Letters in 2019

Tidally locked exoplanets with oceans are supposed to be globally frozen except around the substellar point, where the water stays liquid—known as the eyeball state. However, if sea-ice drift is included in climate models, the open water region shrinks and can even disappear, resulting in a snowball state.

Pockets of iron-rich melt within asteroids get progressively enriched in sulfur while the asteroid is cooling, generating excess pressure that can push the melt up to the surface. This ‘ferrovolcanism’ can be the origin of the stony-iron pallasite meteorites and can explain the contradicting results from asteroid 16 Psyche.

Seventeen molecular clouds are identified in a Milky Way progenitor at z = 1.036, with higher masses, surface densities and supersonic turbulence than present-day analogues. Their properties reflect the hostile ambient interstellar conditions prevalent in distant galaxies and suggest they formed by fragmentation of a turbulent galactic gas disk.

Cromartie et al. have probably found the most massive neutron star discovered so far by combining NANOGrav 12.5-yr data with radio data from the Green Bank Telescope. Millisecond pulsar J0740+6620 has a mass of 2.14 M_⊙, ~0.1 M_⊙ more massive than the previous record holder, and very close to the upper limit on neutron star masses from Laser Interferometer Gravitational-Wave Observatory measurements.

K2-18 b is a planet with a mass around eight times that of the Earth that lies within the standard habitable zone of its star. Hubble spectra show the presence of an atmosphere around K2-18 b containing significant amounts of water vapour (up to a few tens of per cent, depending on the spectral model), but also a non-negligible amount of H₂–He.

The X-ray pulsar wind nebula around PSR B0540–69 brightened by about a third after a change in the pulsar’s spin-down rate. This phenomenon, which has not previously been seen in this way in other pulsars, is likely to be due to a sudden change in the pulsar’s magnetosphere.

Phase curves from a sample of 12 hot Jupiters show that this type of planet keeps the same nightside temperature (~1,100 K) regardless of the irradiation they receive from their star. This effect is due to an optically thick layer of the same species of clouds on the nightside hemisphere.

Pinte et al. report the kinematic detection of a few-Jupiter-mass planet orbiting at 130 au from the young star HD 97048. The radial position of the planet coincides with a gap in both the gas and dust components of the protoplanetary disk, showing that at least some gaps can be linked to the presence of planets.

Graphene doped to the Dirac point can be used to detect signals from 90 to 700 GHz, and prospectively across the entire terahertz range, with high sensitivity. Such a sensor could be used in next-generation space-based telescopes, promising quantum-limited detection that surpasses superconducting technologies.

By using a superconducting integrated circuit to filter incoming millimetre, submillimetre and far-infrared light from distant galaxies, a prototype spectrometer holds promise for wideband spectrometers that are small, sensitive and scalable to wideband spectroscopic imagers.

The Transiting Exoplanet Survey Satellite (TESS) has identified a nearby, bright, quiescent M dwarf star that hosts two sub-Neptune-sized planets and one super-Earth-sized planet. The system is eminently suitable for follow-up studies of transit timing variations, radial velocity measurements and transmission spectroscopy.

Stars in the Milky Way halo are older than those in its thick disk, with their ten-billion-year age distribution cutoff marking the accretion of Gaia-Enceladus to the Milky Way. The red-sequence halo stars are those formed first in the Milky Way progenitor, constituting its long-sought in situ halo.

Of the 72 known fast radio burst (FRB) sources only two have been observed to emit repeat bursts. By calculating the volumetric occurrence rate of non-repeating FRBs, Vikram Ravi shows that there are not enough candidate cataclysmic progenitor events for most FRBs to be one-off phenomena, and therefore most FRBs must repeat.

A combination of laboratory experiments and numerical modelling shows that a 2–3 cm-thick layer of silica aerogel deployed over the temperate regions of Mars could maintain a surface environment conducive to liquid water all year round. Such an approach would create a habitable setting for photosynthetic life.

The MASCOT lander observed a boulder on the surface of asteroid Ryugu up close. The boulder’s low thermal inertia is closer to fine regolith or comets rather than stony boulders, indicating high porosity and low tensile strength. Orbit measurements confirm that Ryugu’s surface is covered with similar boulders.

The Juno spacecraft’s observations of magnetic field perturbations in Jupiter’s polar regions show Birkeland currents associated with aurorae that are weaker than anticipated and filamentary in nature. An asymmetry is observed between the northern and southern auroras.

The superbubble containing the Sun is filled with sparse, very hot matter. Creating a three-dimensional map of this Local Bubble using the diffuse interstellar bands (DIBs), Farhang et al. show that, surprisingly, two DIB carriers exist within the bubble itself. The λ5,780 DIB carrier, compared with λ5,797, is the hardier.

The measurement of faint fluxes of photons from across the Universe requires sensitive terahertz detectors, cooled to minimize noise. By using a photomixer with plasmonics-based semiconductors to downconvert the high frequencies, terahertz detection can be accomplished at room temperature.

Combining gravitational-wave and electromagnetic data with new radio observations of GW170817, an improved measurement of \(H_0 = 70.3_{-5.0}^{+5.3}\, {\mathrm{km}}\, {\mathrm{s}}^{-1}\,{\mathrm{Mpc}}^{-1}\) is derived. Fifteen more GW170817-like events with radio data could resolve the Hubble constant tension.

Contractional features such as scarps, ridges and fractures from thrust faults are typical of large silicate bodies such as Mercury and Mars. Evidence for similar features on Ceres suggests the possibility of large-scale contraction on this icy body.