Planets Still Forming Detected in a Protoplanetary Disk

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An artist rendering of infant star HD 163296 with three protoplanets forming in its disk  The planets were discovered using a new mode of detection — identifying unusual patterns in the flow of gas within a protoplanetary disk. (NRAO/AUI/NSF; S. Dagnello)

Just as the number of planets discovered outside our solar system is large and growing — more than 3,700 confirmed at last count — so too is the number of ingenious ways to find exoplanets ever on the rise.

The first exoplanets were found by measuring the “wobble” in their host stars caused by the gravitational pull of the planets, then came the transit technique that measured dips in the light from stars as planets passed in front of them, followed by the direct imaging of moving objects deemed to be planets, and numerous more.

A new technique can now be added to the toolkit, one that is useful only in specific galactic circumstances but is nonetheless ingenious and intriguing.

By detecting unusual patterns in the flow of gas within the protoplanetary disk of a young star, two teams of astronomers have confirmed the distinct, telltale hallmarks of newly formed planets orbiting the infant star.

In other words, the astronomers found planets in the process of being formed, circling a star very early in its life cycle.

These results came thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), and are presented in a pair of papers appearing in the Astrophysical Journal Letters.

Richard Teague, an astronomer at the University of Michigan and principal author on one of the papers, said that his team looked at “the localized, small-scale motion of gas in a star’s protoplanetary disk. This entirely new approach could uncover some of the youngest planets in our galaxy, all thanks to the high-resolution images coming from ALMA.”

ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. ( ALMA: ESO/NAOJ/NRAO; A. Isella; B. Saxton NRAO/AUI/NSF.

To make their respective discoveries, each team analyzed the data from various ALMA observations of the young star HD 163296, which is about 4 million years old and located about 330 light-years from Earth in the direction of the constellation Sagittarius.

Rather than focusing on the dust within the disk, which was clearly imaged in an earlier ALMA observation, the astronomers instead studied the distribution and motion of carbon monoxide (CO) gas throughout the disk.

As explained in a release from the National Radio Astronomy Observatory, which manages the American operations of the multi-national ALMA, molecules of carbon monoxide naturally emit a very distinctive millimeter-wavelength light that ALMA can observe. Subtle changes in the wavelength of this light due to the Doppler effect provide a glimpse into the motion of the gas in the disk.

If there were no planets, gas would move around a star in a very simple, predictable pattern known as Keplerian rotation.

“It would take a relatively massive object, like a planet, to create localized disturbances in this otherwise orderly motion,” said Christophe Pinte of Monash University in Australia and lead author on the other of the two papers. 

And that’s what both teams found.

ALMA is a radio astronomy array located in Chile and set 16,000 feet above sea level. It’s a partnership between the European Southern Observatory (ESO), the National Science Foundation (NSF) of the United States and the National Institutes of Natural Sciences (NINS) of Japan in collaboration with the Republic of Chile. ALMA, which began operations in 2013, is used to observe light from space in comparatively long radio wavelengths. ((ESO/José Francisco Salgado )

Detecting planets within a protoplanetary disk — or finding theorized planets within those disks — is a big deal. 

That’s because information about the characteristics of very young planets orbiting young stars can potentially add substantially to one of the long-debated questions of planetary science:  How exactly did those billions upon billions of planets out there form?

The leading theory of planet formation, the “core accretion model,” has planets forming slowly — with dust, small objects and then planetesimals smashing into a rocky core and leaving matter behind.  In this model, the planet building takes place in a region close to the protoplanet’s stars.

Another theory looks to gravitational instabilities in the disk, arguing that giant planets can form quickly and far from their host stars.

The distribution of current solar system planets and beyond can give some clues based on the size, type and distribution of those planets.  But planets migrate and evolve, and they have never been studied before they had a chance to do much of either.

The techniques currently used for finding exoplanets in fully formed planetary systems — such as measuring the wobble of a star or how a transiting planet dims starlight — don’t lend themselves to detecting protoplanets.

With this new method for looking into those early protoplanetary disks, the hunt for infant planets becomes possible.  And the results in terms of understanding planet formation look to be very promising.

“Though thousands of exoplanets have been discovered in the last few decades, detecting protoplanets is at the frontier of science,” said Pinte.

 

These earlier images from ALMA reveal details in the planet-forming disk around a nearby sun-like star, TW Hydrae, including an intriguing gap at the same distance from the star as the Earth is from the sun. This structure may mean that an infant version of our home planet is beginning to form there, although these dust gaps are considered to be suggestive rather than conclusive. ( S. Andrews; Harvard-Smithsonian CfA, ALMA (ESO/NAOJ/NRAO)}ALMA

This is not the first time that ALMA images of protoplanetary disks have been used to identify what seem to be protoplanets.

