‘Inside-out’ planetary system confuses astronomers
Four worlds around a small, dim star are challenging theories of planet formation
An artist’s impression of four known planets around the star LHS 1903. The sizes and orbits of the planets are not drawn to scale.
Our familiar, idealized solar system consists of hot, rocky worlds like Mercury and Earth orbiting close to their stars and gas giants like Jupiter and Saturn in more distant orbits. Researchers have found that the same pattern holds for many other planetary systems, and they generally explain it as outer worlds that are filled with ice, gas and dust that are more abundant than those from baby stars. But now a global team of astronomers led by astrophysicist Thomas Wilson of the University of Warwick in England has discovered a planetary system that appears to be built from the inside out, with larger worlds closer together and smaller worlds farther apart. 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, about seven billion years old, and has only half the mass of our Sun, but at first glance its planetary arrangement appears somewhat similar to ours.
NASA’s Transiting Exoplanet Survey Satellite (TESS) spotted three planets there, named 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-Neptunian, 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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Analyzing CHEOPS data, researchers found a fourth planet, LHS 1903 e, hiding at the edge of the system. “Planets with large separations are thought to have formed in cold regions with lots of gas and ice that would create gas-rich worlds with large atmospheres,” explains Wilson. But by cross-referencing data from several world-class observatories, the team found that LHS 1903e has a naked, rocky core with no sign of a gaseous atmosphere. The existence of rocky outer worlds was a puzzle. Did it once have a dense atmosphere that was later lost in a cosmic disaster such as a giant impact? Was it smaller in size, closer to the star, and somehow moved outward?
To explain its existence, researchers proposed a mechanism called gas-attenuated formation. Their hypothesis suggests that the planets around LHS 1903 formed sequentially, one after another, starting with the innermost worlds. “The sequential formation mechanism would mean that the inner planets were formed early in a resource-rich environment, while the outer planets were formed last in a poor region,” says Wilson. According to the team, the outermost planets may have been fused together from leftover rocky debris, called pebbles. Based on dynamical simulations and the fact that the orbits of the planets in the system appear to be stable, Wilson and his colleagues found more attractive scenarios such as collision or migration to be unlikely. But such possibilities do not appear complete.
“The team that carried out this work includes experts at the top of the field, and they have done a great job with the data,” says Lauren Weiss, an astrophysicist at the University of Notre Dame who was not involved in the study. She adds, “As for their conclusion that LHS 1903e formed in a gas-free atmosphere, I would like to see a more detailed experiment exploring the giant-impact scenario.”
However, if the team’s gas-free formation hypothesis is correct, the discovery would add a key piece to our understanding of the difference in the size distribution of exoplanets – the so-called “radius valley” that separates smaller rocky worlds from larger gaseous worlds. While the astrophysical mechanism behind this difference is well understood for Sun-like stars, it has been the subject of fierce debate for M dwarfs. LHS 1903 could be a natural laboratory for finding answers because it has planets on both sides of this “valley”. Because these different worlds 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 astronomer Kevin Hardgree-Ullman of 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.”
However, before looking at other similar systems, Wilson wants to explore LHS 1903 a little more. “The James Webb Space Telescope will be important here, because it allows us to study how planets’ atmospheres form, which could be an important piece of evidence into their formation,” he says.
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