The Q-parameter is used extensively to quantify the spatial distributions of stars and gas in star-forming regions as well as older clusters and associations. It quantifies the amount of structure using the ratio of the average length of a minimum spanning tree,m, to the average length within the complete graph,s. The interpretation of the Q-parameter often relies on comparing observed values of Q,m, ands to idealised synthetic geometries, where there is little or no match between the observed starforming regions and the synthetic regions. We measure Q,m, ands over 10 Myr in N -body simulations which are compared to IC 348, NGC 1333, and the ONC. For each star-forming region we set up simulations that approximate their initial conditions for a combination of different virial rations and fractal dimensions. We find that dynamical evolution of idealised fractal geometries can account for the observed Q,m, ands values in nearby star-forming regions. In general, an initially fractal star-forming region will tend to evolve to become more smooth and centrally concentrated. However, we show that initial conditions, as well as where the edge of the region is defined, can cause significant differences in the path that a star-forming region takes across thē m −s plot as it evolves. We caution that the observed Q-parameter should not be directly compared to idealised geometries. Instead, it should be used to determine the degree to which a star-forming region is either spatially substructured or smooth and centrally concentrated.

Gravitational interactions in star-forming regions are capable of disrupting and destroying planetary systems, as well as creating new ones. In particular, a planet can be stolen, where it is directly exchanged between passing stars during an interaction; or captured, where a planet is first ejected from its birth system and is free-floating for a period of time, before being captured by a passing star. We perform sets of direct N-body simulations of young, substructured star-forming regions, and follow their evolution for 10 Myr in order to determine how many planets are stolen and captured, and their respective orbital properties. We show that in high density star-forming regions, stolen and captured planets have distinct properties. The semimajor axis distribution of captured planets is significantly skewed to wider orbits compared to the semimajor axis distribution of stolen planets and planets that are still orbiting their parent star (preserved planets). However, the eccentricity and inclination distributions of captured and stolen planets are similar, but in turn very different to the inclination and eccentricity distributions of preserved planets. In low-density star-forming regions these differences are not as distinct but could still, in principle, be used to determine whether observed exoplanets have likely formed in situ or have been stolen or captured. We find that the initial degree of spatial and kinematic substructure in a star-forming region is as important a factor as the stellar density in determining whether a planetary system will be altered, disrupted, captured or stolen.

Exoplanets display incredible diversity, from planetary system architectures around Sun-like stars that are very different from our Solar system, to planets orbiting post-main-sequence stars or stellar remnants. Recently, the B-star Exoplanet Abundance STudy (BEAST) reported the discovery of at least two super-Jovian planets orbiting massive stars in the Sco Cen OB association. Whilst such massive stars do have Keplerian discs, it is hard to envisage gas giant planets being able to form in such hostile environments. We use N-body simulations of star-forming regions to show that these systems can instead form from the capture of a free-floating planet or the direct theft of a planet from one star to another, more massive star. We find that this occurs on average once in the first 10 Myr of an association’s evolution, and that the semimajor axes of the hitherto confirmed BEAST planets (290 and 556 au) are more consistent with capture than theft. Our results lend further credence to the notion that planets on more distant (>100 au) orbits may not be orbiting their parent star.

Simulations show that the orbits of planets are readily disrupted in dense star-forming regions; planets can be exchanged between stars, become free-floating and then be captured by other stars. However, dense star-forming regions also tend to be populous, containing massive stars that emit photoionising radiation, which can evaporate the gas in protoplanetary discs. We analyse N-body simulations of star-forming regions containing Jovian-mass planets and determine the times when their orbits are altered, when they become free-floating, and when they are stolen or captured. Simultaneously, we perform calculations of the evolution of protoplanetary discs when exposed to FUV radiation fields from massive stars in the same star-forming regions. In almost half (44 per cent) of the planetary systems that are disrupted – either altered, captured, stolen or become free-floating, we find that the radius of the protoplanetary disc evolves inwards, or the gas in the disc is completely evaporated, before the planets’ orbits are disrupted. This implies that planets that are disrupted in dense, populous star-forming regions are more likely to be super Earths or mini Neptunes, as Jovian mass planets would not be able to form due to mass loss from photoevaporation. Furthermore, the recent discoveries of distant Jovian mass planets around tightly-packed terrestrial planets argue against their formation in populous star-forming regions, as photoevaporation would preclude gas giant planet formation at distances of more than a few au.

Simulations show that the orbits of planets are readily disrupted in dense star-forming regions; planets can be exchanged between stars, become free-floating and then be captured by other stars. However, dense star-forming regions also tend to be populous, containing massive stars that emit photoionising radiation, which can evaporate the gas in protoplanetary discs. We analyse 𝑁-body simulations of star-forming regions containing Jovian-mass planets and determine the times when their orbits are altered, when they become free-floating, and when they are stolen or captured. Simultaneously, we perform calculations of the evolution of protoplanetary discs when exposed to FUV radiation fields from massive stars in the same starforming regions. In almost half (44 per cent) of the planetary systems that are disrupted -either altered, captured, stolen or become free-floating, we find that the radius of the protoplanetary disc evolves inwards, or the gas in the disc is completely evaporated, before the planets' orbits are disrupted. This implies that planets that are disrupted in dense, populous star-forming regions are more likely to be super Earths or mini Neptunes, as Jovian mass planets would not be able to form due to mass loss from photoevaporation. Furthermore, the recent discoveries of distant Jovian mass planets around tightly-packed terrestrial planets argue against their formation in populous star-forming regions, as photoevaporation would preclude gas giant planet formation at distances of more than a few au.