Our galaxy is full with planets. We now know that planets and planetary systems are diverse and come with different sizes, masses and compositions, as well as various orbital architectures. Although there has been great progress in understanding plan
et formation in the last couple of decades, both observationally and theoretically, several fundamental questions remain unsolved. This might not be surprising given the complexity of the process that includes various physical and chemical processes, and spans huge ranges of length-scales, masses, and timescales. In addition, planet formation cannot be directly observed but has to be inferred by gluing together different pieces of information into one consistent picture. How do planets form? remains a fundamental question in modern astrophysics. In this review we list some of the key open questions in planet formation theory as well as the challenges and upcoming opportunities.
I investigate the nature of the transient nebulous companions to the sungrazing comet C/1882 R1, known as the Great September Comet. The features were located several degrees to the southwest of the comets head and reported independently by four obse
rvers, including J. F. J. Schmidt and E. E. Barnard, over a period of ten days nearly one month after perihelion, when the comet was 0.7 AU to 1 AU from the Sun. I conclude that none of the nebulous companions was ever sighted more than once and that, contrary to his belief, Schmidt observed unrelated objects on the four consecutive mornings. Each nebulous companion is proposed to have been triggered by a fragment at most a few tens of meters across, released from the comets nucleus after perihelion and seen only because it happened to be caught in the brief terminal outburst, when its mass was suddenly shattered into a cloud of mostly microscopic debris due possibly to rotational bursting triggered by sublimation torques. The fragments motion was affected by a strong outgassing-driven nongravitational acceleration with a significant out-of-plane component. Although fragmentation events were common, only a small fraction of nebulous companions was detected because of their transient nature. The observed brightness of the nebulous companions is proposed to have been due mainly to C2 emissions, with a contribution from scattering of sunlight by the microscopic dust. By their nature, the fragments responsible for the nebulous companions bear a strong resemblance to the dwarf Kreutz sungrazers detected with the coronagraphs aboard the SOHO space probe. Only their fragmentation histories are different and the latter display no terminal outburst, a consequence of extremely short lifetimes of the sublimating icy and refractory material in the Suns corona.
Dust grains are aligned with the interstellar magnetic field and drift through the interstellar medium (ISM). Evolution of interstellar dust is driven by grain motion. In this paper, we study the effect of grain alignment with magnetic fields and gra
in motion on grain growth in molecular clouds. We first discuss characteristic timescales of internal alignment (i.e., alignment of the grain axis with its angular momentum, ${bf J}$) and external alignment (i.e., alignment of ${bf J}$ with the magnetic field) and find the range of grain sizes that have efficient alignment. Then, we study grain growth for such aligned grains drifting though the gas. Due to the motion of aligned grains along the magnetic field, gas accretion would increase the grain elongation rather than decrease, as in the case of random orientation. Grain coagulation also gradually increases grain elongation, leading to the increase of elongation with the grain size. The coagulation of aligned grains can form dust aggregates that contain the elongated binaries comprising a pair of grains with parallel short axes. The presence of superparamagnetic iron clusters within dust grains enhances internal alignment and thus increases the maximum size of aligned grains from $sim 2$ to $sim 10mu m$ for dense clouds of $n_{rm H}sim 10^{5}rm cm^{-3}$. Determining the size of such aligned grains with parallel axes within a dust aggregate would be important to constrain the location of grain growth and the level of iron inclusions. We find that grains within dust aggregates in 67P/Churyumov-Gerasimenko obtained by {it Rosetta} have the grain elongation increasing with the grain radius, which is not expected from coagulation by Brownian motion but consistent with the grain growth from aligned grains.
The four directly imaged planets orbiting the star HR 8799 are an ideal laboratory to probe atmospheric physics and formation models. We present more than a decades worth of Keck/OSIRIS observations of these planets, which represent the most detailed
look at their atmospheres to-date by its resolution and signal to noise ratio. We present the first direct detection of HR 8799 d, the second-closest known planet to the star, at moderate spectral resolution with Keck/OSIRIS (K-band; R~4,000). Additionally, we uniformly analyze new and archival OSIRIS data (H and K band) of HR 8799 b, c, and d. First, we show detections of water (H2O) and carbon monoxide (CO) in the three planets and discuss the ambiguous case of methane (CH4) in the atmosphere of HR 8799b. Then, we report radial velocity (RV) measurements for each of the three planets. The RV measurement of HR 8799 d is consistent with predictions made assuming coplanarity and orbital stability of the HR 8799 planetary system. Finally, we perform a uniform atmospheric analysis on the OSIRIS data, published photometric points, and low resolution spectra. We do not infer any significant deviation from to the stellar value of the carbon to oxygen ratio (C/O) of the three planets, which therefore does not yet yield definitive information about the location or method of formation. However, constraining the C/O ratio for all the HR 8799 planets is a milestone for any multiplanet system, and particularly important for large, widely separated gas giants with uncertain formation processes.
Context. Due to our increasing knowledge on the Galactic and stellar neighborhood of the Solar System, modern long-period comet motion studies have to take into account both stellar perturbations and the overall Galactic potential. Aims. Our aim is t
o propose algorithms and methods to perform numerical integration of a Solar System small body equations of motion much faster and at the same time with greater precision. Methods. We propose a new formulation of the equations of motion formulated in the Solar System barycentric frame but accurately accounting for the differential perturbations caused by the Galactic potential. To use these equations effectively we provide numerical ephemerides of the Galactic positions of the Sun and a set of potential stellar perturbers. Results. The proposed methods offer the precision higher by several orders of magnitude and simultaneously greatly reduce the necessary CPU time. The application of this approach is presented with the example of a detailed dynamical study of the past motion of comet C/2015 XY1.
