Simon P. Goodwin

The rapidly growing census of exoplanets now includes over a thousand multi-planet systems, revealing architectures that span more than four orders of magnitude in mass and five in orbital scale. Explaining the origin of this striking diversity remains a central challenge for planet formation theory, compounded by observational biases that limit access to Solar System-like regimes. Here we introduce architecture-agnostic metrics to uniformly analyse four independently defined samples of multi-planet systems, uncovering unifying patterns across their diversity. We identify a clear bimodality in system masses, dividing known architectures into two distinct families. The lower-mass family, predominantly hosted by subsolar-to-solar stars, exhibits tightly packed configurations with mass distributions consistent with the ‘peas-in-a-pod’ pattern and modal dynamical excitation comparable with that of the Solar System. By contrast, the high-mass family forms in metal-rich environments around solar-to-supersolar stars, displays gravitationally compact architectures and is an order of magnitude dynamically hotter than both the Solar System and the lower-mass counterparts. Their high level of dynamical excitation points to violent evolutionary pathways driven by global instabilities and planet–planet scattering. Together, these results show that the architectures of planetary systems are born from the combined influence of formation processes and host star properties. Signatures of these roots are preserved across the post-formation dynamical evolution that reshaped most known multi-planet systems.

Third Angel’s show 600 People explores the possibility of humans finding intelligent alien life in our galaxy, and the implications of such a discovery (or lack of it) on our future evolution and our capacity to believe in there being something else out there. In this interview/conversation the show’s creators [Third Angel Co-artistic Directors Rachael Walton and Alexander Kelly, and astrophysicist Simon Goodwin] remember the initial meeting that inspired the show [in 2006] and reflect on its development from a 30-minute spoken word piece [2013] to a more theatrical ‘full’ show [2015–2023], and the ways in which the science progressed alongside the 17 year life of the project. They discuss the overlaps in form and intention between a science talk, astrophysics lecture and theatrical performance lecture, and the inclusion of emotion in science.

Close encounters between stars in star forming regions are important as they can perturb or destroy protoplanetary discs, young planetary systems, and stellar multiple systems. We simulate simple, viralised, equal-mass $N$-body star clusters and find that both the rate and total number of encounters between stars varies by factors of several in statistically identical clusters due to the stochastic/chaotic details of orbits and stellar dynamics. Encounters tend to rapidly `saturate' in the core of a cluster, with stars there each having many encounters, while more distant stars have none. However, we find that the fraction of stars that have had at least one encounter within a particular distance grows in the same way (scaling with crossing time and half-mass radius) in all clusters, and we present a new (empirical) way of estimating the fraction of stars that have had at least one encounter at a particular distance.

Close encounters between stars in star-forming regions are important as they can perturb or destroy protoplanetary disks, young planetary systems, and multiple-star systems. We simulate simple, virialized, equal-mass N-body star clusters and find that both the rate and the total number of encounters between stars vary by factors of several in statistically identical clusters due to the stochastic/chaotic details of the orbits and stellar dynamics. Encounters tend to “saturate” rapidly in the core of a cluster, with stars there each having many encounters, while more distant stars have none. However, we find that the fraction of stars that has had at least one encounter within a particular distance grows in the same way (scaling with the crossing time and half-mass radius) in all clusters, and we present a new (empirical) way of estimating the fraction of stars that has had at least one encounter at a particular distance.

We present results of a high angular resolution survey of massive OB stars in the Cygnus OB2 association that we conducted with the Near-Infrared Imager camera and ALTAIR adaptive optics system of the Gemini North telescope. We observed 74 O- and early-B-type stars in Cyg OB2 in the JHK infrared bands in order to detect binary and multiple companions. The observations are sensitive to equal-brightness pairs at separations as small as , and progressively fainter companions are detectable out to mag at a separation of 2″. This faint contrast limit due to read noise continues out to 10″ near the edge of the detector. We assigned a simple probability of chance alignment to each companion based upon its separation and magnitude difference from the central target star and upon areal star counts for the general star field of Cyg OB2. Companion stars with a field membership probability of less than 1% are assumed to be physical companions. This assessment indicates that 47% of the targets have at least one resolved companion that is probably gravitationally bound. Including known spectroscopic binaries, our sample includes 27 binary, 12 triple, and 9 systems with 4 or more components. These results confirm studies of high-mass stars in other environments that find that massive stars are born with a high-multiplicity fraction. The results are important for the placement of the stars in the Hertzsprung–Russell diagram, the interpretation of their spectroscopic analyses, and for future mass determinations through measurement of orbital motion.

We show that dynamical evolution in a strong (Galactic Centre-like) tidal field can create clusters that would appear to have very top-heavy initial mass functions (IMFs). The tidal disruption of single star-forming events can leave several bound ‘clusters’ spread along 20 pc of the orbit within 1–2 Myr. These surviving (sub)clusters tend to contain an overabundance of massive stars, with low-mass stars tending to be spread along the whole ‘tidal arm’. Therefore, observing a cluster in a strong tidal field with a top-heavy IMF might well not mean the stars were formed with a top-heavy IMF.

Candidate visual binary systems are often found by identifying two stars that are closer together than would be expected by chance. However, in regions with non-trivial density distributions, the ‘random’ distances between stars varies because of the background distribution, as well as the presence of binaries. We show that when no binaries are present, the distribution of the ratios of the distances to the nearest and tenth nearest neighbours, d1/d10, is always well approximated by a Gaussian with mean 0.2–0.3 and variance 0.16–0.19 for any underlying density distribution. The introduction of binaries causes some (or all) nearest neighbours to become closer than expected by random chance, introducing a component to the distribution where d1/d10 is much lower than expected. We show how a simple single or double Gaussian fit to the distribution of d1/d10 can be used to find the binary fraction in any underlying density distribution quickly and simply.