jupiter: Solar System Oddball
Astrophysicists think they know why the gas giant’s evolution was delayed for two million years
What do we know about Jupiter? Rather a lot, as it happens. We know that it’s a giant ball made up of mostly hydrogen and helium, that it is the fifth planet from the Sun, that it’s over 11-times the diameter of Earth and that if you could mash all of the other seven planets into a big splodge Jupiter would still be 2.5-times more massive.
After the Sun it’s the largest body in the Solar System, and it is also very old. But just how old has been the subject of a lot of research over the years, and only recently has the answer emerged. An international group of scientists say Jupiter is roughly as old as the Solar System itself, with the planet’s solid core forming about a million years after the Solar System came into being. And yet there is also much evidence, thanks to a fascinating new study, that it suffered some growing pains along the way.
Lead author of the study, Dr Thomas Kruijer, now of the Lawrence Livermore National Laboratory in California, United States, dated Jupiter at some 4.5 billion years old in 2017. Formerly of the University of Münster, Germany, Kruijer sought to study meteorites in close detail. The team looked at tungsten and molybdenum isotopes on iron meteorites found on Earth, knowing that these dense metals would have been in great abundance within the core of bodies many hundreds of kilometres in size at around the time the giant planets formed.
In doing so, they discovered something rather startling. By using molybdenum and tungsten isotope measurements the scientists were able to determine a meteorite's birthplace and age. But they also found that meteorites, which are mostly derived from the collision of asteroids and date back to the early Solar System, could be separated into two genetically distinct groups. These, they discovered, coexisted in different nebular reservoirs between a million and ~3–4 million years after the Solar System formed – and it led to the theory that the formation of Jupiter was the reason why they were kept apart for so long.
“The idea is that the early Solar System contained two types of solid material: one located closer to the Sun than Jupiter’s orbit and one further away,” explains astrophysicist John Chambers, from the Carnegie Institution for Science. “A leading theory for how Jupiter formed is that it started small and grew larger over time, beginning as a solid planet like Earth, eventually becoming massive enough for its gravity to pull in gas from the solar nebula that surrounded the young Sun. As Jupiter grew it became massive enough that its gravity prevented small [millimetre-to-metre sized] solid particles called ’pebbles’ in the solar nebula from crossing Jupiter’s orbit. This gave rise to the two separate populations of particle seen in meteorites.”
For Jupiter to have produced this effective barrier, Dr Kruijer said the planet's core had to be close to 20 Earth masses within a million years. It then reached 50 Earth masses until at least ~3–4 million years, and eventually grew to 317.8-times the mass of the Earth as it continued to accrete gas around its core. Therein, however, lay another puzzle.
“Dr Krujier et al showed a chronology for Jupiter’s growth based on the meteoritic record, but no explanation was given on how it could have taken two million years for Jupiter to pass from ~10 Earth masses to ~50 Earth masses,” Julia Venturini tells us. With that in mind, the astrophysicist at the University of Zürich began to delve deeper with
“A leading theory for how Jupiter formed is that it started small and grew larger over time, beginning solid like Earth” John Chambers
Yann Alibert, science officer of PlanetS. “Kruijer et al demonstrated the time sequence, but did not explore the implication in terms of the formation process. This is what we did,” Alibert affirms.
Three models tend to explain how planets form. The most accepted, the core accretion theory, states that planets grow a small rock-ice core and then gravitationally acquire additional mass. They do so by gathering material left over from the creation of the Sun, with the solar wind causing hydrogen and helium to form gas giants and rocky material producing terrestrial planets. But there is an issue with giant planets needing to form fast, since the gas disc around the Sun only lasted for around three to four million years.
This latter issue has led to the disc instability model which posits that dust and gas clump together early on, then compact quickly to form the giant planets. But there is also the pebble accretion model, which shows that tiny rocks combine quickly to form the large-scale planets. In 2015 it was suggested that the gas-giant planets accreted pebble-sized rocky material formed from dust grains and that this allowed them to build rapidly. Indeed, Dr Harold Levison, an astronomer at the Southwest Research Institute in Colorado, said it was possible for objects such as Jupiter and Saturn to form within a 10-million-year time frame if they gradually accumulated planetary pebbles.
