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Super-puffs are gas giant sized planets with masses only a few earth masses.
What causes planets to form with such low density and why are they so rare?
‘Super-puff’ exoplanets are like cosmic cotton candy
Artist’s concept of the three “super-puff” giant planets orbiting the star Kepler-51. Image via NASA/ ESA/ L. Hustak/ J. Olmsted/ D. Player/ F. Summers (STScI)/ Hubblesite.
Exoplanets – planets orbiting distant stars – come in a wide variety of sizes, masses and compositions. Most found so far bear at least some resemblance to planets in our own solar system, either large and gaseous or smaller and rocky. Variations, so far anyway, have been more or less based on those two general themes. But another more enigmatic and exotic type of planet has been discovered, which looks like nothing in our solar system. Astronomers call them “super-puff” planets or cotton candy planets. They’re more lightweight (less dense) than any planets seen before.
A few of them have been known about since 2014, but researchers have now been able to take a closer look at three of these planets, orbiting the sun-like star Kepler-51, which is estimated to be 2,600 light-years away. The intriguing findings were made and announced by scientists working with the Hubble Space Telescope. The new peer-reviewed paper will be published soon in The Astronomical Journal.
These bizarre worlds are nearly the size of Jupiter, yet they are only 1/100th as massive.
Artist’s impression of the 3 known Kepler-51 planets in contrast to some of the planets in our own solar system. Image via NASA/ ESA/ STScI/ CU Boulder Today.
Simply put, these planets are weird. As astronomer Jessica Libby-Roberts at the University of Colorado Boulder – who led the new study – stated:
Astronomer Zachory Berta-Thompson, also at UC Boulder and coauthor of the study, said:
This is an extreme example of what’s so cool about exoplanets in general. They give us an opportunity to study worlds that are very different than ours, but they also place the planets in our own solar system into a larger context.
The three planets orbiting Kepler-51 were first discovered in 2014, and fewer than 15 super-puff or cotton candy planets have been found in total so far. According to Berta-Thompson:
Their discovery was straight-up contrary to what we teach in undergraduate classrooms.
Cotton candy in space … do these planets actually look like this? No, they’re round like other planets. But their extremely lightweight composition is analogous to the lightness and fluffiness of the popular carnival treat of spun sugar. Image via CU Boulder Today.
The super-puff or cotton candy planets were found to have much lower densities than anything in our solar system, less than 0.1 grams per cubic centimeter of volume – almost identical to cotton candy, hence the nickname. Even lightweight Saturn, which could theoretically float in water, is still a lot denser than these worlds. As Libby-Roberts said:
We knew they were low density. But when you picture a Jupiter-sized ball of cotton candy – that’s really low density.
The researchers wanted to study the planets’ atmospheres. But instead of having transparent atmospheres, the planets were found to be enshrouded by opaque haze or clouds. Libby-Roberts said:
It definitely sent us scrambling to come up with what could be going on here. We expected to find water, but we couldn’t observe the signatures of any molecule.
The gas giant planet Saturn as seen at equinox by the Cassini spacecraft. Its density is so low that it could, theoretically, float in water. But the super-puff planets have even lower densities than that. Image via NASA/ JPL/ Space Science Institute/ The Atlantic.
The planets seem to be composed mostly of the lightweight gases hydrogen and helium, covered by a thick layer of methane haze. This would make their atmospheres resemble that of Saturn’s moon Titan, which is also enshrouded by a thick methane haze. Libby-Roberts added:
If you hit methane with ultraviolet light, it will form a haze. It’s Titan in a nutshell.
But the researchers noticed something else about these planets, too. They are rapidly – cosmologically speaking, anyway – losing material to space. This means that they may not retain their cotton candy-like characteristics for any longer than the next billion years or so, as they continue to shrink in size. The innermost of the planets is losing billions of tons of material into space every second. Eventually, these worlds may look more like mini-Neptunes, so they may simply be in a transitory phase of planetary evolution. Libby-Roberts said:
People have been really struggling to find out why this system looks so different than every other system. We’re trying to show that, actually, it does look like some of these other systems.
