Jupiter’s Stripes Run Deep, But Hopefully Juno’s Problems Do Not

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Though on holiday, I wanted to share these images and a bit of the Juno at Jupiter news.

This composite image depicts Jupiter's cloud formations as seen through the eyes of Juno's microwave radiometer (MWR) instrument as compared to the top layer, a Cassini imaging science subsystem image of the planet. The MWR can see a couple of hundred miles into Jupiter's atmosphere with the instrument's largest antenna. The belts and bands visible on the surface are also visible in modified form in each layer below. Credit: NASA/JPL-Caltech/SwRI/GSFC
This composite image depicts Jupiter’s cloud formations as seen through the eyes of Juno’s microwave radiometer (MWR) instrument as compared to the top layer, a Cassini imaging science subsystem image of the planet. The MWR can see a couple of hundred miles into Jupiter’s atmosphere with the instrument’s largest antenna. The belts and bands visible on the surface are also visible in modified form in each layer below. (NASA/JPL-Caltech/SwRI/GSFC)

Because telescopes have never been able to see clearly down through the thick clouds of Jupiters– the ones that together form the planet’s glorious stipes– it has remained a mystery how deep they may be.

Based on the Juno spacecraft’s August pass, we now know via its microwave radiometer that the stripes reflect dynamics that occur deep into the planet.

Scott Bolton, leader of the Juno mission reported the team’s conclusions during a press conference at the 2016 meeting of the American Astronomical Society’s Division for Planetary Sciences.

“The structure of the zones and belts still exists deep down,” Bolton said.  “So whatever’s making those colors, whatever’s making those stripes, is still existing pretty far down into Jupiter. That came as a surprise to many of the scientists. We didn’t know if this was [just] skin-deep.”

The new images penetrate to depths of about 200 to 250 miles below the surface cloud layer, Bolton said. While the bands seen on the cloud tops are not identical to the bands identified further down, there is a strong resemblance. “They’re evolving. They’re not staying the same,” Bolton said.

The findings have intriguing implications for exoplanet research.  Bolton said that the hint at “the deep dynamics and the chemistry of Jupiter’s atmosphere. And this is the first time we’ve seen any giant planet atmosphere underneath its layers. So we’re learning about atmospheric dynamics at a very basic level.”

Outer jets and belts composed largely of ammonia and hydrogen sulfide gas can block study of the inner atmosphere. Winds blow the cloud regions in different directions. (NASA)
Outer jets and belts composed largely of ammonia and hydrogen sulfide gas can block study of the inner atmosphere. Winds blow the cloud regions in different directions. (NASA/JPL-Caltech)

These early Juno findings came as it was also reported that the spacecraft had two malfunction that caused it to go into safe mode, just as it was approaching Jupiter for an October 19 flyby.

Right now, Juno makes one orbit every 53 days. Juno was scheduled to fire its engines on Oct. 19 and reduce its orbit to every 14 days. But because of a problem with the engine valves, the Juno team delayed that engine firing for now.

Then, NASA officials said,  a second problem, apparently related to the  “software-performance monitor,” caused the probe’s onboard computer to reboot.  Officials said the problem was not related to that earlier propulsion issue.

“At the time safe mode was entered, the spacecraft was more than 13 hours from its closest approach to Jupiter,” said Rick Nybakken, Juno project manager from NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “We were still quite a ways from the planet’s more intense radiation belts and magnetic fields. The spacecraft is healthy and we are working our standard recovery procedure.”

In safe mode, all unneeded subsystems were shut down and instructions were relayed by controllers. Juno did not collect any data during the flyby, which was to take place as it passed 3,000 miles above Jupiter’s clouds.

The root causes of the problem have not been made public and apparently remain unresolved.  But Bolton that no long-term problems were anticipated, and that the team expected the spacecraft to be ready to turn on all science instruments at the next close flyby, on December 11.

Artist rendering of Juno spacecraft in orbit around Jupiter. NASA
Artist rendering of Juno spacecraft in orbit around Jupiter. (NASA)

Just as Juno was approaching Jupiter this summer, researchers at the University of California, Berkeley, reported that whirling ammonia flows below the sutface clouds help form the planet’s distinctive features.

Researchers used the upgraded Very Large Array radio telescope in New Mexico to probe 60 miles below the top of the clouds.  They reported a correlation between the colorful whirls and spots on the visible surface and the movement of gas below, which is driven by Jupiter’s internal heat source.