In 2016, a team led by Andrea Isella of Rice University reported the possible detection of two planets, each the size of Saturn, orbiting the same star that is the subject of this week’s report, HD 163296.

These possible planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star’s protoplanetary disk.

But at the time that paper was published, in Physical Review Letters, Isella said the team was focused primarily on the dust in the disks and the gaps they created, and as a result they could not be certain that the features they found were created by a protoplanet.

Teague’s team also studied the dust gaps in the disk of HD 163296, and concluded they provided only  circumstantial evidence of the presence of protoplanets.  What’s more, that kind of detection could not be used to accurately estimate the masses of the planets.

“Since other mechanisms can also produce ringed gaps in a protoplanetary disk,” he said, “it is impossible to say conclusively that planets are there by merely looking at the overall structure of the disk.”

But studying the behavior of the gas allowed for a much greater degree of confidence.

 

Composite image of the protoplanetary disk surrounding the young star HD 163296. The inner red area shows the dust of the protoplanetary disk. The broader blue disk is the carbon monoxide gas in the system. ALMA observed dips in the concentration and behavior of carbon monoxide in outer portions of the disk, strongly suggesting the presence of planets being formed. ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)

The team led by Teague identified two distinctive planet-like patterns in the disk, one at approximately 80 astronomical units (AU) from the star and the other at 140 AU. (An astronomical unit is the average distance from the Earth to the sun.)  The other team, led by Pinte, identified the third at about 260 AU from the star. The astronomers calculate that all three planets are similar in mass to Jupiter.

The two teams used variations on the same technique, which looked at anomalies in the flow of the gas – as seen in the shifting wavelengths of the CO emission — that would indicate it was interacting with a massive object.

Teague and his team measured variations in the gas’s velocity. This revealed the impact of several planets on the gas motion nearer to the star.

Pinte and his team more directly measured the gas’s actual velocity, which is better precise method when studying the outer portion of the disk and can more accurately pinpoint the location of a potential planet.

“Although dust plays an important role in planet formation and provides invaluable information, gas accounts for 99 percent of a protoplanetary disks’ mass,” said coauthor Jaehan Bae of the Carnegie Institute for Science.

So while those images of patterns within the concentric rings of a protoplanetary disk are compelling and seem to be telling an important story, it’s actually the gas that is the key.

This is all an important coup for ALMA, which saw its first light in 2013.  The observatory was not designed with protoplanet detection and characterization as a primary goal, but it is now front and center.

Coauthor Til Birnstiel of the University Observatory of Munich said the precision provided by ALMA is “mind boggling.” In a system where gas rotates at about 5 kilometers per second, he said,  ALMA detected velocity changes as small as a few meters per second.

“Oftentimes in science, ideas turn out not to work or assumptions turn out to be wrong,” he said. “This is one of the cases where the results are much more exciting than what I had imagined.

 

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Can You Overwater a Planet?

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Water worlds, especially if they have no land on them, are unlikely to be home to life, or at least lifewe can detect.  Some of the basic atmospheric and mineral cycles that make a planet habitable will be absent. Cool animation of such a world. (NASA)

Wherever we find water on Earth, we find life. It is a connection that extends to the most inhospitable locations, such as the acidic pools of Yellowstone, the black smokers on the ocean floor or the cracks in frozen glaciers. This intimate relationship led to the NASA maxim, “Follow the Water”, when searching for life on other planets.

Yet it turns out you can have too much of a good thing. In the November NExSS Habitable Worlds workshop in Wyoming, researchers discussed what would happen if you over-watered a planet. The conclusions were grim.

Despite oceans covering over 70% of our planet’s surface, the Earth is relatively water-poor, with water only making up approximately 0.1% of the Earth’s mass. This deficit is due to our location in the Solar System, which was too warm to incorporate frozen ices into the forming Earth. Instead, it is widely — though not exclusively — theorized that the Earth formed dry and water was later delivered by impacts from icy meteorites. It is a theory that two asteroid missions, NASA’s OSIRIS-REx and JAXA’s Hayabusa2, will test when they reach their destinations next year.

But not all planets orbit where they were formed. Around other stars, planets frequently show evidence of having migrated to their present orbit from a birth location elsewhere in the planetary system.

One example are the seven planets orbiting the star, TRAPPIST-1. Discovered in February this year, these Earth-sized worlds orbit in resonance, meaning that their orbital times are nearly exact integer ratios. Such a pattern is thought to occur in systems of planets that formed further away from the star and migrated inwards.