A good bit of their weirdness is coming from the fact that we’re seeing them at a time in their development where we’ve rarely gotten the chance to observe planets. This system offers a unique laboratory for testing theories of early planet evolution.
Jessica Libby-Roberts at the University of Colorado Boulder led the new study. Image via University of Colorado Boulder.
Scientists still want to determine these planets’ actual atmospheric compositions. NASA’s upcoming James Webb Space Telescope may be able to do that, since its sensitivity to longer wavelengths of light means it could possibly peer through the deep cloud layers. Until then, these planets are a tantalizing mystery, providing new clues as to how planets evolve, even the similar – but not quite as lightweight – giant planets in our own solar system.
Bottom line: New observations of “super-puff” exoplanets by the Hubble Space Telescope have provided scientists with more clues about these enigmatic and strange worlds.
“Super puff” exoplanet is as big as Jupiter but 10 times lighter, confusing astronomers
About 212 light years from Earth, a gas giant light enough to be nicknamed a “super-puff” or “cotton candy” planet is circling extremely close to its host star. The exoplanet is so light, it’s left astronomers questioning everything we previously knew about how gas giants form.
This super-puff exoplanet, known as WASP-107b, is about the same size as Jupiter, but only about one-tenth the mass — or about 30 times more massive than Earth. According to a new study published Monday in The Astronomical Journal, its core mass is significantly smaller than astronomers thought necessary for the creation of a gas giant planet like Jupiter and Saturn.
The discovery, made by Ph.D. student Caroline Piaulet under the supervision of professor Björn Benneke at the University of Montreal, indicates that gas giants form much more easily than previously believed.
“This study pushes the boundaries of our theoretical understanding of how giant-sized planets form. WASP-107b is one of the puffiest planets out there, and we need a creative solution to explain how these tiny cores can build such massive gas envelopes,” coauthor Eve Lee said in a statement.
WASP-107b isn’t a brand new discovery — astronomers first detected it in the Virgo constellation in 2017. The planet is very close to its star, over 16 times closer than Earth is to the sun, completing one orbit every 5.7 days.
WASP-107b is one of the least dense exoplanets scientists have ever found. They have nicknamed similar types of planets — gas giants with the density of cotton candy — super-puffs.
To find the planet’s surprising mass, astronomers studied observations obtained at the Keck Observatory in Hawaii. They used a technique called the radial velocity method, which studies the wobbling motion of a planet’s star caused by a planet’s gravitational pull, in order to calculate the mass.
Scientists were shocked to conclude that the solid core of WASP-107b has a mass that is no more than four times that of the Earth, meaning more than 85% of its mass stems from the thick gaseous layer surrounding the core. This is a dramatically different breakdown from Neptune, which has a similar mass but holds just 5% to 15% of it within its gas layer.
Based on their knowledge of Jupiter and Saturn, scientists previously believed that a solid core at least 10 times the mass of Earth would be needed to acquire enough gas for a gas giant planet to form. WASP-107b challenges that theory.
“This work addresses the very foundations of how giant planets can form and grow,” Benneke said. “It provides concrete proof that massive accretion of a gas envelope can be triggered for cores that are much less massive than previously thought.”
Lee posits that, “The most plausible scenario is that the planet formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very quickly. The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”
While studying the planet, the team stumbled upon another in the same system, WASP-107c. It has a mass that is about one-third that of Jupiter and takes three years to orbit its host star once.
The planet’s oval-shaped orbit suggests that the astronomers’ new hypothesis is on the right track.
“WASP-107c has, in some respects, kept the memory of what happened in its system,” said Piaulet. “Its great eccentricity hints at a rather chaotic past, with interactions between the planets which could have led to significant displacements, like the one suspected for WASP-107b.”
