Recently I've done posts about exploding lakes and colossal solar storms and places where continents are being torn in two, so it seems fitting to end the week on an appropriately cataclysmic note with the discovery of the remnants of a collision between two giant ice planets.
The coolest part of all of this is that one of the people who first realized something weird was going on was an amateur astronomer named Arttu Sainio, of Järvenpää, Finland (who is listed as an author and credited as "Independent Researcher" in the paper that appeared in Nature this week -- how awesome is that?). Matthew Kenworthy, of Leiden Observatory, was scouring the data looking for evidence of rings around stars that might be involved in planet formation, which would be indicated by a periodic dimming and brightening of the light from the parent star. Kenworthy found a candidate -- a sunlike star called ASASSN-21qj, 1,800 light years from Earth -- and posted his find on Twitter, saying, "It's amazing, this star is fading!" Sainio saw his tweet and responded, "But did you know that it is brightening in the infrared?"
Sainio had been looking at data from NASA's NEOWISE orbiting telescope, and found that nine hundred days before the star had begun dimming, it had shown a strong uptick in the infrared region of the spectrum. This clued in Kenworthy that his hopes of finding a ring were dashed -- but that maybe there was something even cooler going on here.
He assembled a team of astronomers to analyze the data, including Sainio's peculiar discovery, and they came to the conclusion that the best explanation for the anomalous brightening in the infrared and dimming in the visible light region was the collision of two Neptune-sized planets -- leaving an incandescent cloud of debris orbiting the star which radiated in the infrared as the heat from the collision dissipated, but partially blocked the star's visible light at the same time.
What will happen to the debris cloud next is a matter of speculation, because this is the first time anyone's seen an event like this occur. While planetary collisions aren't uncommon -- our own Moon, for example, is thought to have formed when a huge protoplanet slammed into the Earth, blowing a blob of molten rock into space that eventually coalesced as the Moon -- no one's ever watched it happen more-or-less in real time. It's probable that the debris will pull together gravitationally and eventually form one or more planets, but there's no certainty about how long that'll take."It will be fascinating to observe further developments," said Zoe Leinhardt, of the University of Bristol, who co-authored the paper. "Ultimately, the mass of material around the remnant may condense to form a retinue of moons that will orbit around this new planet, but whether that will take ten years or a thousand, we don't yet know."