 

Trappist-1 and some of its seven orbiting planets.  They would have been sterilized by high levels of radiation in the early eons of that solar system — unless they were formed far out and then migrated in.  That scenario would also allow for the planets to contain substantial amounts of water. (NASA)

The TRAPPIST-1 worlds currently orbit in a temperate region where the levels of radiation from the star are similar to that received by our terrestrial worlds. Three of the planets orbit in the star’s habitable zone, where a planet like the Earth is most likely to exist.

However, if these planets were born further from the star, they may have formed with a high fraction of their mass in ices. As the planets migrated inwards to more clement orbits, this ice would have melted to produce a deep ocean. The result would be water worlds.

With more water than the Earth, such planets are unlikely to have any exposed land. This does not initially sound like a problem; life thrives in the Earth’s seas, from photosynthesizing algae to the largest mammals on the planet. The problem occurs with the planet itself.

The clement environment on the Earth’s surface is dependent on our atmosphere. If this envelope of gas was stripped away, the Earth’s average global temperature would be about -18°C (-0.4°F): too cold for liquid water. Instead, this envelope of gases results in a global average of 15°C (59°F).

Exactly how much heat is trapped by our atmosphere depends on the quantity of greenhouse gases such as carbon dioxide. On geological timescales, the carbon dioxide levels can be adjusted by a geological process known as the “carbon-silicate cycle”.

In this cycle, carbon dioxide in the air dissolves in rainwater where it splashes down on the Earth’s silicate rocks. The resulting reaction is termed “weathering”. Weathering forms carbonates and releases minerals from the rocks that wash into the oceans. Eventually, the carbon is released back into the air as carbon dioxide through volcanoes.

Continents are not only key for habitability because they sources of minerals and needed elements but also because they allow for plate tectonics — the movements and subsequent crackings of the planet’s crust that allow gases to escape.  Those gases are needed to produce an atmosphere.  (National Oceanic and Atmospheric Administration)

The rate of weathering is sensitive to temperature, slowing when he planet is cool and increasing when the temperature rises. This allows the Earth to maintain an agreeable climate for life during small variations in our orbit due to the tug of our neighboring planets or when the sun was young and cooler. The minerals released by weathering are used by all life on Earth, in particular phosphorous which forms part of our DNA.

However, this process requires land. And that is a commodity a water world lacks. Speaking at the Habitable Worlds workshop, Theresa Fisher, a graduate student at Arizona State University, warned against the effects of submerging your continents.

Fisher considered the consequences of adding roughly five oceans of water to an Earth-sized planet, covering all land in a global sea. Feasible, because weathering could still occur with rock on the ocean floor, though at a much reduced efficiency. The planet might then be able to regulate carbon dioxide levels, but the large reduction in freed minerals with underwater weathering would be devastating for life.

Despite being a key element for all life on Earth, phosphorus is not abundant on our planet. The low levels are why phosphorous is the main ingredient in fertilizer. Reduce the efficiency with which phosphorous is freed from rocks and life will plummet.

Such a situation is a big problem for finding a habitable world, warns Steven Desch, a professor at Arizona State University. Unless life is capable of strongly influencing the composition of the atmosphere, its presence will remain impossible to detect from Earth.

“You need to have land not to have life, but to be able to detect life,” Desch concludes.

However, considerations of detectability become irrelevant if even more water is added to the planet. Should an Earth-sized planet have fifty oceans of water (roughly 1% of the planet’s mass), the added weight will cause high pressure ices to form on the ocean floor. A layer of thick ice would seal the planet rock away from the ocean and atmosphere, shutting down the carbon-silicate cycle. The planet would be unable to regulate its surface temperature and trapped minerals would be inaccessible for life.

Add still more water and Cayman Unterborn, a postdoctoral fellow at Arizona State, warns that the pressure will seal the planet’s lid. The Earth’s surface is divided into plates that are in continual motion. The plates melt as they slide under one another and fresh crust is formed where the plates pull apart. When the ocean weight reaches 2% of the planet’s mass, melting is suppressed and the planet’s crust grinds to a halt.

A stagnant lid would prevent any gases trapped in the rocks during the planet’s formation from escaping. Such “degassing” is the main source of atmosphere for a rocky planet. Without such a process, the Earth-sized deep water world could only cling to an envelop of water vapor and any gas that may have escaped before the crust sealed shut.

Unterborn’s calculations suggest that this fate awaits the TRAPPIST-1 planets, with the outer worlds plausibly having hundreds of oceans worth of water pressing down on the planet.

So can we prove if TRAPPIST-1 and similarly migrated worlds are drowning in a watery grave? Aki Roberge, an astrophysicist at NASA Goddard Space Flight Center, notes that exoplanets are currently seen only as “dark shadows” briefly reducing their star’s light.

However, the next generation of telescopes such as NASA’s James Webb Space Telescope, will aim to change this with observations of planetary atmospheres. Intertwined with the planet’s geological and biological processes, this cloak of gases may reveal if the world is living or dead.

