Juno, Jupiter and Exo-Jupiters

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.”

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.

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.

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.

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.

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.