Categories Of Planets Research Articles

Split Peas in a Pod: Intra-system Uniformity of Super-Earths and Sub-Neptunes

Abstract The planets within compact multiplanet systems tend to have similar sizes, masses, and orbital period ratios, like “peas in a pod.” This pattern was detected when considering planets with radii between 1 and 4 R ⊕. However, these same planets show a bimodal radius distribution, with few planets between 1.5 and 2 R ⊕. The smaller “super-Earths” are consistent with being stripped rocky cores, while the larger “sub-Neptunes” likely have gaseous H/He envelopes. Given these distinct structures, it is worthwhile to test for intra-system uniformity separately within each category of planets. Here, we find that the tendency for intra-system uniformity is twice as strong when considering planets within the same size category than it is when combining all planets together. The sub-Neptunes tend to be 1.7 − 0.3 + 0.6 times larger than the super-Earths in the same system, corresponding to an envelope mass fraction of about 2.6% for a 5 M ⊕ planet. For the sub-Neptunes, the low-metallicity stars are found to have planets with more equal sizes, with modest statistical significance (p ∼ 0.005). There is also a modest (∼2σ) tendency for wider-orbiting planets to be larger, even within the same size category.

Predicate Change

Like belief revision, conceptual change has rational aspects. The paper discusses this for predicate change. We determine the meaning of predicates by a set of imaginable instances, i.e., conceptually consistent entities that fall under the predicate. Predicate change is then an alteration of which possible entities are instances of a concept. The recent exclusion of Pluto from the category of planets is an example of such a predicate change. In order to discuss predicate change, we define a monadic predicate logic with three different kinds of lawful belief: analytic laws, which hold for all possible instances; doxastic laws, which hold for the most plausible instances; and typicality laws, which hold for typical instances. We introduce predicate changing operations that alter the analytic laws of the language and show that the expressive power is not affected by the predicate change. One can translate the new laws into old laws and vice versa. Moreover, we discuss rational restrictions of predicate change. These limit its possible influence on doxastic and typicality laws. Based on the results, we argue that predicate change can be quite conservative and sometimes even hardly recognisable.

Habitability and biogenic effects on planetary atmospheres

The physical characteristics of planets have been critically examined considering the various properties of solar-system with special reference to Earth. The consequences of abiotic chemical evolution have been focused with an emphasis to the theories of life formation. Also roadmaps for the habitability of planets have been sketched for a re-look of the biogenic effects. Objects of planetary size at the distances of the closest stars cannot be identified optically, even under the most favorable conditions. The usual practice is therefore to implement, wherever possible, available factual material and known relationships. Reasonable inferences in many times are based on models constructed from observations of conditions in the solar system. Dependence on existing theories becomes necessary where no direct data exist, as all known chemical and physical laws must be obeyed elsewhere in the universe. At the beginning of the last century scientists have emphasized by speculation about the existence of life on other planets but only in a qualitative manner (Cameron, 1963, Sagan, 1963, Rood and Trefil, 1981). In order to establish reasonable quantitative limits from which the prevalence of planets can be estimated, it is essential to define more precisely what is meant by a planet. One can imagine different categories of planets that could support some unknown form of life, planets that could support microscopic forms of carbon-based life and planets that could support various extreme forms of terrestrial life like organisms living in seas of liquid ammonia or breathing gaseous sulfur or existing under any other such alien conditions (Goldsmith et al. , 1993, Ulmschneider, 2002). However, by the term habitable we essentially mean a planet with surface conditions suitable for population. The environmental conditions required by population as well as the astronomical circumstances that produce these requisite environmental conditions are very important factors. Many of the questions raised here cannot be answered definitively due to the deficiencies or limitations of our knowledge of the universe at the present time. It is true that our solar system is not an extraordinarily rare assemblage of bodies, but is a typical planetary system and its members can be treated as a good sample of the types of bodies that exist in proximity to other stars (Bhattacharya et al. , 2012). Even in full recognition of the incompleteness of our knowledge, we can extend the range of the planets so that future studies can apply it conceptually to estimate the probabilities that these astronomical concatenations will be found elsewhere in the Galaxy and also to the other galaxies in the immediate neighborhood of the Sun.

Classification of celestial bodies within planetary systems

This paper suggests a set of general criteria for classifying celestial bodies that are elements of planetary systems. An additional criterion based on the phenomenological formula of the distribution of planets in the Solar system is suggested for the Solar system for ascertaining whether celestial bodies belong to the category of planets or dwarf planets.

Characterizing Super-Earths in reflected light

We review the investigation of Super-Earths. After defining this category of planets, we briefly defend the paradigm that they may be good targets for the search of Life, since they may be abundant and all terrestrial concepts are valid for them. We then describe the scientific benefits of their characterization by analyzing (by spectroscopy and polarimetry) the light they reflect from the parent star. We finally compare the performances of space missions and Dome C approaches.

Statistical properties of exoplanets

Interesting emerging observational properties of the period-mass distribution of extra-solar planets are discussed. New recent detections confirm the already emphasized lack of massive planets () on short-period orbits ( days). Furthermore, we point out i) a shortage of planets in the 10–100 day period range as well as ii) a lack of light planets () on orbits with periods larger than ~100 days. The latter feature is shown not to be due to small-number statistics with Monte-Carlo simulations. These observational period-related characteristics are discussed in the context of the migration process of exoplanets. They are found to be in agreement with recent simulations of planet interactions with viscous disks. The observed valley at a few tens of days in the period distribution is interpreted as a transition region between two categories of planets that suffered different migration scenarios. The lack of light planets on longer-period orbits and the corresponding intriguing sharp limit in mass is tentatively explained by the runaway migration process recently studied by Masset & Papaloizou (2003). The observed properties also have implications for the observation strategies of the on-going surveys and of future higher-precision searches.