 

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Exoplanet Clouds; Friend and Foe

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Different colotd pedicted of Hot Jupiters based on their temperatures and the compounds in their atmospheres.
An illustration representing how hot Jupiters of different temperatures and different cloud compositions might appear to a person flying over the day side of these planets on a spaceship, based on computer modeling.  (NASA/JPL-Caltech/University of Arizona/V. Parmentier)

 

Understanding the make-up and dynamics of atmospheric clouds is crucial to our interpretations of how weather and climate behave on Earth, and so it should come as no surprise that clouds are similarly essential to learning the nature and behavior of exoplanets.

On many exoplanets, thick clouds and related, though different, hazes have been impediments to learning what lies in the atmospheres and on surfaces below.  Current technologies simply can’t pierce many of these coverings, and scientists have struggled to find new approaches to the problem.

One class of exoplanets that has been a focus of cloud studies has been, perhaps unexpectedly, hot Jupiters — those massive and initially most surprising gas balls that orbit very close to their suns.

Because of their size and locations, the first exoplanets detected were hot Jupiters.  But later work by astronomers, and especially the Kepler Space Telescope, has established that they are not especially common in the cosmos.

Due to their locations close to suns,  however, they have been useful targets of study as the exoplanet community moves from largely detecting new objects to trying to characterize them, to understanding their basic features.  And clouds are a pathway to that characterization.

For some time now, scientists have understood that the night sides of the tidally-locked hot Jupiters generally do have clouds, as do the transition zones between day and night.  But more recently, some clouds on the super-hot day sides — where temperatures can reach 2400 degrees Fahrenheit –have been identified as well.

Vivien Parmentier, a Sagan Fellow at the University of Arizona, Tucson, as well as planetary scientist Jonathan Fortney of the University of California at Santa Cruz have been studying those day side hot Jupiter clouds to see what they might be made of, and how and why they behave as they do.

“Cloud composition changes with planet temperature,” said Parmentier, who used a 3D General Circulation Model (GCM) to track where clouds form in hot Jupiter atmospheres, and what impact they have on the light emitted and reflected by the planets.  “The offsetting light curves tell the tale of cloud composition. It’s super interesting, because cloud composition is very hard to get otherwise.”

The paper by Parmentier, Fortney and others was published in The Astrophysical Journal.

 

Artist's impression of a hot Jupiter. Image Credit: NASA
Artist’s impression of a hot Jupiter. (NASA)

 

Solid observational evidence of clouds on the days sides of hot Jupiters has been collected for only a short time, and is done by measuring parent starlight being reflected off the atmosphere.  Enough information has accumulated by now, Fortney said, to begin to offer theoretical explanations of the measurements being made.

“What this suggests is that the cloud behavior is quite complex — there is no ‘uniform planet-wide cloud,’ for these tidally locked planets,” he said in an email.

“The hot day side may sometimes lack clouds, compared to the cooler night side, where many clouds form.  Energy redistribution, via winds, leads to gas that is moving into “sunset” from day to night being cloud-free, but gas going into “sunrise,” moving from night to day is full our cloud material that will evaporate when the gas warms up.

The atmospheres are way too hot for water clouds. Instead, the cloud material detected has been iron and silicate rocks (well-known from brown dwarf atmospheres), and manganese sulfide (which has been suggested for brown dwarf as well.)

The different elements and compounds in the clouds give hints about the appearance of the planets, and Parmentier used the GCM model to predict what these planets would look like to the human eye.

The differences in color, said Fortney, are a function of the amount of heat coming off the planet and the stellar scattered light coming off of atmospheric gases and clouds.  “Not all clouds are the same color, which is fun.”

He also said that “this is the first in what will be a longer study to better understand the transport of cloud material around the planets.

For this first study, we only suggest that clouds will form when the temperature is right, but we didn’t track how the cloud material moves with the flow.  That is the next step for a more comprehensive and accurate model.”

 

Hot Jupiters, exoplanets around the same size as Jupiter that orbit very closely to their stars, often have cloud or haze layers in their atmospheres. This may prevent space telescopes from detecting atmospheric water that lies beneath the clouds, according to a study in the Astrophysical Journal. (NASA/JPL-Caltech)
Hot Jupiters often have cloud or haze layers in their atmospheres. This may prevent space telescopes from detecting atmospheric water that lies beneath the clouds, according to an earlier study in the Astrophysical Journal. (NASA/JPL-Caltech)

 

The new insights into hot Jupiter clouds via the GCM allowed the team to draw conclusions about wind and temperature differences.

