“Unveiling Celestial Spectacle: Detecting Explosive Planetary Collisions in Remote Star Systems”

Illustration depicts a vivid portrayal of the colossal planetary confluence resulting from a planetary collision. In the foreground, shards of ice and rock disperse from the collision, destined to traverse the space between Earth and the distant host star discernible in the background. Credit: Mark Garlick

The aftermath of a momentous collision between two massive planets has potentially been observed in a distant star system. This remarkable discovery hints at the possibility of witnessing the genesis of a nascent world in real-time, affording us a unique glimpse into the enigmatic process of planetary formation.

Anomalies in Stellar Illumination

In December 2021, astronomers scrutinizing an ostensibly ordinary sun-like star witnessed an intriguing phenomenon. Over the course of several months, the visible light emanating from this celestial body underwent perplexing fluctuations, occasionally dwindling to near invisibility before resuming its former luminosity.

This particular star, located approximately 1,800 light years from Earth, was designated ASASSN-21qj, bearing the name of the ASASN-SN astronomy survey responsible for initially detecting its luminous fluctuations.

Such variations in stellar brightness are not uncommon and are typically ascribed to interposing celestial material obstructing the star’s radiance. ASASSN-21qj might have simply joined a growing list of analogous occurrences were it not for the vigilance of amateur astronomer Arttu Sainio, who astutely noted that approximately two and a half years prior to the star’s luminosity fluctuations, there was a discernible surge in the emission of infrared light originating from its location.

Infrared radiation primarily emanates from objects boasting relatively elevated temperatures, typically in the range of several hundred degrees Celsius. This revelation raised a fundamental query: Could these two observations be interconnected, and if so, what intricate celestial event was unfolding within the vicinity of ASASSN-21qj?

Planetary Catastrophe

In our publication featured in Nature, we propose that both sets of observations can be elucidated by a cataclysmic planetary collision, commonly referred to as a giant impact. These monumental events are believed to be commonplace during the concluding stages of planetary formation, dictating the ultimate dimensions, compositions, and thermal conditions of planets, as well as influencing the orbital dynamics of celestial objects within these planetary systems.

Within our own solar system, giant impacts have been implicated in various phenomena, such as the peculiar axial tilt of Uranus, the unusual density of Mercury, and the formation of Earth’s Moon. However, until now, direct evidence of such colossal impacts within the broader galaxy had remained elusive.

Implications of the Celestial Collision

To account for the observed phenomena, the collision must have unleashed a prodigious amount of energy within the initial hours following the impact, surpassing the radiant energy output of the star itself. Material from the colliding planetary bodies would have undergone superheating, melting, vaporization, or a combination thereof.

The outcome would have been the formation of a searing, luminous mass of material, dwarfing the original planets by hundreds of times. The elevation in infrared luminosity of ASASSN-21qj was discerned by NASA’s WISE space telescope. However, WISE observes the star only at intervals of approximately 300 days, potentially missing the initial burst of radiant energy resulting from the collision.

Nonetheless, the expanded planetary entity engendered by the collision will require an extended period, conceivably spanning millions of years, to cool and contract into a recognizable new planet. During its zenith, the emissions from this “post-impact body” could still constitute a substantial fraction of the star’s radiance, thus accounting for the observed infrared intensification.

Additionally, the collision would have ejected substantial plumes of debris into diverse orbits surrounding the star. A portion of this debris would have vaporized in the wake of the impact, subsequently coalescing to form clusters of minuscule ice and rock crystals. Over time, segments of this amorphous cloud would have traversed the space between ASASSN-21qj and Earth, intermittently obscuring a fraction of the star’s visible light and engendering the erratic luminosity fluctuations.

Extracting Insights from the Celestial Collision

If our interpretation holds true, the study of this star system holds the potential to illuminate a pivotal facet of planetary formation. Even from the limited data at our disposal, we have gleaned invaluable insights.

Firstly, the post-impact entity, to emit the observed energy, must have surpassed the size of Earth by several orders of magnitude. Consequently, the colliding planets must have been substantially more massive than Earth, possibly comparable in scale to the “ice giant” planets Uranus and Neptune.

Secondly, our estimations place the temperature of the post-impact entity at approximately 700°C. Such a relatively low temperature implies that the colliding bodies could not have consisted solely of rock and metal.

The outer regions of at least one of the planets involved must have harbored elements with lower boiling points, such as water. This suggests that the collision most likely involved two Neptune-like worlds, rich in icy constituents.

The temporal lag between the emission of infrared radiation and the observation of debris traversing the star suggests that the collision transpired at a considerable distance from the star, exceeding the Earth-Sun separation. This configuration, featuring ice giants distanced from the star, bears a closer resemblance to our own solar system than to the densely packed planetary systems often observed around other stars.

The most tantalizing aspect of this discovery is the prospect of continued observation, potentially spanning several decades, allowing us to corroborate our conclusions. Future observations, facilitated by instruments like NASA’s JWST, will furnish critical data concerning the size and composition of debris within the cloud, unveil the chemistry of the upper strata of the post-impact entity, and trace the gradual cooling of this searing debris mass. In this ongoing cosmic spectacle, we might even witness the emergence of new moons.

These observations hold the potential to enrich our theoretical frameworks, enhancing our comprehension of how giant impacts sculpt planetary systems. Hitherto, our insights into such phenomena have primarily been derived from the echoes of impacts within our own solar system. Now, we stand poised to witness the birth of a novel celestial body, an unprecedented event in real-time.

Authors:

• Simon Lock – NERC Research Fellow, School of Earth Sciences, University of Bristol
• Matthew Kenworthy – Associate professor in Astronomy, Leiden University
• Zoe Leinhardt – Associate Professor, School of Physics, University of Bristol

Adapted from an article originally published in The Conversation.

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• Nature Article
• Original Article on The Conversation