Wandering Jupiter accounts for our unusual solar system

Our solar system may have once harbored super-earths
This snapshot from a new simulation by Caltech and UC Santa Cruz researchers depicts a time early in the solar system's history when Jupiter likely made a grand inward migration (here, Jupiter's orbit is represented by the thick white circle at about 2.5 AU). As it moved inward, Jupiter picked up primitive planetary building blocks, or planetesimals, and drove them into eccentric orbits (turquoise) that overlapped the unperturbed part of the planetary disk (yellow), setting off a cascade of collisions that would have ushered any interior planets into the sun. Credit: K. Batygin/Caltech

Long before Mercury, Venus, Earth, and Mars formed, it seems that the inner solar system may have harbored a number of super-Earths—planets larger than Earth but smaller than Neptune. If so, those planets are long gone—broken up and fallen into the sun billions of years ago largely due to a great inward-and-then-outward journey that Jupiter made early in the solar system's history.

This possible scenario has been suggested by Konstantin Batygin, a Caltech planetary scientist, and Gregory Laughlin of UC Santa Cruz in a paper that appears the week of March 23 in the online edition of the Proceedings of the National Academy of Sciences (PNAS). The results of their calculations and simulations suggest the possibility of a new picture of the early solar system that would help to answer a number of outstanding questions about the current makeup of the solar system and of Earth itself. For example, the new work addresses why the terrestrial planets in our solar system have such relatively low masses compared to the planets orbiting other sun-like stars.

"Our work suggests that Jupiter's inward-outward migration could have destroyed a first generation of planets and set the stage for the formation of the mass-depleted terrestrial planets that our solar system has today," says Batygin, an assistant professor of planetary science. "All of this fits beautifully with other recent developments in understanding how the solar system evolved, while filling in some gaps."

Thanks to recent surveys of exoplanets—planets in solar systems other than our own—we know that about half of sun-like stars in our galactic neighborhood have orbiting planets. Yet those systems look nothing like our own. In our solar system, very little lies within Mercury's orbit; there is only a little debris—probably near-Earth asteroids that moved further inward—but certainly no planets. That is in sharp contrast with what astronomers see in most planetary systems. These systems typically have one or more planets that are substantially more massive than Earth orbiting closer to their suns than Mercury does, but very few objects at distances beyond.

"Indeed, it appears that the solar system today is not the common representative of the galactic planetary census. Instead we are something of an outlier," says Batygin. "But there is no reason to think that the dominant mode of planet formation throughout the galaxy should not have occurred here. It is more likely that subsequent changes have altered its original makeup."

According to Batygin and Laughlin, Jupiter is critical to understanding how the solar system came to be the way it is today. Their model incorporates something known as the Grand Tack scenario, which was first posed in 2001 by a group at Queen Mary University of London and subsequently revisited in 2011 by a team at the Nice Observatory. That scenario says that during the first few million years of the solar system's lifetime, when planetary bodies were still embedded in a disk of gas and dust around a relatively young sun, Jupiter became so massive and gravitationally influential that it was able to clear a gap in the disk. And as the sun pulled the disk's gas in toward itself, Jupiter also began drifting inward, as though carried on a giant conveyor belt.

"Jupiter would have continued on that belt, eventually being dumped onto the sun if not for Saturn," explains Batygin. Saturn formed after Jupiter but got pulled toward the sun at a faster rate, allowing it to catch up. Once the two massive planets got close enough, they locked into a special kind of relationship called an orbital resonance, where their orbital periods were rational—that is, expressible as a ratio of whole numbers. In a 2:1 orbital resonance, for example, Saturn would complete two orbits around the sun in the same amount of time that it took Jupiter to make a single orbit. In such a relationship, the two bodies would begin to exert a gravitational influence on one another.

