Astronomers Catch Planets Forming Out Of Order

Astronomers have discovered a planetary system that defies the usual rules of how worlds form, with a rocky planet sitting alone at the outer edge while larger, gassy planets crowd the inner region. The finding, published Thursday in the journal Science, challenges the long-held assumption that gas giants always form far from their star while rocky planets huddle close.

LHS 1903: The Solar System That Defies Logic

A global team of astronomers led by Thomas Wilson, an astrophysicist at the University of Warwick in England, made the discovery while studying a faint, cool star called LHS 1903. The star is ancient, about seven billion years old, and has roughly half the mass of the sun. At first glance, its arrangement looked familiar. Data from NASA’s Transiting Exoplanet Survey Satellite, or TESS, revealed three planets there. The innermost, LHS 1903 b, is a dense, rocky super-Earth.

Next came planets c and d, both sub-Neptunes with thick, gaseous atmospheres. But the picture shifted when the team used the European Space Agency’s Characterizing Exoplanet Satellite, or CHEOPS, to take a closer look. Parsing through the new data, the astronomers found a fourth planet, LHS 1903 e, lurking at the system’s edge. That alone was not surprising. But what they found next turned expectations upside down.

Naked Rocky Planet Found Far From Home

“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 explained. Instead, the team discovered that LHS 1903 e is a naked, rocky core with no atmosphere at all. The existence of such a world posed a puzzle. Did it once have a thick atmosphere that a giant impact stripped away? Did it form closer to the star and somehow migrate outward?

The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating it formed from gas-depleted material.

To answer those questions, the astronomers proposed a new mechanism called gas-depleted formation. Their hypothesis suggests the planets around LHS 1903 formed sequentially, one after another, starting with the innermost worlds. The inner planets built up early in a resource-rich environment. By the time the outermost planet began to form, the gas had already dissipated. It coalesced, pebble by pebble, from whatever rocky debris remained.

Exoplanet Discovery Illuminates Radius Valley Mystery

“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,” Wilson said. Based on dynamical simulations and the fact that the orbits in the system appear stable, the astronomers ruled out flashier scenarios like collisions or migration.

But they acknowledged that those possibilities remain on the table. Other experts praised the work while calling for more research. Lauren Weiss, an astrophysicist at the University of Notre Dame who was not involved in the study, said the team consisted of top experts who handled the data well. Still, she wanted to see a deeper exploration of the giant-impact scenario.

If the gas-depleted formation hypothesis holds, the discovery could help astronomers understand a long-standing mystery known as the “radius valley.” That term describes a gap in the size distribution of exoplanets that separates smaller rocky worlds from larger gaseous ones. For sunlike stars, the mechanisms behind this gap are well understood. For M-dwarf stars like LHS 1903, the topic has sparked fierce debate.

A New Chapter For Exoplanet Science

LHS 1903 offers a natural laboratory because it contains planets on both sides of that valley. Because all the worlds orbit the same star, variables such as stellar age and metallicity remain constant. That allows astronomers to better constrain how the system formed. Kevin Hardegree-Ullman, an astronomer at the NASA Exoplanet Science Institute who was not involved in the study, said the research opens new insights into planet formation around M-dwarf stars.

Finding more systems like this one, he added, will help refine formation models in the near future. Before searching for similar systems, Wilson wants to explore LHS 1903 further. He pointed to the James Webb Space Telescope as a crucial tool. “It allows us to study how planetary atmospheres are built, which can be a key piece of evidence in their formation,” Wilson said.

For now, the discovery stands as a reminder that nature does not always follow the blueprint laid out in textbooks. The astronomers who found this inside-out system say it may force a rethink of how planets take shape across the galaxy. And as more powerful telescopes turn their gaze toward distant stars, they expect to find even more surprises hiding in the dark.