Comment in 2021

There is an ongoing debate as to whether ultra-diffuse galaxies are regular dwarf galaxies with low star formation or massive ‘failed’ objects that quenched their star formation in the early Universe. Current evidence seems to support the former scenario.

The Milky Way, Andromeda and Centaurus A host flattened arrangements of satellite dwarf galaxies with correlated kinematics. The rarity of similar structures in cosmological simulations constitutes a major problem for the ΛCDM model, with no obvious solution in sight.

The existence of planes of satellites has been a cosmological surprise. However, evidence for stable, long-lived satellite planes is generally tenuous, and they are unexpected in all current models of galaxy formation.

In the past 20 years, the discovery and characterization of the smallest galaxies have pushed the edges of observational endeavours and theoretical advancements alike, and they will continue to be at the forefront of this field for years to come.

Alongside its role in astronomical research, the newly commissioned 32-m radio telescope is being used as an outreach tool for astronomy education in Ghana. The observatory also serves as a facility for international postgraduate programmes.

Averting the imminent climate crisis requires large reductions in greenhouse gas emissions within this decade. To provide a benchmark for reduction and to identify the main sources, we estimate the carbon footprint of astronomy research in the Netherlands over 2019.

Recent work has questioned whether nature can extract the rotational energy of a black hole via electromagnetic fields. Although we show that the Blandford–Znajek effect is sound, the deeper physics of the electric nature of black holes remains unresolved.

The question of the possibility of life beyond Earth, as framed by the Drake equation, can be quantified to show that habitable environments for life as we know it are commonplace in the Galaxy.

Without a proper accounting of known and unknown systematics and uncertainties, combining information across multiple surveys, wavelengths, and detectors may be risky. Realizing the true potential of multi-messenger and panchromatic astrophysics requires getting data integration right.

Astronomers are used to advocating for (financial) support for their future endeavours, but how should they go about lobbying for support for issues such as the climate emergency? Join forces with those experienced in effecting policy change.

Astronomers are trusted voices in the communication of science; our community should resist inundating people with facts and figures but use its advantage to encourage the public to listen to climate change experts and encourage the need for urgent cross-sectoral systemic change.

When do we stop an ongoing science project to make room for something new? Decision-making is a complex process, ranging from budgetary considerations and tension between ongoing projects, to progress assessments and allowance for novel science developments.

The climate crisis is no longer a prediction for the future, it is happening here and now. Astronomers have realized that they need to become part of the solution and are working towards reducing their own carbon footprint as well as communicating an astronomical perspective.

The missing baryon problem may now be resolved, but the exact location and physical properties of the diffuse component remains unclear. This problem could be tractable, but requires a combination of new galaxy redshift surveys and new X-ray and radio facilities.

Identifying the sources of high-energy cosmic neutrinos has been a challenge. Considering frequentist and Bayesian arguments, as well as the special conditions found in neutrino astronomy, we discuss whether to believe current 3σ observations.

That the interstellar object 1I/2017 U1 (‘Oumuamua) could be an extraterrestrial artefact is an unorthodox hypothesis. From a philosophical perspective, however, the structure of the underlying argument for the hypothesis is flawed.

Astronomy and astrophysics in Vietnam are still in their infancy. Current efforts to stimulate interest in the general public, to foster the teaching of basic knowledge, and to develop research on frontline topics are described.

Exploring the hypothesis that life is present on Mars today is key to informing planetary protection issues at a pivotal time, with the clock ticking to return pristine samples before humans irrevocably alter the environment.

The 2020 COVID-19 pandemic forced a string of cancelled conferences, causing many organizers to shift meetings online, with mixed success. Seizing the opportunity, a group of researchers came together to rethink how the conference experience and collaboration in general can be improved in a more virtual-centric future.

The Pan-African School for Emerging Astronomers (PASEA) is an innovative short course for African university students, held by an African-led international collaboration. PASEA aims to build a critical mass of astronomers in Africa and exchange ideas about teaching across continents.