An unexpected solar system

 An unexpected solar system detected by Astronomers 

An exoplanetary system about 116 light-years from Earth could flip the script on how planets form, according to researchers who discovered it using telescopes from NASA and the European Space Agency, or ESA. A small, dim star in the Milky Way appears to host a planetary lineup which flips a bedrock rule of planetary science, placing a rocky world on the system’s outer edge instead of a gas-rich planet. The discovery strengthens the case for a little-seen mode of planet building and raises fresh questions about how young planetary systems evolve and use up their raw materials. Four planets orbit LHS 1903, a red dwarf star, the most common type of star in the universe, and are arranged in a peculiar sequence. The innermost planet is rocky, while the next two are gaseous, and then, unexpectedly, the outermost planet is also rocky. This arrangement contradicts a pattern commonly seen across the galaxy and in our own solar system, where the rocky planets (Mercury, Venus, Earth and Mars) orbit closer to the sun and the gaseous ones (Jupiter, Saturn, Uranus and Neptune) are farther away. Astronomers suspect this common pattern arises because planets form within a disk of gas and dust around a young star, where temperatures are much higher close to the celestial body. In these inner regions, volatile compounds such as water and CO2 are vaporized while only materials which can withstand extreme heat, like iron and rock-forming minerals, can clump together into solid grains. The planets that form there are therefore primarily rocky.

Conventional formation models predict small, rocky planets near a star and gas-laden planets farther out. Close-in regions are hot and harsh; stellar radiation strips light gases, leaving behind dense cores. Farther from the heat, cooler temperatures allow forming planets to capture hydrogen and helium, building sub-Neptunes and giants. LHS 1903 e disrupts that script: it orbits in the colder reaches yet presents as a bare, rocky body. This combination is difficult to engineer with the usual explanations. If the planet once possessed a thick atmosphere, something would have had to remove it long after the protoplanetary disk dispersed, a major impact or sustained erosion. If it started closer in and migrated outward, the system would likely bear dynamical scars. Neither scenario fits comfortably with the evidence. In our solar system, the rocky planets are closer to the sun, followed by the gas giants. Farther from the star, beyond what scientists call the “snow line,” temperatures are low enough for water and other compounds to condense into solid ice, a process which allows planetary cores to grow quickly. Once a forming planet reaches about 10 times the mass of Earth, its gravity is strong enough to pull in vast amounts of hydrogen and helium, and in some cases, this runaway growth produces a giant gas planet such as Jupiter or Saturn. “The paradigm of planet formation is that we have rocky inner planets very close to the stars, like in our solar system,” said Thomas Wilson, an assistant professor in the department of physics at the University of Warwick in England and first author of a study on the discovery. “This is the first time in which we have a rocky planet so far away from its host star, and after these gas-rich planets.”

The star, LHS 1903, is a cool M-dwarf with roughly half the Sun’s mass and an age of around 7 billion years. Initial detections revealed three planets hugging close to the star, a configuration familiar across the galaxy: an innermost rocky super-Earth followed by two sub-Neptunes swaddled in thick atmospheres. This tidy pattern broke when a fourth world emerged at a wider orbit. Follow-up observations with a precision photometry mission sharpened the view of the system. The added world, designated LHS 1903 e, appears to be compact and dense, with no sign of a puffy gaseous envelope. The system’s architecture, rocky, gaseous, gaseous, then rocky, reads like planetary physics in reverse. The unexpected rocky planet, called LHS 1903 e, has a radius about 1.7 times that of Earth, making it what astronomers call a “super Earth”, a larger version of our planet with similar density and composition. But why is it there, defying logic and previous observations? “We think that these planets formed in very different environments from each other, and that is what’s kind of unique about this system,” Wilson said. “This outer planet, which is rockier compared to the middle two planets, shouldn’t have happened, based on the standard formation theory. But what we think happened is that it formed later than the other planets.”

The planetary system was first discovered using a Transiting Exoplanet Survey Satellite, or TESS, a NASA space telescope launched in 2018 to discover new exoplanets. The system was then analyzed using ESA’s CHaracterising ExOPlanet Satellite, or Cheops, which was launched in 2019 to study stars which are already known to host exoplanets. The researchers also used data from other telescopes across the world, leading to a large international collaboration. After they confirmed the odd finding of an “inside out” planetary system, the scientists tested some hypotheses to explain the presence of the outermost rocky planet, hoping to understand whether it could have formed via a collision between other planets, or if it could be the remnant of a gas-rich planet that had lost its outer envelope. “We ran a lot of dynamical analysis in this study, basically throwing these planets at each other and throwing other planets at these planets, seeing if you could remove the atmosphere, if you could create these planets via impacts,” Wilson said, referring to two possible formation processes. “But we cannot make these planets this way.”