The Juno findings certainly suggest that the correlation goes much deeper.

 

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Juno Now Orbiting Jupiter

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Artist illustration of Juno as it approaches Jupiter. NASA
Artist illustration of Juno as it approaches Jupiter. (NASA)

It took a while — almost five years since launch — but the Juno spacecraft is now at Jupiter and orbiting the giant planet. A 35-minute rocket burn to slow Juno down from its record-breaking 130,000 mph entry speed led to a successful insertion into orbit just minutes before midnight, making it another July 4th NASA spectacular.

During its mission, Juno will orbit the planet 37 times, dipping as low as 2,600 miles above the planet’s upper clouds of ammonia and water.  Primary goals of the mission are to determine whether Jupiter has a solid rocky core or is made up of gases all the way through, to learn about its extraordinarily powerful magnetic forces, and to determine better the components of those upper clouds and what might lie beneath them.

The overriding purpose is to better understand how Jupiter — the first planet formed in our solar system — came to be, and consequently how our solar system was formed. Considering that Jupiter contains more matter than the rest of the solar system planets, moons, asteroids and comets combined, it clearly is the place to look to understand the origins of the solar system.

But another goal, and a significant one at that, is to learn about the big gas giant as a way to learn about similar planets orbiting other stars.  Woven into the Juno mission from the beginning was a requirement that the two years of orbiting be designed and operated with distant solar systems and exo-Jupiters in mind.

I had the opportunity to speak with Juno principal investigator Scott Bolton just the day before Juno’s arrival, and he made clear that providing information and insights that will help understand exo-Jupiters is a high priority, indeed.

Scott Bolton, principal investigator of the Juno mission. (NASA)
Scott Bolton, principal investigator of the Juno mission. (NASA)

“We know that our Jupiter is quite different from many of the other Jupiter-sized planets found, and so there will be differences,” he said.  “But the dynamics we find, the presence of a rocky core or not, the water abundances, the structure of the planet — I think that will all be extremely useful to exoplanet modelers and theorists.”

He also made the intriguing observation that there may well be links between Juno discoveries and the search for Earth-size planets around other stars.

“It may be that finding a system with a Jupiter of a size like ours,  and in  a location {in its solar system} similar to ours, would be a strong signal that there is also an Earth-sized planet in the system.”

Many Worlds carried a column about Juno, Jupiter and exo-Jupiters a few weeks ago, and you can find it here.

But I wanted to also celebrate the spacecraft’s arrival, as well as share more of the conversation with Bolton.

Image of Jupiter and its moons taken during the approach. (NASA)
Image of Jupiter and its moons taken during the approach. (NASA)

First a little more about Juno:  Its body is 11.5 feet tall and 11.5 feet in diameter. But with its three solar panels open, it spans about 66 feet — more than two-thirds of the distance of an NBA basketball court.

The spacecraft will pass as close as 2,900 miles from the upper levels of the Jovian atmosphere during some orbits.  The previous record for spacecraft proximity to Jupiter was 27,000 miles from the atmosphere’s top when Pioneer 11 passed by in 1974.

The only other spacecraft to orbit Jupiter was the Galileo, which arrived in 1995 and was intentionally directed into the planet in 2003.  (NASA is concerned that a spacecraft potentially contaminated with Earthly life could hit Europa or one of the other moons considered possibly habitable, and so the agency ends missions with these death plunges.) While Galileo traveled out from Jupiter to some of its moons, Juno will be all Jupiter all the time.

Unlike Galileo, Juno’s orbit will take it over the planet’s poles, providing a first close look at those volatile regions — where radiation and magnetic forces are especially strong.  In terms of radiation, for instance, the background level on Earth is .4 rads.  During its mission, Juno will be exposed to 20 million rads.

Repeated close passes over the poles and the cloud tops are expected to provide answers to unresolved questions regarding its core – is it solid or gaseous – and the abundance of water at different levels.  The water abundances are expected to provide answers to when in astronomical time and where in the solar system Jupiter was formed.

That’s the upside of Juno’s close encounters.  The downside is that the extreme radiation present at the poles and elsewhere around Jupiter will stress the spacecraft enormously.  To minimize the radiation risk, a 400-pound titanium vault at the heart of the spacecraft protects the computers and most essential components of the instruments onboard.