Just before the hotter planets passed behind their stars, a blip in the planet’s optical light curve revealed a “hot spot” on the planet’s eastern side. And on cooler eclipsing planets, a blip was seen just after the planet re-emerged on the other side of the star, this time on the planet’s western side.

The early blip on hotter worlds was interpreted as being powerful winds that were pushing the hottest, cloud-free part of the day side atmosphere to the east. Meanwhile, on cooler worlds, clouds could bunch up and reflect more light on the “colder,” western side of the planet, causing the post-eclipse blip.

“We’re claiming that the west side of the planet’s day side is more cloudy than the east side,” Parmentier said in a JPL release.

While the puzzling pattern has been seen before, this research was the first to study all the hot Jupiters showing this behavior.

This led to another first. By teasing out out how clouds are distributed, which is intimately tied to the planet’s overall temperature, scientists were able to determine the compositions of the clouds — likely formed as exotic vapors condense to form minerals, chemical compounds like aluminum oxide, or even metals, like iron.

The science team found that manganese sulfide clouds probably dominate on “cooler” hot Jupiters, while silicate clouds prevail at higher temperatures. On these planets, the silicates likely “rain out” into the planet’s interior, vanishing from the observable atmosphere.

So while exoplanet clouds can and do mask important information about what lies below in a planet’s atmosphere, scientists are learning ways to use the information that clouds provide to push forward on that process of characterizing the vast menagerie of exoplanets being found.

 

Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet. Courtesy of NASA (edited by Jose-Luis Olivares/MIT)
Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet. (NASA, edited by Jose-Luis Olivares/MIT)

 

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Rethinking The Snow Line

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This image of the planet-forming disc around the young star V883 Orionis was obtained by ALMA in long-baseline mode. This star is currently in outburst, which has pushed the water snow line further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snowline, the point from the star where the temperature and pressure dip low enough for water ice to form.
This planet-forming disc around the young star V883 Orionis was obtained by the European Southern Observatory’s  Atacama Large Millimeter/submillimeter Array (ALMA), a prime site for radio astronomy. The star is a state of “outburst,” which has pushed the water snow line further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snowline, the point from the star where the temperature and pressure dip low enough for water ice to form.  ALMA (ESO/NAOJ/NRAO)/L. Cieza

In every planet-forming disk there’s a point where the heat from a host star needed to keep H2O molecules as vapor peters out, and the H2O be becomes a solid crystal.  This is the snow line, and it looms large in most theories of planet formation.

Most broadly, planets formed inside the snow line will generally be rocky and small — a function of the miniscule dust grains that begin the planet forming process.  But outside the snow line the grains get coated by the icy H2O and so are much bigger, leading to gas and ice giant planets.

The existence of water snow lines (and for other molecules, too) is nothing new, but an image of a water snow line would be.  And now an international team led by Lucas Cieza of Universidad Diego Portales in Santiago, Chile, has found the water vapor/ice line around a very young star 1,350 light-years away. The results were published in 2016 journal Nature.

Using a high-precision radio astronomy array in Chile’s Atacama Desert, the team had been looking into whether the massive bursts of young stars might be caused by a theorized collapse into them of fragments of the disk.  But instead they detected and imaged the water snow line instead.

The image itself is an achievement, but what makes the finding especially intriguing is that the snow line was found at an entirely unexpected and enormous distance from the star — more than 42 astronomical units, or forty-two times the distance from our sun to Earth.

That would it was warm enough for H2O to remain a vapor roughly as far out as the orbit of the dwarf planet Pluto around the sun.  A more typical early star snow line is expected to be around 3 AU, an region between the orbits of Mars and Jupiter.

Brenda Matthews, an astronomer at the National Research Council of Canada not involved in the study, wrote in an accompanying column that the snow line finding challenges some traditional models of planet formation.

“The fact that the location of the snow line can evolve with time has strong implications for planet formation,” she wrote. A rapid heating and cooling of the planet-forming disk “would confound models that predict the slow formation of rocky planets within the snow line, and rapid gas-giant formation outside it.”

 

An artist's illustration shows the water snow line detected around the young star V883 Orionis — the delineation between where the hot star vaporizes all water, leaving rocky dust and debris, and where ice and snow exist in the disk. Credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)
An artist’s illustration shows the water snow line detected around the young star V883 Orionis — the delineation between where the hot star vaporizes all water, leaving rocky dust and debris, and where ice and snow exist in the disk. A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

 

Asked his view of those challenges to the traditional planet forming models Cieza gave our Jupiter and, the widely accepted version of how it formed at 5 AU, as an example:

“If the water snow-line stays far out — let’s say at 10 AU — for very long periods of time, then one would think that forming Jupiter at 5 AU would be very difficult given the importance of water ice for the planet formation process.”