The team hopes to continue studying the strange planet with the launch of the James Webb Space Telescope this year.
banner image: Artistic rendition of the exoplanet WASP-107b and its star, WASP-107. Some of the star’s light streams through the exoplanet’s extended gas layer. ESA/HUBBLE, NASA, M. KORNMESSER
‘Super-puff’ exoplanets put a ring on it
The apparent “puffiness” of some exoplanets could be due to Saturn-like rings, rather than envelopes of gas as was previously thought. That’s the view of astronomers Anthony Piro at the Carnegie Institution for Science and Shreyas Vissapragada at the California Institute of Technology, US, who came to this conclusion after simulating the transits of several “super-puff” exoplanets. Their analysis exposes two such exoplanets as likely candidates for having rings – a finding that could be confirmed after the upcoming launch of the James Webb Space Telescope (JWST).
As the list of known exoplanets expands, astronomers are identifying a growing number of bodies that appear to have remarkably large radii, given their relatively low masses. Nicknamed “super-puffs”, these seemingly ultra-low-density planets are typically anomalously cool, and are found in star systems with widely varying ages – meaning that most of them probably aren’t just young planets that haven’t yet fully formed.
To explain these enigmatic objects, some astronomers have proposed that they are surrounded by thick envelopes of gas. If this were the case, these envelopes could be expected to leave diverse absorption dips in the spectra of starlight passing through them. However, the super-puff spectra observed so far have been frustratingly featureless.
Not so puffy
Piro and Vissaptragada propose a different explanation. In their view, super-puffs aren’t actually puffy, but are instead surrounded by rings. These rings dim the light of the planets’ host stars as they pass between the star and observers on Earth, creating the illusion of exoplanets with far larger radii. They tested this theory by simulating observations of Saturn transiting the Sun, from the perspective of a distant star system. This revealed that Saturn would appear to be half as dense as it actually is if its rings weren’t accounted for.
The duo also simulated the transits of a variety of known super-puffs, aiming to determine whether the transit observations could have been distorted by rings. They found that their hypothesis was consistent with the transits of some exoplanets, but not all of them: given their proximity to their host stars, many of the bodies would need to have heavier, rocky rings instead of ice, which would limit the rings’ radii. In addition, the planets would need to spin fast enough to prevent warping in their rings – but this is often hindered by tidal locking with host stars.
These effects didn’t rule out every planet the duo considered. Of the exoplanets they analysed, Piro and Vissaptragada concluded that Kepler 87c and 177c have the best chance of appearing puffy due to rings. Confirming this will require more accurate photometric techniques than are currently available, but these improved measurements should be within reach of the long-awaited JWST, which is now scheduled for launch in March 2021. If such predictions are confirmed, they could greatly improve astronomers’ understanding of how planetary systems form and evolve.
Evidence suggests some super-puffs might be ringed exoplanets
Transit of a ringed planet and the resulting light curves. The upper panel shows an example of the images considered for the transit calculations with the planet at seven different positions. The middle panel shows the resulting transit (normalized to the stellar flux) with the points corresponding to each of the planet positions shown in the upper panel. The bottom shows the difference between a ringed transit and a bare star with the same surface area. Credit: arXiv:1911.09673
A pair of researchers from the Observatories of the Carnegie Institution for Science and the California Institute of Technology have reported evidence that some super-puff exoplanets might be ringed exoplanets. Anthony Piro and Shreyas Vissapragada have written a paper describing their theory and the evidence supporting it and have posted it on the arXiv preprint server.
As Piro and Vissapragada note, space scientists have found a host of exoplanets in recent years with a wide range of attributes. One such group is called "super-puffs," because they have very large radii for their masses, and thus very low densities. Such exoplanets seem to have exceptionally extended atmospheres without explanation. What scientists find odd about them is that that they do not seem to be susceptible to photoevaporation. Piro and Vissapragada suggest that this is because they do not have extended atmospheres at all instead, they have rings like Saturn.
