Stars, Planets and Origins

We think we know how stars live and die, but our picture of how stars form to begin with is incomplete. Although astronomers have discovered more than 1000 planets in other solar systems, we do not really know what conditions actually produce life.

Ultracool Stellar Magnetism

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"Ultracool dwarfs" are the very smallest, coldest stars in the Universe. Some of them aren't even massive enough to power themselves with hydrogen fusion in their cores, making them technically "brown dwarfs" rather than stars, destined to slowly cool off for the rest of the Universe's lifetime. These stars are not capable of generating magnetic fields the same way that our Sun does, and conventional wisdom used to be that they would display only weak signs of magnetism.

Merging White Dwarfs

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Smithsonian scientists and colleagues are discovering extremely low mass white dwarfs in extremely close pairs. In the system called J0651, for example, a pair of Earth-sized white dwarfs, separated by a third of the Earth-Moon distance, go around each other every 12.75 minutes. This system is a gravitational wave source 10,000 times stronger than the Hulse-Taylor pulsar; the white dwarfs will merge and possibly explode as a supernova in about 1 million years.

Computational Stellar Astrophysics

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Star-planet Interaction in HD 189733

The Computational Stellar Astrophysics (CSA) group uses numerical models to study:

Resonance Overlap and Planetary Stability

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motion resonances

Motivated by the population of observed multi-planet systems with orbital period ratios 1 < P2=P1 . 2, TA researchers Kat Deck, Matt Payne & Matt Holman have studied the long-term stability of packed two planet systems.

Kozai-Lidov Mechanism

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Many gravitational triple systems are in a hierarchical configuration, where two objects orbit each other in a relatively tight inner binary while the third object is on a much wider orbit. If the third object is sufficiently distant, an analytic, perturbative approach can be used to calculate the evolution of the system. In the secular approximation , the two orbits torque each other and exchange angular momentum, but not energy. Therefore the orbits can change shape and orientation (on timescales much longer than their orbital periods), but not semimajor axes.

Planetary Dynamics and Celestial Mechanics

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A large number of TA researchers work on projects connected to celestial mechanics, with many looking at the stability and evolution of planetary systems in general, as well as the specific evolution of the Solar System.

Highlights to (and links to) some current research in this area by department members include:

Chaos \& Resonance

Transit Timing Variations

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If an exoplanet transits it's host star along our line of sight, then a planet on an unperturbed Keplerian orbit will transit with a fixed, predictable period. When an exoplanet orbit is perturbed by interactions with other planets, the perturbed orbits can give rise to transit timing variations (TTVs).These TTVs can be used to infer the presence of unseen companions, or if the companion also transits, can be used to provide precise information on the mass of the companion planet.

The Origins of Stellar Spin - Planetary Orbit Misalignments

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The accelerating quantification of exo-planetary orbital obliquity (i.e. the misalignment between the stellar spin axis and the planetary orbit normal) has been interpreted as being among the most information-rich relics left over from the epoch of planet formation. Unexpectedly, the recurrent identification of large spin-orbit misalignments have called into question our understanding of planet-disk interactions and planet-formation in general.

Magnetohydrodynamic Effects in Hot Planetary Atmospheres;

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The first discoveries of extrasolar planets around main-sequence stars revealed the wide-spread presence of planetary bodies that reside in close proximity to their host stars. Through the process of thermal ionization, intense stellar irradiation renders close-in planetary atmospheres electrically conductive. Accordingly, our efforts have been focused on understanding the consequences of magnetic effects on large-scale atmospheric circulation as well as thermal evolution of the interiors of close-in giant planets.

Dynamical Architecture of Planetary Systems

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Driven by the rapid pace of observational detection of novel planetary systems, as well as advances in computational capabilities, the interest in the study of planetary orbital dynamics has been reinvigorated. In turn, the formulation of a substantial aggregate of new models has shed light on the dramatic processes responsible for shaping planetary systems including our own.