A new spacecraft is en route to the king of planets. NASA’s Juno mission will arrive at Jupiter July 4 to study our solar system’s largest world up close and personal. Once its primary mission starts around November, Juno will spend at least a year and a half examining the planet's interior and weather. But some scientists are interested not in what Juno can tell us about Jupiter but in what it could reveal about planets much farther away. They hope that in gathering such detailed information about our own gas-giant planet, Juno will help reveal how giant worlds beyond our own solar system were born and behave.
Scientists have already discovered hundreds of Jupiter-size planets circling other stars, and suspect those are just the tip of the iceberg. Some are known as “hot Jupiters,” because their tight orbits around their parent stars raise them to scorching temperatures. Other Jupiters circle in highly eccentric—that is, oblong—orbits, which is again unlike our own neighborhood. No one knows why some planets end up in such eccentric or close orbits whereas others—like Jupiter—revolve in relatively circular paths and from farther distances.
One theory is that planets start in different orbits and migrate over time. Juno will look for hints that Jupiter may have formed somewhere else than it is now by determining if there is less water and oxygen inside the planet than is likely if its building blocks came from its current location in the solar system. If so, the planet may have formed farther out from the sun where the environment is colder and later traveled inward. Such a finding would have implications for models predicting the formation of other gas giants in other systems. It is unlikely, however, that Jupiter has migrated very far, Juno principal investigator Scott Bolton of the Southwest Research Institute says, and he cautioned that the spacecraft will not be able to do a direct test of orbital migration, because whatever scientists learn about Jupiter’s interior, several formation scenarios may still be in play. “One theorist may update the model based on that [new] data by moving Jupiter out,” he says, “but somebody else may change the [formation] conditions and keep Jupiter where it's at.” [See a slide show of Juno’s mission to Jupiter]
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Scientists have similar questions about whether most of the gas-giant exoplanets they see formed in their current locations or moved around. Hot Jupiters, for example, do not seem likely to have formed where they are because at such proximity to stars, most of the planet-building material would have been scarce, theories suggest. Likewise, researchers have trouble understanding how gas giants can form extremely far away from their stars because of a similar lack of building blocks. Juno's data could therefore help scientists better understand how exoplanetary systems got their layouts, says Jack Lissauer, a staff scientist at NASA Ames Research Center. Any evidence that Jupiter has migrated could support the idea that other giant planets are also likely to migrate.
Such investigations will be just part of Juno’s efforts to learn more about what Jupiter is made of. Despite 400 years of telescopic observations of the giant planet and 40 years of periodic close-up spacecraft studies, researchers still understand little about Jupiter's formation history. Basic mysteries on whether the planet has a core—and if it does have one, the size—as well as how much water the Jovian interior holds stymie scientists.
Unlike past spacecraft sent to Jupiter, Juno is equipped with a microwave radiometer, an instrument that can measure how water vapor contributes to refracting the atmosphere at microwave frequencies, to look beneath the cloud tops to study water content and weather processes. The spacecraft will also measure Jupiter's magnetic and gravity fields with greater precision than its predecessor, Galileo, which studied the planet and its moons between 1995 and 2003, because it will circle the planet in a polar orbit that allows it to get closer to the planet.
Juno’s measurements of Jupiter’s magnetic field could also give insights into the planet’s internal structure. Scientists hope to learn whether deep down a core of heavy elements exists or whether the hydrogen and helium atmosphere goes “all the way down” until the elements compress at the center, Bolton says. Such a core could be helping to generate Jupiter’s magnetic field, but is not required. Scientists can also tackle the question of a core via Juno’s measurements of the planet’s gravitational field because gravitational structure can reflect the convection of heat deep inside, where the pressure is so high that compressed hydrogen acts like a molten metal.
If there is no core at all, Jupiter may have formed similarly to the sun—that is to say it may have gradually coalesced from the “protoplanetary nebula” of gas and dust that birthed our solar system. If researchers do find a core, however, that might indicate that heavy elements floating around when the solar system formed first coalesced into planet-size chunks, which then could have attracted floating gas molecules to create the giant planets Jupiter, Saturn, Uranus and Neptune.
Of course, just because scientists may get clues into how Jupiter formed does not mean they will necessarily know how other gas-giant exoplanets formed, but it is likely that our own giant planet is fairly representative, Bolton says. Jupiter, for instance, is mostly composed of hydrogen and helium, which are the elements that make up most of our solar system as well as most interstellar clouds that collapse to form other solar systems.
For the foreseeable future Juno’s observations will provide the best possible look at what a giant planet's atmosphere is made of, said Raymond Jeanloz, an astronomy professor at the University of California, Berkeley, who studies planetary interiors. For exoplanetary researchers, however, Juno has a key limitation: It is only looking at a single world. There are thousands of known exoplanets too far away for a spacecraft to visit. Scientists are looking forward to two forthcoming space telescopes to measure the atmospheres of many giant planets: the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope (WFIRST). "We are right now just barely getting our first glimpses at atmospheres” with current telescopes, says Heather Knutson, an assistant professor at the California Institute of Technology who studies exoplanetary atmospheres. “With JWST, we will see everything in beautiful detail.” WFIRST's advantage is it will be able to view planets without the overwhelming shine of their stars drowning their own light out, due to a “coronograph” that blocks the stars’ light, she adds.
Juno will spend its first 107 days at Jupiter completing two long orbits to calibrate its instruments and then maneuver to adjust its orbital period to 14 days. The probe will then complete at least 33 of these orbits, which will allow mission scientists to eventually create a full map of Jupiter's cloud tops and also probe beneath their surface. Funding could extend the mission slightly but the intense radiation environment of Jupiter will gradually damage Juno’s instruments and eventually force scientists to deliberately plunge the spacecraft into Jupiter before it is debilitated to the point it cannot be controlled. This measure will prevent any accidental impacts on icy and potentially life-friendly moons nearby, such as Europa, protecting them from chemical contamination by propellants as well as Earth microbes that may have hitched a ride on the spacecraft.