The researchers report that they came to their theory when considering what Saturn would look like to space scientists from a distant world if they did not know about the rings—they found it likely that such aliens would likely see the planet as having just half of its true density. That got them to wondering if some discovered exoplanets, such as super-puffs, might have miscalculated densities due to orbital rings.
The researchers started by considering possible attributes of an exoplanet with rings. They noted that exoplanets close to their star would have rocky rings, because ice would melt. Also, some of the exoplanets would not be able to form rings wide enough to change density estimations due to distant rocks likely clumping, forming moons. Next, they used their assumptions to create simulations that showed what ringed planets might look like to us as they pass across their star. They report that their simulations showed that some of the super-puffs could be ringed exoplanets. They suggest that new data from the James Webb Space Telescope, set for launch in 2012, might confirm their theory.
“Super-Puff” Exoplanets Aren’t Like Anything We’ve Got in the Solar System
The study of extrasolar planets has really exploded in recent years. Currently, astronomers have been able to confirm the existence of 4,104 planets beyond our Solar System, with another 4900 awaiting confirmation. The study of these many planets has revealed things about the range of possible planets in our Universe and taught us that there are many for which there are no analogs in our Solar System.
For example, thanks to new data obtained by the Hubble Space Telescope, astronomers have learned more about a new class of exoplanet known as “super-puff” planets. Planets in this class are essentially young gas giants that are comparable in size to Jupiter but have masses that are just a few times greater than that of Earth. This results in their atmospheres having the density of cotton candy, hence the delightful nickname!
The only known examples of this planet reside in the Kepler 51 system, a young Sun-like star located about 2,615 light-years away in the Cygnus constellation. Within this system, three exoplanets have been confirmed (Kepler-51 b, c, and d) that were first detected by the Kepler Space Telescope in 2012. However, it was not until 2014 that the densities of these planets was confirmed, and it came as quite the surprise.The three giant planets orbiting the Sun-like star Kepler 51 compared to some of the planets in our solar system. Credits: NASA, ESA, and L. Hustak and J. Olmsted (STScI)
While these gas giants have atmospheres that are composed of hydrogen and helium and are about the same size as Jupiter, they are also about a hundred times lighter in terms of mass. How and why their atmospheres would balloon the way they do remains a mystery, but the fact remains that the nature of their atmospheres makes super-puff planets a prime candidate for atmospheric analysis.
That is precisely what an international team of astronomers – led by Jessica Libby-Roberts from the Center for Astrophysics and Space Astronomy (CASA) at the University of Colorado, Boulder – sought to do. Using data from Hubble, Libby-Roberts and her team analyzed spectra obtained from the atmospheres of Kepler-51 b and d to see what components (including water) were there.
As these planets passed in front of their star, light absorbed by their atmospheres was examined in the infrared wavelength. To the team’s surprise, they found that the spectra of both planets did not have any telltale chemical signatures. This they attributed to the presence of clouds of salt crystals or photochemical hazes in their atmospheres.
As such, the team relied on computer simulations and other tools to theorize that the Kepler-51 planets are mostly hydrogen and helium by mass, which is covered up by a thick haze made up of methane. This is similar to what goes on in Titan’s atmosphere (Saturn’s largest moon), where the predominantly nitrogen-atmosphere contains clouds of methane gas that obscure the surface.
Artist’s illustration of the newfound gas-giant planet GJ 3512b, which circles a red dwarf star. Credit: Guillem Anglada-Escude—IEEC/Science-wave, using SpaceEngine.org (CC BY 4.0)
“This was completely unexpected,” said Libby-Roberts. “We had planned on observing large water absorption features, but they just weren’t there. We were clouded out!” However, these clouds provided the team with valuable insight into how Kepler-51 b and d compare to other low-mass, gas-rich exoplanets observed by astronomers. As Libby-Roberts explained in a CU Boulder press statement:
“We knew they were low density. But when you picture a Jupiter-sized ball of cotton candy – that’s really low density… It definitely sent us scrambling to come up with what could be going on here. We expected to find water, but we couldn’t observe the signatures of any molecule.”
