9 research outputs found
Evidence for post-nebula volatilisation in an exo-planetary body
The loss and gain of volatile elements during planet formation is key for
setting their subsequent climate, geodynamics, and habitability. Two broad
regimes of volatile element transport in and out of planetary building blocks
have been identified: that occurring when the nebula is still present, and that
occurring after it has dissipated. Evidence for volatile element loss in
planetary bodies after the dissipation of the solar nebula is found in the high
Mn to Na abundance ratio of Mars, the Moon, and many of the solar system's
minor bodies. This volatile loss is expected to occur when the bodies are
heated by planetary collisions and short-lived radionuclides, and enter a
global magma ocean stage early in their history. The bulk composition of
exo-planetary bodies can be determined by observing white dwarfs which have
accreted planetary material. The abundances of Na, Mn, and Mg have been
measured for the accreting material in four polluted white dwarf systems.
Whilst the Mn/Na abundances of three white dwarf systems are consistent with
the fractionations expected during nebula condensation, the high Mn/Na
abundance ratio of GD362 means that it is not (>3 sigma). We find that heating
of the planetary system orbiting GD362 during the star's giant branch evolution
is insufficient to produce such a high Mn/Na. We, therefore, propose that
volatile loss occurred in a manner analogous to that of the solar system
bodies, either due to impacts shortly after their formation or from heating by
short-lived radionuclides. We present potential evidence for a magma ocean
stage on the exo-planetary body which currently pollutes the atmosphere of
GD362
Are exoplanetesimals differentiated?
Metals observed in the atmospheres of white dwarfs suggest that many have
recently accreted planetary bodies. In some cases, the compositions observed
suggest the accretion of material dominantly from the core (or the mantle) of a
differentiated planetary body. Collisions between differentiated
exoplanetesimals produce such fragments. In this work, we take advantage of the
large numbers of white dwarfs where at least one siderophile (core-loving) and
one lithophile (rock-loving) species have been detected to assess how commonly
exoplanetesimals differentiate. We utilise N-body simulations that track the
fate of core and mantle material during the collisional evolution of planetary
systems to show that most remnants of differentiated planetesimals retain core
fractions similar to their parents, whilst some are extremely core-rich or
mantle-rich. Comparison with the white dwarf data for calcium and iron
indicates that the data are consistent with a model in which
have accreted the remnants of differentiated planetesimals, whilst
have Ca/Fe abundances altered by the effects of heating
(although the former can be as high as , if heating is ignored). These
conclusions assume pollution by a single body and that collisional evolution
retains similar features across diverse planetary systems. These results imply
that both collisions and differentiation are key processes in exoplanetary
systems. We highlight the need for a larger sample of polluted white dwarfs
with precisely determined metal abundances to better understand the process of
differentiation in exoplanetary systems
Exocomets from a Solar System Perspective
Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred
from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently,
they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris
disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the
potential to give us information about early stage formation and evolution conditions of extra solar systems. In the solar
system, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed,
providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential
compositional properties between solar system comets and exocomets to allow for the development of new observational
methods and techniques. The paper aims to highlight commonalities and to discuss differences which may aid the
communication between the involved research communities and perhaps also avoid misconceptions. The compositional
properties of solar system comets and exocomets are summarized before providing an observational comparison between
them. Exocomets likely vary in their composition depending on their formation environment like solar system comets do,
and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of solar system comets. Observations of gas around main sequence stars, spectroscopic observations of “polluted” white
dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show
compositional similarities with solar system comets. The recent interstellar visitor 2I/Borisov showed gas, dust and
nuclear properties similar to that of solar system comets. This raises the tantalising prospect that observations of
interstellar comets may help bridge the fields of exocomet and solar system comets
Clades of huge phages from across Earth's ecosystems
Bacteriophages typically have small genomes and depend on their bacterial hosts for replication. Here we sequenced DNA from diverse ecosystems and found hundreds of phage genomes with lengths of more than 200 kilobases (kb), including a genome of 735 kb, which is-to our knowledge-the largest phage genome to be described to date. Thirty-five genomes were manually curated to completion (circular and no gaps). Expanded genetic repertoires include diverse and previously undescribed CRISPR-Cas systems, transfer RNAs (tRNAs), tRNA synthetases, tRNA-modification enzymes, translation-initiation and elongation factors, and ribosomal proteins. The CRISPR-Cas systems of phages have the capacity to silence host transcription factors and translational genes, potentially as part of a larger interaction network that intercepts translation to redirect biosynthesis to phage-encoded functions. In addition, some phages may repurpose bacterial CRISPR-Cas systems to eliminate competing phages. We phylogenetically define the major clades of huge phages from human and other animal microbiomes, as well as from oceans, lakes, sediments, soils and the built environment. We conclude that the large gene inventories of huge phages reflect a conserved biological strategy, and that the phages are distributed across a broad bacterial host range and across Earth's ecosystems
A new class of Super-Earths formed from high-temperature condensates: HD219134 B, 55 cnC E, WASP-47 E
We hypothesise that differences in the temperatures at which the rocky
material condensed out of the nebula gas can lead to differences in the
composition of key rocky species (e.g., Fe, Mg, Si, Ca, Al, Na) and thus planet
bulk density. Such differences in the observed bulk density of planets may
occur as a function of radial location and time of planet formation. In this
work we show that the predicted differences are on the cusp of being detectable
with current instrumentation. In fact, for HD 219134, the 10 % lower bulk
density of planet b compared to planet c could be explained by enhancements in
Ca, Al rich minerals. However, we also show that the 11 % uncertainties on the
individual bulk densities are not sufficiently accurate to exclude the absence
of a density difference as well as differences in volatile layers. Besides HD
219134 b, we demonstrate that 55 Cnc e and WASP-47 e are similar candidates of
a new Super-Earth class that have no core and are rich in Ca and Al minerals
which are among the first solids that condense from a cooling proto-planetary
disc. Planets of this class have densities 10-20% lower than Earth-like
compositions and may have very different interior dynamics, outgassing
histories and magnetic fields compared to the majority of Super-Earths
Bayesian constraints on the origin and geology of exoplanetary material using a population of externally polluted white dwarfs
White dwarfs that have accreted planetary bodies are a powerful probe of the
bulk composition of exoplanetary material. In this paper, we present a Bayesian
model to explain the abundances observed in the atmospheres of 202 DZ white
dwarfs by considering the heating, geochemical differentiation, and collisional
processes experienced by the planetary bodies accreted, as well as
gravitational sinking. The majority (>60%) of systems are consistent with the
accretion of primitive material. We attribute the small spread in refractory
abundances observed to a similar spread in the initial planet-forming material,
as seen in the compositions of nearby stars. A range in Na abundances in the
pollutant material is attributed to a range in formation temperatures from
below 1,000K to higher than 1,400K, suggesting that pollutant material arrives
in white dwarf atmospheres from a variety of radial locations. We also find
that Solar System-like differentiation is common place in exo-planetary
systems. Extreme siderophile (Fe, Ni or Cr) abundances in 8 systems require the
accretion of a core-rich fragment of a larger differentiated body to at least a
3sigma significance, whilst one system shows evidence that it accreted a
crust-rich fragment. In systems where the abundances suggest that accretion has
finished (13/202), the total mass accreted can be calculated. The 13 systems
are estimated to have accreted masses ranging from the mass of the Moon to half
that of Vesta. Our analysis suggests that accretion continues for 11Myrs on
average