Did we lose our inner planets?

New Scientist: Mercury may be sole survivor of planetary pile-up

Reporting on a presentation given at the AAS (the abstract can be found on the website but it doesn't seem to support direct links to individual abstracts, the relevant session is in the Tuesday "Extrasolar Planets: Dynamics and Stability of Planetary Systems" session)

One problem with forming the inner solar system is that the initial condition that works best is a relatively narrow ring from ~0.7 to ~1.1 AU. Extending outwards gives versions of Mars that are too massive but this can be solved with the Grand Tack. Extending inwards tends to give intra-Venusian planets that are far more massive than Mercury.

This shows that it is possible that our solar system did form with several planets within the orbit of Venus but the orbits were unstable, destroying our solar system's innermost planets and leaving Mercury as a remnant of high-speed planetary collisions. If so, this would make the solar system look more consistent with the typical mode of planetary formation which seems to produce lots of inner-system planets quite efficiently.

Now on arxiv.

Highly interesting read, relevant to both studies of the solar system and extrasolar systems. The model successfully gives solutions for a few issues surrounding the early solar system, and explains the relatively low number of currently surviving compact exoplanet systems compared to the theoretical initial value.

And now a somewhat different option for destroying the inner solar system: the migration of Jupiter pushed the primordial system of super-Earths into the Sun during the Grand Tack. This would imply that the inner planets are the result of an unusual initial state (rather than a relatively normal initial state that was progressively destroyed) and that Earth-like planets are potentially quite rare, with dense hydrogen-rich atmospheres being the norm.

And where is the paper for that? Also consider me a skeptic.

That strikes me as more than a few years out of date. It should have been published in 2011 and been debunked. It is very difficult to reduce earth to rubble and destroying a super earth is harder still. If earths are uncommon then Kepler should have found none. It found very many. Just because super earths exist in a system dosen't mean earthlike planets can't. "Throwing planets into the sun I don't like" is bad science. Haven't research suggested that stars hardly ever consume their planets?

When kepler died in 2013 I knew that theories like this would sprout. It needed four more years to better sample the habitable zones of G stars. Now people are just banking that smaller stars build planets similarly to sunlike stars. Or like this, making assumptions based on the interior planets kepler did find.

The depletion of the inner solar system of his material is one of the most interesting result of the so called "modèle de Nice" or Nice model (Nice, the town, not the word ), almost every paper and study talking about early solar system is talking about the details of this huge and very interesting model.

I have a detailed explanation of this model in one of my Pour La Science magazine, but I can't promice to find it in the chaos of my room.

Relevant paper on arXiv.

Jupiter's Decisive Role in the Inner Solar System's Early Evolution
http://arxiv.org/abs/1503.06945

It looks to me like they had a idea they liked than played with their models until they got the result they wanted.

I say you can't destroy a super earth without leaving something behind. Instead of waiting for future exoplanets to prove their hypothesis ,they should looking for Vulcan asteroids instead.

I think that's an unfair characterisation of the work, frankly. Just because you disagree with a work does not mean it was scientific fraud.

Shepherding material ahead of an inwardly-migrating giant planet is a pretty common prediction of disc migration models, so if there was a Grand Tack then there's no reason to believe that our solar system managed to magically avoid it. Whether inner system super-Earths had been produced by the time of the Grand Tack is another matter, but it does provide some insight into how you might be able to go from a relatively typical system (if the derived occurrence rates of super-Earths are correct, then these multi-super Earth systems with planets closer to the star than Mercury are the typical outcomes of typical initial conditions), to the rather specific initial conditions required by accretion models to produce the terrestrial planets.

And as for stuff being left behind, there's been 4.6 billion years for Poynting-Robertson drag to help clear the Vulcanoid region...

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It should have been published in 2011 and been debunked.

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When kepler died in 2013 I knew that theories like this would sprout.

Your hateful, unfounded words reveal your own ignorance, which I feel I need to exemplify. Like so.

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It looks to me like they had a idea they liked than played with their models until they got the result they wanted.

I look forward to seeing your own freaking analysis.

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It is very difficult to reduce earth to rubble and destroying a super earth is harder still.

You just decided this was true based on your own intuition. Almost as if you had an idea an idea you liked. Have you performed dynamical simulations on the matter?

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If earths are uncommon then Kepler should have found none. It found very many. Just because super earths exist in a system dosen't mean earthlike planets can't.

At no point in the paper is anything directly compatible to your first point said. As for your second point, at no point is it implied that low-mass planets with and without substantial atmospheres are mutually exclusive, which is what you seem to have read.