The team was also able to better constrain the size and mass of these planets by measuring their timing effects. In all systems, slight changes occur in a planet’s orbital period due to their gravitation pull, which can be used to derive a planet’s mass. The team’s results agreed with previous estimates for Kepler-51 b while the estimates for Kepler-51 d indicated that it is slightly less massive (aka. puffier) than previously thought.
The team also compared the spectra of the two super-puffs to those of other planets and obtained results that indicated that cloud/haze formation is linked to the temperature of a planet. This supports the hypothesis that the cooler a planet is, the cloudier it will be, which is something astronomers have been pondering thanks to the recent spate of exoplanet discoveries.
Mini Neptunian planets range in size from about 1.5 to 4 times the size of Earth and have a rocky core and puffy gaseous shell of varying thickness. Credit: Geoff Marcy
Last, but not least, the team observed that both Kepler-51 b and d appear to be losing gas rapidly. In fact, the team estimates that the former planet (which is the closest to its parent star) is dumping tens of billions of tons of material into space every second. If this trend continues, the planets will shrink considerably over the next few billion years and could become mini-Neptunes.
In this respect, this would suggest that the exoplanets are not so uncommon after all, giving that mini-Neptunes appear to be very common. It also suggests that the low densities of the super-puff planets are attributed to the age of the system. Whereas the Solar System is roughly 4.6 billion years old, Kepler-51 has been around for a mere 500 million years.
The planetary models used by the team indicate that the planets likely formed beyond Kepler-51s Frost Line – the boundary beyond which volatile elements will freeze – and then migrated inward. Rather than being oddball planets, then, Kepler-51 b and d may be the first examples astronomers have seen of one of the most common types of planets in our Universe in the early stages of development.
As Zach Berta-Thompson (an assistant APS professor and a co-author of the new research) explained, this makes Kepler-51 a “unique laboratory” for testing theories of early planet evolution:
“This is an extreme example of what’s so cool about exoplanets in general. They give us an opportunity to study worlds that are very different than ours, but they also place the planets in our own solar system into a larger context.”
Illustration of NASA’s James Webb Space Telescope. Credits: NASA
In the future, the deployment of next-generation instruments like the James Webb Space Telescope (JWST) will help astronomers examine the atmosphere of the Kepler-51 planets and other super-puffs. Thanks to the JWST’s sensitivity to longer infrared wavelengths, we may be able to look through their dense clouds yet and determine what these “cotton-candy” planets are actually composed of.
It is also another feather in the cap of the venerated Hubble, which has been in continuous operation for about thirty years now (since May of 1990) and continues to shed light on cosmic mysteries! It is only fitting that it is still making finds that will very-shortly be the subject of follow-up investigations by James Webb, its spiritual successor.
The study that details the team’s research recently appeared online and will appear in The Astrophysical Journal.
The mysterious formation of WASP-107b
Using the Keck data, the team did an analysis to identify what the internal structure of the planet might be. They came to the astonishing conclusion that there must be a solid core that is no more than four times the mass of Earth. This means that over 80% of its mass comes from its dense layer of gas. To put this in perspective, Neptune, a solar system analog when it comes to mass, only derives 10% from its mass from its surrounding gas layer!
W. M. Keck Observatory in Hawaii - Image Credit: Warren Metcalf / HDR tune by Universal-Sci
The fact that WASP-107b is so incredibly light naturally gives rise to additional questions. How is it possible that an exoplanet of such low formed at all? And why it is not losing its enormous layer of gas given the proximity to its host star? Contemporary models for gas-giant formation are founded on the familiar gas-giants found in our own solar system like Saturn. The hypothesis is that a core of at least ten times the mass of Earth is necessary to collect enough gas from the early planet-forming disc surrounding a star.