Because of the unprecedented and extreme conditions Juno will face close in to Jupiter, it is expected to have an orbiting lifetime of 20 months, significantly less than Galileo.

 

ASA's Hubble Space Telescope captured images of Jupiter's auroras on the poles of the gas giant. The observations were supported by measurements taken by Juno. (NASA)
Jupiter’s enormous polar auroras — created by its intense magnetic fields — as captured by NASA’s Hubble Space Telescope.  While in transit, Juno collected data that supported and deepened knowledge of what was occurring when the images were collected. (NASA)

The potential usefulness for exoplanet research of the data from a mission within our solar system data is not unique to Juno, although the role of exo-Jupiters of of particular importance.  Shawn Domogal-Goldman, a research space scientist at the Goddard Space Flight Center who is active in planning exoplanet exploration as well as being part of it, said in an email that “we should now view every solar system mission as a close-up encounter with an exoplanet. We can make measurements on Jupiter that we will likely never (at least in our lifetimes or that of our children or grandchildren) make for exoplanets.”

He is especially interested in what Juno learns about Jupiter’s center.

“We have lots of theories on how planets form, and are beginning to gain an understanding of how planets of different sizes form and then migrate to produce the systems we see today,” he wrote. “But a lot of those theories hinge upon core formation for gas giants. Detecting a core of Jupiter would provide major support of those theories, and do so in a way no exoplanet mission ever could.

Many of the Jupiters discovered thus far are quite close to their suns, and so are very different from our Jupiter.  Some are also much larger — nearing the mass of a star.  So few, if any, Jupiters particularly like ours have been found.

But that’s not because they’re not there, Bolton said.  They’re just harder to find because of the limits of our current methods and technology for observing.  But like our Jupiter, many doubtless loom large in the formation of their solar systems.

Bolton likes to talk about Jupiter, and its role as our solar system’s first planet, in terms of a recipe.  While the sun is almost all hydrogen and helium, Jupiter has carbon, oxygen, sulfur and other “heavy” elements (that is to say, heavier than hydrogen or helium)  at levels much higher than the sun.  How did that happen?

“We have to constrain the recipe by understanding what elements are there below the top clouds, and how abundant they are.  We know some of this, but there’s a long way to go.”

“This is an essential first step, but then we still have to form the rest of the solar system from there… Keep in mind that the process that led to life on Earth really begins at Jupiter.”

By piecing together the story of how Jupiters form, he said, scientists will inevitably gain essential knowledge about how solar systems form, our own and those billions more very far away.

 

 

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Juno, Jupiter and Exo-Jupiters

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Artist rendering of Juno spacecraft in orbit around Jupiter. NASA
Artist rendering of Juno spacecraft in orbit around Jupiter. NASA

The last NASA mission to orbit Jupiter, the Galileo, was designed, flown and its data analyzed as if it was circling the only Jupiter in the sky.

This is hardly surprising since the spacecraft launched in 1989, before the exoplanet era had arrived.  Ironically, Galileo entered its Jupiter orbit in late 1995,  just a few months after the first exoplanet was detected.

That planet, 51 Pegasi b, was a Jupiter-sized planet shockingly close to its host star, and its location and white-hot temperatures turned upside down many then-current theories about gas giant planets and their roles in the formation of solar system.  Scientists are still struggling to make sense of what 51 Pegasi b, and the 250 or so Jupiters found after it, are telling us.

So the Juno mission, which is scheduled to begin orbiting Jupiter on July 4, will arrive at a planet understood quite differently than when Galileo made its appearance.  Juno was built first and foremost to unravel some of the enduring mysteries of the planet:  When and where was it formed?  Does it have rocky core?  Is there water deep in the atmosphere?

But the spacecraft and its instruments will do their unraveling within our current, very different galactic context, where exoplanet scientists will be waiting for results with nearly as much eagerness and anticipation as solar system and planetary scientists.  And the findings from Juno may well have as much impact on the subsequent study of the many, many Jupiter-like planets known to exist in other solar systems as it does on the study of our solar system and its formation history.

Scott Bolton, principal investigator for Juno, recently told a NASA gathering that one of the primarily goals of Juno is to learn, through exploration of Jupiter, “the recipe” for the formation of our planets, our solar system, and those solar systems and planets well beyond Earth.

This is possible because Jupiter was the first planet formed after our sun, which is made almost entirely of hydrogen and helium.  Jupiter is also largely made up of those two elements, but it does have some additional heavy elements that somehow got there — carbon, nitrogen, phosphorus, important gases.

