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Register a new userA Quantitative Criterion for Defining Planets
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Authors
Jean-Luc Margot
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A simple metric can be used to determine whether a planet or exoplanet can clear its orbital zone during a characteristic time scale, such as the lifetime of the host star on the main sequence. This criterion requires only estimates of star mass, planet mass, and orbital period, making it possible to immediately classify 99% of all known exoplanets. All 8 planets and all classifiable exoplanets satisfy the criterion. This metric may be useful in generalizing and simplifying the definition of a planet.
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We derive a criterion for the onset of chaos in systems consisting of two massive, eccentric, coplanar planets. Given the planets masses and separation, the criterion predicts the critical eccentricity above which chaos is triggered. Chaos occurs where mean motion resonances overlap, as in Wisdom (1980)s pioneering work. But whereas Wisdom considered only nearly circular planets, and hence examined only first order resonances, we extend his results to arbitrarily eccentric planets (up to crossing orbits) by examining resonances of all orders. We thereby arrive at a simple expression for the critical eccentricity. We do this first for a test particle in the presence of a planet, and then generalize to the case of two massive planets, based on a new approximation to the Hamiltonian (Hadden, in prep). We then confirm our results with detailed numerical simulations. Finally, we explore the extent to which chaotic two-planet systems eventually result in planetary collisions.
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The mass and distance functions of free-floating planets (FFPs) would give major insights into the formation and evolution of planetary systems, including any systematic differences between those in the disk and bulge. We show that the only way to measure the mass and distance of individual FFPs over a broad range of distances is to observe them simultaneously from two observatories separated by $Dsim {cal O}(0.01,AU)$ (to measure their microlens parallax $pi_{rm E}$) and to focus on the finite-source point-lens (FSPL) events (which yield the Einstein radius $theta_{rm E}$). By combining the existing KMTNet 3-telescope observatory with a 0.3m $4,{rm deg}^2$ telescope at L2, of order 130 such measurements could be made over four years, down to about $Msim 6,M_oplus$ for bulge FFPs and $Msim 0.7,M_oplus$ for disk FFPs. The same experiment would return masses and distances for many bound planetary systems. A more ambitious experiment, with two 0.5m satellites (one at L2 and the other nearer Earth) and similar camera layout but in the infrared, could measure masses and distances of sub-Moon mass objects, and thereby probe (and distinguish between) genuine sub-Moon FFPs and sub-Moon ``dwarf planets in exo-Kuiper Belts and exo-Oort Clouds.
We derive a semi-analytic criterion for the presence of chaos in compact, eccentric multiplanet systems. Beyond a minimum semimajor-axis separation, below which the dynamics are chaotic at all eccentricities, we show that (i) the onset of chaos is determined by the overlap of two-body mean motion resonances (MMRs), like it is in two-planet systems; (ii) secular evolution causes the MMR widths to expand and contract adiabatically, so that the chaotic boundary is established where MMRs overlap at their greatest width. For closely spaced two-planet systems, a near-symmetry strongly suppresses this secular modulation, explaining why the chaotic boundaries for two-planet systems are qualitatively different from cases with more than two planets. We use these results to derive an improved angular-momentum-deficit (AMD) stability criterion, i.e., the critical system AMD below which stability should be guaranteed. This introduces an additional factor to the expression from Laskar and Petit (2017) that is exponential in the interplanetary separations, which corrects the AMD threshold toward lower eccentricities by a factor of several for tightly packed configurations. We make routines for evaluating the chaotic boundary available to the community through the open-source SPOCK package.
We describe an online database for extra-solar planetary-mass candidates, updated regularly as new data are available. We first discuss criteria for the inclusion of objects in the catalog: definition of a planet and several aspects of the confidence level of planet candidates. {bf We are led to point out the conflict between sharpness of belonging or not to a catalogue and fuzziness of the confidence level.} We then describe the different tables of extra-solar planetary systems, including unconfirmed candidates (which will ultimately be confirmed, or not, by direct imaging). It also provides online tools: histogrammes of planet and host star data, cross-correlations between these parameters and some VO services. Future evolutions of the database are presented.
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