Astronomers discover late bloomer planet that challenges theories of planet formation

An international scientific team has discovered an unusual feature in the planetary system around the star called LHS 1903. The order of its planets challenges current planet formation theories. Scientists believe that the most distant outer planet in the system might be rocky—instead of gaseous, as would normally be expected—and was formed later and in a different environment than the other planets around the star. The team, including researchers from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE-CSIC), has detailed these surprising results in an article published yesterday in the journal Science.

The eight planets of our Solar System—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune—can be sorted into two different types: rocky and gaseous. The inner planets that are closest to the Sun—Mercury to Mars—are rocky, and the outer planets—Jupiter to Neptune—are gaseous.

“This general pattern, that planetary systems form with rocky planets closer to their star, followed by gaseous planets as the outer bodies, has been commonly observed across the Universe. It is what our current planet formation theories predict and what observations have widely confirmed to be true,” explains Ignasi Ribas, IEEC researcher at the ICE-CSIC and one of the authors of the study.

That was until scientists took a closer look at the planetary system around the star LHS 1903 with the CHaracterising ExOPlanet Satellite (Cheops) of the European Space Agency (ESA). What they have just discovered might flip our understanding of how planets form upside down.

LHS 1903 is a small red M-dwarf star that is cooler and shines less brightly than our Sun. Thomas Wilson from the University of Warwick in the UK and his international team combined the efforts of various telescopes in space and on Earth to classify three planets that they had spotted orbiting LHS 1903. They were able to conclude that the innermost planet seemed to be rocky, and the two that followed it gaseous.

So far, so normal. It wasn’t until Wilson and his colleagues were analysing observations made by ESA’s Cheops, that they discovered something strange: the data showed a small fourth planet, furthest from LHS 1903. And upon closer inspection, the scientists were surprised to discover that this planet seems to be rocky. 

“That makes this a system with a planet order of rocky-gaseous-gaseous-and then rocky again. Rocky planets don’t usually form so far away from their home star,” says Wilson.

Current planet formation theories predict that the inner planets in a system are small and rocky, because close to the star the radiation is so powerful that it sweeps away most of the gas around the planets’ rocky core. Further away from the star, in the outer part of a planetary system, the conditions are cool enough for a thick atmosphere to gather into a gaseous planet. 

ESA’s Cheops project scientist Maximilian Günther is enthusiastic: “Much about how planets form and evolve is still a mystery. Finding clues like this one for solving this puzzle is precisely what Cheops set out to do.”

Scientists are not quick to say that an established theory needs to be reconsidered, based on a single contradictory observation. So, Wilson and his colleagues set out to explore various explanations for why this strange rocky planet breaks the familiar pattern.

Was the planet, for example, at some point in its past hit by a giant asteroid, comet, or another big object, that blew away its atmosphere? Or had the planets around LHS 1903 swapped places at some point during their evolution? After testing these scenarios through simulations and calculations of the planets’ orbital times, the team of scientists ruled them out. 

Instead, their investigation led them to a more intriguing explanation: the planets may have formed one after the other, instead of at the same time. According to our current understanding, planets form from discs of gas and dust (protoplanetary discs) by clumping into planetary embryos at roughly the same time. These clumps then evolve into planets of different sizes and compositions over millions of years. 

In contrast, here the researchers discovered a planetary system where the star might have given birth to its four planets one after the other, instead of bearing quadruplets at once. This idea, known as inside-out planet formation, was proposed by scientists as a theory about a decade ago, but until now, never has the evidence been so strong.

This conclusion comes with an additional catch: Much like how our younger siblings are growing up in a world that is different from the one of our childhoods, this small rocky planet seems to have evolved and formed in a very different environment than its older sibling-planets. 

“By the time this outer planet formed, the system may have already run out of gas, which is considered vital for planet formation. Yet here is a small, rocky world, defying expectations. It seems that we have found first evidence for a planet which formed in what we call a gas-depleted environment,” says Wilson. 

The small rocky world is either an odd outlier, or the first evidence for a trend we hadn’t known about yet. Either way, its discovery begs for an explanation that lies beyond our usual planet formation theories.

“Historically, our planet formation theories are based on what we see and know about our Solar System,” Isabel Rebollido who is currently a Research Fellow at ESA points out. “As we are seeing more and more different exoplanet systems, we are starting to revisit these theories.”

As our instruments improve, we continue to discover more and more ‘weird’ planetary systems in the vastness of space. They force us to question our understanding and make us reconsider established theories of planet formation. Ultimately, these discoveries are helping us learn about how our Solar System fits into the big family of diverse planetary systems. Maybe, after all, the weird one is our Solar System.