Astronomers have spotted a planetary system that defies conventional planet-formation ideas, featuring a rocky world that formed outside the orbits of its gaseous neighbors—perhaps after much of the available material for building planets had already been consumed.
This system, observed with the European Space Agency’s Cheops telescope, comprises four planets—two rocky and two gaseous—orbiting a relatively small, dim red dwarf star located about 117 light-years away toward the Lynx constellation.
To recall, a light-year is the distance light travels in a year, about 9.5 trillion kilometers.
The star, named LHS 1903, has about half the mass of the Sun and radiates only around 5 percent of the Sun’s luminosity.
What startled scientists was the arrangement of the planets. The closest planet is rocky, the next two are gaseous, and the outermost planet—where current theories would expect a gas-rich world—turns out to be rocky as well.
“The planet-formation model tells us that planets near their star should be small and rocky, with little or no gas or ice,” explained Thomas Wilson of the University of Warwick in England, the study’s lead author published in Science.
“This environment is too hot to sustain substantial gas or ice, and any atmospheres that do form are likely stripped away by the star’s radiation. In contrast, planets farther out are expected to form in colder regions with abundant gas and ice, leading to gas-rich worlds with thick atmospheres. This system upends that view by giving us a rocky planet in a zone where gas-rich planets would usually dominate,” Wilson said.
He described the system as “an inside-out construction.”
In our own Solar System, the inner four planets are rocky and the outer ones are gaseous. Even the dwarf planets beyond the gas giants, like Pluto, are much smaller than the major planets.
Since the 1990s, astronomers have identified roughly 6,100 exoplanets beyond our Solar System.
All four planets in this newly studied system orbit closer to their star than Mercury does to the Sun. In fact, the farthest planet completes its orbit at roughly 40% of Mercury’s distance from the Sun. This compact arrangement is typical of planets around red dwarfs, which are far less luminous than the Sun.
Both rocky worlds are classified as super-Earths—rocky planets larger than Earth but not exceeding ten times Earth’s mass. The two gaseous planets are called mini-Neptunes, meaning they possess substantial atmospheres but are smaller than Neptune.
The team suspects the system did not form all at once from a single, giant disk of gas and dust. Instead, the planets likely formed in sequence, with the gas that would have formed the fourth planet’s atmosphere being consumed or blocked by its siblings before it could coalesce.
Wilson described the fourth planet as probably a “late bloomer.”
“It formed later than the others in a gas-poor environment. There wasn’t a lot of material left to build it,” Wilson noted.
An alternative scenario is that the fourth planet began with a substantial atmosphere that was later stripped away by a dramatic event, leaving behind only a rocky core.
“We might ask: did the fourth planet arrive just as the gas supply ran out, or did it suffer a collision that removed its atmosphere? The latter is a fanciful idea, but recall that the Earth–Moon system itself is thought to have arisen from a colossal collision,” added Andrew Cameron of the University of St Andrews in Scotland, a co-author of the study.
The fourth planet is especially intriguing from a habitability standpoint. With a mass about 5.8 times that of Earth and a surface temperature near 60°C, it sits near a realm that could be hospitable under certain conditions.
“A 60°C temperature is only slightly higher than the hottest temperature ever recorded on Earth (about 57°C), so it’s conceivable that this planet could support life-friendly environments under the right circumstances,” Wilson explained.
Future observations with the James Webb Space Telescope could probe the planet’s atmosphere and surface conditions, offering clues about its potential habitability.
