jupiter: So­lar Sys­tem Odd­ball

As­tro­physi­cists think they know why the gas gi­ant’s evo­lu­tion was de­layed for two mil­lion years

All About Space - - Contents - Re­ported by David Crookes

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

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