Astronomers have observed a planetary system that challenges current planet formation theories, with a rocky planet that formed beyond the orbits of its gaseous neighbors, possibly after much of the planet-forming material had been used up.The system, observed using the European Space Agency’s Cheops space telescope, consists of four planets — two rocky and two gaseous — orbiting a relatively small and dim star called a red dwarf about 117 light-years from Earth in the direction of the Lynx constellation.A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). The star, named LHS 1903, is about 50 per cent as massive and 5 per cent as luminous as our sun. The order of the planets is what caught the attention of scientists.The innermost planet is rocky, the next two are gaseous and the fourth one, which current planetary formation theory suggests should be gaseous, instead is rocky.“The planet-formation paradigm states that planets close to their host star should form small and rocky, with little-to-no gas or ice,” said astronomer Thomas Wilson of the University of Warwick in England, lead author of the study published in the journal Science. “This is because this environment is too hot to maintain substantial gas or ice, and any atmospheres that do form are likely removed via irradiation from their host star. Conversely, 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. This system challenges that by giving us a rocky planet outside of gas-rich planets,” Wilson said. Wilson called it “a system built inside-out”.In our solar system, the four inner planets are rocky and the four outer planets are gaseous. The rocky dwarf planets like Pluto that orbit beyond the gas planets are much smaller than any of the solar system’s planets. Astronomers have detected about 6,100 planets beyond our solar system, called exoplanets, since the 1990s.All four in the newly observed system orbit closer to the star than our solar system’s innermost planet Mercury orbits the sun. In fact, the outermost planet orbits at only about 40 per cent of the orbital distance between Mercury and the sun.This is typical for planets orbiting red dwarf stars that are so much less powerful than the sun. The two rocky planets are categorized as super-Earths, meaning rocky like Earth but two to 10 times more massive.