‘That is just a speculation,” Cieza wrote.  “What we really need is planet formation models that take into consideration the very variable accretion rates of proto-stars and the drastic changes in the location of the water snow-line. That is one of the main conclusion of our paper.”

He also described the potential difficulty of delivering the proper amount of water to rocky planets like our own.

“The current thinking is that terrestrial planets form inside the snow line,” he wrote. “We are used to thinking that the Earth is full of water because 70% of its surface  is covered by oceans. However, water represents only 0.02% of the total mass of the planet. Strictly speaking, the Earth is an extremely dry planet because it formed inside the water snow-line, where the temperature in the protoplanetary disk was too hot for water ice to exist.”

The result:  “If the water snow-line had stayed at 10 AU for very long periods of time, perhaps the Earth wouldn’t had received enough water-rich comets and asteroids to form the ocean and we wouldn’t have any life on Earth.”

Cieza and others on the team are not saying that all planet-forming disks necessarily had very distant snow lines at some point, but that some did and that needs to be taken into account.

This view shows several of the ALMA antennas and the central regions of the Milky Way above. In this wide field view, the zodiacal light is seen upper right and at lower left Mars is seen. Saturn is a bit higher in the sky towards the centre of the image. The image was taken during the ESO Ultra HD (UHD) Expedition.
This view shows several of the ALMA antennas and the central regions of the Milky Way above.  The V883 Orionis viewing was done in four different array configurations with baselines ranging from 30 feet apart to almost 8 miles apart.  (ESO)

As explained by Cieza and co-author Zhaohuan Zhu of  Princeton University, the ALMA image does not measure H2O per se, but rather it measures the grain size present in the disk.  It is well established that inside the snow line — where water is in vapor form — the dust grains are small.  But beyond the snow line — where the water is in the form of ice and snow — the dust grains get bigger because the ice adheres to the grains.

So the dust grain size is a stand-in for the phase of the H2O, and can be captured in an image.

The fact that the snow line is so far out also made it possible to make an image. For technical reasons,  current telescopes cannot distinguish characteristics such as grain size that are close to the suns.  ALMA has previously detected the “snow line” for carbon monoxide for a different star at 12 AU, but it cannot get measurements of any molecules for the intriguing “hot Jupiters” that orbit within one AU of their stars.

The heat and luminosity from V883 Orionis –which was detected in 1993 — is pretty remarkable.  The star has only 30 percent more mass than our sun, but it is sending out 400 times more heat and other radiation.  This is all because of those solar bursts that seem to accompany the early evolution of many (or is it most?) stars and their disks.

Zhu, a NASA Hubble Postdoctoral Fellow, said that the early stellar outbursts are expected to last only about 100 years — which makes the detection of  the one at V883 Orionis quite special.  But the short time frame makes it difficult to see the bursts as common, potentially planet-defining phenomenon.

“So far maybe 20 to 30 outbursts have been found,” Zhu said.  “So we aren’t at all sure whether every young star should have an outburst.  It’s very controversial in the community.”

As theorized, the outbursts are caused by material from the disk falling onto the star.  How exactly that might happen is not understood, and was actually the focus of the observations that led to the discovery of the 42 AU water snow line.

Young and highly variable stars like V883 Orionis are included in a class of objects first identified in 1937, with the detection of suddenly very luminous FU Orionis.  V883 Orionis was identified as a FU Orionis, or FUors, star in 1993 based on similarities between its spectrum and that 1937 prototype.

Joel Green, a research scientist at the Space Telescope Science Institute in Baltimore, has studied that original FUor star’s more recent behavior and found that it was still consuming disk material at a remarkable rate.  Since the feeding frenzy started 80 years ago, the star has eaten the equivalent of 18 Jupiters, Green’s team concluded.

In a release last month from NASA’s Jet Propulsion Laboratory, Green said that  “By studying FU Orionis, we’re seeing the absolute baby years of a solar system. Our own sun may have gone through a similar brightening, which would have been a crucial step in the formation of Earth and other planets in our solar system.”

And if it did go through a similar and substantial brightening, what does that say about the snow line in our very early solar system?  And about how and where our planets might have been formed?

 

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Big Bangs

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Collisions between planets, planetesimals and other objects are common in the galaxies and essential for planet formation. Researchers are focusing on these collisions for clues into which exoplanets have greater or lesser potentials habitability. (NASA)
Collisions between planets, planetesimals and other objects are common in the galaxies and essential for planet formation. Researchers are focusing on these collisions for clues about which exoplanets have greater or lesser potential habitability. (NASA)

What can get the imagination into super-drive more quickly than the crashing of really huge objects?

