To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure firstname.lastname@example.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
We present preliminary astrometric results aimed at understanding the lifetime of circumstellar disks and potential for planet formation. We have obtained parallaxes to stars in the TW Hydrae, Upper Scorpius, and Chamaeleon I stellar associations. These enable new estimates for the ages of the stars. We are also performing the Carnegie Astrometric Planet Search of nearby low mass stars for gas giant planets on wide orbits. We have our first candidate around a mature brown dwarf.
M dwarf stars are attractive targets in the search for habitable worlds as a result of their relative abundance and proximity, making them likely targets for future direct detection efforts. Hot super-Earths as well as gas giants have already been detected around a number of early M dwarfs, and the former appear to be the high-mass end of the population of rocky, terrestrial exoplanets. The Carnegie Astrometric Planet Search (CAPS) program has been underway since March 2007, searching ~ 100 nearby late M, L, and T dwarfs for gas giant planets on orbits wide enough for habitable worlds to orbit interior to them. The CAPSCam-N camera on the 2.5-m du Pont telescope at the Las Campanas Observatory has demonstrated the ability to detect planets as low in mass as Saturn orbiting at several AU around late M dwarfs within 15 pc. Over the next decade, the CAPS program will provide new constraints on the planetary census around late M dwarf stars, and hence on the suitability of these nearby planetary systems for supporting life.
Every ten years the astronomy and astrophysics community in the United States undertakes a survey intended to prioritize plans for major ground- and space-based astronomical facilities for the coming decade. New Worlds, New Horizons (NWNH) was released in August 2010 and represents the community's advice to the United States' funding agencies about the top priorities for 2010-2020. Here we focus on the recommendations of NWNH for space-based exoplanet missions to be considered by NASA, and on the plans developed to date for how NASA will respond to the science goals and missions set out for them by NWNH.
The meeting was opened by Ted Bowell, president, at 11 am. The 2006 Division III meetings were reviewed by Guy Consolmagno, secretary; as the minutes of those meetings have already been published, they were assumed to be approved.
Trained as a physicist, George W. Wetherill (1925-2006) made seminal contributions to the fields of geochemical dating, meteoritical and asteroidal science, and the theory of the formation of terrestrial planets, evolving along the way into one of the first astrobiologists.
Planets typically are considerably more metal-rich than even the most metal-rich stars, one indication that planet formation must differ greatly from star formation. There is general agreement that terrestrial planets form by the collisional accumulation of solids composed of heavy elements in the inner regions of protoplanetary disks. Two competing mechanisms exist for the formation of giant planets, core accretion and disk instability, though hybrid combinations are possible as well. In core accretion, a higher metallicity in the protoplanetary disk leads directly to larger core masses and hence to more gas giant planets. Given the strong correlation of gas giant planets detected by Doppler spectroscopy with stellar metallicity, this has often been taken as proof that core accretion is the mechanism that forms giant planets. Recent work, however, implies that the formation of gas giants by disk instability can be enhanced by higher metallicities, though not as dramatically as for core accretion. In both scenarios, the ongoing accretion of planetesimals by gas giant protoplanets leads to strong enrichments of heavy elements in their gaseous envelopes. Both scenarios also imply that gas giant planets should have significant solid cores, raising questions for gas giant interior models without cores. Exoplanets with large inferred core masses seem likely to have formed by core accretion, while gas giants at distances beyond 20 AU seem more likely to have formed by disk instability. Given the wide variety of exoplanets found to date, it appears that both mechanisms are needed to explain the formation of the known population of giant planets.
Commission 53 on Extrasolar Planets was created at the 2006 Prague General Assembly of the IAU, in recognition of the outburst of astronomical progress in the field of extrasolar planet discovery, characterization, and theoretical work that has occurred since the discovery of the pulsar planets in 1992 and the discovery of the first planet in orbit around a solar-type star in 1995. Commission 53 is the logical successor to the IAU Working Group on Extrasolar Planets WG-ESP, which ended its six years of existence in August 2006. The founding president of Commission 53 is Michael Mayor, in honor of his seminal contributions to this new field of astronomy. The vice-president is Alan Boss, the former chair of the WG-ESP, and the members of the Commission 53 Organizing Committee are the other former members of the WG-ESP.
Bioastronomy: Search for Extraterrestrial Life was established as Commission 51 of the IAU in 1982. The objectives of the commission included: the search for planets around other stars; the search for radio transmissions, intentional or unintentional, of extraterrestrial origin; the search for biologically relevant interstellar molecules and the study of their formation processes; detection methods for potential spectroscopic evidence of biological activity; the coordination of efforts in all these areas at the international level and the establishment of collaborative programs with other international scientific societies with related interests. In 2006, Commission 51 was renamed simply Bioastronomy at the IAU General Assembly in Prague, and approved for the next six years, the default extension for an IAU Commission.
Human beings have long thought that planetary systems similar to our own should exist around stars other than the Sun. However, the astronomical search for planets outside our Solar System has had a dismal history of decades of discoveries that were announced, but could not be confirmed. All that changed in 1995, when we entered the era of the discovery of extrasolar planetary systems orbiting main-sequence stars. To date, well over 130 planets have been found outside our Solar System, ranging from the fairly familiar to the weirdly unexpected. Nearly all of the new planets discovered to date appear to be gas giant planets similar to our Jupiter and Saturn, though with very different orbits about their host stars. In the last year, three planets with much lower masses have been found, similar to those of Uranus and Neptune, but it is not yet clear if they are also ice giant planets, or perhaps rock giant planets, i.e., super-Earths. The long-term goal is to discover and characterize nearby Earth-like, habitable planets. A visionary array of space-based telescopes has been planned that will carry out this incredible search over the next several decades.
