Iron planet

About: Iron planet is a research topic. Over the lifetime, 96 publications have been published within this topic receiving 4961 citations.

Mass-Radius Relationships for Solid Exoplanets

Abstract: We use new interior models of cold planets to investigate the mass-radius relationships of solid exoplanets, considering planets made primarily of iron, silicates, water, and carbon compounds. We find that the mass-radius relationships for cold terrestrial mass planets of all compositions we considered follow a generic functional form that is not a simple power law: log10 Rs = k1 + log10(Ms) - k2M for up to Mp ≈ 20 M⊕, where Ms and Rs are scaled mass and radius values. This functional form arises because the common building blocks of solid planets all have equations of state that are well approximated by a modified polytrope of the form ρ = ρ0 + cPn. We find that highly detailed planet interior models, including temperature structure and phase changes, are not necessary to derive solid exoplanet bulk composition from mass and radius measurements. For solid exoplanets with no substantial atmosphere we have also found the following: with 5% fractional uncertainty in planet mass and radius it is possible to distinguish among planets composed predominantly of iron or silicates or water ice but not more detailed compositions; with ~5% uncertainty water ice planets with 25% water by mass may be identified; the minimum plausible planet size for a given mass is that of a pure iron planet; and carbon planet mass-radius relationships overlap with those of silicate and water planets due to similar zero-pressure densities and equations of state. We propose a definition of "super-Earths" based on the clear distinction in radii between planets with significant gas envelopes and those without.

Astrophysics of Planet Formation

Abstract: Preface 1. Observations of planetary systems 2. Protoplanetary disk structure 3. Protoplanetary disk evolution 4. Planetesimal formation 5. Terrestrial planet formation 6. Giant planet formation 7. Early evolution of planetary systems Appendixes References Index.

A sub-Mercury-sized exoplanet

Abstract: Since the discovery of the first exoplanets, it has been known that other planetary systems can look quite unlike our own. Until fairly recently, we have been able to probe only the upper range of the planet size distribution, and, since last year, to detect planets that are the size of Earth or somewhat smaller. Hitherto, no planets have been found that are smaller than those we see in the Solar System. Here we report a planet significantly smaller than Mercury. This tiny planet is the innermost of three that orbit the Sun-like host star, which we have designated Kepler-37. Owing to its extremely small size, similar to that of the Moon, and highly irradiated surface, the planet, Kepler-37b, is probably rocky with no atmosphere or water, similar to Mercury.

Ocean planet or thick atmosphere: On the mass-radius relationship for solid exoplanets with massive atmospheres

Abstract: The bulk composition of an exoplanet is commonly inferred from its average density. For small planets, however, the average density is not unique within the range of compositions. Variations of a number of important planetary parameters—whicharedifficultorimpossibletoconstrainfrommeasurementsalone—produceplanetswiththesame averagedensitiesbutwidelyvaryingbulkcompositions.Wefindthataddingagasenvelopeequivalentto0.1%Y10% of the mass of a solid planet causes the radius to increase 5%Y60% above its gas-free value. A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet), or alternatively alarge rocky iron coreandsomeHand/orHe.Forexample,awidevarietyof compositionscanexplaintheobservedradiusof GJ436b, althoughallmodelsrequiresomeH/He.Weconcludethattheidentificationof waterworldsbasedonthemass-radius relationship alone is impossible unless a significant gas layer can be ruled out by other means. Subject headingg planets and satellites: general — planetary systems — stars: individual (GJ 436) Online material: color figures

A rocky composition for an Earth-sized exoplanet

Abstract: Planets with sizes between that of Earth (with radius R⊕) and Neptune (about 4 R⊕) are now known to be common around Sun-like stars. Most such planets have been discovered through the transit technique, by which the planet’s size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet’s mass—and hence its density, which is a clue to its composition—is more difficult. Planets of size 2–4 R⊕ have proved to have a wide range of densities, implying a diversity of compositions, but these measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star every 8.5 hours (ref. 6). Given a radius of 1.20 ± 0.09 R⊕ and a mass of 1.69 ± 0.41 M⊕, the planet’s mean density of 5.3 ± 1.8 g cm^(−3) is similar to Earth’s, suggesting a composition of rock and iron.