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 the journal Nature this week.

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 and water vapor can not be accreted efficiently by small rocky planets.

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 recently published 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 exosystems.

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, 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 H2O, 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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