A coreless planet is a theoretical type of
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...
that has no metallic core, i.e. the planet is effectively a giant rocky
mantle. It can be formed in cooler regions and far from the star.
Origin
According to a 2008 paper by
Sara Seager
Sara Seager (born 21 July 1971) is a Canadian-American astronomer and planetary scientist. She is a professor at the Massachusetts Institute of Technology and is known for her work on extrasolar planets and their atmospheres. She is the aut ...
and Linda Elkins-Tanton, there are probably two ways in which a coreless planet may form.
In the first, the planet accretes from
chondrite
A chondrite is a stony (non- metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form p ...
-like fully oxidized water-rich material, where all the metallic iron is bound into silicate mineral crystals. Such planets may form in cooler regions farther from the central star.
In the second, the planet accretes from both water-rich and iron metal-rich material. However, the metal iron reacts with water to form iron oxide and release hydrogen before differentiation of a metal core has taken place. Provided the iron droplets are well mixed and small enough (<1 centimeter), the predicted end result is that the iron is oxidized and trapped in the mantle, unable to form a core.
Magnetic field
Earth's magnetic field results from its flowing liquid metallic core, according to the
dynamo theory
In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can ...
, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles.
Magnesium oxide
Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...
, which is rocky on Earth, can be liquid at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.
Characteristics
The predicted sizes of coreless and cored planets are similar within a few percent, which makes it difficult to interpret the interior composition of
exoplanets based on measured planetary masses and radii.
A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions
L. A. Rogers, S. Seager, (Submitted on 16 Dec 2009 (v1), last revised 4 Jun 2010 (this version, v2))
See also
*Chthonian planet
Chthonian planets (, sometimes 'cthonian') are a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric st ...
References
The Role of Carbon in Extrasolar Planetary Geodynamics and Habitability
Cayman T. Unterborn, Jason E. Kabbes, Jeffrey S. Pigott, Daniel R. Reaman, Wendy R. Panero, (Submitted on 31 October 2013 (v1), last revised 9 November 2013 (this version, v3))
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Hypothetical planet types