Like when a Mars-sized planet did a head-on into the Earth and, the scientific consensus says, created the moon.  Or when a potentially dinosaur-exterminating asteroid heads towards Earth, or when what are now called  “near-Earth objects” seems to be on a collision course.  (There actually aren’t any now, as far as I can tell from reports.)

But for scientists, collisions across the galaxies are not so much a doomsday waiting to happen, but rather an essential commonplace and a significant and growing field of study.

The planet-forming centrality of collisions — those every-day crashes of objects from grain-sized to planet-sized within protoplanetary disks — has been understood for some time; that’s how rocky planets come to be.  In today’s era of exoplanets, however, they have taken on new importance: as an avenue into understanding other solar systems, to understanding the composition and atmospheres of exoplanets, and to get some insight into their potential habitability.

And collision models, it now seems likely, can play a not insignificant role in future decision-making about which planetary systems will get a long look from the high-demand, high-cost space telescopes that will launch and begin observing in the years ahead.

“We’re learning that these impacts have a lot of implications for habitability,” said Elisa Quintana, a NASA Ames Research Center and SETI Institute research scientist who has been modeling space collisions.  Her paper was published in 2016 in the Astrophysical Journal, and took the modeling into new realms.

“When you think of what we know about impacts in general, we know they can effect a planet’s spin rate and rotation and consequently its weather,  they can bring water and gases to a planet or they can destroy an atmosphere and let the volatiles escape.  They effect the relationship between the planet’s core and mantle, and they determine the compositions of the planets.  These are all factors in increasing or decreasing a planet’s potential for habitability.”

 

An artist rendering of a protoplanetart disk around a newly-formed star. Tiny grains of dust grow over millions of years into planets through collisions and the accretion of matter. (NASA)
An artist rendering of a protoplanetary disk around a newly-formed star. Tiny grains of dust grow over millions of years into planetesimals and planets through collisions and the accretion of matter. (NASA)

 

To better understand the logic of impacts, Quintana ran two-billion year simulations of the protoplantary disk-to-mature solar system process, and produced a unique look at how those systems and their planets form.  What’s more, she did it 280 times, which is many more simulations than has been done in the past, and came up with the creation of 164 Earth-like worlds.

Starting about 10 million years after the solar system formed, she recreated dynamics from our own solar system to make the simulations, but tweaked the starting points to make them applicable to other extra solar systems.

In the past, impact simulations like these generally used a “perfect accretion” model, which meant that all the material from one planet or moon-sized body would stick to the larger one it hit.  But we know that is actually not what always happens  — that both the impacted and impactor can fragment and eject rock into the sky. But this scenario, as well as a less dramatic “hit and run” impacts, is hard to model.

Yet the new Quintana et al model does indeed add these kinds of break-ups into the equation.  The result is a model that is governed by the known physical and protoplanetary rules, but with a large dose of chaos.

Elisa Quintana is a research scientist at the SETI Institute and at the NASA Ames Research Center. (SETI Institute)
Elisa Quintana is a research scientist at the SETI Institute and at the NASA Ames Research Center. (SETI Institute)

As Quintana explained:  “One simulation could produce 3 final terrestrial planets. If you run the same simulation again, but move one rock in the disk by 1 meter (keeping everything else exactly the same) the simulation could produce 5 planets. The butterfly effect!”

What this means is that the architecture of any solar system is but one of many that could have been produced by the same protoplanetary disk.

What she reported in that paper and from subsequent work is that:

  • The models generally produce three to four rocky inner planets, as in our solar system.
  • In a system with giant planets like Jupiter and Saturn, the process of increasingly large bodies colliding is roughly 200 million years long.  At that point, the rocky inner planets of the system would have been formed, and the material to add significantly to the planets (or the system) would be largely depleted.
  • But in a system without a Jupiter or Saturn, the process of accreting material onto planets takes much longer and moon-sized objects and smaller planetesimals remain prevalent at 500 million years.  Indeed, she said, these substantial but not planetary orbiting objects would probably be present in an inner solar system even 4.5 billion years later (the age of our solar system.)  In other words, the inner solar systems would be filled with objects like the crowded Oort Cloud of our system — the regions some 100,000 times further from the sun than Earth.
  • Histogram of the total number of giant impacts received by the 164 Earth-like worlds produced in the authors’ fragmentation-inclusive simulations. [Quintana et al. 2016]
    Histogram of the total number of giant impacts received by the 164 Earth-like worlds produced in the authors’ fragmentation-inclusive simulations. [Quintana et al. 2016]
    While impacts are common in the first model they generally end quite abruptly — and with a very big bang.  Since the inner solar system has been largely cleared of smaller objects by this time, what’s left is large ones that exert increasingly great gravitational pulls.  The result is the kind of giant impact that formed our moon.  Every simulation run with a Jupiter or Saturn in place delivered at least one giant impact to each inner rocky planet.

