A new paper says that a Super-Earth may have formed in our Solar System and been swallowed by the Sun. Image Credit: ESA/Hubble, M. Kornmesser

Our Sun May Have Eaten A Super Earth For Breakfast

Article written: 20 Apr , 2016
Updated: 5 May , 2016
by

Our Solar System sure seems like an orderly place. The orbits of the planets are predictable enough that we can send spacecraft on multi-year journeys to them and they will reliably reach their destinations. But we’ve only been looking at the Solar System for the blink of an eye, cosmically speaking.

The young Solar System was a much different place. Things were much more chaotic before the planets settled into the orbital stability that they now enjoy. There were crashings and smashings aplenty in the early days, as in the case of Theia, the planet that crashed into Earth, creating the Moon.

Now, a new paper from Rebecca G. Martin and Mario Livio at the University of Nevada, Las Vegas, says that our Solar System may have once had an additional planet that perished when it plunged into the Sun. Strangely enough, the evidence for the formation and existence of this planet may be the lack of evidence itself. The planet, which may have been what’s called a Super-Earth, would have formed quite close to the Sun, and then been destroyed when it was drawn into the Sun by gravity.

In the early days of our Solar System, the Sun would have formed in the centre of a mass of gas and dust. Eventually, when it gained enough mass, it came to life in a burst of atomic fusion. Surrounding the Sun was a protoplanetary disk of gas and dust, out of which the planets formed.

What’s missing in our Solar System is any bodies, or even rocky debris in the zone between Mercury and the Sun. This may seem normal, but the Kepler mission tells us it’s not. In over half of the other solar systems it’s looked at, Kepler has found planets in the same zone where our Solar System has none.

A key part of this idea is that planets don’t always form in situ. That is, they don’t always form at the place where they eventually reach orbital stability. Depending on a number of factors, planets can migrate inward towards their star or outwards away from their star.

Martin and Livio, the authors of the study, think that our Solar System did form a Super-Earth, and rather than it migrating outward, it fell into the Sun. According to them, the Super-Earth most probably formed in the inner regions of our Solar System, on the inside of Mercury’s orbit. The fact that there are no objects there, and no debris of any kind, suggests that the Super-Earth formed close to the Sun, and that its formation cleared that area of any debris. Then, once formed, it fell into the Sun, removing all evidence of its existence.

The authors also note another possible cause for the Super-Earth to have fallen into the Sun. They propose that Jupiter may have migrated inward to about 1.5 AUs from the Sun. At that point, it got locked into resonance with Saturn. Then, both gas giants migrated outward to their current orbits. This process would have shepherded a Super-Earth into the Sun, destroying it.

Some of the thinking behind this whole theory involves the size of the inner terrestrial planets in our Solar System. They’re very small in comparison to other systems studied by the Kepler Mission. If a Super-Earth had formed in the inner part of our System, it would have dominated the accretion of available material, leaving Mercury, Venus, Earth and Mars starved for matter.

A key idea behind this study is what’s known as a dead zone. In terms of a solar system and a protoplanetary disk, a dead zone is a zone of low turbulence which favors the formation of planets. A system with a dead zone would have enough material to allow Super-Earths to form in-situ, and they would not have to migrate inward from further out in the system. However, since large planets like Super-Earths take a long time to fully form, this dead zone would have to be long-lived.

If a protoplanetary disk lacks a dead zone, it is likely too turbulent for the formation of a Super-Earth close to the star. A turbulent protoplanetary disk favors the formation of Super-Earths further out, which would then migrate inwards towards the star. Also, a turbulent disk allows for quicker migration of planets, while a pronounced dead zone inhibits migration.

As the authors say in the conclusion of their study, “The lack of Super–Earths in our solar system is somewhat puzzling given that more than half of observed exoplanetary systems contain one. However, the fact that there is nothing
inside of Mercury’s orbit may not be a coincidence.” They go on to conclude that in our Solar System, the likely scenario is the in situ formation of a Super-Earth which subsequently fell into the Sun.