The Working Group was formed at the request of the Board of DivisionIII and approved by the IAU Executive committee in March 2004. This was in recognition of the fact that discoveries in the Trans Neptunian region were repeatedly raising the question of “what is a planet”. The task of the WG was to investigate the options available and give indications of the level of support and opposition for each if more than one option was emerging.
The Working Group on Extrasolar Planets (hereafter the WGESP) was created at a meeting of the IAU Executive Council in 1999 as a Working Group of IAU Division III and was renewed for three more years at the IAU General Assembly in 2003. The charge of the WGESP is to act as a focal point for international research on extrasolar planets. The membership of the WGESP has remained unchanged for the last three years.
The discovery of gas giant planets around nearby stars has launched a new era in our understanding of the formation and evolution of planetary systems. However, none of the over four dozen companions detected to date strongly resembles Jupiter or Saturn: their inferred masses range from sub-Saturn-mass to 10 Jupiter-masses or more, while their orbits extend from periods of a few days to a few years. Given this situation, it seems prudent to re-examine mechanisms for gas giant planet formation. The two extreme cases are top-down or bottom-up. The latter is the core accretion mechanism, long favored for our Solar System, where a roughly 10 Earth-mass solid core forms by collisional accumulation of planetesimals, followed by hydrodynamic accretion of a gaseous envelope. The former is the long-discarded disk instability mechanism, where the protoplanetary disk forms self-gravitating, gaseous protoplanets through a gravitational instability of the gas, accompanied by settling and coagulation of dust grains to form solid cores. Both of these mechanisms have a number of advantages and disadvantages, making a purely theoretical choice between them difficult at present. Observations should be able to decide the dominant mechanism by dating the epoch of gas giant planet formation: core accretion requires more than a million years to form a Jupiter-mass planet, whereas disk instability is much more rapid.
Searches for very low mass objects in young star clusters have uncovered evidence for free-floating objects with inferred masses possibly as low as 5 to 15 Jupiter masses (MJup), similar to the masses of several extrasolar planets. We show here that the process which forms single and multiple protostars, namely collapse and fragmentation of molecular clouds, might be able to produce self-gravitating objects with initial masses less than ˜ 1MJup. Models are calculated with a three dimensional, finite differences code which solves the equations of hydrodynamics, gravitation, and radiative transfer in the Eddington and diffusion approximations. Magnetic pressure is added to the gas pressure, magnetic tension is approximated by gravity dilution once collapse is well underway, and ambipolar diffusion is treated approximately as well. Initially oblate clouds fragment into multiple protostar systems containing a small number (of order four) of fragments. If such fragments can be ejected from an unstable quadruple protostar system, prior to gaining significantly more mass, protostellar collapse might then be able to explain the formation of free-floating objects with masses below 13MJup. These objects might then be best termed “sub-brown dwarfs”.
Fragmentation is the leading explanation for the formation of binary and multiple stars. However, nearly all three dimensional calculations of the collapse and fragmentation of dense molecular cloud cores have ignored the effects of magnetic fields, whereas magnetic fields are generally regarded to be a dominant force in molecular clouds. Three dimensional models of the collapse of clouds with frozen-in magnetic fields have shown that such clouds cannot fragment for a range of initial conditions. However, calculations that allow for magnetic field loss by am-bipolar diffusion have shown that fragmentation is possible for initially prolate or oblate, rotating, magnetically-supported cloud cores. The latter calculations rely on approximations that should be verified by more detailed, traditional magnetohydrodynamical codes. The most obvious effect of magnetic fields is to delay the onset of the collapse phase, but once collapse begins in earnest, fragmentation proceeds in much the same manner as in nonmagnetic clouds, with initially prolate clouds tending to form binary protostars, and with initially oblate clouds tending to form multiple protostars.
Binary and multiple star systems may form by fragmentation, that is, through break-up of a dense molecular cloud core during the dynamical collapse phase that leads to the formation of protostellar objects. This review concentrates on theoretical models of fragmentation based on numerical hydrodynamical calculations in three spatial dimensions, using both finite-difference and smoothed particle hydrodynamics techniques. A variety of recent results are described, including calculations of the fragmentation of bar-like (prolate) clouds, fragmentation in clouds with initial power-law density and angular velocity distributions, tidally-induced fragmentation, fragmentation in cooling clouds, formation of hierarchical systems, and the dividing line between clouds that fragment and those that appear to form single protostars. A brief comparison of the predicted physical and dynamical properties of the theoretical fragments with the observed properties of main-sequence and pre-main-sequence binary stars lends supports to the hypothesis that fragmentation is the dominant formation mechanism for binary and multiple star systems. The major uncertainties regarding fragmentation are the extent to which precollapse clouds are susceptible to fragmentation, and the degree to which binary fragments undergo orbital decay and possibly mergers through interactions with the enveloping disk.
Email your librarian or administrator to recommend adding this to your organisation's collection.