Professor Eve Lee, a renowned authority on super-puff exoplanets, stated in an interview that there are several hypotheses, the most plausible of which is that the exoplanet began its life further away from its host star. According to Lee, the gas in a planet-forming disc is cold enough that gas buildup can happen a lot faster at more considerable distances. WASP-107b would have migrated towards the center of its star system at a later moment, probably following interactions with other planets or with the planet-forming disc itself.
'Super-puff' exoplanets with the density of cotton candy discovered
Astronomers have discovered a number of “super-puff” exoplanets in the Kepler 51 star system that are as dense as cotton candy.
Utilizing data from the Hubble Space Telescope, researchers found fewer than 15 of the planets that are almost as big as Jupiter, but have extremely low density, at less than 100 times the gas giant's mass or less than 0.1 grams per cubic centimeter of volume.
“They’re very bizarre,” said the study's lead author, Jessica Libby-Roberts, in a statement.
An artist's depiction of the Kepler 51 star system. (Credit: NASA/ESA/STScI)
“This is an extreme example of what’s so cool about exoplanets in general,” said Zachory Berta-Thompson, one of the study's co-authors, said. “They give us an opportunity to study worlds that are very different than ours, but they also place the planets in our own solar system into a larger context.”
The three "super-puff" exoplanets in the Kepler 51 system were “straight-up contrary to what we teach in undergraduate classrooms,” Berta-Thompson added.
The Kepler 51 system is approximately 2,400 light-years from Earth and is approximately 500 million years old. A light-year, which measures distance in space, equals 6 trillion miles.
Using the Hubble, the researchers also attempted to look at the planets' atmospheres, but ran into issues, as the atmospheres were opaque, rather than transparent.
“It definitely sent us scrambling to come up with what could be going on here,” Libby-Roberts continued. “We expected to find water, but we couldn’t observe the signatures of any molecule.”
Kepler 51's three planets compared to the size of planets from our solar system. (Credit: NASA/ESA/STScI)
Libby-Roberts and the other researchers theorized that the exoplanets are likely mostly comprised of hydrogen and helium, using computer simulations. It's also probable that it is covered by a "thick haze made up of methane," which makes them reminiscent of Saturn's moon, Titan.
“If you hit methane with ultraviolet light, it will form a haze,” Libby-Roberts said. “It’s Titan in a nutshell.” In June, NASA unveiled a mission that will explore Titan, which could potentially host extraterrestrial life.
The researchers also discovered that the exoplanets are losing gas rapidly, with the innermost of the three exoplanets putting an estimated "tens of billions of tons of material into space every second." Should that trend continue, these planets could shrink considerably over the next billion years and might wind up looking similar to "mini-Neptune" exoplanets.
“People have been really struggling to find out why this system looks so different than every other system,” Libby-Roberts said. “We’re trying to show that, actually, it does look like some of these other systems.”
“A good bit of their weirdness is coming from the fact that we’re seeing them at a time in their development where we’ve rarely gotten the chance to observe planets," Berta-Thompson concurred.
The research, which is set to be published in The Astronomical Journal, can be read here.
A 'super-puff' planet like no other
The core mass of the giant exoplanet WASP-107b is much lower than what was thought necessary to build up the immense gas envelope surrounding giant planets like Jupiter and Saturn, astronomers at Université de Montréal have found.
This intriguing discovery by Ph.D. student Caroline Piaulet of UdeM's Institute for Research on Exoplanets (iREx) suggests that gas-giant planets form a lot more easily than previously believed.
Piaulet is part of the groundbreaking research team of UdeM astrophysics professor Björn Benneke that in 2019 announced the first detection of water on an exoplanet located in its star's habitable zone.
Published today in the Astronomical Journal with colleagues in Canada, the U.S., Germany and Japan, the new analysis of WASP-107b's internal structure "has big implications," said Benneke.
"This work addresses the very foundations of how giant planets can form and grow," he said. "It provides concrete proof that massive accretion of a gas envelope can be triggered for cores that are much less massive than previously thought."
