‘Inside-out’ planetary system perplexes astronomers

‘Upside-down’ planetary system leaves astronomers perplexed

By Jacek Krywko edited by Lee Billings

Our familiar, archetypal solar system includes hot, rocky worlds like Mercury and Earth orbiting close to their stars and gas giants like Jupiter and Saturn spread out in more distant orbits. Researchers have found that this same pattern applies to many other planetary systems, and they usually explain it by the fact that outer worlds cluster into ice, gas, and dust, which are more abundant the farther away 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 appears to have been built in reverse, with larger, closer worlds capped by a smaller, more distant world. 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 about half the mass of our sun, but at first glance the arrangement of its planets seemed somewhat similar to ours.

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 sub-Neptunes, worlds with thick gaseous atmospheres. But the situation changed when the international team used the European Space Agency’s Exoplanet Characterization Satellite (CHEOPS) to take a closer look.

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By analyzing CHEOPS data, researchers discovered a fourth planet, LHS 1903 e, hidden at the edge of the system. “Planets at larger separations are thought to be built in colder regions with lots of gas and ice, which would create gas-rich worlds with large atmospheres,” says Wilson. But by cross-referencing data from several world-class observatories, the team discovered that LHS 1903 e is a bare rocky core, with no sign of a gaseous atmosphere. The existence of a rocky outer world was an enigma. Did it once harbor a thick atmosphere that was then lost in a cosmic catastrophe, such as a giant impact? Did it form small, closer to the star, and migrate outwards?

To explain its existence, researchers proposed a mechanism called gas-depleted formation. Their hypothesis suggests that the planets around LHS 1903 formed sequentially, one after the other, starting with the innermost worlds. “The sequential formation mechanism would mean that the inner planets were built very early, in a resource-rich environment, while the outer body was built last in a poorer region” after the abundant gas was swept away, Wilson says. The more distant planet, the team says, must have coalesced, pebble by pebble, from the remaining rocky debris. Based on dynamic simulations and the fact that planetary orbits in the system appear stable, Wilson and his colleagues found flashier scenarios such as collisions or migrations unlikely. But such possibilities are not completely excluded.

“The team that did this work are experts at the top of the field, and they did a very good 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-poor environment, I would have liked to see a more detailed experiment exploring the giant impact scenario,” she adds.

If the team’s hypothesis about the gas-depleted formation is true, the discovery would add a crucial element to our understanding of a gap in the size distribution of exoplanets – the so-called “ray valley” that separates small rocky worlds from larger gaseous worlds. Although the astrophysical mechanisms behind this discrepancy are well understood for Sun-like stars, it is hotly debated for M dwarfs. LHS 1903 could be a natural laboratory for answers because it contains planets on both sides of this “valley.” Since 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 process of formation of multiplanet systems orbiting M dwarf stars,” says Kevin Hardegree-Ullman, an astronomer at NASA’s Exoplanet Science Institute, who was not involved in the study. “Finding more of these systems will really help us refine and constrain models of planetary formation in the near future.”

Before searching for other similar systems, Wilson wants to explore LHS 1903 a little more. “The James Webb Space Telescope will be crucial here, because it will allow us to study how planetary atmospheres are built, which may be a key piece of evidence in their formation,” he says.

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