"That resonance allowed the two planets to open up a mutual gap in the disk, and they started playing this game where they traded angular momentum and energy with one another, almost to a beat," says Batygin. Eventually, that back and forth would have caused all of the gas between the two worlds to be pushed out, a situation that would have reversed the planets' migration direction and sent them back outward in the solar system. (Hence, the "tack" part of the Grand Tack scenario: the planets migrate inward and then change course dramatically, something like a boat tacking around a buoy.)

In an earlier model developed by Bradley Hansen at UCLA, the terrestrial planets conveniently end up in their current orbits with their current masses under a particular set of circumstances—one in which all of the inner solar system's planetary building blocks, or planetesimals, happen to populate a narrow ring stretching from 0.7 to 1 astronomical unit (1 astronomical unit is the average distance from the sun to Earth), 10 million years after the sun's formation. According to the Grand Tack scenario, the outer edge of that ring would have been delineated by Jupiter as it moved toward the sun on its conveyor belt and cleared a gap in the disk all the way to Earth's current orbit.

But what about the inner edge? Why should the planetesimals be limited to the ring on the inside? "That point had not been addressed," says Batygin.

He says the answer could lie in primordial super-Earths. The empty hole of the inner solar system corresponds almost exactly to the orbital neighborhood where super-Earths are typically found around other stars. It is therefore reasonable to speculate that this region was cleared out in the primordial solar system by a group of first-generation planets that did not survive.

Batygin and Laughlin's calculations and simulations show that as Jupiter moved inward, it pulled all the planetesimals it encountered along the way into orbital resonances and carried them toward the sun. But as those planetesimals got closer to the sun, their orbits also became elliptical. "You cannot reduce the size of your orbit without paying a price, and that turns out to be increased ellipticity," explains Batygin. Those new, more elongated orbits caused the planetesimals, mostly on the order of 100 kilometers in radius, to sweep through previously unpenetrated regions of the disk, setting off a cascade of collisions among the debris. In fact, Batygin's calculations show that during this period, every planetesimal would have collided with another object at least once every 200 years, violently breaking them apart and sending them decaying into the sun at an increased rate.

The researchers did one final simulation to see what would happen to a population of super-Earths in the inner solar system if they were around when this cascade of collisions started. They ran the simulation on a well-known extrasolar system known as Kepler-11, which features six super-Earths with a combined mass 40 times that of Earth, orbiting a sun-like star. The result? The model predicts that the super-Earths would be shepherded into the sun by a decaying avalanche of planetesimals over a period of 20,000 years.

"It's a very effective physical process," says Batygin. "You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the sun."

Batygin notes that when Jupiter tacked around, some fraction of the planetesimals it was carrying with it would have calmed back down into circular orbits. Only about 10 percent of the material Jupiter swept up would need to be left behind to account for the mass that now makes up Mercury, Venus, Earth, and Mars.

From that point, it would take millions of years for those planetesimals to clump together and eventually form the terrestrial planets—a scenario that fits nicely with measurements that suggest that Earth formed 100-200 million years after the birth of the sun. Since the primordial disk of hydrogen and helium gas would have been long gone by that time, this could also explain why Earth lacks a hydrogen atmosphere. "We formed from this volatile-depleted debris," says Batygin.

And that sets us apart in another way from the majority of exoplanets. Batygin expects that most exoplanets—which are mostly super-Earths—have substantial hydrogen atmospheres, because they formed at a point in the evolution of their planetary disk when the gas would have still been abundant. "Ultimately, what this means is that planets truly like Earth are intrinsically not very common," he says.

The paper also suggests that the formation of gas giant planets such as Jupiter and Saturn—a process that planetary scientists believe is relatively rare—plays a major role in determining whether a planetary system winds up looking something like our own or like the more typical systems with close-in super-Earths. As planet hunters identify additional systems that harbor gas giants, Batygin and Laughlin will have more data against which they can check their hypothesis—to see just how often other migrating giant planets set off collisional cascades in their planetary systems, sending primordial super-Earths into their host stars.