“We don’t know exactly how that happened, but we know that it’s really important,” Bolton said.  “That’s because the stuff that Jupiter has more of is what we’re all made of made of, and is what Earth is made out of, and what life comes from.  So really learning about that history is critical if we’re going to figure out how we got here…and how we find other systems like the Earth elsewhere.”

Artist rendering of the formation of a solar system in its early stages. (NASA/JPL-Caltech
Artist rendering of the formation of a solar system in its early stages. (NASA/JPL-Caltech

Jonathan Lunine, Director of the Cornell Center for Astrophysics and Planetary Sciences,at Cornell University, is a Juno team member with a background in planet and exoplanet formation.  He said that while Juno was not designed “with exoplanets in mind, per se,” its findings will have inherent and significant relevance for exo-Jupiters elsewhere.

“Juno was designed to tell us of the origin and evolution of Jupiter,” he said. “But, clearly, one should think of our solar system’s giant planets as the touchstone to be used, with exoplanet observations, to understand how planetary systems form in general.”

One of those scientists who will be looking to Juno for insight into Jupiter-like exoplanets is Hannah Wakeford, a fellow at Goddard Space Flight Center who studies the atmospheres of hot Jupiters like 51 Pegasi b.

“Juno may well answer some of the outstanding big questions about Jupiter, and that new information will be enormously helpful in studying other gas giants similar to Jupiter,” she said.  “What Juno finds certainly won’t apply directly to all Jupiter-mass planets,  but it will give real world data that can go into our models and very much help limit the possible explanations.”

One of the most important goals of the Juno mission is to determine whether Jupiter has a rocky/icy core or is gaseous, or mostly gaseous, all the way through.  This issue has been hotly debated for years, and Juno should provide data to settle the issue.

Hannah Wakeford, a research fellow at Goddard specializing in the atmospheres of hot Jupiters.
Hannah Wakeford, a research fellow at Goddard specializing in the atmospheres of hot Jupiters.

Then there is the effort to measure how much water and oxygen Jupiter has in its lower atmosphere, below the thick top layer of clouds.   The spacecraft has instruments that can tease out some answers, and they, too, will become central to future Jupiter science.

The same issues loom large when it comes to extra solar gas giants. Understanding water abundances and the presence (or absence) of a solid core is considered essential to characterizing exo-Jupiters, and to learning about their histories.

As Wakeford explained it, the question of a core is key to understanding how and where a Jupiter-sized planet formed.  If there is a rocky core, then the planet most likely began as a small planetesimal and was in the right place in the protoplanetary disk to pull in and keep massive amounts of gas and dust.

But if there is no solid or rocky material detected — by measuring the gravitational and magnetic fields of the interior — then Jupiter would be more like a failed star that formed through a gravitational collapse that didn’t have the mass to become a star.

“That information {about whether there is a core or not} would give us one data point for understanding other Jupiter-sized planets, and there are definitely problems with that.  But that data point would be one more than we have now,” Wakeford said.

Juno should answer the long-debated question of whether or not Jupiter has a rocky, solid core. If it does, the implications for understanding the planet -- and many exo-Jupiters of similar sizes -- are great.
Juno should answer the long-debated question of whether or not Jupiter has a rocky, solid core. If it does, the implications for understanding the planet — and many exo-Jupiters of similar sizes — are great. (NASA)

The issue of water abundance is also key.  Juno has a microwave instrument that can see deep inside the planet, piercing through the many layers of clouds.  The amount of water present (likely in crystal or vapor form) provides an important clue about where the planet was formed in its disk —  inside the ice line of the solar system, or outside.

Morever, the abundance of water has implications for Jupiter and exo-Jupiter migrations. Wakeford said that if Jupiter turns out to have significantly more, or significantly less, water than what is predicted for a planet that formed at its current location in the solar system, that would suggest the planet migrated to that orbit at some point in its history.  And if that method succeeds in nailing whether or not Jupiter has migrated significantly during the eons, then it could be used for exo-Jupiters, too.

For Lunine, the issue of water abundance is particularly compelling.   He said that in the years ahead, he plans to use some of his dedicated time on the James Webb Space Telescope (to be launched in 2018) to observe and analyze exo-Jupiter atmospheres from the perspective of whether they, like our Jupiter, have increased amounts of oxygen (from water) and carbon compounds relative to their host stars.  The information has the potential to explain a lot about the planet’s formation and history.  He said others in the field will surely be focused on this potentially revealing exo-Jupiter enrichment as well.

