Jupiter and Saturn - Back and Forth in the Early Solar System?

[lpetrich]

The Extrasolar Planets Encyclopaedia
Exoplanet Orbit Database | Exoplanet Data Explorer
Numerous planets outside the Solar System have been discovered, and for some curious reason, a large number of them are around Jupiter's mass, but much closer in than one would expect. The usual theory nowadays is that they formed at a larger distance, then spiraled in as a result of their interaction with the still-remaining protoplanetary nebula.

But what happens to the inner planets? Would they still form?
[1004.0971] The Compositional Diversity of Extrasolar Terrestrial Planets: I. In-Situ Simulations -- no migration
[1209.5125] The Compositional Diversity of Extrasolar Terrestrial Planets: II. Migration Simulations

Apparently, they can, at least if not too close to one of these wandering giants. However, their composition tends to be more mixed, because of the giants' mixing up the protoplanetary nebula, and Earthlike planets can get much more water than the Earth has, making much deeper oceans.

But if wandering giant planets are so common, then why is it that the Solar System's ones did not migrate? Or did they?

Some planetary scientists are proposing that Jupiter and Saturn spiraled in, then spiraled out again.

http://www.nature.com/nature/journal/v475/n7355/full/nature10201.html

Jupiter and Saturn formed in a few million years from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ~100,000 years. Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth–Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30–50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.

http://www.gps.caltech.edu/classes/ge133/reading/grand_tack_nature.pdf
How Did Jupiter Shape Our Solar System?
NASA - Jupiter's Youthful Travels Redefined Solar System
Here's a nice presentation: The Grand Tack Hypothesis, after a sailing maneuver

In it, Jupiter starts out at 3.5 AU, Saturn at 4.5 AU, Uranus at 6 AU, and Neptune at 8 AU. Jupiter spirals into about 1.5 AU in 100 thousand years (kyr), Saturn quickly follows at about 100 kyr, while Uranus and Neptune don't move very much.

Along with the giant planets are lots of planetesimals, small asteroid-like objects that condensed out of the solar nebula. From 0.3 to 3 AU are S-type (stony) ones, and from 3.5 to 13 AU are C-type (carbonaceous-chondrite) ones. The C-type ones contain water, from where they formed.

Jupiter and Saturn push the S-type objects together, while mixing up S-type and C-type ones as they go. Some S-type ones end up in the outer Solar System, while some C-type ones end up in the inner Solar System.

Then Jupiter and Saturn get locked in a 3:2 resonance, with Jupiter at 1.5 AU and Saturn at 2 AU, and their interactions with the protoplanetary disk push them outward. As they go outward, they push Uranus and Neptune outward as those planets get into resonances with them. They also leave behind the asteroid belt as they go.

Inside 3.5 AU, it's mostly S-type asteroids, while outside 3.5 AU, it's mostly C-type asteroids.

Mars ends up relatively small, since it does not have as much starting material as the Earth.

The C-type planetesimals supply water to the inner planets, making the Earth's oceans.

It's also a good setup for the Nice model of outer-planet migration. Saturn, Uranus, and Neptune keep going further out, and they scatter lots of planetesimals outward to form the Kuiper Belt. The Nice here is not the English word, but Nice, France, where the model was developed.

The origin of the giant planets is still not very well understood, it must be said.


For some somewhat technical background on planetary-system formation, check out Scott Tremaine's home page

 

[D H]

The origin of the giant planets is still not very well understood, it must be said.

There's a problem with this jumpin' Jupiter hypothesis: When simulated, one of the outer gas giants tend to get expelled. A recent competing hypothesis that claims to overcome this problem is that a gas giant was expelled: A fifth gas giant.

David Nesvorný, "Young Solar System's Fifth Giant Planet?", 2011 ApJ 742 doi:10.1088/2041-8205/742/2/L22
http://arxiv.org/abs/1109.2949

 

[lpetrich]

That's interesting. Close encounters could also explain the sizable number of exoplanets in very eccentric orbits.

Using the data in exoplanets.org, about 1/3 of all known exoplanets have orbit eccentricities > 0.2, and 1/2 with e's > 0.1.

The very close ones tend to have nearly circular orbits, however. It's those with a major axis of 1 - 3 AU's that often have large eccentricities.

 

[mfb]

Well, statistics based on known exoplanets are tricky - you always have selection effects. For the same major axis, a larger eccentricity leads to larger differences in the radial velocity of the star, for example. I would expect that it increases the transit probability, too. The effects might be of second order (?), but they are there. And most exoplanets were found with those two methods.

 

[pmacias]

at the Institute for Advanced Study: http://www.sns.ias.edu/~tremaine/

Thank you for your question! It's clear that you have a strong interest in planetary science and the formation of our solar system. To answer your question, let's first take a closer look at the current theories for the formation of Jupiter and Saturn and their potential migration in the early solar system.

As you mentioned, the current theory is that Jupiter and Saturn formed at a larger distance from the sun, but eventually spiraled in towards their current orbits due to interactions with the protoplanetary nebula. This process, known as gas-driven migration, is thought to have occurred relatively quickly, on a timescale of only 100,000 years.

But what about the inner planets? Would they still have formed if this migration had occurred? The answer is yes, but their composition would likely be different. Studies have shown that the mixing of materials from the protoplanetary nebula by the migrating giant planets would have resulted in a more mixed composition for the inner planets, with potentially deeper oceans.

However, some planetary scientists are now proposing a new theory that suggests Jupiter and Saturn may have undergone a two-stage migration, first spiraling in towards the sun and then back out again. This theory, known as the "Grand Tack Hypothesis," suggests that Jupiter started out at around 3.5 AU, Saturn at 4.5 AU, Uranus at 6 AU, and Neptune at 8 AU. Jupiter and Saturn then spiraled in towards the sun, pushing the inner planets closer together and mixing up the composition of the planetesimals in the process.

But why did Jupiter and Saturn eventually spiral back out again? The Grand Tack Hypothesis suggests that they got locked in a 3:2 resonance, with Jupiter at 1.5 AU and Saturn at 2 AU, and their interactions with the protoplanetary disk pushed them back outwards. This process also pushed Uranus and Neptune outwards, leaving behind the asteroid belt as they went.

In this scenario, the inner planets still formed, but their composition was likely influenced by the mixing of materials from the outer solar system. This theory also helps explain the compositional differences we see in the asteroid belt, with S-type (stony) asteroids closer to the sun and C-type (carbonaceous-chondrite) asteroids further out.

However, it's important to note that the formation of the giant