Letters in 2019

A comprehensive set of Hubble and Spitzer observations reveal a hydrogen-rich, low-metallicity atmosphere on the sub-Neptune exoplanet GJ 3470 b. Water vapour is detected, but the planet is surprisingly depleted in methane, possibly because of photochemical or thermal processes. Sub-millimetre-sized Mie-scattering cloud particles partially attenuate the molecular signatures at short wavelength, but are largely transparent beyond 3 µm.

The uniformly high ¹³C/¹²C ratios of the Tagish Lake meteorite, whose parent body is a D-type asteroid, indicate that at least some D-type asteroids were formed in the cold outer Solar System from a ¹³C-rich CO₂ ice reservoir and subsequently migrated inwards.

Theoretical modelling of velocity maps of high-redshift Lyman α emitters indicates sufficient gas inflow to fuel the central galaxy’s star-formation rate and angular momentum, implying that cold gas accretion is building galaxies at the peak of star formation.

All trans-Neptunian objects larger than 1,000 km have satellites. A hydrodynamic simulation shows that such satellites were formed by giant impacts that happened before the migration of Neptune and that kept the satellites in a fluid-like state for 10⁴–10⁶ yr.

A unique merger shock is found in an early phase of two clusters undergoing a major merger, propagating outward along the equatorial plane of the merger. Owing to the rapid approach of the cluster pair, the gas along the merger axis is strongly compressed.

A study of 105 nearby galaxy clusters from the Sloan survey shows that barred galaxies appear more frequently in interacting clusters, indicating that cluster–cluster interaction can play an important part in bar formation.

The predominantly prograde orientation and broad inclination distribution of trans-Neptunian binary objects is reproduced by a three-dimensional hydrodynamical simulation of planetesimal formation driven by the streaming instability, showing evidence of the activation of the streaming instability in the solar protoplanetary disk.

X-ray observations of the Coma Cluster show enhanced plasma collision rates due to either particle scattering off microfluctuations caused by plasma instabilities or transport processes that are anisotropic with respect to the local magnetic field.

A minor but important fraction of silicate stardust believed to come from red giant stars is shown to have a supernova origin instead, making the supernova dust fraction among >200-nm-sized presolar silicates significantly higher than previously inferred.

An exceptionally low delay of 83 minutes between variability in the accretion disk and Hα emission is reported from the nucleus of the dwarf galaxy NGC 4395. The implied black hole mass of about 10,000 solar masses is consistent with the mass–velocity dispersion relation.

Two Hα emission peaks are detected within the disk of the T Tauri star PDS 70: one corresponds to protoplanet PDS 70 b, and the other is associated with a second accreting planet of few Jupiter masses at ~35 au. The two protoplanets are near 2:1 mean motion resonance, supporting migration scenarios of giant planets during planetary formation.

Chandra X-ray Observatory spectral observations of the active star HR 9024 provide evidence of plasma motions that indicate a stellar flare and subsequent coronal mass ejection. This event provides critical information on non-solar coronal mass ejections and a point of comparison to the Sun, a much less active star.

Measurements of Mo in meteorites constrain the time when the Earth accreted carbonaceous material from the outer Solar System (a likely source of Earth’s water and volatiles) to late in the Earth’s growth history—probably in the same event that formed the Moon.

A systematic change in Jupiter’s magnetic field can be detected by collating all data obtained in the last 45 years by multiple spacecraft, from Pioneer 10 to Juno. Such variation can be attributed to the zonal winds, which advect the magnetic field from the deep atmospheric layers.

Carbon-rich and oxygen-rich material coexist in a presolar grain originated by an outburst of a CO nova and found within the LaPaz Icefield 031117 meteorite. Condensation of both carbon and oxygen can thus happen in the same circumstellar environment, contrary to previous assumptions.

The Milky Way is thought to have accreted dwarf galaxies and their stellar content. Here, Xing et al. study a Galactic halo star enhanced in r-process elements and depleted in α-elements, indicating that it likely formed in a recently accreted dwarf galaxy similar to Ursa Minor.

PlanetNet, a new deep learning algorithm, can quickly and accurately map spatial and spectral features across large, heterogeneous areas of a planet. The major components of the 2008 storm on Saturn are delineated, indicating regions to be probed more deeply with radiative transfer models.

Three mysterious features at Titan’s northern polar region appear as lakes when observed with Cassini’s radar during winter but as land when observed in the infrared during spring, providing evidence of liquid removal on Titan at seasonal scale.

Molecular emission lines originating in Titan’s relatively unexplored upper mesosphere and thermosphere reveal a strong prograde jet that reaches speeds of 340 m s^–1 at 1,000 km altitude.

Using large-aperture Cherenkov telescopes, Benbow et al. have measured the angular sizes of two stars through timely occultations by asteroids, achieving an order of magnitude improvement in resolution over the lunar occultation method.