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Inside Rocky Exoplanets

We’ve gone from discovering the presence of exoplanets to studying their atmospheres by analyzing the spectra produced when a planet transits in front of its star. We’re even in the early stages of deducing weather patterns on some distant worlds. Now we’re looking at probing the inside of planets to learn whether their internal structure is something like that of the Earth.

The work is led by Li Zeng (Harvard-Smithsonian Center for Astrophysics), whose team developed a computer model based on the Preliminary Reference Earth Model (PREM), the standard model for the Earth’s interior. Developed by Adam Dziewonski and Don L. Anderson for the International Association of Geodesy, PREM attempts to model average Earth properties as a function of radius. Zeng adjusted the model for differing masses and compositions and applied the revised version to six known rocky exoplanets with well understood characteristics.

The work shows that rocky worlds should have a nickel/iron core that houses about thirty percent of the planet’s mass, with the remainder being mantle and crust. “We wanted to see how Earth-like these rocky planets are,” says Zeng. “It turns out they are very Earth-like.”

From the paper:

These dense exoplanets between 2 and 5 M so far appear to agree with the mass-radius relation with CMF [Core Mass Fraction] ≈ 0.26, suggesting that they are like the Earth in terms of their proportions of mantle and core. But their surface conditions are utterly different as they are much too hot. This is due to observational bias that currently it is much easier for us to detect close-in planets around stars. The fact that we now see so many of them suggests there may be abundant Earth-like analogs at proper distances from their stars to allow existence of liquid water on their surfaces.

base

Image: This artist’s illustration compares the interior structures of Earth (left) with the exoplanet Kepler-93b (right), which is one and a half times the size of Earth and 4 times as massive. New research finds that rocky worlds share similar structures, with a core containing about a third of the planet’s mass, surrounded by a mantle and topped by a thin crust. M. Weiss/CfA.

The paper points out that these conclusions fit with studies of disintegrated planet debris in polluted white dwarf spectra. Here the remnant of a star like the Sun has evidently swallowed up planetary materials, usefully producing spectra that can be analyzed to see what elements emerge. Such studies indicate debris that resembles the composition of the Earth, where over 85 percent of the mass is composed of oxygen (O), magnesium (Mg), silicon (Si), and iron (Fe). Moreover, we find similar ratios of iron to silicon and magnesium to silicon. These ratios indicate formation processes similar to those we find in our own Solar System:

Current planet formation theory suggests that the solar nebula was initially heated to very high temperatures to the extent that virtually everything was vaporized except for small amount of presolar grains… The nebula then cools to condense out various elements and mineral assemblages from the vapor phase at different temperatures according to the condensation sequence… Fe-Ni [nickel] metal alloy and Mg-silicates condense out around similar temperatures of 1200-1400K (depending on the pressure of the nebula gas) according to thermodynamic condensation calculation… Oxygen, on the other hand, does not have a narrow condensation temperature range, as it is very abundant and it readily combines with all kinds of metals to form oxides which condense out at various temperatures as well as H [hydrogen], N [nitrogen], C [carbon] to form ices condensing out at relatively low temperatures… As supported by the polluted white dwarf study, we expect other exoplanetary systems to follow similar condensation sequence as the solar system in a H-dominated nebular environment for the major elements: Fe, Mg, Si, and O…

The assumption of chemical compositions similar to the Earth’s may come into question in parts of the galaxy that are less rich in metals, in which case different planetary interior structures could evolve, a subject Zeng and team plan to investigate in future research.

The paper is Zeng et al., “Mass-Radius Relation for Rocky Planets based on PREM,” accepted at The Astrophysical Journal (preprint).

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Comments on this entry are closed.

  • Andrew LePage February 9, 2016, 10:28

    Interestingly, exoplanets with masses less than ~6 times that of Earth for which we have reasonably accurate radius and mass measurements (and that are not obviously low-density mini-Neptunes) all seem to fall along a trend in mass-density space consistent with a roughly Earth-like composition.

    http://www.drewexmachina.com/2015/01/03/the-composition-of-super-earths/

    It will be interesting to see if this continues to be true as we characterize more and more roughly Earth-size exoplanets.

  • Brett February 9, 2016, 13:37

    I read some stuff arguing that rocky super-earths might not have plate tectonics because of greater internal heat and pressure. I hope that’s not true.

  • Ashley Baldwin February 9, 2016, 14:49

    Characterisation is the name of the game. But how to do it ? Extrapolation and simulation can only go so far. With Kepler,TESS ,WFIRST and PLATO there will be thousands of such planets ( around varying stars ) discovered in the next decade or so but with practically no characterisation , with most of what has been done being based on improvisation with Hubble and Spitzer on hot gas giants and with JWST maybe helping a bit during the small time (15%) it has allotted to exoplanets .
    Even just a 2-3 m transit spectroscopy telescope ( basically a spectroscope and sensor array behind a mirror -not exactly complicated ) could improve things many fold . But no middle of the way NASA astrophysics budget. Only multi billion dollar “once in a decade” flagship missions can cover astrophysics missions if they cost more than an the Explorer level $250 million. Actual cost probably about $1.5 billion. Hardly JWST level , but simple design guarantees far longer service and far greater science return. Service even. When the improvisation and innovation of Explorer missions is looked at , it makes you wonder what could be done if a $9 billion flagship fund was instead divided into two every decade , with half spent on one big mission and the remainder divided amongst say two or three intermediate missions . ( allowing for a free launch per mission too) . All stringently capped, like the Explorer concepts, to avoid ruinous JWST cost overruns . That’s three or four decent , big missions per decade to interest everyone throughout the period , instead of just one , to say nothing of stretching the limited funds further. Necessity is the mother of innovation after all.
    The best hope after all that ground based ELTs.