 

The models were run on the Pleaides supercomputer at NASA Ames Research Center, which allowed for additional factors (the fragmenting of planets, those hit-and-runs) to be included. And then its enormous capacity allowed for so many more models to be run, and run quickly.

Below are animations of the first scenario (with a Jupiter and Saturn already in place, as it would be at 10 million years after disk formation) and the second without the giant planets.

The green lines are orbits of moon-sized planetesimals, the blue lines Mars-sized planet embryos, and the red is “fragmented material” the size of half a moon kicked during impacts.  (More on this later.)  The Jupiter is purple and the Saturn is yellow.  (Pop-up button on upper right allows for full screen view.)

As you’ll see, the planet formation process ends quite early where a Jupiter and Saturn are in the system — they dominate the gravitational dynamics and clear out smaller objects quickly.  But with a sun only and no larger planets, the smaller objects remain for billions of years.

 

 

Animations by Chris Henze, NASA Ames

 
Simon Lock is a doctoral student at Harvard (and the University of California, Davis) and is working on impacts as well with planetary scientist Sarah Stewart and her group.  They are especially interested in how impacts effect atmospheres on exoplanets, and have found the consequences can be both catastrophic and constructive.
 
The catastrophic is easy to picture:  a huge impact strips the atmosphere from a planet and leaves it barren, with none of the water and compounds needed for potential life.  Lock said that Stewart’s group has modeled giant impacts and found the shock wave of the really big ones can travel through a planet and actually do much of its atmosphere destroying on the far side of the impacted planet.
 
But models from both Stewart’s group and Quintana show that these giant impacts are pretty rare — only 1 percent experienced an atmosphere stripping impact, according to Quintana.  Collisions that could strip 50 percent of an atmosphere, however, were far more common.   The average Earth-like planet in Quintana’s model would experience around three of these in a two billion year period.
 
With future exoplanet research and discoveries in mind, Lock has focused on what happens when a planet with a super-thick atmosphere collides with something much larger than an asteroid.  The result, he said, could be disaster or it could be the creation of an atmosphere far more conducive to life than what existed before.
 
 Using Venus as an example of a planet with a very heavy atmosphere of carbon dioxide, he said it is commonly held that the surface once had water but now is bone dry.  A less heavy atmosphere would potentially keep its water, and an atmosphere-thinning collision is what could bring that about.   With many exoplanets now seen as having heavy atmospheres, the dynamics are significant.
 
“There’s a narrow tightrope when it comes to planets and their atmospheres,” he said.  “Not enough atmosphere and you lose water and other volatiles” like methane, ammonia, nitrogen, sulfur dioxide.  “But too much and nothing can survive either.  So some atmosphere loss is needed for a planet to be habitable, but not too much. And impacts play a big role here.”
 
What’s more, planets are fed H20, organic compounds and other essential for life elements via impacts.  There’s much debate about how and when that happened on Earth, but it definitely did happen.
 
 
This artist concept illustrates how a massive collision of objects, perhaps as large as the planet Pluto, smashed together to create the dust ring around the nearby star Vega. New observations from NASA's Spitzer Space Telescope indicate the collision took place within the last one million years. Astronomers think that embryonic planets smashed together, shattered into pieces, and repeatedly crashed into other fragments to create ever finer debris.

In the image, a collision is seen between massive objects that measured up to 2,000 kilometers (about 1,200 miles) in diameter. Scientists say the big collision initiated subsequent collisions that created dust particles around the star that were a few microns in size. Vega's intense light blew these fine particles to larger distances from the star, and also warmed them to emit heat radiation that can be detected by Spitzer's infrared detectors.
This artist concept illustrates how a massive collision of objects, at 1,200 miles in diametert perhaps as large as the dwarf planet Pluto, smashed together to create the dust ring around the nearby star Vega. New observations from NASA’s Spitzer Space Telescope indicate the collision took place within the last one million years. Astronomers think that embryonic planets smashed together, shattered into pieces, and repeatedly crashed into other fragments to create ever finer debris.

  (NASA)
 
So are major collisions friend or foe to life?
 
It certainly seems that the downside can be pretty great in terms of violently re-arranging the planet and its atmosphere, and stripping both to some extent of necessary compounds and elements.  And, of course, a small planet or planetesimal can just be smashed entirely apart.
 
But the one planet that we know for sure supports life not only survived the giant moon-forming impact, but also millions of years before that of what is known as “heavy bombardment,”  and smaller but almost life-eliminating impacts since.
 
Is that just coincidence?  Good luck?  Or perhaps the result of a necessary series of transformations brought about by those in-coming large objects?
 
There’s no consensus now, but there’s an intriguing body of work making the case that impacts really matter.
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