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The University of Birmingham in the U.K. published this original story on February 12, 2026. Edits by EarthSky.<\/p>\n
In our solar system, the inner planets (Mercury to Mars) are rocky, and the outer planets (Jupiter to Neptune) are gaseous. This pattern \u2013 rock to gas \u2013 has been consistently observed across the Milky Way \u2026 until now. <\/p>\n
Using a European Space Agency telescope, an international research team said on February 12, 2026, that it\u2019s taken a closer look at a star called LHS 1903. This is the furthest observable planetary system still in the Milky Way, at some 116 light-years away from Earth. And the researchers said this system of four planets breaks the standard convention. <\/p>\n
The planets around LHS 1903 \u2013 a cool faint red dwarf star \u2013 begin as expected with a rocky planet orbiting close by and then two gas worlds. ESA\u2019s CHaracterising ExOPlanet Satellite (CHEOPS) then reveals a surprising fourth planet at the system\u2019s outer edge that is rocky, rather than gaseous. <\/p>\n
Co-author Ancy Anna John is a member of the Birmingham research team, which made a key contribution toward confirming the existence and properties of the planets. Reflecting on the discovery, John said: <\/p>\n
\nConfirming the uniqueness of this remarkable system using my specialized analysis pipeline was incredibly exciting. It truly felt like standing at the forefront of scientific discovery. I\u2019m grateful to be part of an international team of outstanding astronomers working together on a discovery that challenges convention.<\/p>\n<\/blockquote>\n
The researchers published their observations on February 12, 2026, in the peer-reviewed journal Science<\/em>. <\/p>\n
EarthSky\u2019s 2026 lunar calendar is available now. Get yours today! Makes a great gift.<\/p>\n
A rocky planet far from its star<\/h3>\n
Traditional models suggest the closest planets to stars are rocky because stellar radiation sweeps away their gaseous atmospheres. This leaves just the dense, solid cores behind. Gas giants form farther out in cooler regions where gas can accumulate and planets can hold onto it. Yet the rocky world orbiting LHS 1903 appears to have either lost its gaseous atmosphere or never formed one. <\/p>\n
Lead author Thomas Wilson from the University of Warwick said: <\/p>\n
\nThis strange disorder makes it a unique inside-out system. Rocky planets don\u2019t usually form far away from their home star, on the outside of the gaseous worlds.<\/p>\n<\/blockquote>\n
How did this inside out planetary system come to be?<\/h3>\n
The research team set out to explore various explanations for the rogue rocky planet. Could the rocky and gaseous planets have swapped places? Had the rocky planet lost its atmosphere in a collision? They ruled out those theories, but found evidence that the four planets did not form at the same time but instead formed one after another. This is a process called inside-out planet formation<\/em>, which could explain the rocky planet. <\/p>\n
Current understanding says that planets form from disks of gas and dust around stars. These disks are called protoplanetary disks. Gas and dust clumps together into planetary embryos at roughly the same time. These clumps evolve into planets of different sizes and compositions over millions of years. <\/p>\n
Co-author Annelies Mortier, from the University of Birmingham, said: <\/p>\n
\nThe most compelling theory to explain the rocky planet says that LHS 1903 gave birth to its four planets one after another from the inner to outermost planet, instead of bearing quadruplets at once. This means that each planet evolved separately, sweeping up nearby dust and gas, with further out worlds waiting their turn for the next planet to form.<\/p>\n<\/blockquote>\n
Much like how younger siblings grow up in a world that is different from their elders, the fourth small rocky planet seems to have evolved in a very different environment than its sibling-planets in an outer area of the protoplanetary disc where gas had already run out. <\/p>\n
An environment depleted of gas<\/h3>\n
Wilson explained what this meant for the rocky planet: <\/p>\n
\nBy the time this final outer planet formed, the system may have already run out of gas, which is considered vital for planet formation. Yet here is a small, rocky world, defying expectations. It seems that we have found first evidence for a planet that formed in a gas-depleted environment.<\/p>\n<\/blockquote>\n
Isabel Rebollido, a research fellow at ESA, said: <\/p>\n
\nHistorically, our planet formation theories are based on what we see and know about our solar system. As we are seeing more different exoplanet systems, we are starting to revisit these theories.<\/p>\n<\/blockquote>\n
Scientists are unsure whether this small rocky outer planet might be an odd exception or the first clue to a new pattern in how planetary systems evolve. The discovery hints at an explanation that lies beyond our typical understanding of how planets form. <\/p>\n
Bottom line: Scientists have found an inside out planetary system. In this system, the planets leading away from their star are rocky, gaseous, gaseous, and rocky. How did the rocky planet get so far out in this system?<\/p>\n
Source: Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903<\/p>\n
Via University of Birmingham<\/p>\n<\/div>\n
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