Astronomers believe they have caught one of the rarest sights in the universe: two planets colliding around a distant star, in a scenario reminiscent of the giant collision believed to have given rise to the Moon. The event would have occurred about 11,000 light years from Earth, around Gaia20ehk, a solar-type star close to the constellation Puppis, and has just been described in The Astrophysical Journal Letters.
Until 2016, Gaia20ehk looked like an ordinary, stable star, emitting regular light like our Sun. But by exploiting observation archives, in particular those of NASA’s SPHEREx mission, doctoral student Anastasios “Andy” Tzanidakis spotted suddenly chaotic behavior: around 2021, the brightness of the star began to dip briefly three times before going “into a spin” in a completely unprecedented way for this type of star.
These first drops in brightness indicated that an object or cloud regularly passed in front of the star, obscuring part of its light. However, it is impossible, with this data alone, to decide between a simple cloud of dust, a stellar eruption or a more violent phenomenon, such as a massive body being sucked in and destroyed by a black hole. The team then turned to infrared to look for the heat signature of what might be blocking the light.
Surprise: at the moment when Gaia20ehk seemed to turn off and behave erratically in the visible spectrum, its infrared emission exploded. In other words, the system was becoming hotter overall: the material obscuring the star was not cold, but so hot that it began to radiate in the infrared, summarizes Live Science. Estimating the cloud’s temperature at around 900 kelvins and its minimum size at a significant fraction of an astronomical unit, the researchers concluded that they were likely dealing with debris fresh from a planetary shock.
Stellar crash
The scenario they propose is that of a progressive collision between two planetary-type bodies, orbiting approximately one astronomical unit of Gaia20ehk, or roughly the same distance that separates the Earth from the Sun. The first three drops in brightness would be the traces of successive brushes, impacts which release only a little infrared energy; then would come the catastrophic collision, this frontal crash which pulverizes the two planets and projects around the star a gigantic cloud of dust and incandescent rocks.
This type of event is not exceptional in young planetary systems, where embryonic planets collide and merge constantly, but it is almost impossible to observe live: for this to happen, the orbit of the debris must pass precisely in front of the star, and telescopes must monitor the scene at the right time, in several wavelengths. All the conditions are met here, which explains the enthusiasm of James Davenport, co-author of the study, who emphasizes that Anastasios Tzanidakis’ method – tracking slow variations in decades of data – opens the door to a whole category of past events that we tended to miss.
If Gaia20ehk is so fascinating, it is because its configuration closely resembles the dominant hypothesis on the formation of our own moon. According to this model, our satellite would have been born from a giant impact between proto-Earth and a body the size of Mars, called Theia, around 4.5 billion years ago: the shock would have projected into orbit a disc of debris which, by cooling and aggregating, would have ended up giving rise to today’s Earth and its only Moon. The fact that the Gaia20ehk cloud is also located one astronomical unit from its star reinforces the analogy.
Astronomers will now follow the system over years, even millennia on a cosmic scale, to see how this incandescent debris evolves: do they dissipate, do they fragment, or do they begin to condense into one or more massive bodies? The future large telescope of the Vera C. Rubin Observatory, capable of carrying out a time lapse of the sky over ten years, could help detect other collisions of this type; James Davenport estimates that around a hundred similar events could be spotted in the next decade, providing a finally solid statistic on the frequency of impacts capable of making moons.
This would help astronomers know how rare the accident that shaped our Earth-Moon pairing is: if giant collisions in temperate zones are common, then worlds with a large stabilizing satellite – useful for tides, protection against asteroids and climate balance – could be more widespread than previously thought. If, on the contrary, they are exceptional, our case would remain an extraordinary opportunity.