Although many of the early exoplanets detected were Jupiter size and larger, with a fair number of those “hot” Jupiters orbiting surprisingly close to their host stars, that turned out to be an artifact of the observing techniques.  These large bodies orbiting close to their stars simply were the easiest to detect.

Scott Bolton, principal investigator of the Juno mission. (NASA)
Scott Bolton, principal investigator of the Juno mission. (NASA)

But the Kepler Space Telescope and other planet-finding instruments have identified more than 3,000 planets smaller than the Jupiters, and the expectation is that future discovery methods will show that Jupiter-size exoplanets are relatively rare and that planets smaller than any detected so far are most common.  Nonetheless, Jupiters will remain central to exoplanet research because, as Juno principal investigator Bolton said, they contain that astrophysical and chemical recipe for all that came later.

Adding to the interest (and challenge), Jupiter is, as Bolton described it, a “planet on steroids.”  It 300 times more massive than Earth, and at the planet’s center the temperature is several times hotter than the surface of the sun.  The pressure is tens of millions times the air pressure of Earth.  In this environment, scientists have concluded that the abundant hydrogen is in a liquid metallic form.

In addition to its focus on the formation and evolution of the planet (through the search for a core and measure that water (oxygen), the Juno mission will also study Jupiter’s magnetic fields, which is 20,000 times more powerful than Earth’s and by far the strongest in the solar system.  The extreme magnetism is a function, scientists believe, of the presence of that metallic hydrogen and the speedy rotation of the planet, which is day is but 10 hours long.

Outer jets and belts composed largely of ammonia and hydrogen sulfide gas can block study of the inner atmosphere. Winds blow the cloud regions in different directions. (NASA)
Outer jets and belts composed largely of ammonia, ammonium hydrosulfide and water clouds can block study of the inner atmosphere. Winds blow the cloud regions in different directions. (NASA)

The Juno mission will include 37 passes closer to the Jupiter than any previous spacecraft — 2,600 miles above the upper clouds.  Those layers are made up of largely ammonia and hydrogen sulfide, with the H2O clouds much deeper in the atmosphere.  Working out ways to see through or around those upper clouds into the far more scientifically important atmospheres is another high priority task for Juno.

Opaque clouds and hazes are common to exoplanets, too, and especially the larger ones.  Some hot Jupiters, for instance, have clouds of iron oxides surrounding them, blocking efforts to look into their far more important atmospheres.  Developing techniques to pierce through those outer layers of hazes and clouds will be another potential Juno boon to exoplanet study.

 

 

 

 

 

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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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The Borderland Where Stars and Planets Meet

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Brown dwarfs -- like the one illustrated here - are more massive and hotter than planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to Jupiter's, with wind-driven, planet-size clouds. (NASA/JPL-Caltech)
Brown dwarfs — like the one illustrated here – are more massive and hotter than planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to Jupiter’s, with wind-driven, planet-size clouds. (NASA/JPL-Caltech)

Results from two very different papers in recent weeks have brought home one of the more challenging and intriguing aspects of large exoplanet hunting:  that some exoplanets the mass of Jupiter and above share characteristics with small, cool stars.  And as a result, telling the two apart can sometimes be a challenge.

This conclusion does not come from new discoveries per se and has been a subject of some debate for a while.  But that borderland is becoming ever more tangled as  discoveries show it to be ever more populated.

The first article in The Astrophysical Journal described the first large and long-lasting “spot” on a star, a small and relatively cool star (or perhaps “failed star”) called an L dwarf.  The feature was similar enough in size and apparent type that it was presented as a Jupiter-like giant red spot.  Our solar system’s red spot is pretty well understood and the one on star W1906+40 certainly is not.  But the parallels are nonetheless thought-provoking.

“To my mind, there are important similarities between what we found and the red spot on Jupiter,” said astronomer John Gizis of the University of Delaware, Newark.  “Both are fundamentally the result of clouds, of winds and temperature changes that create huge dust clouds.  The Jupiter storm has been going for four hundred years and this one, well we know with Hubble and Spitzer that it been there for two years, but it’s probably more.”