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"Throwing planets into the sun I don't like" is bad science.

"I don't like this, so it must be wrong" is a logical fallacy. Moreover, to quote, "This form is especially unsound when there is no indication that A [the doubting party] is aware of the evidence given by B [the evidencing party]". As you have evidently not properly read the paper you have set out to dismiss, you only further prove this point.

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Now people are just banking that smaller stars build planets similarly to sunlike stars.

I am aware that this is not relevant to the current subject, but I must point out that this is again not true. Are you unaware that there are several thousand M-dwarfs in the Kepler field? Have you read anything on the subject? You should know that there are enough M-dwarf KOIs to make comparisons of the planet populations of different parts of the main sequence.

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Or like this, making assumptions based on the interior planets kepler did find.

What is there to assume?! Kepler has shown that the rate of incidence of short-period, low-mass planets exceeds one per star. How can one not arrive at the assumption that such planets are normal?! Why must you continue touting your exceptionally misguided belief of authority?!?

Since you make it excessively clear you do not properly understand what you are ridiculing, I must demand that you either cease presenting unfounded, untested opinions as fact or actually attempt to read this paper with an open mind. If, for whatever reason, neither is appealing, then I will be happy to educate you myself.

Now, can I present my own thoughts without it being mired by such hostile slander?

So. This work is an interesting alternative scenario compared to the work at the top of the page, since they operate at very different points in the history of the solar system. Both are made on sound footing - this one is a natural consequence of the well-accepted grand tack model, while the other is a sensible, properly-founded consequence of dynamical chaos that matches observations. One wonders which happened here.

I posit - could they both have occurred? It may be that close-in planet formation in protoplanetary disks like the solar one within the first megayear is inevitable, but such a system may or may not be able to fully form on such a short timescale. Perhaps, after the removal of the young inner solar system planets, enough mass remained to form another low-mass inner planetary system, that then destabilised itself at a later date? This would lead to the suspicion that the three outer modern terrestrial planets formed not too long after the outward migration of Jupiter, but Mercury was formed at a point significantly later. This would be compatible with the inevitability of material cascading into the inner solar system evidenced by this work, and the peculiar nature of Mercury and its orbit discussed by the other one.

One also wonders how common this scenario could be elsewhere. Due to the constraints of current observational timescales it is not clear how common Jupiter-Saturn-like systems are, and given Saturn's pivotal role in the grand tack model in stopping Jupiter from continuing to migrate inwards, a solar system-like current architecture may be uncommon.

So, a few points of further study relevant to refinement of these models, and understanding of the early solar system:

• When did Mercury form? Given its small size, it is possible that Mercury has been geologically stagnant since it formed, or at least that some of its surface composition is comparable to the age of the planet itself. In-situ (i.e on-surface) dating of rocks on the surface of Mercury seems like a good goal in this respect. Knowledge of Mercury's age will allow us to definitively determine the validity of the idea of auto-destabilisation of a hypothetical inner solar planetary system, and fill in some blanks in the early history of the modern solar system planets.
  
• Do planetary systems with long-period giant planets less often contain multiple short-period, low-mass planets like the norm? If something like the grand tack model is common in other young planetary systems, then it ought to manifest itself in the architecture of their extant interior planets, if they survived. A few good systems for study like this are already known, so dedicated radial velocity observations would be enlightening. If "normal" close in, packed, low-mass planetary systems are less common in such systems compared to those without exterior giant planets, this could be interpreted as evidence in favour of the scenario currently in question.
  
• When, in relation to other giant planets, did Jupiter migrate? Currently, the timescale of planetary migration is not well-known. Observationally, one can point out three extant populations of Jovian planets around sun-like stars - Hot Jupiters, "AU Jovians" (as I refer to them by), and giant planets which likely remain close to where they formed. Giant planet migration requires a significant interior mass of material, and because the star depletes mass from the inner region of its disk, it is implied that giant planets that are currently in successively closer-in of the three populations began migrating earlier than those currently further out. Establishing the timescale at which each population of planets forms, begins to migrate, and approximately reaches their modern positions, will bring information about Jupiter's time of of formation and time of migration, allowing for refinement of models of the early solar system as a whole.
  

As a final point to the… previous matter, I am sympathetic with Lazarus' viewpoint and agree with his statements.

As much as I enjoy the debate, let's keep it civil, everyone.

Shellface wrote:

"I don't like this, so it must be wrong" is a logical fallacy.

Argument from Incredulity/Ignorance is probably a better fit.

One of SETI's weekly lecture videos is a rather fantastic presentation about this topic.
https://www.youtube.com/watch?v=pZF4021xDyo