Juno at Jupiter
On this coming Forth of July, NASA’s Juno spacecraft will arrive at Jupiter and enter polar orbit. Over the next 20 months, it will make 37 orbits skimming to within 5,000 kilometers of the polar cloud tops. In Greco-Roman mythology, Hera/Juno had the ability to see through Zeus/Jupiter’s clouds and
discover what secrets he was keeping under them: Juno’s main mission objective is to probe beneath the cloud layer and to determine whether Jupiter has a solid core, like the other gas giants in the solar system. We think Jupiter formed in the same way as other giants planets, by first accreting a solid core and then sucking down nebular gas onto it. But we have never yet flown a spacecraft close enough to Jupiter to measure the distribution of mass deep inside the planet. Juno will fill that gap in our observations and allow us to fit Jupiter (and by extension, Jupiter-sized exoplanets) into our models more accurately.
It’s hard to get the public excited about the interiors of planets, but understanding them is essential
for understanding how planets form. Today we know of over 3,000 exoplanets, most of them in the one patch of sky observed by the Kepler telescope. Next year NASA’s TESS mission will begin an all-sky survey of exoplanets in the solar neighborhood. The year after that, the James Webb Space Telescope will begin examining exoplanets in individual depth, not merely adding to catalogs of orbital parameters, but gleaning their individual chemical and physical properties. But here in our own Solar System there are still gaps in our knowledge of how planets and smaller bodies formed, and these objects we can actually visit, at least robotically.
Since the Juno mission is focused on Jupiter’s formation and interior, I thought a few diaries on planet formation would be a good lead-in to to next month’s July 4 arrival at Jupiter. This first diary is an overview of the planet formation process. Future diaries (if written!) may focus on giant planets, terrestrial planets, the smaller bodies of the solar system, and exosolar planetary systems with hot Jupiters and other exotica.
Planet Formation ACCOMPANIES STAR FORMATION
Planets form together with stars, in dense molecular clouds called star forming regions. Star forming regions abound in our galaxy: the spiral arms of a typical spiral galaxy like our own Milky Way are
delineated by their bright patches; these are conspicuous when you look at a spiral. Not all galaxies have them: most giant elliptical galaxies do not. It seems to take a certain amount of ongoing gravitational disturbance to a galaxy to sustain star forming regions, as well the right (cooler) temperatures in a galaxy’s gas. In the Milky Way, it may be the gravitational disturbances are caused by orbiting dwarf galaxies and dark matter clouds, some of which pass through the galactic disk. Exactly why star formation shuts down in most elliptical galaxies is imperfectly understood and is an ongoing area of research. However, the basic recipe for making stars is that you need gas, cool enough to condense, and you need to stir the pot a little. Gas is not permanently used up by star formation, because stars put gas back into the interstellar medium at the end of their lives — the returned gas is enriched with heaver elements by stellar nucleosynthesis.
When stars form, they are accompanied by circumstellar disks of dense nebular material. This material is
composed of gas and interstellar grains — tiny needles of carbon, metals or silicates onto which volatile ices have condensed. The needles are formed in the atmospheres of giant stars and in material ejected from supernova.
When stars form in a region, some are more massive than others and the more massive stars run through their main sequence lifetime (hydrogen burning phase) more quickly than less massive stars. The most massive stars burn hydrogen for only a few million years before swelling into giant stars and then exploding as supernova. The molecular cloud is enriched by the elements produced by nucleosynthesis in these more massive stars, which can then go into forming planets. Chemical reactions occur in molecular clouds, driven by uv light from hot stars and particle radiation; these reactions can produce many kinds of molecules.
PROTOPLANETS FORM and GROW
Within 100 AU of a young star, a circumstellar disk can reach temperatures > 50,000 K. Chondritic meterorites (aka “chondrites”) contain calcium-aluminium inclusions (CAIs) that melt above 1300 K, and are dated to 4568.22 ± 0.17 million years old — which is when the solar nebula had cooled enough for them to condense. CAIs are the oldest solid material known in the solar system. Chondrites are named after the
spherical chondrules they contain, which condensed a few million years later than the CAIs, when the nebula had cooled further. At this point the solar nebula — at least in the region of the chondrules — contained vast numbers of these tiny round pellets. They were traveling in very circular orbits at the same speed as their neighbors, and rapidly clumped together into protoplanetary bodies and planetesimals: basically small asteroids, and in the region with ices, comets.