A far cry from 400 years, but the other similar storms and spots identified have been on brown dwarfs — failed stars that start hot and burn out over a relatively short time.  Gizis said some large storms have been detected on them but that they’re gone in a few days.

The dust and wind storm on the L dwarf W1906+40 rotates around the cool star every nine hours and is large enough to hold three Earths. L-dwarfs mark the boundary between real stars and “failed stars” only the most massive L dwarfs fuse hydrogen atoms and generate energy like our sun. Most L dwarfs known are brown dwarfs, also known as “failed stars,” because they never sustain atomic fusion. (JPL/NASA-Caltech)
The dust and wind storm on the L dwarf W1906+40 rotates around the cool star every nine hours and is large enough to hold three Earths. L-dwarfs mark the boundary between real stars and “failed stars.” . Most known L dwarfs are brown dwarfs, also known as “failed stars” because they never sustain atomic fusion, but the most massive L dwarfs can fuse hydrogen atoms and generate energy like our sun.  (JPL/NASA-Caltech)

 

The second article came from Alexandre Santerne of the Instituto de Astrofísica e Ciências do Espaço, Portugal and Aix Marseille University, France, and was shared and widely discussed at the recent Extreme Solar Systems meeting in Hawaii.  In the paper, the researchers report that a high percentage (55 percent) of the very large exoplanet “candidates” listed by the Kepler mission are in fact not exoplanets. Santerne and colleagues spent a year’s worth of nights between 2010 and 2015 observing, via the radial velocity method, 129 of Kepler’s more than 4,000 planet candidates.  Their tool was the SOPHIE spectrograph at Haute-Provence Observatory  in southeastern France.

The Kepler science team has long predicted that the “false positive” rate for these very large radii planets would be high — a projected 30-40 percent rate for candidates larger than Jupiter versus less than 10 percent false positive rate for candidates smaller than Jupiter.  But this even higher percentage came initially as something of a worrisome surprise.

Many of what the Santerne team described as “false positives” were determined to be multi-star systems (rather than a star with planets)  while three were identified as brown dwarfs, those  small, cool failed suns.

Said team member Vardan Adibekyan of the Centre for Astrophysics of the University of Porto:  “Detecting and characterizing planets is usually a very subtle and difficult task. In this work, we showed that even big, easy to detect planets are also difficult to deal with.”

While finding many false positives, the Santerne team also confirmed 45 Kepler very large planet candidates, fifteen more than had been confirmed before.

Size comparison of stellar vs substellar objects. (Credit: NASA/JPL-Caltech/UCB).
Size comparison of celestial objects from our sun to Earth. (NASA/JPL-Caltech/UCB)

Natalie Batalha, Mission Scientist for the Kepler Space Telescope mission,  said that at first glance the reported false positive rates seemed higher than expected based on predictions by the Kepler team,  in particular the modeling work of astrophysicist Timothy Morton of Princeton.

But after a careful read and some number crunching, Batalha said she came away confident that the new results do not reflect any flaws in the planet identification process itself and, in fact, agree with predictions.  The apparent rise in the false positive rate, she said, can be attributed to a more liberal inclusion of larger exoplanet “candidates” initiated in 2014 by the Kepler mission.

Previously, planet candidates more than twice the radius of Jupiter were all discarded because no planets above that line had ever been detected — they were deemed “astrophysical false positives”.   But they were returned to the “candidate” list a year ago so that scientists could explore the transition between giant planets and brown dwarfs and small stars.   Once these larger-than-two-Jupiter “candidate” planets were folded back into the Jovian planet group, Batalha says, the false positive rate for the group naturally shot up. Which is predictable, since no two-Jupiter planets were identified by the Santerne group.

Nonetheless, she said, the results reflect and illustrate the complex nature of large exoplanet detection and characterization. “The truth is that we don’t know a lot about the transition from giant planets to stars.  It’s an important subject and this team is one of the few working on it.”