The researchers describe their work in a paper titled "Jupiter's Decisive Role in the Inner Solar System's Early Evolution."


Explore further

'Hot Jupiters' provoke their own host suns to wobble

More information: Jupiter's decisive role in the inner Solar System's early evolution, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1423252112
Citation: Wandering Jupiter accounts for our unusual solar system (2015, March 23) retrieved 23 August 2019 from https://phys.org/news/2015-03-jupiter-accounts-unusual-solar.html
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Mar 23, 2015
A capture event (or a series of them) could also explain our oddball SS. Although it has little to do with gravity and far more to do with EM. Our ancestors seemed to have reported such events that are intertwined in religion and mythology worldwide.

Mar 23, 2015
Every time I read something like this, I am reminded of something I heard on that overnight radio show, Coast To Coast AM. It had to be at least 15 years ago but it stuck with me. An elderly sounding gentleman with a Velikovskian scenario that he derived from his interpretation of an ancient Sanskrit text, the Rig Veda. So, once upon a time, the inner solar system was only Earth, Mars and Jupiter, orbiting the Sun in synchronous alignment. Mars had a core and a magnetic field. And then something happened and Venus was born of Jupiter. She headed towards the Sun, and dragged Mars close enough to Earth that the two planets began to orbit each other geosynchronously as they continued around the Sun. There were lightning discharges (thunderbolts of the gods) and mass transfer (mainly water and loose material) from Mars to Earth. Meanwhile, Venus took a trip around the Sun... (continued)

Mar 23, 2015
(continued) Eventually Venus returned for a second pass and this time, poor Mars was so weakened by his gravity war with Earth, that Venus was able to pull his heart (core) out of his body. This disrupted everything and all the participants settled into new orbits around the Sun, including the core of Mars, which we now call Mercury...

Mar 23, 2015
Jupiter came in like a wrecking ball!

Mar 23, 2015
Read that too, but all these just-so stories are attempts to explain how and why our strange inner rocky and outer gaseous 2-system came to be.
Scientists have looked at surrounding systems for similarities and did not find anything that looks like us.
I am not convinced.

Mar 23, 2015
A hypothesis to explain the absence of that which is not there. Notwithstanding that, how does it explain the Krypton isotope ratios on Earth? The usual explanation is that there was hydrodynamic drag due to the original gas from the nebula being blown away by extreme solar UV, but that only works if there is a lot of hydrogen, etc, in the original atmosphere, and that only works if Earth formed when the accretion disk gases were still here. That won't be the case with this scenario. There are a number of other isotope problems as well, not the least of which is, why do the planets and asteroids have different isotope ratios, but the Moon the same as Earth, if the rocky planets formed from a jumble of planetesimals on highly angular orbits? Why do the atmospheres of the rocky planets have different compositions? The scenario just does not work.

Mar 23, 2015
I forgot to turn on the email alert. This is a dummy, just to get it.

Mar 24, 2015
This sounds a lot like Velikosky's book that described numerous planetary impacts within the early solar system. Something like this must have accounted for the formation of our solar system which cannot be accounted for by a single theory. The current theories for solar system formation have been continually contradicted. At least this accounts for the heterogenous nature we observe today.

Mar 24, 2015
I'm surprised that no one has noticed the effect on the Drake equation.

Mar 25, 2015
Here's what bothers me.

I don't have a problem with the studies as much as I have a problem with the extrapolation.

Our primary technique in detecting exoplanets thus far has been the transmit method; as a large planet crosses over a star, the star dims correspondingly to the planet. I really shouldn't have to explain precisely why this is biased towards finding big things (better to block light) near their stars (small orbits good enough to be detected in the time we've been looking). Additionally our methods at finding planets do far better when the orbits are aligned to our line of sight.

I really do think our tendency to find super Earths and hot Jupiters is a methodological artifact...