The research team favors a different pathway: sequential, inside-out assembly within a gas-depleted disk. Planets formed one after another, starting near the star while the natal disk was still rich in gas and dust. The inner rocky super-Earth and its two gaseous neighbors took shape early, when material was abundant. Over time, the growing planets and stellar radiation cleared out the remaining gas. By the time the outermost world began to coalesce, the system had largely run out of the light gases needed to build a sub-Neptune, leaving only solids, pebbles and dust, to pile up into a compact rocky planet. This “gas-depleted formation” mechanism naturally yields an inside-out order: bigger, more volatile-rich worlds inside, capped by a smaller rocky planet outside. It also offers a snapshot of disk evolution in action, emphasizing how timing, the order in which planets claim their share of the disk, can sculpt the final architecture. Once they ruled out these possibilities, the researchers landed on what Wilson calls a “gas-depleted” formation mechanism in which the planets formed one after another and in the opposite order to our own solar system, starting with the innermost planet and moving outward. “This formation mechanism, where you start with the inner one and then you move away from the host star, means the outermost planet formed millions of years after the innermost one,” Wilson said. “And because it formed later, there was actually not that much gas and dust in the disk left to build this planet from.”

In our solar system, the gas giants formed first and quickly, followed by the four inner rocky planets. There are also rocky bodies beyond the orbit of Neptune, such as Pluto, but compared with LHS 1903 e, Wilson said, they are far smaller, ice-rich and likely formed much later than the other solar system planets, as a result of collisions. The finding may offer “some of the first evidence for flipping the script on how planets form around the most common stars in our galaxy,” according to Sara Seager, professor of planetary science and physics at the Massachusetts Institute of Technology and a coauthor of the study. However, she added, the study is centered around a difficult interpretation, so the debate remains open. “Even in a maturing field, new discoveries can remind us that we still have a long way to go in understanding how planetary systems are built,” she said. LHS 1903 is an intriguing planetary system which can teach scientists a lot about how small planets form and evolve, according to Heather Knutson, a professor of planetary science at the California Institute of Technology who was not involved with the study. “Planet e is particularly intriguing, as it can potentially host many different kinds of atmospheres and may be cool enough for water to condense,” she said. “This would be a fascinating planet to observe with the James Webb Space Telescope, which might be able to tell us more about its atmospheric properties.”

According to Ana Glidden, a postdoctoral researcher at MIT’s Kavli Institute for Astrophysics and Space Research, the four-planet LHS 1903 system can serve as a natural laboratory for studying how small planets form around a star different than our own sun. The formation hypothesis outlined in the paper is exciting, but planet formation is a complex process that scientists are still trying to understand, warned Néstor Espinoza, an astronomer at the Space Telescope Science Institute in Baltimore. How planets form around small stars such as LHS 1903 is now a matter of debate, Espinoza said. “This system adds a very interesting datapoint that will have planet formation models trying to explain it for years to come, and I’m sure we will learn something new about the process of planet formation once they are compared against each other! Alternative explanations were stress-tested. A giant impact capable of stripping an atmosphere would likely inject noticeable dynamical chaos or create other signatures; modeling indicates the planetary orbits appear stable. Large-scale migration, where planets swap positions after formation, can also scramble a system, but the observed layout and inferred masses favor a calmer history. While those pathways cannot be completely dismissed, they require added complications that the sequential-formation scenario does not. The observational trail began with a transit survey which flagged the inner trio, then sharpened with targeted measurements which uncovered the outer fourth. Cross-checks with multiple facilities strengthened the case that LHS 1903 e is indeed rocky and not quietly harboring a hidden atmosphere.

“The authors reasonably conclude that the outermost planet likely formed in a region with little gas rather than losing its atmosphere through a violent collision,” Glidden said, adding that future observations may allow scientists to probe their atmospheres and better understand how different types of planets form and evolve. LHS 1903 sits in the Milky Way’s thick disk and has endured for billions of years, suggesting the system’s unusual blueprint is dynamically durable. If gas-depleted, inside-out formation can happen here, it may be more common than current planet catalogs imply, especially around M-dwarfs, where disks evolve quickly and planets often huddle close to their stars. Next steps are straightforward. Precision radial-velocity campaigns can tighten mass estimates and probe for any additional, unseen companions. Atmospheric spectroscopy of the two sub-Neptunes could reveal how much gas they retained and how their compositions diverged from their rocky neighbors. Longer-baseline monitoring may also search for a debris belt or outer bodies which could further contextualize the system’s history. For now, the message is clear: astronomers detect a solar system they say should not be possible, and in doing so, they spotlight the clockwork of planet formation, where timing, not just location, can decide a world’s fate in our universe.