CLOSE ENCOUNTERS: Compressed planets may be the remnants of ice giants stripped of their outer layers by a close encounter with their suns.
Image: NASA/ESA/C.Carreau
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Mysterious dense bodies outside the Solar System could be the remnants of ice giants similar to Neptune that wandered too close to their suns, according to results presented this week at a meeting on exoplanets at the Royal Society in London.
Among the most puzzling finds of NASA’s Kepler space mission to find exoplanets, which launched in 2009, are bodies too heavy for their size. In some of the rare cases in which astronomers can estimate both the mass and the size of distant planets discovered by the probe, the objects have radiuses similar to that of Earth but are denser than pure iron.
No conventional theories about planet formation can account for such densities in planets of this size. “There is no way to explain that in the Solar System,” says Olivier Grasset, a geophysicist at the University of Nantes in France.
Fossil worlds
Grasset and his collaborators now say that the strange bodies could be the “fossil cores” of planets that were once much larger, an idea that was first proposed by researchers in 2011. These planets would have been ice giants that formed in the outer parts of a star system and then migrated inwards — as their orbits were affected by interactions with surrounding gas and dust — perhaps getting as close to their suns as Mercury is to ours.
The hotter temperatures closer to the stars, Grasset explains, would evaporate the outer layers of the planets, which are made mainly of volatile components such as hydrogen, helium and water. The leftover cores would consist of rock and metal, just like the bulk of Earth, and could weigh up to several times as much as our planet, making them what scientists call super-Earths.
But these cores formed under the weight of their planets’ outer layers, under pressures of around 500 gigapascals — 5 million times atmospheric pressure on Earth — and typical temperatures of about 6,000 kelvin. As a result, the materials in these cores should be more compacted, and denser, than Earth.
Quick change
Together with his colleagues Antoine Mocquet, a planetary scientist also at Nantes, and Christophe Sotin, a planetary geologist at NASA’s Jet Propulsion Laboratory in Pasadena, California, Grasset created a computer simulation to test the idea.
The team found that if the outer layers of an ice giant are removed over billions of years, the materials would ‘relax’, expanding back to more ordinary densities. But if the stripping occurred over a geologically short time, the sudden cooling would keep the core locked into its dense state essentially forever. “If the process is short, you end up with a very compressed super-Earth,” says Grasset.
Lars Stixrude, a geologist at University College London, calls the idea “fascinating” — although he warns that science’s understanding of the behavior of materials under the extreme temperatures and pressures of an ice-giant core is still incomplete. Grasset agrees that there are large uncertainties in his team’s calculations, especially in the rate of relaxation of the naked cores. But, he adds, he and his colleagues made conservative assumptions.
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10 Comments
Add CommentOr, perhaps, they formed in regions that had an abundance of elements heavier than iron. What is the elemental composition of their sun and are they native to that sun?
Reply | Report Abuse | Link to thisTheir stars cannot be composed of iron... Please see
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Supernova#Core_collapse
I think you misunderstood the OP's question.
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Supernova#Source_of_heavy_elements
Maybe so - my reasoning is that if it's implied that the planets formed directly from a protoplanetary disk that contained mostly elements heavier than iron, the 'star' would never have had any fusible material - iron and heavier elements are not fusible. Besides that, I think that even supernovae release elements lighter than iron, but even if not I think it'd be extremely difficult to locate a large molecular cloud consisting mostly of elements heavier than iron. I hope this helps...
Reply | Report Abuse | Link to thisActually, jtdwyer, I was thinking that there are a number of heavier elements that can be ejected from a supernova that may have formed an enriched cloud of material that could have formed a new planetary system. The heavier elements may have formed into a planetary body prior to the formation of that particular sun or may just have had a different orbital trajectory that kept an appreciable amount of it from condensing into the core of the new star. It should be statistically possible if the expulsion of matter during the nova event was not distributed evenly. That was why I asked if the stars composition might have shown a difference in elemental distribution.
Reply | Report Abuse | Link to thisAnother possibility is the partial vaporization of planets from the star that went nova may have left a planet or planets consisting only of the heaviest elements of it's core that were later captured by this star.
I know our earth's core is mostly iron but I also know that the heat generated deep in the earth is in theory generated by radioactive decay which implies a measure of heavier elements there. I suspect there is no standard mix of elements so it is possible that the remaining core of a planet may have a higher concentration of heavier elements than we would expect for our own planet.
You do have to admit that these articles do not contain very much information which is why we are always asking questions.
Meantime, maybe that iron ball we call Mercury is the remainder of a former ice giant in Sol system?
Reply | Report Abuse | Link to thisYes, I'm basing my reasoning somewhat on chapter 3 of "The Formation of Stars" (Stahler & Palla, 2004) - I don't read books but this one is interesting. Please see
Reply | Report Abuse | Link to thishttp://www.scientificamerican.com/article.cfm?id=theory-explains-how-star-clusters-form-evolve
It seems that most stars are condensed in clusters from enormous molecular clouds. I'm presuming that those giant clouds are generally formed from the debris of many generations of supernovae & other miscellaneous sources...
Very interesting thought - thanks!
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Mercury_(planet)#Internal_structure
Indeed, these super-dense exoplanets of KIC 05807616 may have been volatilely depleted by deep immersion in the expanded solar envelope of the red-giant phase of their dying star.
Reply | Report Abuse | Link to thisVolatile 'evaporation' of planets includes chemical reduction to metallic elements by the highly-reduced stellar plasma. In the case of iron, its density would cause it to sink in plumes toward the core while more electronegative metallic elements act as sacrificial anodes, protecting the iron from re-oxidation on its descent. Sequestering metallic iron in the core would protect it from volatilization from further exposure by stellar plasma, vaporizing more refractory lithophile elements like aluminum and calcium continuously exposed at the surface.
An planet composed of metallic iron and other siderophile elements would indeed have a density greater than iron, but its high density would be completely stable and unrelated to the speed of its volatilization.
Our own terrestrial planets may similarly have been volatilized by by the Sun during the violent spiral-in-common-envelope/LRN-merger/flare-star phases of our former binary star, similarly forming their chemically-reduced metallic-iron cores and the 'terrestrial volatility trend'.
Wonder if this has anything to do with those fancy new mini super novas.
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