We present comprehensive orbital analyses and dynamical masses for the substellar companions Gl~229~B, Gl~758~B, HD~13724~B, HD~19467~B, HD~33632~Ab, and HD~72946~B. Our dynamical fits incorporate radial velocities, relative astrometry, and most impo
rtantly calibrated Hipparcos-Gaia EDR3 accelerations. For HD~33632~A and HD~72946 we perform three-body fits that account for their outer stellar companions. We present new relative astrometry of Gl~229~B with Keck/NIRC2, extending its observed baseline to 25 years. We obtain a $<$1% mass measurement of $71.4 pm 0.6,M_{rm Jup}$ for the first T dwarf Gl~229~B and a 1.2% mass measurement of its host star ($0.579 pm 0.007,M_{odot}$) that agrees with the high-mass-end of the M dwarf mass-luminosity relation. We perform a homogeneous analysis of the host stars ages and use them, along with the companions measured masses and luminosities, to test substellar evolutionary models. Gl~229~B is the most discrepant, as models predict that an object this massive cannot cool to such a low luminosity within a Hubble time, implying that it may be an unresolved binary. The other companions are generally consistent with models, except for HD~13724~B that has a host-star activity age 3.8$sigma$ older than its substellar cooling age. Examining our results in context with other mass-age-luminosity benchmarks, we find no trend with spectral type but instead note that younger or lower-mass brown dwarfs are over-luminous compared to models, while older or higher-mass brown dwarfs are under-luminous. The presented mass measurements for some companions are so precise that the stellar host ages, not the masses, limit the analysis.
In this article, theory-based analytical methodologies of astrophysics employed in the modern era are suitably operated alongside a test research-grade telescope to image and determine the orbit of a near-earth asteroid from original observations, me
asurements, and calculations. Subsequently, its intrinsic orbital path has been calculated including the chance it would likely impact Earth in the time ahead. More so specifically, this case-study incorporates the most effective, feasible, and novel Gausss Method in order to maneuver the orbital plane components of a planetesimal, further elaborating and extending our probes on a selected near-earth asteroid (namely the 12538-1998 OH) through the observational data acquired over a six week period. Utilizing the CCD (Charge Coupled Device) snapshots captured, we simulate and calculate the orbit of our asteroid as outlined in quite detailed explanations. The uncertainties and deviations from the expected values are derived to reach a judgement whether our empirical findings are truly reliable and representative measurements by partaking a statistical analysis based systematic approach. Concluding the study by narrating what could have caused such discrepancy of findings in the first place, if any, measures are put forward that could be undertaken to improve the test-case for future investigations. Following the calculation of orbital elements and their uncertainties using Monte Carlo analysis, simulations were executed with various sample celestial bodies to derive a plausible prediction regarding the fate of Asteroid 1998 OH. Finally, the astrometric and photometric data, after their precise verification, were officially submitted to the Minor Planet Center: an organization hosted by the Center for Astrophysics, Harvard and Smithsonian and funded by NASA, for keeping track of the asteroids potential trajectories.
Observations in the UV-regime are very important for exoplanet research, because many diagnostically important lines for studying stellar activity are in this regime. Studying stellar activity is not only important because of its negative effects on
the determination planetary parameters, but also because the XUV-radiation from the host stars affects the photochemistry and the erosion of planetary atmospheres . Unfortunately, the XUV-region is only accessible from space. However, since the XUV-radiation is correlated with the CaII,HK-lines, we can use these lines to study the XUV radiation indirectly. The CaIIHK lines for relatively bright stars can be observed with PLATOspec, a new high-resolution echelle spectrograph in development for the ESO 1.5m telescope at La Silla. One advantage compared to instruments on larger telescopes will be that large programs can be carried out. There will be two modes for obtaining precise RV-measurements. In the future, the CUBES instrument on the VLT will be able to study the same lines to probe the XUV-radiation in much fainter targets.
We conducted a project of reinvestigating the 2017--2019 microlensing data collected by the high-cadence surveys with the aim of finding planets that were missed due to the deviations of planetary signals from the typical form of short-term anomalies
. The project led us to find three planets including KMT-2017-BLG-2509Lb, OGLE-2017-BLG-1099Lb, and OGLE-2019-BLG-0299Lb. The lensing light curves of the events have a common characteristic that the planetary signals were produced by the crossings of faint source stars over the resonant caustics formed by giant planets located near the Einstein rings of host stars. For all planetary events, the lensing solutions are uniquely determined without any degeneracy. It is estimated that the host masses are in the range of $0.45lesssim M/M_odot lesssim 0.59$, which corresponds to early M to late K dwarfs, and thus the host stars are less massive than the sun. On the other hand, the planets, with masses in the range of $2.1lesssim M/M_{rm J}lesssim 6.2$, are heavier than the heaviest planet of the solar system, that is, Jupiter. The planets in all systems lie beyond the snow lines of the hosts, and thus the discovered planetary systems, together with many other microlensing planetary systems, support that massive gas-giant planets are commonplace around low-mass stars. We discuss the role of late-time high-resolution imaging in clarifying resonant-image lenses with very faint sources.
Stars produce explosive flares, which are believed to be powered by the release of energy stored in coronal magnetic field configurations. It has been shown that solar flares exhibit energy distributions typical of self-organized critical systems. Th
is study applies a novel flare detection technique to data obtained by NASAs TESS mission and identifies $sim10^6$ flaring events on $sim10^5$ stars across spectral types. Our results suggest that magnetic reconnection events that maintain the topology of the magnetic field in a self-organized critical state are ubiquitous among stellar coronae.