As big as Jupiter but 10 times lighter
WASP-107b was first detected in 2017 around WASP-107, a star about 212 light years from Earth in the Virgo constellation. The planet is very close to its star -- over 16 times closer than the Earth is to the Sun. As big as Jupiter but 10 times lighter, WASP-107b is one of the least dense exoplanets known: a type that astrophysicists have dubbed "super-puff" or "cotton-candy" planets.
Piaulet and her team first used observations of WASP-107b obtained at the Keck Observatory in Hawai'i to assess its mass more accurately. They used the radial velocity method, which allows scientists to determine a planet's mass by observing the wobbling motion of its host star due to the planet's gravitational pull. They concluded that the mass of WASP-107b is about one tenth that of Jupiter, or about 30 times that of Earth.
The team then did an analysis to determine the planet's most likely internal structure. They came to a surprising conclusion: with such a low density, the planet must have a solid core of no more than four times the mass of the Earth. This means that more than 85 percent of its mass is included in the thick layer of gas that surrounds this core. By comparison, Neptune, which has a similar mass to WASP-107b, only has 5 to 15 percent of its total mass in its gas layer.
"We had a lot of questions about WASP-107b," said Piaulet. "How could a planet of such low density form? And how did it keep its huge layer of gas from escaping, especially given the planet's close proximity to its star?
"This motivated us to do a thorough analysis to determine its formation history."
A gas giant in the making
Planets form in the disc of dust and gas that surrounds a young star called a protoplanetary disc. Classical models of gas-giant planet formation are based on Jupiter and Saturn. In these, a solid core at least 10 times more massive than the Earth is needed to accumulate a large amount of gas before the disc dissipates.
Without a massive core, gas-giant planets were not thought able to cross the critical threshold necessary to build up and retain their large gas envelopes.
How then do explain the existence of WASP-107b, which has a much less massive core? McGill University professor and iREx member Eve Lee, a world-renowned expert on super-puff planets like WASP-107b, has several hypotheses.
"For WASP-107b, the most plausible scenario is that the planet formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very quickly," she said. "The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system."
Discovery of a second planet, WASP-107c
The Keck observations of the WASP-107 system cover a much longer period of time than previous studies have, allowing the UdeM-led research team to make an additional discovery: the existence of a second planet, WASP-107c, with a mass of about one-third that of Jupiter, considerably more than WASP-107b's.
WASP-107c is also much farther from the central star it takes three years to complete one orbit around it, compared to only 5.7 days for WASP-107b. Also interesting: the eccentricity of this second planet is high, meaning its trajectory around its star is more oval than circular.
"WASP-107c has in some respects kept the memory of what happened in its system," said Piaulet. "Its great eccentricity hints at a rather chaotic past, with interactions between the planets which could have led to significant displacements, like the one suspected for WASP-107b."
Beyond its formation history, there are still many mysteries surrounding WASP-107b. Studies of the planet's atmosphere with the Hubble Space Telescope published in 2018 revealed one surprise: it contains very little methane.
"That's strange, because for this type of planet, methane should be abundant," said Piaulet. "We're now reanalysing Hubble's observations with the new mass of the planet to see how it will affect the results, and to examine what mechanisms might explain the destruction of methane."
The young researcher plans to continue studying WASP-107b, hopefully with the James Webb Space Telescope set to launch in 2021, which will provide a much more precise idea of the composition of the planet's atmosphere.
"Exoplanets like WASP-107b that have no analogue in our Solar System allow us to better understand the mechanisms of planet formation in general and the resulting variety of exoplanets," she said. "It motivates us to study them in great detail."
