The formation of super-Earths and mini-Neptunes with giant impacts

The majority of discovered exoplanetary systems harbour a new class of planets, bodies that are typically several times more massive than the Earth but that orbit their host stars well inside the orbit of Mercury. The origin of these close-in super-Earths and mini-Neptunes is one of the major unanswered questions in planet formation. Unlike the Earth, whose atmosphere contains less than 10[superscript −6] of its total mass, a large fraction of close-in planets have significant gaseous envelopes, containing 1–10 per cent or more of their total mass. It has been proposed that close-in super-Earths and mini-Neptunes formed in situ either by delivery of 50–100 M⊕ of rocky material to the inner regions of the protoplanetary disc or in a disc enhanced relative to the minimum mass solar nebula. In both cases, the final assembly of the planets occurs via giant impacts. Here we test the viability of these scenarios. We show that atmospheres that can be accreted by isolation masses are small (typically 10[superscript −3]–10[superscript −2] of the core mass) and that the atmospheric mass-loss during giant impacts is significant, resulting in typical post-giant impact atmospheres that are 8 × 10[superscript −4] of the core mass. Such values are consistent with terrestrial planet atmospheres but more than an order of magnitude below atmospheric masses of 1–10 per cent inferred for many close-in exoplanets. In the most optimistic scenario in which there is no core luminosity from giant impacts and/or planetesimal accretion, we find that post-giant impact envelope accretion from a depleted gas disc can yield atmospheric masses that are several per cent the core mass. If the gravitational potential energy resulting from the last mass doubling of the planet by giant impacts is released over the disc dissipation time-scale as core luminosity, then the accreted envelope masses are reduced by about an order of magnitude. Finally we show that, even in the absence of type I migration, radial drift time-scales due to gas drag for many isolation masses are shorter than typical disc lifetimes for standard gas-to-dust ratios. Given these challenges, we conclude that most of the observed close-in planets with envelopes larger than several per cent of their total mass likely formed at larger separations from their host stars

Stealing the Gas: Giant Impacts and the Large Diversity in Exoplanet Densities

Although current sensitivity limits are such that true solar system analogs remain challenging to detect, numerous planetary systems have been discovered that are very different from our own solar system. The majority of systems harbor a new class of planets, bodies that are typically several times more massive than the Earth but orbit their host stars well inside the orbit of Mercury. These planets frequently show evidence for large hydrogen and helium envelopes containing several percent of the planet's mass and display a large diversity in mean densities. Here we show that this wide range can be achieved by one or two late giant impacts, which are frequently needed to achieve long-term orbital stability in multiple planet systems once the gas disk has disappeared. We demonstrate using hydrodynamical simulations that a single collision between similarly sized exoplanets can easily reduce the envelope-to-core-mass ratio by a factor of two and show that this leads to a corresponding increase in the observed mean density by factors of two to three. In addition, we investigate how envelope mass loss depends on envelope mass, planet radius, semimajor axis, and the mass distribution inside the envelope. We propose that a small number of giant impacts may be responsible for the large observed spread in mean densities, especially for multiple-planet systems that contain planets with very different densities and have not been significantly sculpted by photoevaporation

Analysis and implementation of the bilayer microfluidic geometry

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Page 126 blank. Cataloged from PDF version of thesis.Includes bibliographical references (p. 121-125).Microfluidic devices form an important class of analytical platforms that have found wide use in the biomedical sciences. In particular, they have been used in cell culture systems, where they are used to monitor cell behavior in various environments. One challenge that has emerged, however, is the ability for a microfluidic device to uniformly deliver soluble factors to a given culture of cells without subjecting the cells to hydrodynamic shear stresses that could potentially alter their behavior in an unpredictable or undesirable way. This is especially true for a number of cell types, and striking a balance between solute transport and shear stress remains the subject of active research. In this thesis, we will consider a membrane bilayer device configuration in which the transport of a solute to a cell population is achieved by flowing solute through a proximate channel separated from the culture channel by a membrane and seek to characterize some of its hydrodynamic and transport characteristics. It will be shown analytically that this configuration affords greater flexibility over a more traditional single-channel setup, in terms of control over solute transport and applied shear. We will also discuss some topics related to the flow fields within such devices, as well as the fabrication and implementation of the bilayer microfluidic device in an experimental setting.by Niraj K. Inamdar.S.M