Colors plotted here represent the average expected false positive rate for candidates of a given size (radius on the y-axis) and orbital period (x-axis). For candidates smaller than Jupiter, the expected false positive rate is less than 10%. For planets between one and two Jupiter radii, the false positive rate jumps up to 38%. For objects larger than twice the size of Jupiter, the false positive rate increases to 90%. White points show the properties of the candidates observed by the Santerne team. (NASA Exoplanet Archive/N. Batalha, T. Morton
Colors plotted here represent the average expected false positive rate for candidates of a given size (radius on the y-axis) and orbital period (x-axis). For candidates smaller than Jupiter, the expected false positive rate is less than 10%. For planets between one and two Jupiter radii, the false positive rate jumps up to 38%. For objects larger than twice the size of Jupiter, the false positive rate increases to 90%. White points show the properties of the candidates observed by the Santerne team. (NASA Exoplanet Archive/N. Batalha, T. Morton

 

As determined by the International Astronomical Union, any celestial object with a mass greater than 13 Jupiters should be considered a star.But according to Jonathan Fortney, an exoplanet and brown dwarf theorist at the University of California, Santa Cruz, this definition leaves a lot of researchers cold because it doesn’t take into account how the object was formed. Did it form in a giant molecular cloud (like most stars)?  Or in orbit around a parent star, by slowly adding on large amounts of gas, atop a solid core of rock and ice (like most planets)? Or as a result of gravitational instability in a disk (a theory that suggests the formation of massive gas giant planets as the result of a quick pulling together of disk material to form dense clumps)?

“It seems clear that star formation can make objects less massive than ten Jupiters and we can see planets more massive than several Jupiters in disks around stars.  So there’s an overlap here, and we don’t always know when star formation stops and planet formation starts,” Fortney said. “That why it’s so important to learn about the composition and evolution of the objects to figure out what they are.”

Artist's conception of the clouds on Kepler-7b, compared for size with Jupiter (right). Many exoplanets and brown dwarfs have mostly hydrogen-helium atmospheres that are covered in layers of mineral dust, while Jupiter’s hydrogen-helium atmosphere has clouds of ammonia. (NASA/JPL-Caltech/MIT)
Artist’s conception of the clouds on Kepler-7b, compared for size with Jupiter (right). Many exoplanets and brown dwarfs have mostly hydrogen-helium atmospheres that are covered in layers of mineral dust, while Jupiter’s hydrogen-helium atmosphere has clouds of ammonia. (NASA/JPL-Caltech/MIT)

And of particular interest in that borderland are brown dwarfs, convincingly identified only twenty years ago.

As Fortney explained, brown dwarfs are formed in the same vast clouds that produce stars by the hundreds, but don’t have sufficient mass to build the internal pressure needed to begin the nuclear fusion of hydrogen that defines a star.  Still, the gravitational energy of a brown dwarf does get converted into heat and so they can warm their surroundings before cooling like embers leaving a fire.  Some researchers even hold that planets could form around brown dwarf and protoplanetary disks have already been found around a few of them.

What particularly fascinates Fortney about brown dwarfs is that they have atmospheres and winds and weather, and as a result offer some potential insights into larger exoplanets, especially those surrounded by thick dust clouds.

This overlay of suspended minerals (sometimes exotic metals like aluminum oxide and magnesium-rich forsterite — a form of silicate rock — and irons) have made it very difficult if not impossible to look spectroscopically at the atmospheres of many exoplanet.  But depending on the temperatures and compositions of the dust clouds, astronomers sometimes have more luck  looking through the clouds and haze of brown dwarfs.

But still, the process of getting information about distant atmospheres is painstaking and Fortney said his work with brown dwarfs provides “a window into just difficult it is and will be” with exoplanets.  Basic questions like temperatures, what kinds of molecules are present and in what abundances — they’re all veiled by the dust clouds.

Cosmic dust surround a brown dwarf in the making. ALMA (ESO/NAOJ/NRAO)/M. Kornmesser
Artist’s rendering of a brown dwarf surrounded by cosmic dust and gases. ALMA (ESO/NAOJ/NRAO)/M. Kornmesser

Progress, however, is being made, both in terms of technical approaches to “seeing” through the clouds, and the science of these objects.  Even gigantic exoplanets appear to have clouds and dynamic atmospheres, Fortney said, “and I think we’ll see that across the board.”

Batalha also identified a related bit of progress.  The Santerne paper identified three brown dwarfs in the Kepler candidate list, she wrote, and so they produced the beginning of an occurrence rate for brown dwarfs. In addition, the paper published an occurrence rate for warm Jupiter-size planets within one astronomical unit or AU (roughly the distance from the sun to Earth) of their own sun.

Putting the two observations together, and you reach the conclusion that warm Jupiters are 15 times more common than brown dwarfs in similar one AU orbits.

That, she said, is the intriguing  news coming from the giant planet/failed star borderland.

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