Our familiar, archetypal solar system has warm, rocky worlds like Mercury and Earth orbiting close to their star and gas giants like Jupiter and Saturn sprawled out in more distant orbits. Researchers have found that this same pattern holds for many other planetary systems, and they typically explain it as outer worlds bulking up on ice, gas and dust that’s more abundant farther out from baby stars. But now a global team of astronomers led by Thomas Wilson, an astrophysicist at the University of Warwick in England, has discovered a planetary system that seems to have been built inside out, with bigger worlds closer in capped by a smaller one farther out. The result is published today in Science.
Their observations of a faint, cool M-dwarf star called LHS 1903 revealed a system with a rocky world at its outer edge. LHS 1903 is an ancient star, around seven billion years old, and only has about half the mass of our sun, but at first glance its arrangement of planets appeared somewhat similar to our own.
NASA’s Transiting Exoplanet Survey Satellite (TESS) had spotted three planets there called LHS 1903 b, c and d. The innermost world, planet b, is a dense, rocky super-Earth. Next come planets c and d, both of which are sub-Neptunes, worlds with thick, gaseous atmospheres. But the picture changed when the international team used the European Space Agency’s Characterizing Exoplanet Satellite (CHEOPS) to take a closer look.
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Parsing through the CHEOPS data, the researchers found a fourth planet, LHS 1903 e, lurking at the system’s edge. “Planets at larger separations are thought to be built in colder regions with a lot of gas and ice that would create gas-rich worlds with large atmospheres,” Wilson explains. But by cross-referencing data from multiple world-class observatories, the team discovered that LHS 1903 e is a naked, rocky core with no sign of a gaseous atmosphere. The existence of a rocky outer world was a puzzle. Did it once harbor a thick atmosphere that was then lost in some cosmic catastrophe, such as a giant impact? Did it form small, closer to the star, and somehow migrate outward?
To explain its existence, the researchers proposed a mechanism called gas-depleted formation. Their hypothesis suggests the planets around LHS 1903 formed sequentially, one after the other, beginning with the innermost worlds. “The sequential formation mechanism would mean that the inner planets were built early on, in a resource-rich environment, whereas the outer body was built last in a poorer region” after abundant gas had been swept away, Wilson says. The outermost planet, according to the team, must have coalesced, pebble by pebble, from the remaining rocky debris. Based on dynamical simulations and the fact that planetary orbits in the system seem stable, Wilson and his colleagues found more flashy scenarios such as collisions or migration unlikely. But such possibilities are not entirely off the table.
“The team that conducted this work consists of experts at the top of the field, and they have done a very fine job with the data,” says Lauren Weiss, an astrophysicist at the University of Notre Dame, who was not involved in the study. “As for their conclusion that LHS 1903 e formed in a gas-depleted environment, I would have liked to see a more detailed experiment exploring the giant-impact scenario,” she adds.
If the team’s gas-depleted formation hypothesis is right, however, the discovery would add a crucial piece to our understanding of a gap in the size distribution of exoplanets—the so-called “radius valley” that separates smaller rocky worlds from larger gaseous ones. While the astrophysical mechanisms behind this gap are well understood for sunlike stars, it’s been a topic of fierce debate for M dwarfs. LHS 1903 could be a natural laboratory for getting answers because it contains planets on both sides of this “valley.” Because these disparate worlds all orbit the same star, variables such as stellar age and metallicity are controlled, allowing astronomers to better constrain the formation history.
“This study opens new insights into the formation process of multiplanet systems orbiting M-dwarf stars,” says Kevin Hardegree-Ullman, an astronomer at the NASA Exoplanet Science Institute, who was not involved in the study. “Finding more of these systems will really help us refine and constrain planet formation models in the near future.”
Before looking for other, similar systems, though, Wilson wants to explore LHS 1903 a bit further. “The James Webb Space Telescope will be crucial here, as it allows us to study how planetary atmospheres are built, which can be a key piece of evidence in their formation,” he says.