicate mantle and an atmospheric envelope in a process called planetary differentiation -- is critical in determining what volatile elements the rocky planet retains.

Using nitrogen as proxy for volatiles, the researchers showed most of the nitrogen escapes into the atmosphere of protoplanets during differentiation. This nitrogen is subsequently lost to space as the protoplanet either cools down or collides with other protoplanets or cosmic bodies during the next stage of its growth.

This process depletes nitrogen in the atmosphere and mantle of rocky planets, but if the metallic core retains enough, it could still be a significant source of nitrogen during the formation of Earth-like planets.

Dasgupta's high-pressure lab at Rice captured protoplanetary differentiation in action to show the affinity of nitrogen toward metallic cores.

"We simulated high pressure-temperature conditions by subjecting a mixture of nitrogen-bearing metal and silicate powders to nearly 30,000 times the atmospheric pressure and heating them beyond their melting points," Grewal said. "Small metallic blobs embedded in the silicate glasses of the recovered samples were the respective analogs of protoplanetary cores and mantles."

Using this experimental data, the researchers modeled the thermodynamic relationships to show how nitrogen distributes between the atmosphere, molten silicate and core.

"We realized that fractionation of nitrogen between all these reservoirs is very sensitive to the size of the body," Grewal said. "Using this idea, we could calculate how nitrogen would have separated between different reservoirs of protoplanetary bodies through time to finally build a habitable planet like Earth."

Their theory suggests that feedstock materials for Earth grew quickly to around moon- and Mars-sized planetary embryos before they completed the process of differentiating into the familiar metal-silicate-gas vapor arrangement.

In general, they estimate the embryos formed within 1-2 million years of the beginning of the solar system, far sooner than the time it took for them to completely differentiate. If the rate of differentiation was faster than the rate of accretion for these embryos, the rocky planets forming from them could not have accreted enough nitrogen, and likely other volatiles, critical to developing conditions that support life.

"Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation sets a unique pathway to satisfy Earth's nitrogen budget," said Dasgupta, the principal investigator of CLEVER Planets, a NASA-funded collaborative project exploring how life-essential elements might have come together on rocky planets in our solar system or on distant, rocky exoplanets.

"This work shows there's much greater affinity of nitrogen toward core-forming metallic liquid than previously thought," he said.

The study follows earlier works, one showing how the impact by a moon-forming body could have given Earth much of its volatile content, and another suggesting that the planet gained more of its nitrogen from local sources in the solar system than once believed.

In the latter study, Grewal said, "We showed that protoplanets growing in both inner and outer regions of the solar system accreted nitrogen, and Earth sourced its nitrogen by accreting protoplanets from both of these regions. However, it was unknown as to how the nitrogen budget of Earth was established."

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