Mar 25, 2015
Sigh. _No_ system looks anything like another, Kepler showed that. They are all individuals, within a distribution.

So far we have a surplus of packed systems, because they are easy to see while the methods can't yet compare our system with others. (As tovarich noted before me.) But these systems have different mass planets all over the place.

In fact, Kepler has a bimodal distribution, either 0-1 gas giants or 4-8 mixed planets. Our system place well within this distribution, in one of the peaks no less.

[tbctd]


Mar 25, 2015
[ctd]

Nice/Nice 2 simulations can keep the inner planets small, contrary to what this paper claims. "In the case of the original Nice model, the slow approach of Jupiter and Saturn to their mutual 2:1 resonance, necessary to match the timing of the Late Heavy Bombardment, can result in the ejection of Mars and the destabilization of the inner Solar System.[24] A step-wise separation of Jupiter's and Saturn's orbits due to gravitational encounters with one of the ice giants, called the jumping-Jupiter scenario, has been shown to be necessary to avoid these issues.[25]" [ http://en.wikiped...ce_model ]

The proposed scenario may work. But it seems a lot less well constrained than the Nice models that get our system (out to the inner Kuiper Belt edge) straight up.

Mar 25, 2015
I guess it is no use to ask the trolls to keep their respective myths out of a science discussion between adults...

So on to the real questions instead:

@jack: A single planetary system evolution model can't affect the likelihood for habitables, life, et cetera. Especially here, when the field is less well constrained. We need more data, test more of the model space, et cetera.

Meanwhile, our own system has a surprisingly well constrained evolution model in the Nice series. So generic models have to square with that too, before we can tell the effect on habitability. (And out of our one solar system, it has many habitables and much life... =D)

Mar 25, 2015
The simple truth is that there is no way to form a star system through process of accretion. There is no way to form our own solar system and it in such a configuration based on the known physics laws. They do not create order in the universe but only support it for a while.

Mar 25, 2015
Other than Mr. Larsson it appears those with at least a smidgin of intellect and self respect have decided to let the idiots have their circle jerk. Is it really worth the hits, phys.org, to cultivate such a lot of complete imbeciles and be associated with them? There used to be good discussion. Now no one that knows what they're talking about cares to be associated with all the grandiose idiotic legends-in-their-own-mind wastes of humanity.

I have a lot to discuss on this, and one point about the the equations used to solve the orbital mechanics. But not in front of this braying pack of loonies! Cantdrive, you really should publish your identity. It would REALLY pain me beyond belief if you died and I didn't know where to dance on your grave!

Mar 25, 2015
As to the statement by T Larsson : "No_ system looks anything like another, Kepler showed that." That is not exactly true. See Bovaird et al., MNRAS 448, 3608–3627 (2015), in which there is a reasonable number of systems following tolerably the Titius Bode law. Yes, I have published an alternative theory of planetary formation, a statement which I rather suspect will draw all sorts of adverse statements, but it has a simple reason for some systems roughly following the Titius Bode law: initial accretion of dust is chemical in nature, not gravitational, and hence it occurs in different temperature zones, and gives zones of slightly different composition. Our system is a model system, and within the theory, occurs because the stellar clean out occurred earlier than many. (See LkCa 15 for confirming evidence). If the star holds the accretion disk long enough, the planets get big enough they start to gravitationally interact. I can find no evidence to falsify it.

Mar 26, 2015
its hard to believe that orbital resonance between Saturn and Jupiter was intact when colliding inward with super earths. Or am I missing something else here ?

Apr 03, 2015
I have already written on 20th March'2015 in an abstract submitted for Vietnam conference on " Planetary System : A Synergistic View" going to held in July'2015 in response to an abstract call from AAAS

"Jupiter, Saturn, Uranus & Neptune are not a planet; they are junior Sun (Jr. SUN). THEY CAME IN THE SYSTEM during speedy contraction of Sun, just after supernova blast.

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