A planet 10 times larger than the Earth may have collided with Jupiter as planets were first taking shape, a new computer simulation reveals. This impact, happening 4.5 billion years ago, would have stirred the young core of Jupiter, altering the largest planet of the Solar System until our own time.
Unexpected readings from the Juno spacecraft, studying the massive planet since 2016, found the core of the gas giant is both larger, and less dense, than calculations had predicted, altering what we thought of about the inner structure of Jupiter. This observation sparked researchers to investigate the mystery.
“This is puzzling. It suggests that something happened that stirred up the core, and that’s where the giant impact comes into play,” Andrea Isella, astronomer at Rice University, explains.
Jupiter’s Accidental Encounter
Theories of planet formation suggest that Jupiter started as a dense, frozen, rocky world. This body collected vast quantities of gas over time from the primordial disk forming the Solar System. The core, deep beneath the clouds, was thought to have a smooth surface, the result of enormous gravity and pressure.
Juno measured the gravitational field around Jupiter, revealing a different model.
“Instead of a small compact core as we previously assumed, Jupiter’s core is ‘fuzzy.’ This means that the core is likely not made of only rocks and ices but is also mixed with hydrogen and helium and there is a gradual transition as opposed to a sharp boundary between the core and the envelope,” explains Ravit Helled, professor at the University of Zürich, and team member on the Juno mission.
The idea of a massive impact between Jupiter and a large body could explain the observations, researchers concluded. A direct impact between Jupiter and a body of 10 Earth masses would have mixed dense layers deep in the core with less-dense material sitting closer to the surface.
“[A] sufficiently energetic head-on collision (giant impact) between a large planetary embryo and the proto-Jupiter could have shattered its primordial compact core and mixed the heavy elements with the inner envelope,” researchers describe in an article published in the journal Nature.
Such an impact would take “many, many billions of years” to recover, Isella describes.
Getting to the Core of What Happened
Impacts which grazed Jupiter at an angle were caught in the gravitational field of Jupiter and swallowed, while bodies the size of Earth disintegrated in the atmosphere of the massive world.
The model found an impact could have resulted in the core we see today, but it would have required an impact with a body 10 times larger than the Earth, in a head-on collision with Jupiter.
“Because it’s dense, and it comes in with a lot of energy, the impactor would be like a bullet that goes through the atmosphere and hits the core head-on. Before impact, you have a very dense core, surrounded by atmosphere. The head-on impact spreads things out, diluting the core,” Isella said.
The gravitational influence of Jupiter would have altered the course of small planetary embryos in the early Solar System, drawing them toward the rapidly-growing world. The tremendous mass of the world would also focus incoming bodies, greatly increasing the frequency of head-on collisions, the model determined.
A study published in March concluded that Jupiter formed in the outer reaches of the Solar System, and migrated inward, closer to the Sun.
Juno has Long Eyes
Juno launched on August 5, 2011, and arrived at Jupiter on July 4, 2016. The mission was originally scheduled to orbit the massive world once every 14 days. However, concerns about valves in the spacecraft’s fuel system caused mission engineers to raise its orbit to a point where the spacecraft takes 53 days to complete a single orbit. This change in orbit slows down data collection, extending the total length of the mission by 41 months. This extended mission could reveal new data about, and images of, the largest planet in the Solar System.
While in orbit around Jupiter, Juno will measure the amount of water in the Jovian atmosphere, providing data on the formation of that world. The spacecraft will analyse the atmosphere, and the magnetic and gravitational fields of the massive gas giant. And Juno will observe aurora — northern and southern lights — that put those seen on Earth to shame. This study will provide significant data on how the magnetic field of Jupiter reacts with the atmosphere of that giant world.
Juno will orbit Jupiter until July 2021, when the spacecraft will dive into the atmosphere of the gas giant, destroying the vehicle. This terminal command to Juno will ensure any microbes which may have hitched a ride to Jupiter aboard the spacecraft will never land on one of the dozens of moons of Jupiter.
Looking to the Future
Astronomers are currently bathing in new discoveries of worlds around other stars. Since 1992, we have learned of more than 4,000 planets in alien solar systems. Most of these worlds are large, as these worlds (and those close to their parent stars) are the easiest to find.
“We must believe then, that as from hence we see Saturn and Jupiter; if we were in either of the Two, we should discover a great many Worlds which we perceive not; and that the Universe extends so in infinitum.”
— Cyrano de Bergerac
The data collected by Juno at Jupiter could assist astronomers seeking to understand conditions on these far-flung worlds.
One unusual feature seen around a fraction of alien stars are diffuse “flashes” of infrared light, which shine a few years before fading away. These may be debris of planets colliding with each other, Rice researchers suggest.
In Greek and Roman mythology, Jupiter would wrap himself in clouds to hide his misdeeds. His wife, the Goddess Juno, would peer through the clouds to reveal her husband’s deepest secrets. Today, Juno the spacecraft, together with computer modeling, is revealing ancient secrets hiding deep within the history of Jupiter.
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