“If the pebbles form too quickly, pebble accretion would lead to the formation of hundreds of icy Earths," said Dr Katherine Kretke, also of SwRI, who co-authored a paper with Levison and Dr Martin Duncan. "The growing cores need some time to fling their competitors away from the pebbles, effectively starving them. This is why only a couple of gas giants formed."
But just why did it take Jupiter so long to grow from ~10 to ~50 Earth masses only to then rapidly spurt to ~317.8? With no other explanation of the meteoric records, Alibert, Venturini and their colleagues got to work on a computer simulation, drawing upon experts in the fields of astrophysics, cosmochemistry and hydrodynamics. Before long, they found that Jupiter had grown over three stages.
First of all, the planetary embryo accreted tiny pebbles that were little more than a centimetre in size. “The protoplanetary discs are full of these pebbles in the first million years” explains Venturini. “They are slowed down by gas drag and they spiral towards the star. When they do so, a planet growing in the disc can intercept the pebbles very efficiently. Indeed, pebble accretion is the fastest way to grow the core of giant planets, so in only one million years a core can grow up to 10 to 20 Earth masses at the position of Jupiter.”
Once that mass is reached, she continues, the protoplanet is massive enough to perturb the disc. “As a consequence, the pebbles that normally drift from the outside of the disc to the inside stop drifting and are stuck outside the planet’s orbit and cannot be accreted anymore,” Alibert explains. “This is what is called the ’pebble isolation mass’.”
It is at this point that things become even more intriguing. For at least the next two million years, the theory says Jupiter began to accrete large rocks of around a kilometre in size called planetesimals. Like the pebbles they added mass, but only in small measures. Since a lot of heat was released, they also generated high energy. As a result, very little gas was accreted.
“Planetesimals are harder to accrete than pebbles, especially in the first million years of the disc’s lifetime, since planetesimals are still forming and their accretion rate is expected to be low,” Venturini tells us. “However, after pebble accretion stops, planetesimals are the only solids that can be accreted. The accretion of solids releases energy because they bombard the protoplanet, transforming their initial gravitational potential energy into kinetic energy that heats the gas of the protoplanet’s atmosphere.”
This is crucial in explaining why Jupiter’s growth slowed down in its early life. Under normal circumstances a forming planet would accrete gas from the disc in which it is embedded due to its gravity, but the ability to do this also depends on how much the atmosphere is able to cool and contract. “Cooling and contraction allows for more gas to enter in the gravitational influence of the planet, and thus allows for more gas to be accumulated in the form of an atmosphere,” says Venturini. “If the atmosphere is heated, then cooling will take much longer, and that is exactly the mechanism we are proposing to explain the delayed accretion of gas on to Jupiter.”
Only when Jupiter became big enough after three million years of being bombarded by planetesimals was the planet able to accrete large amounts of gas and grow quickly again. That’s because, at this stage, the gaseous atmosphere could cool and contract. The findings fit perfectly with the timescale given by the meteorite data. “Now we will extend our new mechanism to check if it also matches the formation of Uranus and Neptune,” Alibert says. “If we can explain the formation of these planets with our new scenario it will be a strong support to the hybrid accretion mode.”
The academics are certainly thrilled about the results, given the ramifications involved in explaining the formation of Jupiter – the most important event in the formation of the Solar System. “The presence of Jupiter has strong consequences on the way the Earth is today,” explains Alibert. “For example, some studies have shown that, without Jupiter, the Earth may have been much bigger, and not habitable [perhaps like Neptune]. Also, some studies have shown that the amount of water on Earth could have been much higher without Jupiter. In this case the Earth could have been an ocean planet, and probably not habitable. Understanding how many habitable planets there are in the universe requires
“Some studies have shown that, without Jupiter, the Earth may have been much bigger, and not habitable” Yann Alibert
Jupiter’s gravitational influence has helped to shape the Solar System, and it controls numerous asteroids
Objects will have bombarded and been accreted by Jupiter from its very early days