The Formation of Super-Earths and Mini-Neptunes with Giant Impacts

The majority of discovered exoplanetary systems harbour a new class of planets, bodies typically several times more massive than Earth but orbiting their host stars well inside the orbit of Mercury. The origin of these close-in super-Earths and mini-Neptunes is a major unanswered question in planet formation. Unlike Earth, whose atmosphere contains $<10^{-6}$ its total mass, a large fraction of close-in planets have significant gaseous envelopes, containing $1 -10\%$ or more of their total mass. It has been proposed that these close-in planets formed in situ either by delivery of $50-100M_{\oplus}$ of rocky material to the inner disc, or in a disc enhanced relative to the MMSN. In both cases, final assembly of the planets occurs by giant impacts (GIs). Here we test the viability of these scenarios. We show that atmospheres accreted by isolation masses are small ($10^{-3}-10^{-2}$ the core mass) and that atmospheric mass-loss during GIs is significant, with typical post-GI atmospheres that are $8 \times 10^{-4}$ the core mass. Such values are consistent with terrestrial planet atmospheres but more than an order of magnitude below atmospheric masses of $1-10\%$ inferred for many close-in exoplanets. In the most optimistic scenario with no core luminosity, post-GI envelope accretion from a depleted gas disc yields atmospheric masses that are several per cent the core mass. If the gravitational potential energy due to the last mass doubling of the planet by GIs is released over the disc dissipation time-scale as core luminosity, then envelope masses are reduced by about an order of magnitude. Finally we show that radial drift time-scales due to gas drag for many isolation masses are shorter than typical disc lifetimes. Given these challenges, we conclude that most observed close-in planets with envelopes larger than several per cent likely formed at larger separations from their host stars.Comment: 10 pages, 10 figures. Abstract abridged for submission. Fixed typographical error in Eq. 6 exponen

Design and Test of a Deployable Radiation Cover for the REgolith X-Ray Imaging Spectrometer

The REgolith X-ray Imaging Spectrometer (REXIS) instrument contains a one-time deployable radiation cover that is opened using a shape memory alloy actuator (a "Frangibolt") from TiNi Aerospace and two torsion springs. The door will be held closed by the bolt for several years in cold storage during travel to the target asteroid, Bennu, and it is imperative to gain confidence that the door will open at predicted operational temperatures. This paper briefly covers the main design features of the radiation cover and measures taken to mitigate risks to cover deployment. As the chosen FD04 model Frangibolt actuator has minimal flight heritage, the main focus of this paper is the testing, results and conclusions with the FD04 while discussing key lessons learned with respect to the use of the FD04 actuator in this application

Modeling the Expected Performance of the REgolith X-ray Imaging Spectrometer (REXIS)

OSIRIS-REx is the third spacecraft in the NASA New Frontiers Program and is planned for launch in 2016. OSIRIS-REx will orbit the near-Earth asteroid (101955) Bennu, characterize it, and return a sample of the asteroid's regolith back to Earth. The Regolith X-ray Imaging Spectrometer (REXIS) is an instrument on OSIRIS-REx designed and built by students at MIT and Harvard. The purpose of REXIS is to collect and image sun-induced fluorescent X-rays emitted by Bennu, thereby providing spectroscopic information related to the elemental makeup of the asteroid regolith and the distribution of features over its surface. Telescopic reflectance spectra suggest a CI or CM chondrite analog meteorite class for Bennu, where this primitive nature strongly motivates its study. A number of factors, however, will influence the generation, measurement, and interpretation of the X-ray spectra measured by REXIS. These include: the compositional nature and heterogeneity of Bennu, the time-variable Solar state, X-ray detector characteristics, and geometric parameters for the observations. In this paper, we will explore how these variables influence the precision to which REXIS can measure Bennu's surface composition. By modeling the aforementioned factors, we place bounds on the expected performance of REXIS and its ability to ultimately place Bennu in an analog meteorite class.Comment: Presented at the SPIE Optics + Photonics Conference, 18 August 2014, San Diego, C

Formation of Super-Earths

Super-Earths are the most abundant planets known to date and are characterized by having sizes between that of Earth and Neptune, typical orbital periods of less than 100 days and gaseous envelopes that are often massive enough to significantly contribute to the planet's overall radius. Furthermore, super-Earths regularly appear in tightly-packed multiple-planet systems, but resonant configurations in such systems are rare. This chapters summarizes current super-Earth formation theories. It starts from the formation of rocky cores and subsequent accretion of gaseous envelopes. We follow the thermal evolution of newly formed super-Earths and discuss their atmospheric mass loss due to disk dispersal, photoevaporation, core-cooling and collisions. We conclude with a comparison of observations and theoretical predictions, highlighting that even super-Earths that appear as barren rocky cores today likely formed with primordial hydrogen and helium envelopes and discuss some paths forward for the future.Comment: Invited review accepted for publication in the 'Handbook of Exoplanets,' Planet Formation section, Springer Reference Works, Juan Antonio Belmonte and Hans Deeg, Ed

The REgolith X-Ray Imaging Spectrometer (REXIS) for OSIRIS-REx: identifying regional elemental enrichment on asteroids

The OSIRIS-REx Mission was selected under the NASA New Frontiers program and is scheduled for launch in September of 2016 for a rendezvous with, and collection of a sample from the surface of asteroid Bennu in 2019. 101955 Bennu (previously 1999 RQ36) is an Apollo (near-Earth) asteroid originally discovered by the LINEAR project in 1999 which has since been classified as a potentially hazardous near-Earth object. The REgolith X-Ray Imaging Spectrometer (REXIS) was proposed jointly by MIT and Harvard and was subsequently accepted as a student led instrument for the determination of the elemental composition of the asteroid's surface as well as the surface distribution of select elements through solar induced X-ray fluorescence. REXIS consists of a detector plane that contains 4 X-ray CCDs integrated into a wide field coded aperture telescope with a focal length of 20 em for the detection of regions with enhanced abundance in key elements at 50 m scales. Elemental surface distributions of approximately 50-200 m scales can be detected using the instrument as a simple collimator. An overview of the observation strategy of the REXIS instrument and expected performance are presented here.Astronom