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A planetary collision afterglow and transit of the resultant debris cloud

Nature volume 622, pages 251–254 (2023)Cite this article

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An Author Correction to this article was published on 01 December 2023

This article has been updated

Abstract

Planets grow in rotating disks of dust and gas around forming stars, some of which can subsequently collide in giant impacts after the gas component is removed from the disk1,2,3. Monitoring programmes with the warm Spitzer mission have recorded substantial and rapid changes in mid-infrared output for several stars, interpreted as variations in the surface area of warm, dusty material ejected by planetary-scale collisions and heated by the central star: for example, NGC 2354–ID8 (refs. 4,5), HD 166191 (ref. 6) and V488 Persei7. Here we report combined observations of the young (about 300 million years old), solar-like star ASASSN-21qj: an infrared brightening consistent with a blackbody temperature of 1,000 Kelvin and a luminosity that is 4 percent that of the star lasting for about 1,000 days, partially overlapping in time with a complex and deep, wavelength-dependent optical eclipse that lasted for about 500 days. The optical eclipse started 2.5 years after the infrared brightening, implying an orbital period of at least that duration. These observations are consistent with a collision between two exoplanets of several to tens of Earth masses at 2–16 astronomical units from the central star. Such an impact produces a hot, highly extended post-impact remnant with sufficient luminosity to explain the infrared observations. Transit of the impact debris, sheared by orbital motion into a long cloud, causes the subsequent complex eclipse of the host star.

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Fig. 1: Optical and infrared photometry of ASASSN-21qj.
Fig. 2: The light curve of ASASSN-21qj from several different photometric surveys and the derived transverse velocities.
Fig. 3: The size and temporal evolution of a post-impact body.

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Data availability

The datasets generated and analysed during this study are available in the Zenodo repository at https://doi.org/10.5281/zenodo.8344755.

Code availability

All the code for the analysis and the generation of all the figures are available in a showyourwork72 reproducible framework available as a git repository at https://github.com/mkenworthy/ASASSN-21qj-collision/. The source code and documentation for the SWIFT open-source simulation code are available from www.swiftsim.com.

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Acknowledgements

G.K. is supported by the Royal Society as a Royal Society University Research Fellow. S.J.L. acknowledges funding from the UK Natural Environment Research Council (grant NE/V014129/1). L.C. acknowledges funding from the European Union H2020-MSCA-ITN-2019 under grant agreement no. 860470 (CHAMELEON). J.D. acknowledges funding support from the Chinese Scholarship Council (no. 202008610218). Giant impact simulations were carried out using the Isambard 2 UK National Tier-2 HPC Service (http://gw4.ac.uk/isambard/) operated by GW4 and the UK Met Office and funded by EPSRC (EP/T022078/1). We thank K. Stanek and the work of the ASAS-SN team with their survey and for providing public access to the database. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).

Author information

Author notes

  1. These authors contributed equally: Matthew Kenworthy, Simon Lock, Grant Kennedy, Richelle van Capelleveen, Eric Mamajek, Ludmila Carone

Authors and Affiliations

  1. Leiden Observatory, Leiden University, Leiden, The Netherlands

    Matthew Kenworthy & Richelle van Capelleveen

  2. School of Earth Sciences, University of Bristol, Bristol, UK

    Simon Lock

  3. Department of Physics, University of Warwick, Coventry, UK

    Grant Kennedy

  4. Centre for Exoplanets and Habitability, University of Warwick, Coventry, UK

    Grant Kennedy

  5. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Eric Mamajek

  6. Department of Physics and Astronomy, University of Rochester, Rochester, NY, USA

    Eric Mamajek

  7. Space Research Institute, Austrian Academy of Sciences, Graz, Austria

    Ludmila Carone

  8. Vereniging Voor Sterrenkunde, Brugge, Belgium

    Franz-Josef Hambsch

  9. American Association of Variable Star Observers, Cambridge, MA, USA

    Franz-Josef Hambsch

  10. Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V., Berlin, Germany

    Franz-Josef Hambsch

  11. IPAC, California Institute of Technology, Pasadena, CA, USA

    Joseph Masiero & J. Davy Kirkpatrick

  12. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    Amy Mainzer

  13. Las Cumbres Observatory, Goleta, CA, USA

    Edward Gomez

  14. School of Physics and Astronomy, Cardiff University, Cardiff, UK

    Edward Gomez

  15. School of Physics, HH Wills Physics Laboratory, University of Bristol, Bristol, UK

    Zoë Leinhardt & Jingyao Dou

  16. Institute of Astronomy, University of Cambridge, Cambridge, UK

    Pavan Tanna

  17. Independent researcher, Järvenpää, Finland

    Arttu Sainio

  18. Variable Stars South, Nelson, New Zealand

    Hamish Barker

  19. Southern Spectroscopic Project Observatory Team, Chabons, France

    Stéphane Charbonnel, Olivier Garde, Pascal Le Dû, Lionel Mulato & Thomas Petit

  20. Department of Astronomy, The Ohio State University, Columbus, OH, USA

    Michael Rizzo Smith

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  1. Matthew Kenworthy
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Contributions

