If you learned in school that small rocky planets sit near the star while the big gassy ones live farther out, you were not wrong. But a newly-studied planetary system around a cool red dwarf called LHS 1903 has turned that classroom rule on its head, with a tiny rocky world found at the outer edge of a four-planet family.
An international team led by astronomer Thomas G. Wilson at University of Warwick reports that this odd lineup is strong evidence that at least one planet formed late, in a region that had already run out of the gas usually needed to build worlds.
Their measurements combine space and ground-based telescopes and appear in the journal Science.
A planetary system that runs “inside out”
The star LHS 1903 is a small, faint red dwarf in the thick disk of the Milky Way, cooler and dimmer than our Sun. Around it, astronomers have found four planets that circle the star in just a few days to about one month.
Closest in, LHS 1903 b is a dense rocky world. Next come two larger planets, LHS 1903 c and LHS 1903 d, which have puffy gas-rich envelopes more like mini Neptunes. The surprise is LHS 1903 e, the most distant planet, which turns out to be small and rocky again instead of another gas-dominated world.
“This strange disorder makes it a unique inside out system,” Dr. Wilson said, noting that rocky planets usually huddle closer to their star.
In everyday terms, it is like finding a compact studio apartment on the outskirts of town, with two large suburban houses tucked between it and the city center. The pattern “rocky gas gas rocky” simply does not match what most models predict.
How planets are supposed to form
In our own solar system, Mercury through Mars are rocky, while Jupiter through Neptune are gaseous giants or ice-rich worlds. That basic trend also shows up in many other planetary systems, where inner planets are heated so strongly that much of their gas is stripped away, leaving solid cores behind.
Farther out, where starlight is weaker and cooler, planets can hold onto thick atmospheres of hydrogen and helium.
Over the past decade, surveys with space telescopes have revealed a “radius valley” in the sizes of small exoplanets, a gap between compact super Earths and slightly larger sub Neptunes that hints at how atmospheres are lost or never gained in the first place.
The LHS 1903 planets sit right across that valley inside a single system, which makes them a kind of natural laboratory for studying how rocky and gas-rich planets diverge.
On paper, the outer planet should have followed the same basic script as its neighbors and ended up with at least a modest gas envelope. Instead, its measured density suggests bare rock. So something about when or where it formed must have been different.
Cheops and TESS track a late bloomer
The planets around LHS 1903 were first spotted through tiny dips in the star’s brightness using NASA’s Transiting Exoplanet Survey Satellite, better known as TESS.
The European satellite Cheops (for CHaracterising ExOPlanet Satellite) then watched repeated crossings of the outer planet to pin down its size with high precision, while ground-based telescopes measured the star’s wobble to estimate each planet’s mass.
Those combined data show that LHS 1903 b is dense and rocky, c and d are lower density planets wrapped in substantial gas, and e is again dense, with no sign of a thick atmosphere. To a large extent, that points to a formation history that was not one single burst.
Instead, the team finds that the planets likely formed one after another, from the inside outward, in a process known as “inside out” planet formation.
By the time the outer planet came together, Dr. Wilson explains, “the system may have already run out of gas, which is considered vital for planet formation.”
In other words, the final world grew up in a very different neighborhood than its older siblings. It is the first strong candidate for a rocky planet that formed directly in a gas-depleted environment rather than losing an atmosphere later.
Why this rocky outsider matters
Is LHS 1903 a one-off curiosity or a clue that many planetary systems build worlds in stages like this? Right now, scientists simply do not know. As Cheops project scientist Maximilian Günther at the European Space Agency put it, “much about how planets form and evolve is still a mystery,” and finding oddballs like this is exactly why the mission was launched.
Future observations will look for similar “backwards” systems around other red dwarf stars to see whether late-forming rocky planets are rare or surprisingly common. If more examples show up, theorists may need to rethink how long gas-rich disks last around low mass stars and how often planets form in several waves instead of all at once.
That is why this one small outer planet carries so much weight for models of worlds far beyond our own.
At the end of the day, discoveries like LHS 1903 remind researchers that our familiar solar system is only one way to build planets, not the universal template.
The next “inside out” system could already be hiding in the data from TESS and Cheops, waiting for someone to notice that its planets refuse to line up the way the textbooks say they should.
The main study has been published in Science.