There are a lot of variables that have to be fine-tuned for this scenario to happen. The young solar system would need a dead zone, the depth of the turbulence in the protoplanetary disk would have to be just right, and the disk would have to be the right temperature. The fact that these things have to be within a certain range may explain why we don’t have a Super-Earth in our system, while over half of the systems studied by Kepler do have one.

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11 Responses

  1. Helio George says

    Another nice example of how studying things elsewhere helps us with us here.

    If this becomes mainstream, I wonder what name this planet with have — Morsel?

  2. mewo says

    “Om nom nom. Delicious planet.”
    –the Sun.

  3. markbvet says

    Hmmm, nearly 50% of other planetary systems do NOT have a large inner planet… this could be simply due to vagaries of how each planetary system forms. Though the possibility exists of a prior planet that got consumed, it does not appear to be clear at all that this is what in fact occurred. More supposition than fact, at this stage, I would surmise. As a side note, the author of this article misused the term ‘in situ’… at least in the first usage: “…planets don’t always form in situ. That is, they don’t always form at the place where they eventually reach orbital stability.” This is an erroneous usage of the term. “In situ” is a latin-derived term for ‘in the original or natural position’. By definition, ALL planets form in situ (in their original position), regardless of whether they end up in a stable orbit elsewhere. Later in this article, the term is used more appropriately, but it appears the author had no exact idea of what this term actually means.

  4. markbvet says

    Hmmm, nearly 50% of other planetary systems do NOT have a large inner planet… this could be simply due to vagaries of how each planetary system forms. Though the possibility exists of a prior planet that got consumed, it does not appear to be clear at all that this is what in fact occurred. More supposition than fact, at this stage, I would surmise.

  5. Member
    LordJohn says

    Well, until we have amuch better picture of other solar systems than we have at present we need to be cautious. Keppler cannot show us WEarth sized planets and also only shows planets with short periods, say less than 2 years. Our solar system has much longer period planets and so the systems Keppler shows are not like ours. Yet there must be smaller planets and longer period ones in some of these systems.

    There is a great danger basing ideas on too little data, especially when one knows the limits of what one has.

  6. bazbsg says

    “In over half of the other solar systems it’s looked at, Kepler has found planets in the same zone where our Solar System has none.”

    According to Wikipedia, Kepler has found 1013 confirmed planets and 3199 unconfirmed planet candidates out of the over 145,000 stars it monitors. So the above statement is not correct and the true value is far far less than half.

    Also keep in mind that it’s easier for Kepler to find large planets close to the star than to find small planets or planets further from the star, as large planets close to the star will block more light and do so more often. And Kepler has only been looking for seven years, so a planet as far from its star as Jupiter is from the Sun, might not have transited its star yet for Kepler to observe. And Kepler will only find planets around a star if its ecliptic aligns fairly well with Kepler. So while Kepler does give plenty of data, it’s not correct to conclude from it what the typical planetary system should be like. And since Kepler can more easily find large planets close to their star, it’s not correct to conclude that there used to be one around the sun.

  7. BlackWolfStanding says

    Maybe Planet IX outside the Kepler Belt is where the ‘Super Earth’ went too instead of falling into the Sun.

  8. Pasander says

    “In the early days of our Solar System, the Sun would have formed in the centre of a mass of gas and dust. Eventually, when it gained enough mass, it came to life in a burst of atomic fusion. Surrounding the Sun was a protoplanetary disk of gas and dust, out of which the planets formed.”

    This is a bit misleading, I think. It takes millions of years for a protostar to develop into a main-sequence star (hydrogen burning in hydrostatic equilibrium) . So it’s not exactly a “burst” but a very slow and gradual process. Meanwhile, the planetary system around the new star is busy forming.

  9. crocodilebomb says

    I remember reading somewhere (maybe here) that it’s actually very difficult to fall into the sun. You have a lot of angular momentum to kill. I suppose you need two super-earths to do it! One to fall into the sun, and one to be ejected from the solar system. Or perhaps it was Theia herself… bank shot right off proto-earth!

    If it was inside the sun’s Roche limit 5 billion years ago would you still have a ring?

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