M.K. led the writing of the paper, management, obtaining the optical observations and initial models. S.L. led the afterglow modelling and theory. G.K. led the orbital analysis and dust analysis. R.v.C. carried out the optical-light-curve-data reduction and reddening analysis and velocity-constraint analysis. E.M. performed the analysis of the properties of the star. F.-J.H. and E.G. carried out optical monitoring of the star. J.M., A.M., J.D.K. and A.S. were responsible for NEOWISE identification and data reduction. S.L., L.C., J.D., P.T. and Z.L. provided discussion on the ejected material and subsequent evolution. J.D. performed the SPH impact simulations. H.B., S.C., O.G., P.L.D., L.M. and P.T. were responsible for the observation and reduction of observational data. M.R.S. led the discovery of the optical dimming of the star. All co-authors assisted with manuscript writing and proofreading.

Corresponding author

Correspondence to Matthew Kenworthy.

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Extended data figures and tables

Extended Data Fig. 1 Spectrum of ASASSN-21qj components.

The red symbols show the optical and pre-brightening WISE infrared photometry and the blue symbols show the post-brightening WISE and ALMA fluxes. Stellar and 1,000 K components consistent with the pre-brightening and post-brightening fluxes are shown. The dashed line shows an estimated cool-component spectrum for 0.1-μm-sized grains associated with the transiting dust cloud. Downward triangles are upper limits. Error bars are shown at 1σ confidence.

Extended Data Fig. 2 The light curve of ASASSN-21qj from TESS and the periodogram of TESS and ASAS-SN photometry.

a,b, Photometry of ASASSN-21qj from three sectors of TESS. c, Lomb–Scargle analysis of photometry from TESS (coloured blue) and from ASAS-SN V-band data from MJD 57420 to MJD 58386 (light grey) shows a signal at 4.4 days. At lower frequencies, the ground-based photometry shows power and aliasing signals. The TESS signal shows an important signal at 4.43 days, and a similar signal is seen in the ground-based data. The longer time baseline in the ASAS-SN data reveals substructure in the signal.

Extended Data Fig. 3 Deriving the transverse velocity from a light curve.

a, ASAS-SN g′ photometry is shown in units of normalized flux. Straight-line fits (light-blue lines) are made to the photometry in the regions indicated by the light-grey vertical lines. b, Gradient of the light curve as a function of time. c, Transverse velocity derived from the light curve and the gradient of the light curve. Error bars are shown at 1σ confidence.

Extended Data Fig. 4 Blueing of the B–V and V-I colours during the dimming event.

Points show AAVSO data and lines show models. a, The V magnitude versus V-I colour and b, the V magnitude versus V-I colour. The dashed line is a line of Aλ/AV for the value shown in the legend and the solid line is a model that includes an underlying scattered-light component with s = 7.5% of the stellar flux. Error bars are shown at 1σ confidence.

Extended Data Fig. 5 Sketch of the hypothesis for the observations seen towards ASASSN-21qj.

At t = 0, the collision occurs, producing a cloud of debris that expands and cools. Material close to the remnant is heated by its luminosity, generating the 1,000 K infrared emission. Around 1,000 days later, the expanding cloud crosses the line of sight between the star and the Earth, generating the optical light curve.

Extended Data Fig. 6 Simulations of the formation of a post-impact body.

Giant impacts between super-Earths and mini-Neptunes can produce post-impact bodies hundreds of Earth radii across, comparable with that required to produce the observed infrared flux. With the exception of the lower-right panel, particles are coloured by their material (forsterite, water or a H2–He mixture moving outwards in the initial bodies) and whether they came from the impactor or target (see top-left panel). The final two panels show just the mass bound to the primary remnant, which has a mass of 48.4 MEarth. In the final panel, particles that are at the minimum density imposed by the code are coloured in green.

Extended Data Table 1 Properties of ASASSN-21qj
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Extended Data Table 2 Photometric observations of ASASSN-21qj
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Kenworthy, M., Lock, S., Kennedy, G. et al. A planetary collision afterglow and transit of the resultant debris cloud. Nature 622, 251–254 (2023). https://doi.org/10.1038/s41586-023-06573-9

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