"WASP-107b's density is even lower: a case study for the physics of gas envelope accretion and orbital migration," by Caroline Piaulet et al., was posted today in the Astronomical Journal. DOI: 10.3847/1538-3881/abcd3c. In addition to Piaulet (iREx Ph.D. student, Université de Montréal) and professors Björn Benneke (iREx, Université de Montréal) and Eve Lee (iREx, McGill Space Institute, McGill University), the research team includes Daniel Thorngren (iREx Postdoctoral Fellow, Université de Montréal) and Merrin Peterson (iREx M.Sc student), and 19 other co-authors from Canada, the United States, Germany and Japan.
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Astronomers Probe Mysteries of ‘Super Puff’ Planet
(CN) — Scientists exploring space revealed Monday that gas-giant planets such as Jupiter and Saturn form more easily than previously thought.
Researchers at the Université de Montréal found that the core mass of WASP-107b, a giant exoplanet, is much lower than scientists thought necessary to create the immense gas envelope surrounding it and other super-sized planets.
Discovered by Ph.D. student Caroline Piaulet of UdeM’s Institute for Research on Exoplanets, the finding, published in the Astronomical Journal, is based on new analysis of WASP-107b’s internal structure.
Piaulet is part of the research team led by UdeM astrophysics professor Björn Benneke that detected water on an exoplanet located in its star’s habitable zone in 2019.
“This work addresses the very foundations of how giant planets can form and grow,” Benneke said. “It provides concrete proof that massive accretion of a gas envelope can be triggered for cores that are much less massive than previously thought.”
Known as a “super-puff” planet to astrophysicists, WASP-107b is an exoplanet that was first detected in 2017 around WASP-107, a star more than 200 light years from Earth. The planet, which is as big as Jupiter but 10 times lighter, exists near its star at a distance that is 16 times closer than the Earth is to the Sun.
Conducting their work at the Keck Observatory in Hawaii, Piaulet and her team first assessed the mass of WASP-107b using the radial velocity method, which determines a planet’s mass by measuring the wobble of its host star due to the planet’s gravitational pull. Researchers concluded that the mass of WASP-107b is about one-tenth that of Jupiter, or about 30 times that of Earth.
Next, Piaulet and her colleagues determined that WASP-107b’s internal structure must have a solid core of no more than four times the mass of Earth, making about 85% of the planet’s mass included in the thick layer of gas that surrounds this core. By comparison, Neptune has a similar mass to WASP-107b but no more than 15% of its total mass resides in its gas layer.
“We had a lot of questions about WASP-107b,” Piaulet said. “How could a planet of such low density form? And how did it keep its huge layer of gas from escaping, especially given the planet’s close proximity to its star?”
Such questions motivated researchers to analyze WASP-107b’s formation history.
Planets form in the layer of dust and gas surrounding a young star. Historical models of gas-giant planet formation are based on Jupiter and Saturn, where a solid core massive enough to dwarf Earth is needed to build up enough gas before the disc dissipates.
Without a massive core, scientists assumed that gas-giant planets could not create and retain their large gas envelopes. Until Piaulet’s discovery about WASP-107b, that is.
“For WASP-107b, the most plausible scenario is that the planet formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very quickly,” said fellow researcher Eve Lee of McGill University. “The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”
The research team also included Daniel Thorngren and Merrin Peterson, as well as 19 other co-authors from Canada, the United States, Germany and Japan.
Mysteries still surround WASP-107b, inspiring Piaulet to continue studying the planet. Future analysis could involve the soon-to-launch James Webb Space Telescope, providing a more precise idea of the planet’s atmosphere.
“Exoplanets like WASP-107b that have no analogue in our solar system allow us to better understand the mechanisms of planet formation in general and the resulting variety of exoplanets,” Piaulet said. “It motivates us to study them in great detail.”
During the research, Piaulet and her colleagues made a second discovery: a second planet named WASP-107c with a mass considerably denser than its sister planet. But this planet operates differently, existing much farther from the central star and following a trajectory more ovular than circular.
“WASP-107c has in some respects kept the memory of what happened in its system,” Piaulet said. “Its great eccentricity hints at a rather chaotic past, with interactions between the planets which could have led to significant displacements, like the one suspected for WASP-107b.”