The universe has a curious habit: every time we think we have a perfect standard model for how things form, something comes along that forces us to rewrite the textbooks. At the moment, our solar system (like many others) seems to have a logical order with rocky planets like the Earth near the Sun and gas giants far away, but what just published the magazine Science It is the exception that confirms that the rules are meant to be broken.
A new model. An international team, with strong Spanish participation from IEEC-CSIC and the IAChas discovered LHS 1903, a system 120 light years away which presents an “impossible” architecture according to traditional models: rocky, gaseous, gaseous and… rocky again.
The importance of order. This study details the discovery of four exoplanets orbiting a red dwarf starwhich a priori does not seem anything out of the ordinary. But the focus is on how they are placed, as can be summarized in the following list:
- LHS 1903 b: an inner rocky planet.
- LHS 1903 c: a gaseous sub-Neptune.
- LHS 1903 d: another gaseous sub-Neptune.
- LHS 1903 e: an outer rocky planet.
The normal thing in planetary formation is that the outer planets, when formed far from the heat of the star where ice and gas are abundant, accumulate enormous gaseous atmospheres like Jupiter or Neptune. This is why a rocky, bare planet, without a gaseous envelope, in the outermost orbit is an anomaly that has baffled astronomers. It’s like there are two Earths in locations where they shouldn’t be.
How it has been seen. To confirm this strange system, a single telescope was not enough. The finding is the result of the combination of data from TESSNASA’s exoplanet hunter, and the satellite’s surgical precision Cheops of the European Space Agency (ESA).
In this way, while TESS detects the general transit signals when a planet passes in front of the star, Cheops is able to refine those observations to determine the exact size. Combining all this with velocity measurements from ground-based observatories such as the Canary Islands telescope, the team was able to calculate the densities and confirm that planet ‘e’ is indeed a solitary rock on the outside.
How is it possible? A priori, there are two theories to explain why a planet loses its gas and becomes rocky: photoevaporation and the internal heat of the planet. However, neither of these theories work for LHS 1903 e. As the most distant planet, it receives much less radiation than its inner gaseous brethren and is too cold to have lost its atmosphere on its own.
In this way, if the planet did not lose its atmosphere a priori, the only logical explanation that the authors find is that it never had it. The study proposes a training model in a gas-depleted environment where the protoplanetary disk ((the cloud of gas and dust where the planets are born) did not form all the bodies at once. What happened, theoretically speaking, is that the inner planets formed first when there was a lot of gas and the outer planet formed later.
He is left with the crumbs. By the time the last planet finished forming, the gas in the disk had already dissipated or been absorbed by its older brothers. In this way, it was formed from solid “leftovers”, with no gas available to build an atmosphere.
This supports the theory of the “inside-out” formation, where the planets appear sequentially. It is a scenario that has rarely been confirmed with such observational clarity as until now in this system.
Its importance. This discovery forces us to rethink the history of solar systems around red dwarfs, which are the most common stars in our galaxy. And we even thought that the position of a planet determined its destiny, but LHS 1903 teaches us that timing is just as important.
The LHS 1903 system thus becomes a perfect laboratory: four planets, the same star, but completely different birth stories coexisting in a stable orbital balance.
Images | THAT
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