Protoplanets are also moving in circular orbits with low mutual velocities and they gravitate to each other and coalesce. Above a certain size, gravity begins to pull objects into a spherical shape, and radioactive elements (produced by the supernovae in star forming regions) heat them internally, causing iron and siderophile elements to move to the center of the body, forming a metallic core, while silcates and chalcophile elements become a mantle. These have now become differentiated protoplanetary bodies. Some
differentiated protoplanets got smashed up after differentiating, creating nickel-iron meteorites (metallic core material), stony meteorites (silicate mantle material), and strange and rare meteorites called Pallasites, which come from the core-mantle boundary.
core accretion and Isolation masses
The protoplanetary bodies grow in size through further gravitational accretion until a certain limit is reached, at which each one has largely cleared its “feeding zone”. At this point, they are called “isolation masses”. (Some authors call them “oligarchs” — a practice I shall not follow here. Also cf. the related concept of a Hill sphere) Isolation masses range in size from roughly Earth’s moon to Mars. It takes anywhere from 10 to 100 of them to make a terrestrial planet like Earth. The entire process, from CAIs to isolation masses, takes only a few million years — less than 10 million.
In colder parts of the protoplanetary nebula, where ices are solid and abundant, it takes only a few million years to
assemble the cores of giant planets. These cores are >10 Earth masses, but they form quickly in the few million years before the gas dissipates from the nebula, and gas rapidly accumulates on the cores to blow the planets up to Jupiter or Saturn size — and even larger giants have been found in exoplanet systems. The ice giants Uranus and Neptune formed a bit too late to capture much of the nebular gas, which had dissipated by then, but there was enough ice for them to grow to >10 Earth masses.
PLANETARY MIGRATION
It takes longer for a terrestrial planet to assemble than it does for gas giants. By the time the solar system was a few million years old, there might have been ~100 rocky isolation masses rattling around the inner solar system. Jupiter and Saturn first migrated inwards for a short time (this is called the Grand Tack hypothesis) then reversed course and moved out to where they are now. On the way inwards, Jupiter scattered a lot of material and limited how much was available to make Mars, which is why Mars is still basically just a big isolation mass. In some exoplanet systems, the giant planet moves closer to the star than Mercury is to the Sun, forming a “hot Jupiter”; surprisingly, this doesn’t necessarily stop multiple smaller planets from orbiting even closer to the star. But in our system, Saturn saved the day by clearing material behind Jupiter, which reversed the inward migration. On the way back out, Jupiter and Saturn hit a 1:2 orbital resonance which threw the outer solar system into the form it has today (this is called the Nice model.) The Oort Cloud of comets was formed during this orbital resonance period. Recently, evidence for a possibly similar outer zone of comets has been detected around the star HD 181327. Did a migration of giant planets and an episode of orbital resonance also shape this system?
TERRESTRIAL PLANETS AND GIANT IMPACTS
It takes several 10s of millions of years — possibly as much as 100 million years — for Earth-sized terrestrial planets to form from rocky isolation masses. The isolation masses rattle around and occasionally hit each other… not at really high speeds, which would only blow them apart again, but mostly at lower speeds which allow them to coalesce. Mercury seems to have lost most of its mantle in one such giant impact, and Venus may have gotten its slow retrograde rotation from another. Earth got the Moon out of its final giant impact with an isolation mass, probably one that had formed at either the L4 or L5 Lagrange point of Earth’s orbit.
This may not be an extremely rare event: it seems the Pluto/Charon system formed in a similar way to the Earth/Moon system, by a grazing large impact, and one theory for how Neptune got its moon Triton in an unusual retrograde orbit is that Triton was originally yet another binary system like Pluto/Charon, but that Triton’s companion was ejected during the capture at Neptune. (Capturing a moon is impossible if only two bodies are involved, but becomes possible with 3.) Since slow-motion collisions are a common theme of planet formation, grazing collisions occasionally resulting in binary or binary-like planets may not be an exceedingly rare phenomenon -- particularly not if the process of companion isolation masses forming at leading or trailing Lagrange points is not rare. Perhaps more than a few Earthlike planets in the galaxy have large moons in their skies.
Quite a few isolation masses were probably either ejected from the solar system in these early years or are still orbiting out in the Oort cloud. It is also likely that Earth sized planets and even giant planets are often ejected during planet formation; rogue planets — planets not bound to any star — may be quite common in the galaxy.